Chapter 13 – Regolith of Taurus-Littrow: Ejecta Zones, Source Craters, Ages and Implications

“Something Miraculous Happened over 26 Billion Years Ago[1]. We all are living with or studying its wonder, its order, and its consequences.”

 

Chapter 13

Regolith of Taurus–Littrow: Ejecta Zones, Source Craters,
Ages and Implications

A part of a DTM constructed from four Apollo 17 nadir metric mapping photos from rev. 14 and four from rev. 15 of the Taurus-Littrow environs (north is up). The valley extends from the upper middle towards the lower right. The main features are the small Family Mountain at the valley entrance at left; the Lee-Lincoln Scarp crossing the valley from the South to the North Massifs; the avalanche from the northeast facing slope of the South Massif (horseshoe crab shaped mountain in center); the Crater Cluster in the valley center north of Bear Mountain; the Sculptured Hills mostly in shadow to the right of the North Massif; and also the East Massif casting a large shadow at the right end of the valley. (Total DTM triangulated from NASA photos AS17-M-0445 through M-0448 and AS17-M-0594 through M-0597 by the editor (Wells, 2021, Fig. 3.13c)).

Preface

Comparison of the valley of Taurus-Littrow on the Moon with the Grand Canyon of the Colorado on Earth has been irresistible for over half a century. Since I explored this last site of the Apollo lunar landings, I have used this reference repeatedly.

Recently, Basil Tikoff and Thomas Shipley wrote in awe of the Grand Canyon[2]: “Standing on the rim of the canyon, you viscerally experience the name—it feels big in a way that pictures do not capture. The immensity of absence, which is the Grand Canyon, is conveyed across all of the senses. For instance, the soundscape of the rim is not silence; you can hear the wind. Yet the sound quality is unfamiliar; absent are the common echoes from the nearby surfaces. The space of the canyon is so immense that echoes disappear. The Grand Canyon’s ubiquity in geology and popular culture reflects in part, the clarity with which stories of time are written in its space.”

If Tikoff’s and Shipley’s insights about the Grand Canyon were applied to Taurus-Littrow on the Moon, it might be said: “Standing on the floor of the valley of Taurus-Littrow, your brain must assimilate what it knows about size without references to the familiar and without sound other than the life-preserving movement of oxygen in the fragile envelope around you. Brilliant massif walls, illuminated by a morning Sun, silhouetted in reverse against a blacker than black sky, rising above a chasm deeper than the Grand Canyon, and watched over by the seemingly stationary blue and white Earth above the highest of the confining walls. The valley’s geological record of time precedes that of even the magnificent strata of the Grand Canyon, as far into the past as that natural wonder takes us, but its pages of the history of our solar system are far more subtle , but nonetheless reflect the order imposed by Creation.”

The presence of an experienced field geologist as Lunar Module Pilot on Apollo 17’s mission to explore the valley of Taurus-Littrow, as well as over 53 years of reports on analytical studies from the returned samples and geophysical data, provide a unique opportunity for integrated scientific synthesis. Large scale reviews of the vast stores of observational, photographic, petrographic and analytical data for the areas of Apollo exploration have not occurred over the 40 to 50 years since these data were published, documented in “gray” unpublished volumes and abstracts, or burned in to explorers’ memories.

Beginning in the 1950s, funding for lunar science has almost entirely been the purview of the National Aeronautics and Space Administration (NASA) and the National Science Foundation (NSF), with earlier contributions by the Geological Survey and various academic institutions. The understandable tendency of scientists to focus on narrow, definable questions as well as on the equally narrow interests and results-oriented biases of federal agencies, however, almost never have encouraged proposals to undertake broad integration and correlation of the full range of the resulting vast reservoir of available information. Small exceptions to this existing paradigm of closely bounded research efforts have been NASA’s funding of early “consortium” studies of specific mission sample suites and the recent Apollo Next Generation Sample Analysis (ANGSA) program. The latter, however, still largely concentrated on one Apollo 17 drive tube core sample.

Over the last 15 years or so, the author has attempted the synthesis of the observations, imagery and samples related specifically to his Apollo 17 exploration of the valley of Taurus-Littrow. Such research is a dynamic and, of course, a continuous process. As the project progressed, new insights required sequential revisions of previous approaches to portions of the synthesis that, in turn, required alternative approaches to be considered. Eventually, an internally consistent picture of the geology and history of the Taurus-Littrow valley emerged. The iterative nature of the synthesis of many data sets results in early sections of this work referring to later sections so that the reader is aware of the details that led to various integrated findings that supplanted earlier, more tentative results.

The ~50 km long Taurus-Littrow, fault-bounded valley radially penetrates the outer ring of uplifted and augmented massifs that encircle the 740 km diameter, Serenitatis Basin. Taurus-Littrow’s varied, 1600 to 2200 m high valley walls of impact melt and fragmental breccias constitute ejecta from the Crisium, Serenetatis, and Imbrium basin-forming events. Lithoclastic ash, basaltic lava, pyroclastic ash, and subsequent impact-derived debris that partially fill the valley and covered earlier features provide insights into a variety of eruptive, impact and mass wasting activity. The first of such activity closely followed deposition of ejecta from the Imbrium Basin and additionally provide insights into the nature and origin of materials in the lunar interior.

The synthesis of these data has been greatly influenced by early studies that relate to the upper-most 938 million years of a 3.8 billion year accumulation of regolith ejecta that comprise the 294 cm, Apollo 17 deep drill core. Isotopic and maturation data from the core and other samples indicate that the average energy of the solar wind increased about 512 million years ago and then increased again about 200 million years ago, this latter increase by over a factor of ~3.4. A spectrum of impacts, avalanches, debris flows, boulder shadowing, and thrust faults that occurred subsequent to valley volcanism add additional perspectives into recent lunar and solar history and provide samples from which values for the rates of aging of exposed debris (regolith) have been derived. These rates of aging, in turn, allow estimates for the ages of 10 regolith ejecta source craters located throughout the valley.

There is great and unique value of a far-ranging synthesis of data related to each Apollo landing site in increasing understanding of the geological history of specific sites as well as the internal, solar and other dynamic factors that recorded that history.

Harrison H. Schmitt

Apollo 17 Lunar Module Pilot and Field Geologist

Major Findings:

      1. The Apollo 17, 294 cm long, deep drill core sample of Taurus-Littrow regolith (70001/9; see Fig. 13.5↓ for location) is comprised of 11 strata or zones of regolith ejecta derived from ten 400-800 m diameter source craters and one 1400 m source crater, the ages of which span 938 million years of lunar history and represent the upper portion of ~11 m of regolith ejecta deposited on ilmenite basalt, the youngest eruption of which was at ~3.795 Ga.
      2. Growth of exposure-related maturity indices (∆Is/FeO), in the core’s regolith ejecta zones, due to the reduction of structural Fe++ to Feº, is the result of (1) alpha+beta particles from the decay of uranium and thorium (∆Is/FeO / Myr = 0.58 per ppm U+Th); and (2) surface exposure to solar protons (∆Is/FeO / Myr = 0.11 for young regolith and 0.032 for old regolith).
      3. Exposure ages and deposition ages for the deep drill core’s regolith ejecta zones can be calculated from half-cm logging of a zone’s ∆Is/FeO divided by the zone’s total ∆Is/FeO / Myr.
      4. Half-cm logging of the deep drill core, comparison of core zone cosmic ray exposure ages with ∆Is/FeO-based exposure ages, and Is/FeO relationships at Shorty and Van Serg Craters show that impact shock partially resets the “Is” component of maturity indices, ranging from a few percent for small impacts that average one every ~5.7 Myr near the core to over 90% for zone source craters that average one every ~94 Myr.
      5. Relationships between the core’s regolith ejecta zones’ ∆Is/FeO and δ15N‰, and ancient regolith δ15N‰ values <210‰ show: (1) a sharp change in δ15N‰ from ≤210‰ to about –118‰ at ~512 Myr ago; (2) a lunar δ15N‰ accretionary value of ~+80‰; and (3) a factor of ∼3.4 increase in solar wind energy at ~199 Myr ago, with the increase in solar 15N production at ~512 Myr, within probable error limits, being related to warming associated with the Cambrian Explosion of oceanic life on Earth at ~538 Myr.
      6. The absence of Is/FeO values >2 in glassy pyroclastic ash in core 74001/2, particularly values that would result from alpha+beta particles, and the attenuation of Is/FeO growth in deep drill core zones as a function of ash content, indicates that a very small separation of Fe++ from uranium and thorium and from access by solar protons prevents Feº formation.
      7. Positive values for δ15N‰ (+13‰) in buried pyroclastic ash of core 74001/2, versus a current solar wind value of ~118‰ indicate a chondritic lower mantle source for the ash magma and volatiles and suggest a lunar origin by accretion and capture rather than from a giant impact on the Earth.
      8. The young and old light mantle avalanche deposits are two of at least seven mass wasting deposits from the slopes of the Taurus-Littrow massifs and have Is/FeO-based exposure ages, respectively, somewhere between 27-32 Myr and 10-15 Myr and deposition ages, respectively, somewhere between of 27-32 Myr and 37-47 Myr with a deposition age of ~4 Myr for the ancient light mantle.
      9. Mass wasting activity from the slopes of the South and North Massifs and from a peak in the Sculptured Hills is likely the consequence of repeated thrust faults along the Lee-Lincoln Fault caused by continued contraction of the gradually cooling Moon.
      10. Comparison of North and South Massif regolith samples from the valley of Taurus-Littrow indicate that volatile-rich, lithoclastic volcanic eruptions preceded those that produced the ~3.795 Ga mare basalts.
      11. Symplectitic partial coronas in troctolite 76535 between plagioclase and olivine consisting of retrograde symplectitic textures made up of Cr-spinel intergrown with clineoproxene next to plagioclase and with orthopyroxene next to olivine have replaced prograde Cr-rich garnet and indicate this sample (and crushed dunite 72415) originated below ~400 km in the partially differentiated magma ocean as a result of overturn triggered by the Procellarum basin-forming impact at ~4.35 Ga that also resulted in the partial melting of the warm upper mantle to produce Mg-suite plutons.
      12. The synthesis of data from Taurus-Littrow indicates that the Imbrium Basin forming impact occurred between 3.850 and 3.795 Ga.
      13. The addition of water to the early impact-generated regoliths of Earth and Mars would create smectic clay mineral species (phyllosilicates) whose adaptive silicate sheet crystal structures could serve as templates for the organization and stabilization of complex, pre-biotic and biotic organic molecules.

1.0 Introduction

Any discussion of the science of the lunar regolith must acknowledge the foundational observations and interpretations made by Gene Shoemaker (Shoemaker, 1965; Shoemaker et al., 1967), based on his early studies and on the Surveyor III television observations of its landing site in Mare Cognitum. Shoemaker’s insights were followed by the seminal work of Larry Taylor that advanced understanding of the lunar regolith sampled by the Apollo astronauts. Taylor’s and his associates’ vast contributions to deciphering the character and formation of lunar regolith are without peer (e.g., Taylor, 1988; Carrier et al., 1991; Pieters, Taylor and Noble, 2000?; Taylor et al., 2001a, 2001b; Pieters et al., 2010).

Contributing to the studies by Taylor and others, the Apollo 17 mission to the valley of Taurus-Littrow (Schmitt and Cernan, 1972a; Schmitt, 1973, 2021a; Wolfe et al., 1981; Meyer, 2012) provided a uniquely broad and diverse suite of regolith samples and the massif and volcanic components contained within them. Review, integration and synthesis of the geologic context of this sample suite and published analytical research continue to give special insights into the processes of regolith formation as well as indirect clues related to other important issues of lunar geology and solar and Earth history. Of particular importance to the development of various regolith-related hypotheses have been the maturity indexes (Is/FeO) conceived of and measured by Richard Morris (1978, 1979, 1989); the nitrogen isotopic measurements by Robert Clayton, Mark Thiemens, and Richard Becker (1975, 1977, 1980); and John Ketterige (1991); cosmic ray exposure ages by Peter Eberhardt (1974) and Eugster (1977, 1979, 1981, 1985); uranium and thorium analyses by Lee Silver (1974); and, of course, the wonderful Lunar Sample Compendium of Charles Meyer (2012). These pioneers were all assisted by innumerable colleagues of the Apollo lunar science generation.

