clarification of sources of material returned by luna 24 ...lev tarasov. photo taken in 1970s. in...

ISSN 0016�7029, Geochemistry International, 2013, Vol. 51, No. 6, pp. 456–472. © Pleiades Publishing, Ltd., 2013.Published in Russian in Geokhimiya, 2013, Vol. 51, No. 6, pp. 510–528.

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1 INTRODUCTION

In 1976 the automatic station Luna 24 landed in thesoutheastern part of Mare Crisium, drilled a bore�holedown to the depth of about 2 m and returned the core toEarth with the stratification of the sampled depositspreserved (Fig. 1). The material delivered was activelystudied, contributing to a better understanding of thepetrogenesis and history of mare basaltic magmatism ofthe Moon—the discovery of very low�titanium basaltsformed during a relatively late (3.2–3.4 Ga ago) stage ofmare volcanism [e.g., 1–3]. The stratified character ofthe Luna 24 core in these studies was not commonlytaken into account, mostly because of lack of informa�tion on the local geological setting of the landing site.Obtaining images with resolution down to 0.5 m, inwhich the Luna 24 spacecraft and the area around it areseen, allowed us to return to analysis of the geology ofthe landing site, to determine more accurately the coor�dinates of the landing site (12.7146 ± 0.0006° N,

1 The article was translated by the authors.

62.2130 ± 0.0006° E [4]) and to use the newly�gaineddata to reconsider the characteristics of the regolithcore.

It is seen in these images that the lander rests on therim of a crater ~65 m in diameter (Fig. 2). We suggestedthe name “Lev” for this crater to commemorate LevTarasov (1927–1998), the Vernadsky Institute leadingresearcher of the Luna 16, 20 and 24 samples (Fig. 3).The Working Group for Planetary System Nomencla�ture of International Astronomical Union has officiallyadopted this name (http://planetarynames.wr.usgs.gov/Feature/14952). In this paper we brieflydescribe the geology in the close vicinity of the landingsite as it is seen in the portion of the mosaic of imagestaken by the Wide Angle part of Lunar ReconnaissanceCamera (LROC WAC) and image M119449091REtaken by the Narrow Angle part of this camera (LROCNAC) and involve the appropriate model calculations.Then as part of analysis of the local geologic context,stratigraphy and history, we consider both theory andexperiments on impact cratering and the size of Lev

Clarification of Sources of Material Returned by Luna 24 Spacecraft Based on Analysis of New Images of the Landing Site

Taken by Lunar Reconnaissance Orbiter1

A. T. Basilevskya,c, B. A. Ivanovb, A. V. Ivanova, and J. W. Headc

a Vermadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, ul. Kosygina 19, Moscow, 119991 Russia

e�mail: [email protected] Institute for Dynamics of Geospheres, RAS, Moscow, 119334 Russia

c Department of Geological Sciences, Brown University, Providence, RI 02912 USAReceived June 5, 2013; in final form, October 17, 2012

Abstract—We describe the results of the geologic analysis of high�resolution (0.5 m/px) LROC NAC imagesof the landing site of the Luna 24 spacecraft, which landed in the southeastern part of Mare Crisium, drilleda borehole to the depth of ~2 m and recovered a core, which was then delivered to the Earth. LROC NACimages show that the spacecraft landed on the rim of the 65–m crater Lev. Image analysis was aided by cal�culations of the expected thickness of ejecta from crater Lev found to be 0.5–1 m. Using these calculationsand the results of photogeologic analysis, we reconsidered the characteristics of the Luna 24 core material.This led to an understanding of the geologic position of different parts of the Luna 24 core and allowed us todistinguish in the core the intervals dominated by: 1—effects of arrival of ejecta from the 6.5–km crater Fahr�enheit (Zone IV), 2—gradual reworking of the local regolith by small impacts (Zones II and III), and 3—emplacement of ejecta of Lev crater, which is a secondary of the distant 22�km crater Giordano Bruno (ZoneI). This understanding allowed us to propose that some additional new analyses of the Luna 24 material beundertaken, with emphasis on the study of Zone IV and Zone I. Additional analysis of zone IV could searchfor and identify material of ejecta from the crater Fahrenheit and their comparisons with local materials,while new analysis of Zone I could study the material derived from the deepest parts of the local regolith andsearch for material of the impactor (ejecta from the very young Giordano Bruno crater on the lunar farside)that formed Lev crater.