The Lunar Source Book (https://www.nasa.gov/history/alsj/) and the Lunar Sample Compendium (https://curator.jsc.nasa.gov/lunar/lsc/index.cfm) produced by Grant Heiken, David Vaniman, and Bevan French (1991) and Charles Meyer (2012), respectively, and the Professional Papers on the geology of individual missions published by the Geological Survey are invaluable sources of compiled data and references from a nearly infinite number of sources. Most recently, the Apollo Next Generation Sample Analysis (ANGSA) program has added significant data for integration into studies of avalanche deposits (Shearer et al., 2022, 2024) and Thermo-luminescence of shadowed samples (Shelke and Sears, 2022; Sears et al., 2024).

The analytical data are bolstered by the petrographic observations of Grant Heiken and David McKay (1974), Dave Vaniman and Jim Papike (1979), Jeffrey Taylor and his colleagues (1979), and John Nagel (1979), along with the author’s field observations during the Apollo 17 mission (Schmitt, 2023a, Chapters 10-12; 2024a,b,c). Equally valuable to this synthesis is Eric Jones’s Apollo 17 Apollo Lunar Surface Journal with its crew annotated transcripts and referenced Hasselblad camera images (https://history. nasa.gov/alsj/a17/a17.html) and Ed Wolfe and his colleagues’ integration of early Apollo 17 geological interpretations with sample locations and other data.

1.1 General Nature of Lunar Regolith

Particles that make up the Taurus-Littrow regolith (Shoemaker, 1965, 1967; Heiken and McKay, 1974; Pieters et al., 2010) consist of varying proportions of mineral and rock fragments, regolith breccia fragments, volcanic ash beads and shards, and abundant impact generated, vesicular glassy agglutinates containing some or all these other particles. The mineral and rock fragments and bulk compositions of given regolith areas tend to reflect mixtures of regolith developed on near-by bedrock units. Other than spherical beads of volcanic ash and some transported impact glass, almost all particles are irregular in shape with vesicular glassy agglutinates having largely fractal outlines. Particle sizes range from hand-specimen size and larger to less than a micron with up to fifty percent by weight being less than about 70 microns, and they generally show an approximate bell-shaped size-frequency histogram (Graf, 1993), although variations in histograms shape and peak sizes are very useful in some interpretations. Observationally (Schmitt, 2023a, Chapters 10-12; 2024a,b,c), rock fragments >2 cm in diameter tend to be <5% of the total surface area of inter crater areas (Schmitt, 1972, 2023a, 2024a,b,c; Meyer, 2012, sample 70001/9; Hasselblad images).

Broadly distributed, ancient silicate-dominated surface regolith on Earth and apparently on Mars, likely played a role in the origin of complex organic molecules and their organization into replicating life forms (Schmitt, 2015). Once stable and relatively cool planetary rusts had differentiated from an accretionary magma ocean and stabilized in an impact-dominated and water-rich environment, clay minerals (phyllosilicates), particularly of the smectite group, would have been the dominant mineral species at the Earth’s surface. The characteristic of smectitic clays to incorporate many elements and molecules into their hydrous sheet-like crystal structure offered the potential of organizing organic bases and essential elements into increasingly complex forms.

High-energy solar wind ions and micro-meteors impacting the Moon apparently erode plasmas from exposed particle surfaces that re-deposit on local mineral and glass surfaces (Taylor, 1988; Taylor et al., 1994; Burgess and Stroud, 2018; Cymes et al., 2022a; Cymes et al, 2022b; Li, 2022). Protons (H+) in the average solar wind have energies of a few Mev and abundances of about (6 ions/cm3. Protons from solar flares and CMEs, however, arrive at the Moon with Gev energies and abundances of 100s of ions/cm3 (Reames, 2017; Oldensald, 2004) and may dominate the plasma-forming process. The resulting re-deposition of plasma material from this so-called “sputtering” process, probably with some contribution from micro-meteor impact plasmas, produces a nearly ubiquitous but discontinuous, ~50 µ thick patina of alumino-silicate glass patina on regolith particles and rock surfaces. The glass present in the pits formed by micro-meteor impacts on rock surfaces suggests a contribution of impact plasmas to these patinas.

Alpha and beta particles from internal uranium and thorium decay, with minor contribution from solar proton radiation, reduces Fe++ to Feº (npºFe) (Morris et al, 1975; Taylor, 1988). Impact gardening gradually exposes new regolith particles to this surface-specific process and mixes previously exposed particles throughout the zone. The titanium-bearing mineral ilmenite, FeTiO3, appears to be the preferred host for this concentration of nano-phase iron (Cameron, 1990; Taylor et al., 1993). Solar wind volatiles may be concentrated in 5-50 µ diameter vesicles as well as being present in crystal defects and cracks (see Taylor, 1988; Taylor et al., 2005; Burgess and Stroud, 2018; Cymes et al., 2022a; Cymes et al, 2022b; Ao, 2022).

Nano-phase iron production occurs throughout a regolith zone due to ionizing alpha particles, and probably by associated beta radiation, that comes from internal uranium and thorium decay. This source of nano-phase iron has been neglected in past maturation studies; however, Bower et al.’s (2016) analysis of pleochroic halos around uranium- and thorium-bearing zircons in biotite found that radiation from uranium and thorium decay also converts structural Fe++ in biotite to Feº. Alpha+beta radiation, therefore, must be taken into account in the interpretation of variations in regolith maturation, as uranium and thorium exist in amounts of 1-4 ppm in Taurus-Littrow regolith samples. Extremely fine fragments of minerals and glass containing both elements have been broadly disseminated during long-term impact gardening of lunar regolith. Additionally, earlier in lunar history, the concentrations of these continuously decaying elements was significantly greater than today.

2.0 Error Limits Related to Taurus-Littrow Data Synthesis

Much of the following synthesis of over 55 years of data on the Taurus-Littrow regolith involves the integration of different types of measurements (maturity indices, radio-isotopic and cosmic ray ages, petrographic and chemical contents, etc.) each of which have varying estimates of analytical error limits, if any are reported. Accumulated error limits resulting from various calculations, therefore, can grow to numbers that suggest very large uncertainties. On the other hand, the synthesis itself includes various direct and indirect tests on the reasonableness of the results of calculations, as well as, in some cases, confirmation of those results by independent approaches or by their compatibility with clear constraints imposed by the overall geology of the area, or of the Moon, or of the history of the Earth and Solar System. As a consequence, when available, and unless otherwise noted, the error limits on original published data are quoted, but the accumulated hypothetical error limits inherent in various calculations generally are not considered except in terms of the reasonableness of the results.

In this context, the underlying, measurable parameter for initially evaluating the formational history of the lunar regolith consists of the Is/FeO maturity index for returned regolith samples developed and measured by Richard Morris (1976, 1978, 1979, 1989). This objective, maturity data consists of measuring the intensity of the ferromagnetic resonance (Is) of a sample and then normalizing it for comparison with other samples by dividing by each sample’s FeO content. The author of this current narrative originally considered the Is of a sample basically to reflect the amount of nano-phase iron (np°Fe) produced by solar wind proton reduction of regolith Fe++ (Pieters et al., 2000) in proportion to the samples FeO content. It was eventually realized that alpha+beta particle radiation from the decay of uranium and thorium contributes to the formation of np°Fe (Bower et al., 2016).

Morris (1978) states that the precision of the Is measurement used in the Is/FeO determinations is ±5%. As the error limits of FeO determinations used by Morris (usually from R. Warner, unpublished) are not available, measured Is/FeO values used throughout this paper will be assumed to be accurate to be within 5%, that is, Is/FeO = 20 would be ± 1 and Is/FeO = 80 would be ± 4. This assumption implies that inaccuracies in analytical determination of values for FeO are included in the 5% stated by Morris and, nonetheless, would be small enough not to affect the inherent value of Is/FeO as a synthesis tool. On the other hand, consistency in the results of the following synthesis suggest that the precision of Morris’s combined Is and FeO measurements is significantly better than ±5%, possibly about  ±2% or even better.

3.0 Apollo 17 Deep Drill Core

3.1 Introduction

The Apollo 17, ~3 m deep drill core (70001/9; Fig. 13.5 for location) (Baldwin, 1973; Meyer, 2012) constitutes the most complete vertical sample of the lunar regolith obtained to date (Meyer, 2012; Heiken et al., 1991). Synthesis of published analytical data provides new insights into the formational, depositional and exposure history of regolith sampled by this core. Of particular importance are the investigations of maturity indices (Morris et al., 1979); petrographic characteristics (Vaniman and Papike, 1977; Vaniman et al., 1979; Taylor et. al., 1979; Heiken et. al., 1991); bulk compositions (Laul et al., 1978, 1979, 1981), volatile element concentrations (Laul and Papike, 1980), nitrogen isotopic measurements (Thiemans and Clayton, 1980), cosmic ray exposure age determinations (Eberhardt et al., 1974), and uranium and thorium concentrations (Silver, 1974; Laul et al., 1978, 1979, 1981; compilations by Meyer, 2012).

The in situ inter-crater regolith sampled by the Apollo 17 deep drill core is made up of eleven definable zones of regolith ejecta from 400-800 m diameter craters and one 1400 m diameter crater within about 6 km of the core site (Schmitt (2019, 2021, 2023a)). Such zones superficially resemble each other, but they show significant differences in detailed maturity (Morris et al., 1979), lithology and mineralogy (Taylor et al. (1979)), and bulk chemical compositions (Laul et al. (1978, 1979, 1981); Meyer (2012); Silver (1974)). These differences, in turn, reflect the compositions and, in part, the pre-excavation maturities and cosmic ray exposure ages (Eberhardt et al.  (1974)) of the source regoliths.

The 11 distinct regolith ejecta zones comprise the ~294 cm of material in the deep drill core and record, directly or indirectly, a significant portion of the cratering, regolith maturation, and volcanic history of Taurus-Littrow. These zones also contribute to understanding the distribution of geological units and sequence of impact cratering events that created a corresponding 14 potential source craters, 400-1400 m in diameter, that lie within about 6 km of the core site. The maturity of these regolith ejecta zones due to space weathering and internal alpha+beta particle radiation, as well as associated cosmic ray exposure age data, further allow the approximate dating of these large impact events (§6.0). Certain aspects of the history of the solar wind are also recorded in the core.

Much of the following synthesis relies on analyses and observations of portions of the Apollo 17 deep drill core conducted by many researchers; however, the exact length of the returned core, and thus the precise depth of analyses are not absolutely certain within a few centimeters. For example, Morris et al.’s (1979) maturity index data measured at half-cm intervals are available from –1.30 to 285.85 cm (Morris, pers. comm.), while Silver’s (1974) uranium, thorium and lead analyses and Eberhardt et al.’s (1974) cosmic ray exposure ages are reported up to 290 cm, and Meyer’s (2012) compilation of element analyses goes to 294 cm. To complicate matters further, Meyer (2012) states that when the core was opened, there was a “10-12 cm void” at the base of the upper pair of returned drill stems, that is, where core sample 70008 is now defined. This “void” possibly relates to Astronaut Cernan’s statement, made while the core was still in the ground, that the top, stabilizing “plug” for that section ultimately moved about two-thirds of the length of the compacting rammer. This operation compressed the less compact upper portion of cored regolith. A 10-12 cm thickness of low-density regolith at the surface of inter-crater areas is consistent with direct observations during Apollo 17 EVAs.

Top core section, 70009, is listed in Mitchell et al. (1973) as having a length of 25 ± 2 cm, about 15 cm less than the 40 cm of a full section. As the rammer was about 50 cm long, Cernan’s statement probably meant that “two-thirds” of the rammer was still showing outside the core section for his statement to match Mitchell et al.’s information. In light of the above, and, as Mitchell et al. measured the total “drill stem depth” (not core length) at 305 ± 1 cm with 95-97% recovery, the total length of “core” will be assumed here to be ~294 cm, a length which also matches the Meyer report. Morris (pers. comm.; Morris et al., 2022) records 284.65 cm as the depth of the last Is/FeO measurement given in Morris et al. (1979), but also says that the first measurement was at a depth of “minus 1.30 cm.” These figures suggest that there is ~7 cm of core at the bottom for which Is/FeO data was not obtained.

The following probably contributed to the “10-12 cm void” reported by Meyer:

    1. It is certain that the loosely consolidated upper 10-15 cm of the regolith compacted upon insertion and “ramming” of a plug on top of the core in the first drill stem section before exctraction of the drill stem extraction from the regolith.
    2. During handling and transport to the Lunar Receiving Laboratory in Houston, the core in the first of two drill stem as sections, as well as the top plug, slid away (upward) from the bottom cap.

As will be detailed below (§5), the chemical composition of three samples from the upper 20 cm of the compacted core (Laul et al., 1978) and the composition of nearby control sample 70181 are almost identical, indicating that the control sample gives a good representation of the elemental and mineralogical makeup of the top portion of the core.

3.2 Deep Drill Core Maturity Indices and Regolith Ejecta Zones

Morris et al. (1979) have reported, graphically, the maturity indices (Is/FeO) for nearly the full length of the Apollo 17 deep drill core (Fig. 13.1a). These Is/FeO values reflect the degree of reduction of Fe++ to Feo by internal alpha+beta and surface solar proton particle radiation. They have been derived from the ferro-magnetic response of nano-phase iron (npoFe) particles in regolith samples, normalized to reflect the varying FeO content of the measured regolith samples. Morris et al.’s data plotted in Fig. 13.1a, with a representative example of the raw, detailed, half-cm resolution data set (Morris, pers. comm.) in Fig. 13.1b, indicates the existence of zones of regolith with significantly different degrees of exposure to the internal and external radiation as well as having distinct petrographic characteristics after comparison with other data. This conclusion is supported less precisely in Fig. 13.2 by agglutinate content (Taylor et al., 1979) and “cosmic ray track maturity” (Goswami and Lai, 1979). Of particular importance, however, are the variations in Is/FeO measured as a function of depth that provide data on relative age based on superposition.


Fig. 13.1a. Plot of Is/FeO maturity indices and FeO content in the Apollo 17 deep drill core (Morris et al., 1979) with a rough indication of the identified regolith ejecta zones shown in the column on the right.

Fig. 13.1b. Representative portion of the half-cm resolution, raw data set (Morris, pers. comm.) related to the deep drill core’s Is/FeO values graphically represented in Fig. 13.1a. Column 6 indicates zone decreases and increases of Is/FeO noted during logging of this data set.

Fig. 13.2.  Data on the maturity of the Apollo 17 deep drill core comparing Is/FeO maturity indices, left column (Morris et al., 1979); cosmic ray track density, center column (Goswani and Lai, 1979); and agglutinate content, right column (Taylor et al., 1979). Figure is after Heiken et al., (1972). Approximate regolith ejecta zones S-Z** are largely based on maturity data shown in Fig. 13.1a as verified by logging of half-cm raw data (Fig. 13.1b).

Table 13.1a provides the depth intervals for defined regolith ejecta zones (Column 1) and that are used in the subsequent synthesis of Taurus-Littrow geological, petrographical and analytical data. “Is” is a measure of ferromagnetic resonance and thus of the content of nano-phase iron produced through reduction of Fe++, that is, by (1) solar wind weathering, largely by energetic solar protons (Pieters et al. (2000)), and (2) internal alpha+beta particle radiation (Schmitt (2022)). Micro- and macro-meteor impacts are contributing factors as they continuously garden and mix exposed regolith ejecta as well as produce complementary agglutinates. Sorting out the quantitative effects and implications of these factors is a significant focus of this synthesis.

In Table 13.1a and in subsequent discussions, Is/FeOm and ∆Is/FeO refer to the estimated deposition value of Is/FeO of source regolith, as suggested by the half-cm logging, and the post-deposition accumulation of Is/FeO, respectively. As will be discussed below, the actual pre-deposition total Is/FeO of an integrated source regolith is not given by the value of Is/FeOm  in the table, as there is strong evidence that the shock and/or heat of impact partially resets the ferromagnetic component of the near-impact nano-phase iron on which the “Is” component of Is/FeO is based. The extent to which impact reset has affected the logged, estimated Is/FeOm given in Column 3 of Table 13.1a will be examined in §8.0.

With respect to zone S, it appears that the significantly higher velocity regolith ejecta from the large MOCR Crater (§3.4) that constitutes zone S resulted in more mixing with the previously deposited zone T+T* than at other zone boundaries. This mixing zone is about 4 cm thick (22.35 to 18.35 cm) and includes an increase in ∆Is/FeO of ~47 points that is not the consequence of maturation after zone S deposition. The logged post-deposition ∆Is/FeO of zone S is 46, confirmed by the reported ∆Is/FeO = 47 of control sample 70181 (Morris, et al. 1978).

[In the course of this synthesis, the definitions of the various zones have varied. Initially, those definitions were based purely on broad changes in Is/FeO shown in Fig. 13.1b, however, correlations with various other associated data, the half-cm logging of Morris’s detailed data, and improved understanding of source regoliths indicated the need for improvements in these zone definitions. This process first suggested several subzones and then, in some cases, indicated recombination into the original set. This final process is reflected in zones labeled T+T*, V+V*, Y+Y* and Z+Z* in various tables and figures. Subzones W*, X* and Z** proved to be coherent stratigraphic units in their own right, but the asterisk (*) designations have been retained for them.]

Table 13.1a footnotes:


Close examination of the plotted points in Fig. 13.1a shows that the saw tooth patterns present along the length of the core have Is/FeO minima that are roughly constant for various sections of the core. The existence of these minima is best shown in the core sections between 115 and 170 cm. These locally constant minima are probably close to the original deposition maturity index (Is/FeOm). The precise deposition Is/FeOm values are uncertain within about ± 5 in most cases.

In defining stratigraphic zones based on Is/FeO values in the deep drill core, a large, sharp decease or increase in background Is/FeOm is interpreted as the boundary between zone depositions. These defined boundaries are generally supported by petrographic and compositional data as will be discussed in §5. The sum of the positive magnitudes of the small saw tooth spikes, showing the total increase in Is/FeO over Is/FeOm throughout a given zone (∆Is/FeO), is a measure of the post-deposition maturation of the zone before burial by a younger zone.

Table 13.1a gives the regolith ejecta zones identified by examination of Fig. 13.1a, as refined by subsequent analysis that resolved a number of inconsistencies with other data. After preliminary definition, the indicated depths of zones were determined within half a centimeter with reference to the raw Is/FeO data provided by Richard Morris (pers. comm.).

Table 13.1b compares the zone definitions in Table 13.1a with “unit” classifications given by Nagle and Waltz (1979), Vaniman et al. (1979), and Taylor et al. (1979). Although these latter three stratigraphic classifications and their associated descriptions do not line up exactly with the detail in Table 13.1a, they are generally consistent relative to evidence of maturity and source regolith characterization. Nagle and Waltz’s boundaries between zones S and T+T*, V+V* and W, W* X and X* in their sketch of the deep drill core are particularly consistent with the divisions in Table 13.1a Some relevant comparison data have been highlighted in bold.

Table 13.1b footnotes:

3.2.1 Agglutinate Formation in Relation to Is/FeO Maturity Index

Vesicular “agglutinates” composed of mineral and rock particles in a glass matrix accumulate over time at the surface of lunar regolith. These agglutinates are the result of local impact melting and mixing. Their abundance is a rough measure of the relative exposure of a given regolith unit to the geologically steady impact of macro- and micro-meteors. The pattern of abundance of agglutinates (Taylor et al., 1979) in the deep drill core are shown in Fig. 13.3a.

Fig. 13.3a. Graph of percentage variations in agglutinates as measured by modal analysis of 171 thin sections from the deep drill core (Taylor et al., 1979).

As deep drill core values of total Is/FeO (Morris et al., 1979) and total modal-% agglutinates (Taylor et al., 1979) rise with age, a correlation between these two parameters would be expected. Fig. 13.3b gives a visual comparison of this relationship. The indicated graphical relationships zone by zone are given in Table 13.2a with reference to total Is/FeO from Table 13.1a, maximum modal-% agglutinate peak; and the ratio of these two parameters. The ratio of total Is/FeO to maximum agglutinates averages 4.95 ± ~1.7 (lowest and highest ratios not included), verifying the close and constant relationship between nanophase iron production and agglutinate formation.

It would be expected that this relationship would carry over to the ratio between ∆Is/FeO and ∆Agglutinate. Values of ∆Agglutinate shown in Column 4 of Table 13.2b were determined by summing the measured increases with decreasing depth for each zone shown in Fig. 13.3a. Dividing ∆Is/FeO values by ∆Agglutinate values (Table 13.2b) gives a significantly higher ∆Is/FeO to ∆Agglutinate ratio (average of 7.49) than the 4.95 average for total Is/FeO to maximum agglutinate. This discrepancy is likely due to the low resolution of measurements of changes in modal-% of agglutinates, that is, some increases were missed. This conclusion is supported by the fact that the ratios for total Is/FeO to total Agglutinate are generally less than those for ∆Is/FeO to ∆Agglutinate. From personal experience, microscopic “point counting” to determine modal-% of particles is an art of judgment more than of science.

Fig. 13.3b. Comparison of total Is/FeO (Morris et al., 1979) and modal-% agglutinates (Taylor et al., 1979). Red dash lines indicate the zone widths in Table 13.1a.

3.3 Regolith Transport after Large Impacts in Lunar Gravity and Vacuum

Impacts have dominated the evolution of the Moon’s surface for much of its history (Wilhelms, 1987) from primordial accretion, to the creation of the magma ocean, to the creation of the mega-regolith that now constitutes the lunar crust, to the excavation of the large basins, to the evolution of today’s surface regolith since the formation of a region’s last large impact basin. As craters on the floor of the valley of Taurus-Littrow are, at least in large part, underlain by ilmenite basalt, regolith ejecta zones identified in the deep drill core have their origins in ejecta from impact craters formed since regolith began to form and accumulate on mare basalt erupted about 3.82 Ga (39-40Ar, Schaeffer et al. (1977)).

[Further consideration of the potential closure temperatures for the various isotopic systems suggests that the actual age of the youngest lavas would be close to the 39-40Ar age of rapidly cooled, fine-grained samples. One such sample, 70215, has been dated (using old 40K decay constants) by the 39-40Ar method at 3.82 ± 0.05 Ga. (Schaeffer et al., 1977). In this chapter, this date will be used as representative of the most recent age of the ilmenite basalt lava eruption.

As a means to further tighten the chronology of eruptive activity in Taurus-Littrow, it would be useful to precisely date by 39-40Ar methods other rapidly cooled ilmenite basalt samples from the Apollo 17 collection (e. g., 70075, 71537, 74235, 78586, and 78587). A close examination of un-numbered basalt fragments in the rake samples from Stations 1, 2, 6, and 8 also may include very fine-grained, rapidly cooled ilmenite basalts suitable for dating.

The recently reported revision of the 40K decay constant (Renee et al. (2010)); Noumenko-Dezes et al. (2018)); Carter et al. (2020)) would lower previously reported 39-40Ar dates by about 0.025 Ga (Schmitt et al. (2017)). However, in order to incorporate as many previously dated samples in this synthesis on a consistent basis, revisions based on the new constant will not be made.]

The regolith zones from the Taurus-Littrow valley floor that are represented in the deep drill core 70001/9 are distinctive because of the presence of detectable regolith contributions from five diverse geologic units related to the massifs surrounding the valley and volcanic ash deposits that contrast with the low SiO2 and low U+Th ilmenite basalt regolith. Those major units are the orange+black low SiO2 volcanic ash, Very Low Titanium (VLT) and high SiO2 volcanic ash, Sculptured Hills Mg-rich suite regolith, North Massif medium SiO2 and U+Th impact breccias, and South Massif high SiO2 and high U+Th impact breccias (§5.0). If new insights are to be gained about the source areas of these regolith zones, the geological challenge is to tie specific regolith zones to specific craters in the Taurus-Littrow valley.

An LROC image-based temporal study of new lunar craters in regolith (Speyerer et al., 2016), indicated that regolith at the sites of recent impacts has been ejected many crater diameters from the impact points and deposited continuously along their trajectory. In contrast, examination of ejecta from fresh craters in lunar basalt of comparable size to the 744 m diameter Camelot Crater (Schmitt et al., 2017) indicates that, much as on Earth, ejected bedrock, extends only about a crater diameter from the crater rim. Thus, it is very probable that significant amounts of regolith reached the deep drill core location after being ejected within a few micro-seconds of the instant of impact at the sites of large craters in the valley of Taurus-Littrow.

Support for this conclusion also is found in Oberbeck’s (1975) experiments on “hypervelocity impacts against a quartz sand layer resting on an ‘indurated’ quartz sand substrate.” Based on the Oberbeck’s and Speyerer et al.’s observations, and the numerous regolith zones identified in the Apollo 17 deep drill core, the following inferences about the impacts that created the 400 m diameter and larger craters in Taurus-Littrow valley appear to be in order:

    • The geotechnical differences are profound between surface regolith and underlying blocky and solid bedrock, and result in a sharp discontinuity in reactions to impact shock and resulting excavation.
    • At the instant of most impacts and during the subsequent few micro-seconds, the transfer of kinetic energy into overlying regolith creates a dense sheath of particles that are accelerated into an inverted, ~45º, continuously expanding, “conical wedge” (Fig 13.4a) (see also Oberbeck, 1975, Fig. 24). (Less common, low angle oblique impacts appear to bias the shape of the sheath in the direction of impactor flight.)

Fig. 13.4a. Schematic cross-section of a laterally expanding, inverted, conical wedge of ejected regolith formed micro-seconds before a reflected shock wave fully excavates a lunar impact crater.

    • The initial conical wedge containing ejected regolith will have its unit mass and velocity of contained particles decreasing away from their source as a function of the radially outwardly decreasing kinetic energy released by the impact and the original density and effective depth of regolith in the impact area.
    • A proportionally large amount of regolith near the immediate impact point will be accelerated to the highest velocities in the conical wedge. Initial regolith particle velocities, however, are rapidly reduced as the lateral shock wave encounters more and more regolith mass until expansion of the initially flat, nearly cylindrical crater in the regolith ceases to expand due to the lateral resistance of the material encountered.
    • As lateral kinetic shock energy dissipates and the transient flat crater diameter increases, more and more mass of regolith is exposed to a lateral shock wave of less and less energy per unit volume.
    • Both the radial explosive energy and the ejected mass per cylindrical unit of volume decreases to zero as the energetically maximum possible flat crater diameter is reached.
    • The distribution of total ejecta mass and average particle velocity within a unit volume of the conical wedge would be a function of the level of decreasing shock energy at various distances from the point of impact within the forming crater.
    • As a consequence of the above, in the lunar gravity and vacuum environment, the expanding, inverted conical wedge of regolith ejecta curves (“blooms”) into a radially irregular 360º, parabolic ballistic arch ( Fig. 13.4b).
    • Within the ballistic arch, small differences in particle velocities and trajectories and the evolution of solar wind volatiles through agitation, produce ballistically turbulent mixing of the original regolith strata, creating an at least partially integrated chemical composition (§5.0).
    • Deposition of progressively lower velocity regolith ejecta moving outward within the ballistic wedge and parabolic arch would begin at the lowest energy tail of the wedge as it moves across the surrounding surface, away from the rim of the newly formed flat crater. The highest velocity ejecta, of course, would be the last to be deposited down range and would land farthest from the impact point.

Fig. 13.4b. Schematic cross-section of a laterally expanding, inverted conical wedge that gravitationally transitions, in ultra-high vacuum, into a parabolic arch containing the largest proportion of ejected regolith.

    • As the hemispherical shock wave of potential and kinetic energy, equal to “mgh and 1/2mv2,” respectively, delivered at the impact point moves downward into broken and then coherent bedrock that underlies the excavated regolith, a reflected shock excavates the main volume of the final crater and deposits that material around the crater rim to a distance of approximately one crater diameter. This relatively late arriving bedrock-related material falls on the earliest, lowest velocity portion of regolith ejecta.

These considerations suggest that the initial and progressively radial but nearly instantaneous release of kinetic energy is the primary factor in the ejection of regolith at the impact site, with remaining kinetic energy and the release of potential energy being the primary factors in subsequent excavation of the crater as a whole.

3.4 Source Craters for Regolith Ejecta in Deep Drill Core

The temporal study of recent lunar meteor impacts (Speyerer et al., 2016), based on comparative examination of LROC images, show that regolith is ejected long distances from the point of impact. In that context, there exist 10, 400-800 m diameter, definable impact craters (Fig. 13.5; Table 13.3a) in the Taurus-Littrow exploration area, plus 8 more that make up the Crater Cluster in the south central valley area and one 1451 m diameter crater east of the exploration area. At least some of the initial regolith ejecta from craters in this size range would have reached the deep drill core site and be possible sources for the regolith ejecta in the identified stratigraphic zones in the core.

Fig. 13.5. Valley of Taurus Littrow exploration area showing the location of named craters and exploration stations (yellow). The 70009/1 deep drill core site next to the ALSEP deployment area is marked by the yellow X adjacent to “Core”, right of Camelot Crater. (NASA LROC)

The relative geomorphic ages of major potential source craters can be determined by visual comparisons of differences in crater morphology (Melosh, 1977) and in the size-frequencies and distribution patterns of boulders on crater rims and interior walls. The relative ages of crater formation given in Table 13.3a reflect: (1) direct observations (indicated by red entries); (2) surface photography obtained during Apollo 17 EVAs (Chapters 10-12); and (3) diameter to depth ratios measured on QuickMap LROC images. As this report’s synthesis progressed, it became clear that slope mass wasting and nearby crater ejecta affecting some older craters required the use of some geological adjustment of estimated relative ages.

A close study of the large craters in Taurus-Littrow indicates significant variability. The 1451 m diameter MOCR Crater is an exception to other craters in the valley in its much larger size and youngest relative age, as well as being located 6.3 km east of the deep drill core site. Another exception to other craters are four of the major craters in the Crater Cluster (Steno, Emory, Sputnik and Faust as green entries in Table 13.3a) in that they are elliptical in shape rather than circular. Each long axis of these four craters is oriented at a bearing of ~320° ± 5°. This comparable orientation of spatially and apparently temporally related elliptical craters suggests a nearly simultaneous oblique impact of a closely aggregated group of objects. A fifth elliptical crater, Hess, also in the Crater Cluster, is distinct from the other four in its long to short axis ratio and long axis bearing. Finally, two relatively young, circular craters in the Crater Cluster, Sherlock and Powell, may have formed simultaneously.

Using this combination of visual characteristics and diameter to depth ratios, both of which are subject to significant judgment, Column 1 of Table 13.3a contains source crater assignments to each zone as a function of increasing depth in the deep drill core. Although a comparison of these assignments with the petrographic data in Tables 13.1a and 13.1b and compositional modeling in §5 support the sequence indicated, the following caveats are in order:

        1. “Crater Cluster” in Table 13.3a, and subsequently in Table 13.4, is the name given to a grouping of 8 large craters and many associated small craters and a generally lumpy surface (Wolfe et al., 1981; QuickMap). Close examination of the relative ages of the large craters (Table 13.3a), however, indicates that Steno, Emory. Sputnik and Faust were formed by simultaneous, oblique impacts. These four craters are distinct from other craters in the Cluster in being elliptical with a consistent bearing of 320° ± 5° of the long axes.
        2. Post-impact shape distortions of Mackin, Henry, Sputnik, Lara, WC and SWP Craters make diameter to depth ratio determinations subject to significant judgment as to placement of the best diameter line in QuickMap. On the other hand, compositional correlations with deep drill core zones (see §5.0, Tables 13.6aa to 13.6kk) reinforce the judgments listed in Table 13.3a).
        3. The abundance of basaltic lithic clasts throughout the core (Table 13.1a, Column 7) further emphasizes the general absence of massif melt-breccia fragments that resemble boulders at Station 2, 6 and 7 at the base of the massifs and as rocks in avalanches from the South Massif.
        4. Lara Crater (~675 m in diameter) is not included in Table 13.3a since it is probably too far from the drill site (6.0 km) to contribute significant basaltic regolith ejecta. Lara Crater will be discussed separately, below.

Craters in the same size and relative age range as those on the valley floor are difficult to define on the North and South Massifs and in the Sculptured Hills; however, analysis of those elevated craters nearest the valley rim show that their distances from the deep drill core site would limit significant quantities of their ejecta from reaching the core site. Clear petrographic and compositional evidence of ejecta contributions from impacts directly on the massifs, is absent from the deep drill core, with the possible exception of zone V+V*, and will be discussed in §5.0.

Table 13.3b gives a summary evaluation of the apparent source craters for deep drill core zones by comparison of (1) evidence for relative ages in relation to the superposition of regolith zones in core (Table 13.1a); (2) petrographic evidence relative to general geology in source crater area (Tables 13.1a and 13.1b); and (3) thickness of zones relative to distance from rim to core site (Fig. 13.8, below).

3.4.1 Lara Crater as Possible Source Crater

Although Lara Crater’s rim is farther from the deep drill core site than other similarly sized craters (~6.0 km), (Fig. 13.6a), it is of the right diameter (~675 ± 30 m) to be considered as a possible source of one of the core’s relatively thin regolith zones. On the other hand, in addition to its distance from the core site being much greater than 2.6 km (see Fig. 13.8), determination of Lara’s relative age is made more difficult by being partially covered by the light mantle avalanche deposits discussed in §17. Lara also has been distorted, tilted, and moved ~500 m eastward by movement on the Lee-Lincoln thrust fault (Schmitt et al., 2017). These factors make an estimate of its relative age and its diameter to depth ratio problematic and the faulting transport increases its original rim distance from the drill site to ~6.5 km.

Lara’s current diameter to depth ratio is ~11.4 ± 1 (845/74, using N-S diameter). On the other hand, boulders observed on Lara’s walls, and scattered elsewhere nearby, that project through the light mantle deposit (Chapter 11), suggest a relative age comparable to or younger than Horatio (diameter to depth ratio = 9.95). For Lara to match that ratio for Horatio (Fig. 13.6b), the fill on its floor would need to be ~8 m thick to give an 82 m original depth. This is a reasonable thickness for such fill relative to comparable estimates for the combined thickness of two light mantle avalanche deposits that have over-ridden Lara (§17.2). In addition to an initial deposition of young and old light mantles, there was some backflow of young light mantle avalanche material into the crater (not included in the current depth estimates).

Fig. 13.6a. Lara Crater. (LROC Quick Map image).

Fig. 13. 6b. Horatio and Camelot Craters (LROC Quick Map image).

3.4.2 Possible Source Craters on the Massifs

Craters >400 m diameter present on the tops of the North and South Massifs have diameter to depth ratios similar to those on the valley floor; however, such craters are too far away for their parabolic arches to reach the core site. The closest crest of the North Massif, for example, is 6.5 km from the drill site (QuickMap).  Fig. 13.7 indicates that a 45º intersection of the leading portion of a crest crater’s parabolic arch with a point level with the crest would be ~2.6 km from the hypothetical crater rim. Although the massif crest is ~1.6 km above the valley floor, this extra ejecta flight time still would be insufficient to reach the drill site lying 6.5 km from the crest even if its flight maintained an optimum 45º trajectory as illustrated graphically in Fig. 13.7.

Fig. 13.7. Graphical analysis of the possibility of a >400 m diameter crater at the crest of the North Massif (1.6 km higher than the valley floor) contributing regolith ejecta to the deep drill core site lying 6.5 km from the crater rim.

Fig 13.7 shows the parabolic arch from the crest of the North Massif, with the optimum 45º launch-angle replicated as the impact angle. An 1.6 km altitude advantage would miss reaching the core site by ~2.3 km if a 45º impact angle for an 1.6 km altitude were maintained, which, of course, is greater than gravitational physics would allow.

Craters on the slopes of the North Massif that might have contributed significant regolith ejecta to the core have not been identified nor is there petrographic evidence that this occurred (Taylor et al., 1979) beyond the contributions of down-slope mass wasting (§5.0, zones U, V+V*, W and Y+Y*). It would appear that southwestward moving regolith ejecta from those craters visible on the top of North Massif largely impacted the southwestern facing slope of the Massif. This would have contributed to the mass wasting from that slope (such as noted for WC Crater), but not significantly to the regolith accumulating on the valley floor. (The base of the slope is >4.5 km horizontally from >400 m diameter massif craters nearest the crest (QuickMap measurements)).

The same arguments related to the North Massif would apply to craters on the even more distant South Massif. Petrographic and compositional characteristics of zones U (Steno, etc.), W (Hess) and Y (Makin) (§5), however, indicate that their regolith ejecta include material from one or more South Massif slope avalanches (§17.0).

3.4.3 Reach of Each Source Crater’s Regolith Ejecta

The regolith ejecta sheath hypothesis postulates that a hypervelocity impact creates a conical ejecta wedge that evolves into a 360º parabolic arch, with ejecta deposition beginning at the tail of the wedge as it leaves the rim of the now-formed crater. This hypothesis implies that the distribution of mass in the wedge-arch combination at any given time and distance of deposition can be projected onto a roughly circular plane centered at the source crater. The thickness of regolith ejecta deposits forming at the moving tail of the wedge and arch at any given distance from the crater rim would be a function of the initial velocity and mass distribution of ejecta within the wedge-arch and the rate of decrease in its density as outward expansion continued. If, as hypothesized here, regolith ejecta from various 400-800 m diameter craters make up the regolith zones in the deep drill core (except that of zone S), the thickness of those zones should vary systematically with distance from their source crater rim.

From these relationships, it may be possible to empirically determine the approximate distance at which maximum deposition thickness of each source crater’s regolith wedge-arch occurs. Table 13.4a gives the following data:

          • Thicknesses of the regolith ejecta zones in the deep drill core,
          • Potential source crater diameter,
          • Distance from the core to the rim of potential source craters (rim chosen because that is where deposition begins).
          • Estimated excavated regolith volume from the potential source crater from Table 13.17, §8.0.

The volume of ejected regolith was initially thought to be that of an approximate cylinder with a thickness of all the regolith present above bedrock, with an assumption that regolith depth averaged ~10 m. Considerations in §8.0, however, indicated that the actual depth of ejected regolith could be estimated in relation to cosmic ray exposure ages reported for the deep drill core zones as compared to the accumulated exposure age per meter in the deep drill core (37.5 Myr per m). The reader is referred to that Section for these estimates.

Table 13.4a footnotes:

For target materials of the same density, total excavated volume is representative of the total impact energy delivered to the crater site, both kinetic and potential. In attempting to relate deep drill core zone thickness to the release of impact energy, zone thickness (width) is plotted against distance from source crater rims to the core site gives relationships as shown in Fig. 13.8. The plot suggests that maximum deposition occurs at a distance of about 2.6 km from the source crater rim. In Fig. 13.8, there also is a strong hint of two different non-linear distributions of thickness as a function of distance, approximated by dashed line segments. One trend (red) appears to relate to source craters closer than 2.6 km with large zone thicknesses. The other trend (black) relates to source craters closer than 2.6 km but with relatively small thicknesses. The statistical support for these differences is not great, however.

Fig. 13.8. Relationship between the thicknesses of zones of regolith ejecta deposited at the deep drill core site and the distance to the rim of probable source craters. The “distance from core” points for simultaneously formed elliptical craters in the Crater Cluster (Steno, Emory, Faust and Sputnik) are shown in green with their positions a projection from their crater distances to the black dashed curve (see text).

Relationships comparable to those in Fig. 13.8 would be expected if the 360° sheath of ejected regolith follows a parabolic trajectory from which deposition of increasingly energetic particles occurs continuously at the trailing end of the sheath but with the greatest concentration of particles being from the portion of the parabola with the greatest total energy.

As their ellipsoidal shapes, long axis orientations, and diameter/depth ratios indicate that craters Steno, Emory, Faust and Sputnik (green points in Fig. 13.8) formed simultaneously, their contributed thicknesses to the 39.10 cm of zone U would vary relative to their distances from the core site. Projecting source crater rim distance to the dashed trends in Fig. 13.8, the total thickness such a projection predicts for zone U would be, respectively, 6+16+34+28 cm = 84 cm. It is hard to see how the dashed curves could be moved downward enough to significantly lower these estimates, as the curves are based on objective measurements of zone thickness and crater rim distance. It is more likely that interaction of the simultaneously formed ejecta sheaths resulted in less regolith mass reaching the core site than otherwise would be predicted from the trends in Fig. 13.8. Alternatively, an asymmetric concentration of ejected regolith from an oblique impact may have resulted in lesser deposition at the core site. Weighing the excessive predictive thicknesses for Steno, Emory, Faust and Sputnik, however, against an actual zone U thickness of 39.10 cm gives approximately 2.7, 7.3, 15.4 and 13.7 cm, respectively, as the four predicted contributions to zone U’s logged thickness.

Fig. 13.9a is a graphical plot of zone width normalized to distance from its source crater rim (km) vs. estimated ejected regolith volume (Table 13.4a). Some observations related to Fig. 13.9a are as follows:

      1. The points plotted for most craters appear to define a trend (red) indicating that zone thickness has a relationship to source regolith ejected volume.
      2. The intersection of the red trend at the zone thickness-rim distance ratio axis may indicate that as ejected volume approaches zero that ratio approaches a value of ~6. This would imply that 0.1 m away from the rim of a small volume crater the ejecta thickness would be ~0.6 cm, giving some perspective on the nature of the micro-meteor near-surface regolith gardening process.
      3. The improved correlation of zone T+T*’s source craters, Sherlock and Powell, with the red trend through the use of the total rim distance and total regolith volume lends support for the hypothesis that Sherlock and Powell were simultaneous impacts.
      4. The close grouping of green points representing the 4 four source craters for zone U would be expected for simultaneous impacts clustered together in one area with corresponding interference between ejecta sheaths.
      5. The clustering of the green points representing the four source craters for zone U well below the red trend line suggests that the thickness of zone U of 39.10 cm is not the maximum hypothetical zone thickness that could have resulted from those four impacts, and and further suggests ejecta sheath interferences or, alternatively, asymmetric deposition with thicker regolith being deposited further downrange.

Fig. 13.9a. Plot of deep drill core zone thickness normalized for source crater rim distance against ejected regolith volume based on source regolith thickness calculated in §8.0.

Fig. 13.9b is a graphical plot of zone width / km vs. estimated initial crater volume (Table 13.4b), a rough surrogate for total energy of the impactor. The ejected total volume calculation assumes that the initial excavated volume was that of a half sphere with its depth equal to the crater radius. In the case of the ellipsoid craters, the initial depth was assumed to be the same as half the short axis. These first approximations of radius and depth are greater to some limited degree than the current radius and depth, of course, as crater diameters increase and crater depths decrease with age.

Table 13.4b footnotes:

Fig. 13.9b. Relationship between deep drill core zone thickness (width / km) and the estimated initial source crater volume (Table 13.4b), assuming that the radius of the half sphere, or short axis in the case of ellipsoidal craters, approximates the source crater’s initial depth.

Fig. 13.9b indicates the following:

      1. As in the case of the plot for regolith volume ( 13.4a), the points plotted for craters (red o), other than the ellipsoidal source craters for zone U (green o), most craters appear to define a trend (red) indicating that zone thickness has a relationship to ejected volume.
      2. Also, as in the case of the plot for regolith volume ( 13.4a), the intersection of the red trend at the zone thickness-rim distance ratio axis may indicate that as ejected volume approaches zero that ratio approaches a value of ~6.
      3. The improved correlation of zone T+T*’s source craters, Sherlock and Powell, with the red trend through the use of the total rim distance and total volume lends (Table 13.4b) support for the hypothesis that Sherlock and Powell were simultaneous impacts.
      4. As in 13.9a, the close grouping of green points representing the 4 four source craters for zone U would be expected for simultaneous impacts clustered together in one area with corresponding interference between ejecta sheaths.
      5. The clustering of the green points representing the four source craters for zone U well below the red trend line, as in 13.9a, suggests that the thickness of zone U of 39.10 cm is not the maximum hypothetical zone thickness that could have resulted from those four impacts, and further suggests ejecta sheath interferences or, alternatively, asymmetric deposition with thicker regolith being deposited further downrange.
      6. The relatively close association of source crater points along a similar trend line in both 13.9a and Fig. 13.9b gives strong support for the correlations of source craters with deep drill core regolith ejecta zones (Table 13.3a).

In Fig. 13.8 and Figs. 13.9a and 13.9b, points for MOCR Crater are, of course, obvious outliers because of that much larger crater’s greater formational energy than those in the 400-800 m range.

There are many possibilities for error in the correlation of deep drill zones with specific source craters; however, support for the assignments comes from the close correlations shown in Fig. 13.9a and Fig. 13.9b, helping to confirm the support  found in petrographic and compositional details, discussed in §5.0. The plots in these figures and in Fig. 13.8 indicate that the primary controls on zone thickness are distance from the source crater, combined with the source crater’s total formational energy and the thickness of regolith at the pre-source crater impact location. The distinct plots for regolith volume (Fig. 13.9a) versus crater volume (Fig. 13.9b) further support the separation of kinetic energy and potential energy in the physics of impact excavation. This separation also sets limits on the extrapolation of data from explosions craters to impact craters, constringing explosions to be equivalent to the release of potential energy but not the release of initial kinetic energy.

3.4.4 Summary

In summary, if the parabolic arch hypothesis for a 360° ejecta sheath from any given source crater is correct, Fig. 13.8 suggests that the probable limit of significant deposition from such arches would be about 2.6 km from the rim of that crater. Further, there is a strong implication in the data as plotted in Fig. 13.8 that, for craters in the 400-800 m diameter range and up to about 2.6 km from the deep drill core site, the thicknesses of the regolith ejecta zones are primarily functions of the kinetic energy of the source crater impactor as well as the portion of the parabolic arch that finally hits the surface. In that scenario, the thick regolith ejecta zones for Cochise (zone Z+Z*), Hess (zone W), Mackin (zone Y+Y*), and possibly Faust may represent deposition of regolith ejecta contained in densely concentrated portions of its parabolic arches, that is, the velocity and volume of regolith ejected is greatest just after initial impact and, possibly counter-intuitively, less dependent on the total kinetic+potential energy finally released.

Other factors besides kinetic energy and distance potentially come into play with any given impact. These factors include actual thickness and density of source regolith and angle of impact; however, these may have been a positive or negative factor at Taurus-Littrow only for the four related elliptical craters in the Crater Cluster as well as Mackin and Hess that impacted regolith thickened by one or more South Massif avalanches.

Another way of viewing the plots in Fig. 13.8 is that regolith ejecta mass is largely concentrated in the leading volume of the conical wedge and the parabolic arch as they form, and this mass is deposited near the end of the arch’s ballistic trajectory. It appears that 2.6 km constitutes the lunar ballistic limit for the largest volumes of regolith ejected from craters in the 400-800 m diameter size range. The steep drop in delivered regolith thickness documented for Hess Crater (zone W) in Fig. 13.8 supports this conclusion, although a small volume of high velocity ejecta from near the exact point of impact almost certainly goes much greater distances and additional small volumes of lower velocity ejecta are spread between the source crater rim and ~2.6 km from that rim. Clearly, more data from deep drill cores at other locations, selected to maximize the number of potential source craters, are needed before this apparent ballistic limit can be more precisely defined as a function of source crater size and its distance to a deposition site. The ballistic limit for large terrestrial impacts might be determined by consideration of tektite strewn fields, and then used to identify candidate source craters not yet identified.

4.0 Process Sequence That Forms Inter-Crater Regolith

4.1 Introduction

Fig. 13.1a and the log of detailed maturity data from the Apollo 17 deep drill core in half-cm steps from which Fig. 13.1a was derived (Morris et al. (1979); pers. comm.) indicate that major impact events that result in regolith ejecta deposition broadly surrounding the resulting crater are the primary initial contributors to the three-dimensional structure and composition of inter-crater regolith at Taurus-Littrow and other lunar locations. Once deposited as a regolith zone, impact gardening and external and internal radiation act as agents of continuous maturation until the zone is buried by new regolith ejecta.

The maturity parameter Is/FeO, developed by Morris (1976), is based on the ferromagnetic response of nano-phase iron particles (np°Fe) produced by the alpha+beta and proton reduction of increasing amounts of structural Fe++ to Fe°. Dissolved Fe++ in volcanic glass and glass-rich agglutinates is protected from both alpha+beta and proton radiation as a consequence of the dissemination of those ions and their shielding from radiation by surrounding glass.

Throughout regolith that is continuously gardened by impacts, alpha+beta particle radiation of Fe++-bearing minerals produces a steadily increasing contribution to the value for Is/FeO within 20-30 µ of disseminated uranium and thorium radiation sources (Schmitt, 2022; §6.3.2). Once gardening ceases after burial, however, this form of maturation rapidly dies off as local Fe++ ions are consumed. At the surface, solar proton radiation also increases Is/FeO values on exposed crystalline particles (§6.3.3). This exposed surface material is gradually incorporated into deeper units through the impact gardening process.

Close to points of macro- and micro-meteor impacts, nano-phase iron particles have their ferromagnetic properties disturbed by shock and/or heat so that local Is/FeO values are partially reset to lower values (§4.2). These partial resets do not appear to extend beyond the reach of actual regolith excavation and crater wall deformation, based on evidence of such resets in (1) ancient light gray regolith at the rim of Shorty Crater (74240, 74260); (2) regolith breccia sample 79115 and regolith breccias from other Apollo missions; and (3) a sample (73131) from the wall of a small fresh impact crater in mature light mantle regolith discussed further below in §17.0. The extent of “Is” reset by large source craters will be further examined later in §6.4.2.

The general sequence of inter-crater regolith formation, such as that surrounding the Apollo 17 deep drill core, has been illuminated in this synthesis by detailed logging of the Is/FeO profile in the deep drill core (every half-centimeter) and by logical consideration of the constant micro- and small macro-meteor rain of gardening impacts. That sequence of inter-crater regolith development at the deep drill core site was as follows:

          1. Regolith ejecta from 10 large, 400-800 m diameter source craters, and one ~1400 m diameter source crater (MOCR), are deposited as layers up to 50 cm thick that sequentially cover hundreds of square meters around the core site. A given layer is contiguous with a circumferential blanket of comparable regolith ejecta surrounding its source crater that reaches maximum thickness at a distance of ~2.6 km from the rim of the crater (Fig. 13.8). Bedrock-related ejecta is confined to an irregular blanket comprised largely of brecciated bedrock that extends an average of about one crater diameter from the rim of the source crater.
            • It appears that the initial kinetic energy of the impactor creates the far-reaching sheath of regolith ejecta, whereas, its potential energy is largely responsible for excavation of bedrock and overlying rubble and deposited within about one crater diameter from the crater rim.
            • Prior to regolith ejecta deposition, small differences in the velocity and trajectories of individual particles, along with volatiles released due to agitation, create ballistic and volatile turbulence within the parabolic regolith ejecta sheath that emanates from a source crater.
            • Turbulence partially integrates Is/FeO values, cosmic ray exposure ages, and compositional parameters of the finest fractions (>70 wt-%; Graf, 1993) of pre-existing regolith zones.
          2. Micro- and small macro-meteor impacts with an exponential size-frequency distribution increasing toward smaller sizes (Shoemaker et al. (1967); Graf, 1993) begin to modify and garden newly deposited regolith ejecta.
            • Regolith at and near the point of small impacts is subjected to partial resets of their Is/FeO values due to local intense shock or heat or both.
            • Nearby small impacts capable of depositing regolith at the core site but still are the most distant from the site contribute the most significant Is/FeO reset values, as that most distal regolith has received the greatest shock and energy input from impact. The amount of detectable individual resets varies from ~2% to ~20% for mature regolith, up to ~30% for immature regolith (§4.2) , and to over 90% for regolith subject to large impacts(§§6.4.2, 11.2).
            • Complementarily, ejected regolith that has the Is/FeO value of the local regolith may or may not be apparent in the raw data.
            • As small impacts garden the continually reforming regolith surface, radiation from solar protons on exposed surfaces and U+Th alpha+beta radiation throughout the bulk regolith continue to reduce structural Fe++ to nano-phase iron (np°Fe), thus increase post-reset Is/FeO. In varying time spans, this maturation counters, partially counters, or exceeds the effect of impact-related resets, but continues to give an increasing measure of the increase in Is/FeO (∆Is/FeO) with surface exposure.
            • The work of Burgess and Stroud (2018) on solar proton radiation effects on surfaces of exposed ilmenite, chromite, and glass particles confirm solar proton maturation; however, that research did not include consideration of the formation of nano-phase iron related to alpha particle radiation reported by Bower et al. (2016) in terrestrial biotite.
            • The rising Is/FeO values produced by nano-phase iron production appears in Morris’s raw data (pers. comm.) from the deep drill core as gradual increases in Is/FeO values over 1-3 cm that peak at a point where a rapid decrease in those values occurs due to a reset. This process forms the repeated Is/FeO saw tooth peaks, cliffs and valleys that are so apparent in Fig. 13.1a, particularly in deep drill core zones S, V and W.
              1. Surprisingly, Is/FeO decreases and increases in extended saw tooth patterns are roughly equal in most regolith ejecta zones. Exceptions to this pattern provide information on temporal variations in the impact environment affecting the regolith (see discussion of zone U in §5.0).
            • Each original regolith ejecta zone is ultimately gardened by craters with diameters up to ~5 times (Pike, 1974) the thickness of a given zone, until another source crater impact delivers a new regolith ejecta zone of a distinct initial Is/FeO (Is/FeOm) that buries the earlier zone.
              1. Given enough time, an impact that penetrates the entire regolith ejecta zone may reach into the top of the immediately underlying regolith ejecta zone with that zone’s regolith being integrated into the zone that buried it. An anomalous pattern at a contact with an earlier regolith ejecta zone would indicate that gardening had crossed into that underlying zone, but no such anomaly has been noted in the deep drill core core raw data.
              2. Lack of a pronounced saw tooth pattern in a relatively thick zone, e.g, zone U (~40 cm thick), would show that the general process outlined above was interrupted by another temporary process. In the case of zone U, a period of small macro- and micro-meteor impacts of higher energy than normal may have resulted in the steady, reset-driven decline in Is/FeO values for that zone (see Fig. 13.1a).

4.2 Impact Shock Partial Resets of Is/FeO Values

In the course of this synthesis, as noted above, several lines of evidence came to light indicating that very locally, impact shock, and potentially heat, partially resets the ferromagnetic basis for the determination of the “Is” component of Is/FeO values developed by Morris (1976). The evidence for this phenomenon is as follows:

      1. The Is/FeO values of 5 and 5.1 for light gray regolith samples 74241 and 74261, respectively, obtained at the rim of Shorty Crater, are inconsistent (§11.2) with the cosmic ray exposure ages reported for 74241 that range from 150 to 320 Myr (Table 13.5) with large error limit.
      2. A sample of the regolith, 73231, taken from the wall of a small, very fresh impact crater on the young light mantle has an Is/FeO = 16 that is inconsistent with Is/FeOs = 88 for skim sample 74121 as well as Is/FeOm = 48 for bottom trench sample 73141 from ~15 cm depth (§17.4).
      3. Deep drill core zone deposition Is/FeOs (Is/FeOm) are up to 93% lower than they were in the pre-source crater regolith, as indicated by cosmic ray exposure ages on those regoliths (§8.0, Table 13.16).
      4. The Is/FeOs of impact-lithified regolith breccia samples from Apollo 15 and 16 (Table 13.6) are consistently much lower than bulk regolith samples from those missions (McKay et al. (1986); McKay et al. (1989)).
      5. The probable role that impact heating, in addition to shock, plays in resetting Is/FeO to low and very low values relative to bulk regolith is indicated by Apollo regolith breccia samples (Morris in McKay et al. (1986, 1989); Morris, 1976) that have petrographic and/or image evidence (Meyer, 2012) of adhering impact glass or a glass matrix (Table 13.6).

 

The effects of partial resetting of Is/FeO values by impacts are superposed at the half-cm scale on the overall maturity of regolith ejecta zones of the deep drill core shown in Figs. 13.1a and 13.1b. These effects, and the subsequent increases in Is/FeO, create the saw tooth patterns in Fig. 13.1a. These patterns are reflected in the raw data (Morris, pers. comm.) from which Fig. 13.1a was created. The general pattern, identified in half-cm logging, is one of a sharp decrease of about 2-10 Is/FeO points followed by multiple increases in Is/FeO values in the overlying 1-3 cm. The increase in Is/FeO may or may not exceed the amount of the reset. This pattern is interpreted here as the result of gardening ejecta from nearby small impacts that contain resets being spread over the core’s location. This exposed reset regolith then continues normal proton and alpha+beta particle maturation until the arrival of the next spray of reset ejecta.

4.3 Summary of Detailed Is/FeO Log of Apollo 17 Deep Drill Core

Morris et al. (1979) recorded the maturity index, Is/FeO, for every half-cm of the Apollo 17 Deep Drill Core, summarizing their findings with the graph shown in Fig. 13.1a. An example of the author’s detailed log of the raw data (Morris, pers. comm.) underpinning Fig. 13.1a‘s graphical representation is shown in Fig. 13.1b. The half-cm log reveals a systematic sequence of sharp resets of Is/FeO values in less than the measurement thickness of half a centimeter, followed by more gradual increases in those values over ~0.5-3 cm along the core. A summary of the Is/FeO log is given in Table 13.7. The major findings illustrated by Table 13.7 and other data can be summarized as follows:

  1. Due to frequent resets and recoveries, the integrated original Is/FeOm of a regolith ejecta zone after deposition (Column 3) can only be approximated, probably ± 5 Is/FeO points, by consideration of Is/FeO values near the base of a given zone.
  2. Is/FeO resets per centimeter (Column 6) is roughly constant at 0.510 ± ~1 except for zones T+T* and V+V* (italics in Table 13.7).
    • Zone T+T* probably is anomalous due to their very low initial Is/FeOm values of ~10, thus limiting the amount of individual resets that can be resolved by the Is/FeO technique at a half-cm interval. This issue is addressed further below.
    • Zone V+V* probably is anomalous due to its small data set (4 resets over 11.8 cm).
  3. The apparent 8 point Is/FeO reset at the top of zone S does not appear to be the introduction of a new regolith ejecta deposit. This portion of the core also has been disturbed by compaction. Unlike the sharpness of other zone boundaries, the upper 4 Is/FeO data points are spread across 2.65 cm.
    • The bulk chemistry for the control sample 70181 for the deep drill core shows no significant differences from the bulk chemistry of core sample 70009 at 1, 9 or 20 cm depths (§5.2, Table 13.8a, below).
    • The difference in U+Th content of the control sample 70181 (1.380 ppm; Silver (1974)) for the deep drill core and 70009 at 1.18 ppm at ~1 cm depth appears to be only in the uranium measurement, as the thorium amounts of 0.95 ppm at 1 cm and 0.94 ppm at 20 cm (Meyer (2012)) are close to the 1.049 ppm Th measured for 70181 by Silver.
  4. Total decrease in Is/FeO resets for the core (1227) exceeds total maturation increase (1138) by only 89 points (7.3%), suggesting a rough balance between the effect of resets and that of nano-phase iron production after zone deposition. Why, other than coincidence, such a rough balance would exist over about one billion years of core accumulation (§6.0) is not obvious, particularly as the total ∆Is/FeO varies greatly between zones.
    • Other data discussed below indicate that overall regolith maturity in the long term is more a function of alpha+beta particle flux than solar proton flux (§6.0). Although mixed downward by impact gardening over time, the effect of proton maturation initially only affects particles exposed at a post-reset surface rather than continuously throughout the entire zone.
    • Zone U is the major intra-zone exception to the reset-∆Is/FeO balance, as it shows a continuous decline in Is/FeO in Fig. 13.1a as well as in the half-cm log spite of a relatively normal value for resets per centimeter of 0.455. These data suggest that reset impacts affecting zone U were of abnormally high kinetic energy, possibly due to the periodic passage of the Moon through the tail of a long period comet (Long period comet terrestrial impact velocities are as high as 67 km/sec (Hughes and Williams, 2000) while asteroids impact velocities are around 17 km/sec (Koeberl and Sharpton, 2024). Lunar impact velocities would be less, of course, however the relative difference would remain significant.)

Table 13.7 footnotes:

Within this general background, there are rare anomalous patterns, as follows:

        1. teady decrease in Is/FeO over >2 measurement intervals (May be there were multiple ejecta depositions with relatively little time in between or there was core penetration of a buried crater wall or ejecta blanket).
        2. Steady increase in ∆Is/FeO over >2 measurement intervals (May be a period with no ejecta deposition).
        3. Oscillation between (1) and (2), i.e., from steady decrease in Is/FeO over >2 measurement intervals to steady increase in ∆Is/FeO over >2 measurement intervals (May be that impact activity was inside the resolution of the data).
        4. Sharp decrease in Is/FeO that is greater than normal pattern (May be that ejecta in core is from point of impact where reset of Is/FeO is greatest).
        5. Sharp increase in ∆Is/FeO that is greater than normal pattern (May be that ejecta is mature regolith where reset of Is/FeO is least and is from a significant distance away from point of impact).
        6. No decrease in Is/FeO over >2 measurement intervals (May be there were no nearby impacts).
        7. No increase in ∆Is/FeO or decrease in Is/FeO over >2 measurement intervals (May be that deposited regolith has same Is/FeO as preceding redeposit.).
        8. A few apparent impact resets extended upward for up to six, half cm intervals These extended resets may represent where the core penetrated the sloping wall of a small gardening impact crater. In these few instances, the core interval is overturned and records that the level of reset increased downwards in the parent crater, as would be expected relative to the three-dimensional distribution of levels of shock in the final crater wall, that is, shock was greatest at the base of the crater wall.

     

    Zone U constitutes an exception to the general pattern of a sharp decrease in Is/FeO and a variable increase ∆Is/FeO before another sharp decrease. For zone U, the increase that follows a reset is usually less than the preceding reset decrease, resulting in a continuous decrease in zone U’s total maturity index (Fig. 13.1a). As zone U’s frequency of resetting impacts as a function of depth appears consistent with most other zones (Table 13.6, Column 6), this zonal-wide change in pattern suggests that the average energy of resetting impacts increased at the time of the deposition of zone U regolith ejecta, with the increase in impact energy dying out within the overlying zone T+T*. As discussed previously, the source of zone U regolith ejecta appears to be four hemispherical craters (Steno. Emory, Faust and Sputnik) in the Crater Cluster, and these craters may be the result of a cometary aggregate’s impacts. The zone U pattern may be the consequent of periodic passage through a cometary tail of debris. The discussion for zone U in §5.0 examines the compositional evidence for such a possibility.

    5.0 Petrography and Composition of Deep Drill Core Regolith Zones

    5.1 Introduction

    Tables 13.1a and 13.1b include a summaries of the variations in stratigraphic parameters for the Apollo 17 deep drill core zones initially defined by an estimation of significant positive or negative changes in Is/FeOm minima from Fig. 13.1a and then verified both in the raw Is/FeO data supplied by Morris (pers. comm.) and by correlation with other variables. Differences in detailed petrography of each of the identified deep drill core regolith zones (Table 13.1a, Column 6) generally correspond with the definition of zones based on Is/FeO data. Of particular usefulness in correlating petrography with maturity indices has been the work of Taylor et al. (1979) whose summary graphs are reproduced in Figs. 13.3a, 13.10a and 13.10b. Numerous chemical analyses for specific depths reported by Laul et al., (1974, 1978; 1981) and by Meyer (2012) provide additional stratigraphic data that support the indicated definition of deep drill core zones and the identification of their source craters and the geological context of the latter’s local regolith.

    Fig. 13.10a. Graph of percentage variations in basalt lithic clasts as measured by modal analysis (microscope point counting of particle types) of 171 thin sections from the deep drill core (Taylor et al., 1979). See also Fig. 13.3a for agglutinate variations and Fig. 13.10b for black (+orange) glass (volcanic ash) variations.

    Fig. 13.10b. Graph of percentage variations in black (+orange) glass (volcanic ash) as measured by modal analysis of 171 thin sections from the deep drill core (Taylor et al., 1979). See also Fig. 13.3a for agglutinate variations and Fig. 13.10a for mare basalt clast variations..

    5.2 Compositional Modeling of Source Regolith for Deep Drill Core Regolith Ejecta Zones

    Laul et al.’s (1978, 1979, 1981) regolith ejecta zone analyses are a remarkable resource in understanding the nature and derivation of the deep drill core’s regolith ejecta zones. In modeling their data to verify likely zone source craters, the mix of various rake sample chemical components employed are: (1) Station 1 high TiO2 and low U+Th regolith (71501) derived largely from ilmenite basalt (Rhodes et al. (1974)); (2) South Massif high SiO2 and U+Th and low TiO2 regolith (72501) (Laul et al. (1981)); (3) North Massif moderate SiO2 and low U+Th, and TiO regolith (76501); and (4) Sculptured Hills moderate SiO2, U+Th and TiO2 regolith (78501) and (78245) (Rhodes et al. (1974)). In addition, it was found that an added 5-30% of orange+black ash (74220) (Rhodes et al. (1974)) and/or VLT (very low titanium) ash bead from 70007 (Vaniman and Papike (1977)) compositions to several of the modeling mixes was required to approach a reasonable match with Laul et al.’s zone analyses. Uranium and thorium analyses of model components are largely those of Silver (1974) or, if required, from Laul et al. and Meyer (2012) and including Eldridge et al. (1974) for 71501. In most cases, iterative modeling of mixes of these sample compositions came within 0.5 wt % of Laul et al.’s zone analyses or their average. Exceptions are noted in the zone specific discussions.

    As indicated in the mixing proportions given in many modeling tables of this section, there is a strong suggestion that significant orange+black ash (74220) as well as a VLT ash (bead in 70007) (similar to VLT scoria 78526) are present in most regolith zones; however, it is not clear why their compositional contributions are not contained within the other component regoliths unless, due to their very fine grain size, they were not included in Laul et al.’s analyses of the 20-90 µ (nm) size fractions. Ash might be expected to be lost in the <20 µ size fraction where the 20-90 µ is taken to represent bulk composition (see Graf (1973)). On the other hand, there is otherwise no significant difference between the reported 20-90 µ and bulk compositions (Laul et al. (1978; 1979; 1980a)), suggesting that the 20-90 µ fraction and bulk compositions are nearly the same. Indeed, the addition of VLT ash to the models appears to be necessary even when only a bulk analysis is reported.

    The answer to this puzzle may be that loss of very fine ash in the course of curation and/or sample preparation for analysis, that included sieving, did not affect the accuracy of the analyses. Laul et al. (1978), for example, estimate that “material loss in sieving operations was typically 10%, largely from the <20 µm fraction.” As ash like 74220 and 74001/2 erupted on the Moon is concentrated in the small size fractions (Graf (1993)), this reported material loss may indicate that the actual ash content of the regolith ejecta zones as well as the ash content of the samples contributing to the models are higher than that required by the composition models.

    1. Zone S: –1.30 – 19.35 cm (19.65 cm) (MOCR Crater): The major element composition (Table 13.8a) reported for the deep drill control sample 70181 from the top 0-5 cm of the nearby surface (Rhodes et al. (1974)) closely matches that for this zone at its intervals of 0.6-1.1, 9.1-9.6 and 20.1-20.6 cm (Laul et al. (1978)).

    MOCR Crater is 1451 m in diameter and lies in the central portion of the eastern region of the floor of the valley of Taurus-Littrow, about 6.3 km from the core site. It is the largest as well as the youngest large impact crater on the floor of the valley (Table 13.3a). As it is the youngest large crater and zone S is the youngest regolith ejecta zone, MOCR is its most likely source crater. MOCR Crater was formed by an impact many times more energetic than other source craters (as indicated by total excavated volume (Table 13.4b), so its parabolic arch of ejected regolith would have reached significantly beyond the 2.6 km estimated in Fig. 13.8 for 400-800 m diameter source craters. Just the kinetic energy present to eject the mass of thick regolith from MOCR Crater (0.0134 km3; Table 13.4a) was 3-10 times more than that of other source crater impacts (Table 13.4a). The formational energy of MOCR Crater places it in a much higher energy level than other source craters in Taurus-Littrow with the total volume ejected of 0.799 km3 (Table 13.4b) being over 8 times that of 0.0991 km3 for the next largest source crater, Henry Crater.

    Relative to the composition of zone S (Table 13.8a), the South Massif and North Massif regolith samples 72501 and 76501, respectively, clearly do not indicate compositional similarities with many of the oxide components in zone S; however, the mix of 50% 71501 ilmenite basalt regolith, 30% 78501 Sculptured Hills regolith, and 20% 74220 orange+black ash (Table 13.8aa) matches the average of three analysis and of control sample 70181 within a half a percentage point. This match is consistent with MOCR Crater’s location ~2.6 km south of the base of Sculptured Hills (Fig. 13.5, QuickMap) where additional source craters for MOCR pre-impact regolith are located. These correlations of bulk chemistry allow zone S to be a surrogate for the general, inter-crater regolith of the eastern Taurus-Littrow valley floor and support the interpretation that MOCR Crater, 6.3 km from the core site, is the source of zone S’s regolith ejecta.

    [As a side note, the lack of a need to include VLT ash in the model for zone S, suggests that the pyroclastic fissures identified by Schmitt et al. (2017) in the Sculptured Hills to the north of MOCR Crater, are vents for orange+black ash. If true, this possibility would support the conclusion that orange+black ash eruptions largely came from fractures outside the valley floor which had been sealed off by lava eruptions just prior to the initiation of pyroclastic activity.]


    [Note: The 4 cm deep drill core interval between 18.35 and 22.35 has been interpreted here as a thicker than normal zone of mixing due to the high energy of the impacting regolith ejecta coming from the 1451 m diameter MOCR Crater.]

    2. Zone T+T* 22.35-47.05 cm (25.70 cm) (Sherlock and Powell Craters): The very low maturity (Morris et al. (1979)) (Fig. 13.1a), very low agglutinate (<10%) (Fig. 13.3a), and high basalt fragment and orange+black ash content (Taylor et al., (1979) uniquely distinguish zone T+T* from all other zones in the deep drill core and add strong support for the approach used in this synthesis to define other zones in the deep drill core. Table 13.8b gives the reported chemical composition in comparison with the rake sample 71501 from Station 1 on the ejecta blanket of Steno Crater within the Crater Cluster area. Nothing in Table 13.8b stands out that would negate correlation with the Crater Cluster regolith, post-Steno, –Emory, –Faust, and –Sputnik impacts, although some mixing between zones T+T* and U may have reduced the zone’s TiO2 content at its base. On the other hand, the analysis of the 90-20 µ size fraction may have missed some portion of the very fine TiO2-rich orange+black ash present in regolith 71501 that may have comprised the black layer observed in situ at Station 1 (Chapter 10; also in Schmitt (2024a)).

    End of UPLOAD 04

    Fig. 13.11. 16-25 cm portion of the deep drill core (70001/9) showing contrast in large fragment size-frequency between zone S and zone T (NASA photo from Meyer (2012)).

    Table 13.8bb shows that the mix of 75% 71501, 15% 74220 ash, and 10% 70007 VLT ash comes to within half a percentage point of matching the average of three analyses for zone T+T*. Although the U+Th values in the two zones vary between about 0.9 and 1.4 ppm, probably due to poor mixing of U+Th host minerals, the compositional, maturity and petrographic characteristics overall strongly suggest that the source area for their regolith ejecta is from the post-Steno-Emory-Faust-Sputnik area of the Crater Cluster and, specifically, from regolith developed on ilmenite basalt regolith, mixed with significant South Massif regolith, ejected from Steno Crater. Evidence for the presence of pre-Steno, South Massif avalanche material in rake sample 71501 is in the uranium and thorium concentrations (0.23 and 0.75 ppm, respectively) versus their concentrations in ilmenite basalt rock samples from the bedrock rubble at the rim of the near-by ~10 m diameter crater (Station 1, Chapter 10). U and Th contents in basalt samples 71055 and 71136 are 0.132 and 0.32 ppm (Brunfelt, et al., 1974) and 0.22 and 0.46, ppm (Eldridge et al., 1974), respectively. As compared to U+Th = 3.45 ppm in young light mantle sample 72121. These lower bedrock values indicate 10-20% South Massif regolith in 71501.

    The circular Sherlock and Powell Craters, respectively, both appear similar to each other as well as younger in diameter to depth ratios (Table 13.3a) than the elliptical craters of the Crater Cluster. Both craters are potential source craters for zone T+T*. As there is no other zone in the deep drill core that resembles T+T*, it is likely that Sherlock and Powell Craters formed simultaneously as also discussed in §3.4.3 relative to their combined inclusion in Fig. 13.9a and Fig. 13.9b.This zone also may include small contributions from South Massif avalanche regolith (see zone U discussion in item 3, below); however, the rake sample 71501, as well as large rock fragments in regolith developed on the Steno Crater ejecta blanket, appears to be largely derived from an ilmenite basalt parent (Schmitt, 2014b).

    The chemical composition of deep drill core zone T+T*’s sample 70008 (at 28 cm), is distinct (Laul et al., 1978) from all other zones in the core and in many elements distinct from the ranges of analyses of 10084, the representative regolith sample from Apollo 11  high TiO2 basalt (Meyer, 2012). The comparisons are as follows, with 10084 contents given in brackets ([ ]):

    a. FeO is ~3 wt % higher (~18% vs. ~15%). [14.4-16 wt%]

    b. Sc at 72.1 ppm is highest. [55-60 ppm]

    c. Co at 22.3 ppm is lowest. [18-34 ppm]

    d. Ni at 60 ppm is lowest. [200-250 ppm]

    e. Ga at 5.2 ppm is lowest. [4-5 ppm]

    f. Tb at 2.2 ppm is highest. [2.5-3.0 ppm]

    g. Dy at 12. 6 ppm is highest. [17-21 ppm]

    h. Yb at 8.21 ppm is highest. [10-11 ppm]

    i. Lu at 1.16 ppm is highest. [1.4-1.7 ppm]

    j. Hf at 6.5 ppm almost the highest. [9-11 ppm]

    k. Ta at 1.44 ppm is highest. [1.2-1.4 ppm]

    l. Th at 0.766 ppm (26 cm) is lowest (Silver, 1974). [1.6-1.9 ppm]

    m. U at 0.233 ppm (26 cm) is lowest (Silver, 1974). [0.41-0.5 ppm]

    These trace element data tend to indicate that zone T+T+ represents as close to an unmodified, ilmenite basalt regolith as is present in the Taurus-Littrow sample suite. In turn, zone T+T* regolith therefore would provide a close representation of the partial melt produced by reheating the ilmenite basalt’s source region in the upper mantle, presumably a region that has not been affected by the hypothesized, impact induced overturn of the upper mantle beneath the Procellarum basin (§26.0). Distinct contrasts with the ilmenite basalt regolith 10084 are present in lower FeO (but TiO2 is lower as well), Sc, Ni, Hf, Th and U, suggesting different mantle source regions for basalts erupted at the two locations.

    3. Zone U 47.05-86.15 cm (39.10 cm) (Steno, Emory, Faust and Sputnik Craters: As previously noted in §4.3, zone U’s Is/FeO pattern is anomalous relative to other deep drill core zones, From its deposition on zone V+V* to final burial by zone T+T*, it shows a continuous, drawn out decline in Is/FeO. Table 13.8c gives the major element composition of zone U that appears to match what can be inferred about the pre-Steno-Emory-Faust-Sputnik regolith. This correlation is re-enforced by a Th content of 1.095 ppm and an U+Th content of 1.414 ppm that suggests a mix of some non-basaltic material with ilmenite basalt regolith (71501). Sculptured Hills-type regolith ejecta (78501) from older potential sources craters (WC, SWP, Cochise, Shakespeare,) would have reached the area. In addition, as discussed below relative to zone W, there is the possibility that mass wasting avalanche deposits from the slope of the South Massif once covered the area, similar to the light mantle avalanche deposits (§17.0).

    Table 13.8c includes a comparison of zone U’s reported bulk compositions with that of rake samples of ilmenite basalt 71501, Sculptured Hills 78501, and South Massif 72501. For Sculptured Hills regolith 78501, with 1.337 ppm Th, to raise Th in 71501 from 0.75 ppm to 1.095 would require the mix in zone U to be ~81% Sculpture Hills regolith. This percentage is highly unlikely, as it would distort the major element contents of zone U well beyond what is reported. On the other hand, only 15% of South Massif regolith 72501 with Th = 3.106 ppm needs to be mixed to 60% 71501 to give Th = 1.17 and match zone U’s Th = ~1.095 ppm. This percentage, along with 10% VLT ash bead from 70007 (Vaniman and Papike (1977)) and 15% orange+black ash 74220 also gives appropriate adjustments to 71501 major oxide contents to match zone U’s (Column 6 in Table 13.8cc). The match is within half a percentage point except for FeO; however, Laul et al.’s (1978) FeO value for 46.8-47.3 cm depth (FeO = 19.0%) is from the top of zone U and may be mixed with the base of zone T* regolith (FeO =19.0%). Samples from zone U also are reported (as containing 20-24 modal-% orange+black ash (Taylor et al. (1974)) and an estimated ~2 modal-%, yellow-green VLT(?) ash (Vanniman et al. (1979)). (Note that significant amounts of either ash would have particle sizes less than 20 µ and some may have been lost before analysis.).

    It would appear that there was a South Massif avalanche deposit covering the pre-Crater Cluster area that became mixed with ilmenite basalt regolith to become the regolith ejecta now found in zone U as well as other zones discussed below with source craters in the Crater Cluster area. As zone U, with its unusual Is/FeO pattern, may be related to regolith ejected from an area in which elliptical craters Steno, Emory, Faust and Sputnik had formed, one hypothesis suggested above is that these apparently simultaneous oblique impacts relate to a disaggregated, long-period comet. The elliptical shape and consistent northwest (320° ± 5°) orientation of the long axes of the four craters also distinguishes them from other, circular Taurus-Littrow source craters, with the exception of the older, elliptical Hess Crater. Long-period comets also generally have a significant tail of related but much smaller objects. The high kinetic energy, cometary remnants in this tail would have been periodically encountered by the Moon in its orbit around the Sun, increasing the reset effects of small impacts recorded in zone U until the comet’s tail dissipated soon after zone T* was deposited. This possibility may be supported by the pattern in Fig. 13.3b showing a decreasing modal-% of agglutinates with decreasing depth. Even at high velocities, the low-density cometary impactors may destroy fragile glassy agglutinates at a higher rate than they create them.

    Remotely sensed data compiled in Quickmap shows the following contrasts between the area of the Crater Cluster, including Steno, Emory, Faust and Sputnik Craters, and its surroundings:

    1. Clementine color indication is significantly lighter than surrounding basaltic regolith suggesting mixing with South Massif regolith or alteration of the parent ilmenite basalt regolith.

    2. OMAT is brighter (less mature) than the close surroundings, although this is consistent with Station 1 Is/FeO indications of low Is/FeO values (35) for rake sample 71501 (Morris, 1978) which are, in turn, consistent with even lower values for regolith ejecta in zone T+T* derived from the post-impact regolith around Steno, Emory, Faust and Sputnik Craters in the Crater Cluster area.

    3. Diviner H-Parameter is similar to the Taurus-Littrow massifs, suggesting different thermo-physical properties than the area’s close surroundings, consistent with images and samples obtained at and near Station 1 that show significant concentrations of rocks and boulders. Also, in the later discussion of zone W, synthesis of U+Th and Th contents suggest that 60-68% of regolith ejecta from W’s source crater, Hess, consists of light mantle-like regolith from the South Massif. Hess Crater is 2.7 km from the core site, near the optimum regolith ejecta distance of 2.6 km (Fig. 13.8). On the other hand, Hess is only about 2.2 km from the base of the South Massif, in contrast to the 6.5 km separation of Station 1 sample 71501 from that base, with the Steno, Emory, Faust and Sputnik group in between. A Th content comparison of zone W (Th = 1.535 ppm) with South Massif sample 72501 (Th = 3.106 ppm) in Table 13.8ff, a ratio of 0.49, when compared against an estimated 10.4 m pre-Hess regolith depth (Table 13.16, §8.0) suggests that ~5 m of a light mantle-like deposit could have been included in a thick portion of in situ regolith at Hess Crater’s location, roughly consistent with the estimated >3 m thicknesses of the young light mantle (§17.2) at 2.2 km from the base of the South Massif.

    4. UVVIS image is brighter than the close surroundings, indicating greater iron abundance than for those for close surroundings, consistent with the elevated FeO content shown in Fig. 13.1a for zone T+T* whose regolith ejecta is interpreted as coming from ilmenite basalt regolith developed on bedrock ejecta blankets, whereas, surrounding areas have regolith that is less pristine.

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    4. Zone V+V*: 86.15-111.45 cm (25.30 cm) (Camelot Crater): Initially, this 25.30 cm section of the core appeared in Fig. 13.1a to be two zones of 11.80 and 13.50 cm. Laul et al.’s (1979) compositional analyses of the two potential zones, however, show very little distinction between them. The two initial zones, therefore, have been combined.

    The wall rock boulders observed and sampled at Station 5 (Schmitt et al., 2017) initially suggested that the Camelot impact primarily excavated ilmenite basalt; however, in contrast with overlying zones U, T+T*, and S, the relatively low FeO and TiO2 (15.3% and 5.7%) and relatively high SiO2 (42.3%) of both zone V+V* analyses (Laul et al. 1979) indicate a contribution from another, non-basaltic source. The compositions of several regolith samples in comparison to zone V+V* are shown in Table 13.8d. The possibility of significant Sculptured Hills-like regolith being included in zone V+V* is suggested by the thorium (1.090 ppm) and uranium (0.341 ppm) contents that are significantly higher than measured in ilmenite basalt regolith samples such as zone T+T* (0.766 ppm Th and 0.233 ppm U).

    Station 5 data from the Traverse Gravity Experiment indicate relatively low-density bedrock below the Camelot area (Talwani et al., 1973). Further, the existence of hills (kapukas) of Sculptured Hills-like material elsewhere on the valley floor (Bear and Family Mountains), suggest that the composition of zone V+V* regolith ejecta might be consistent with pre-Camelot regolith having been developed in part on Sculptured Hills-like bedrock (30%) with only a thin basalt cover (44%) in this general area. Camelot’s rim boulders now woud be representative of this basalt cover.

    The high model abundance of VLT ash is at odds with Vaniman and Papike’s (1977) report that thin-section analysis of the core at the equivalent depths for zone V+V* has about 5% VLT ash as well as about 18% orange+black ash (Taylor et al., 1979). That amount of orange+black ash would result in an imbalance in both SiO2 and TiO2 relative to the Laul et al. (1979) average.

    Known potential sources for pyroclastic VLT and orange+black ashes are areas in the Sculptured Hills (Schmitt et al., 2017) and near Shorty Crater (Schmitt, 2014a), respectively. In addition, a pyroclastic fissure has been identified on the flank of the North Massif (Schmitt et al., 2017) that is also a potential source of either ash type; although remote sensing data for the apparent debris flow from this fissure, relative to the composition of orange+black ash, suggest the build up of a now collapsed, lower titanium and higher silica ash cone (Petro and Schmitt, 2018).

    As Camelot Crater is within 2 km of the areas of both Shorty Crater and the North Massif fissure, either is a likely source for ash in zone V+V*. Table 13.8dd, column 7, shows the results of mixing 30% Sculptured Hills regolith (78501) with 44% ilmenite basalt regolith (71501) and 26% VLT ash (70007 bead) (Note that significant portions of the ash would have particle sizes less than 20 µ). Although not a perfect match with zone V+V*’s major element composition, all values for the mix fall within a 0.5 percentage point. Using VLT scoria sample 78526 from the Sculptured Hills (column 6 in Table 13.8dd) rather than the 70007 bead from the core would change the match slightly, particularly in SiO2 (higher) and MgO (lower).

    At a thickness of 25.30 cm, the correlation of zone V+V*’s regolith ejecta with Camelot Crater raises a question relative to §3.4.3’s quantitative verification of a close relationship of regolith ejecta thickness with source crater distance from the core site. This zone V+V* thickness makes it anomalous relative to most other zones; however, at a location that is slightly less than a crater diameter from Camelot (0.700 vs. 0.744 km, respectively), late stage, low energy regolith ejecta from just inside the current crater rim probably contributed to the thickness of the zone. This possibility is supported by the much higher percentage (62%) (Taylor et. al., 1979) of basaltic lithic clasts in the upper portion of zone V+V* than is present (32%) in the lower portion (Fig. 13.10a).

    5. Zone W: 111.45-144.05 cm (32.60 cm) (Hess Crater): As discussed above relative to zone U, zone W’s apparent source crater, Hess, resembles Steno, Emory, Faust and Sputnik Craters in having an elliptical shape; however, Hess has a long to short axis ratio of ~1.36 versus 1.03-1.16 for the other four craters. In addition to Hess Crater’s diameter to depth ratio indicates its relative age is significantly greater than those for Steno, etc. (Table 13.3a), the lack of a timing association between Hess Crater and Steno, etc. craters is further indicated by the orientation of its long axis at ~345° versus ~315-325°, respectively, for Steno, etc. Hess Crater, therefore, is an older, low angle impact feature.

    The relatively high U+Th content (1.985 ppm) of zone W (Silver, 1974) suggests that the very mature regolith ejected by Hess Crater near the South Massif had accumulated significant South Massif regolith ejecta like 72501 (U+Th = 3.995 ppm). Comparison of 71501 (U+Th = 0.98, Th = 0.70 ppm) from Station 1 (near Steno) and 72501 from Station 2 on the South Massif suggest that ~30% of zone W is derived from the South Massif (Table 13.8e, Column 6) as compared to 58% from ilmenite basalt regolith and 12% from VLT ash (70007 bead).

    Although the combination of oxide equivalent analyses in Column 6 correlate with that for zone W within somewhat more than the desired 0.5 percentage point, the indicated combination supports the probability that regolith ejecta from Hess Crater included material from a light mantle-like avalanche deposit from the South Massif (§17.0). Zone W has significantly more mature regolith than any of the deep drill core zones (Fig. 13.1a) with an Is/FeOm, averaging about 75. This fact probably indicates that the pre-Crater Cluster avalanche consisted of South Massif slope regolith more mature than that which formed the young light mantle deposit (Is/FeOm = 48). It may be that the accumulation of more mature regolith prior to the pre-Hess Crater avalanche indicates the relative age of the first movement on the Lee-Lincoln thrust fault. Later sections of this synthesis will estimate absolute ages for zone and mass wasting depositions.

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    ENDNOTES:

        1. Gupta, R. P. (2023). JWST early universe observations and ΛCDM cosmology. Mon. Not. Roy. Astron. Soc., 524, 3385-3395. ↑
        2. Tikoff, B. and T. F. Shipley (2025) Grand Canyon, USA: Lumping and splitting to make sense of a (somewhat) predictable world. GSA TODAY, 35, 4, 4.9