Keywords: regolith, Luna 24 spacecraft, regolith core

DOI: 10.1134/S0016702913060025

GEOCHEMISTRY INTERNATIONAL Vol. 51 No. 6 2013

CLARIFICATION OF SOURCES OF MATERIAL RETURNED BY LUNA 24 SPACECRAFT 457

Fig. 1. Left—Telescopic image of the Mare Crisium and highlands around; asterisk shows the Luna 24 landing site; ConsolidatedLunar Atlas, photo C1337. Right—Artistic presentation of the Luna 24 lander; its total height is 4.5 m; the diameter of sphericalcapsule for returned sample core on top of the lander is 50 cm; the two�headed arrow shows the liftoff part of the spacecraft; imagesource: http://www.laspace.ru/rus/luna24.html // http://nssdc.gsfc.nasa.gov/image/spacecraft/luna24.jpg.

500 mC

50 m

Fig. 2. Left—surface of Mare Crisium in the vicinity of Luna 24 site, white box outlines area shown in Figure 2 (left); fragmentof LROC image M119449091RE. Right—crater Lev and the Luna 24 lander (see also inlets); fragment of imageM119449091RE; left inlet is blow�up of the same image, right inlet is blow�up of image M174868307L.

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crater and calculate the expected thickness of the Levcrater ejecta at the place where the lander is seen. Wethen briefly consider the lithology, petrography, stratig�raphy and other characteristics of the Luna 24 core inlight of our calculation of the ejecta thickness and theprevious potential implications of the analyses ofremote sensing data for this area [e.g., 4–7].

GEOLOGY OF THE LUNA 24 LANDING SITEThe geology of the Luna 24 landing site is

described here based on analysis of a portion of theLROC WAC mosaic with a resolution of 100 m/pxand the LROC NAC image M119449091RE having aresolution of 0.5 m/px. The broad regional environ�ment of the Luna 24 site is seen in the left part of Fig. 1:the southeastern part of the circular Mare Crisium. It isseen that the landing site is within the mare surface sothe returned samples should contain mostly marebasalts and products of their impact reworking, a sug�gestion that is confirmed by laboratory studies of thesamples [e.g., 1]. The landing site is relatively close(~40 km) to the highland terrain of the rim of Crisiumbasin, so one may expect an admixture of a highlandcomponent to the predominant mare materials in theLuna 24 samples. It was found that these samples docontain highland materials, but in quantities less than5% [e.g., 2, 8]. Most of the highland materials wereprobably brought by small cratering events from theneighboring Crisium basin rim, but some part of itcould be brought by distant large�scale impact(s): In thevicinity of landing site there was described the presence

of rays and secondary craters of the 22�km craterGiordano Bruno (36° N, 103° E) which is in the far�side highlands, ~1300 km northeast of the landingsite [5, 9–13].

Figure 4 provides a regional context for the Luna 24landing site: the relatively smooth dark surface of MareCrisium, with generally northeast�trending wrinkleridges (part of Dorsa Harker), and the Crisium basinrim highland area. South and west of the landing site areseen faint, slightly brighter spots considered to be rays ofGiordano Bruno [5, 9–12]. About 20 km northwest ofthe landing site is located the crater Fahrenheit, 6.5 kmin diameter and ~1.5 km deep. Settle et al. [14] analyzedthe Apollo 15 panoramic camera images of this craterand found that its ejecta has a rather subdued morphol�ogy and estimated on this basis that its geologic age wasEratosthenian, with an absolute age not less than~1.5 Ga. Through comparisons with the ejecta patternof the younger crater Lichtenberg B (4.9 km in diame�ter) these authors concluded that ejecta of the craterFahrenheit had to reach the Luna 24 area, formingsmall secondary craters (now completely destroyed)and that these ejecta probably become a source of somecomponents of the Luna 24 samples.

We estimated the expected thickness of the Fahren�heit ejecta at the Luna 24 landing site. For that we usedFig. 8 of Housen et al. [15] showing the dependence ofejecta thickness on distance from the source crater andfound that it should be ~0.5 m of solid rock equivalent.In earlier work of Ivanov and Comissarova [16] the esti�mated value was 0.4 m. Of course, these ejecta were not

Fig. 3. Lev Tarasov. Photo taken in 1970s. In the right—Tarasov works in the receiving facility with the Luna 24 sample.



a layer of softly deposited material: According to Settleet al. [14] the ejecta landed there with velocities 165–230 m/s forming secondary craters and mixing withlocal regolith. These authors estimated the depth ofexcavation of Fahrenheit ejecta that arrived at the Luna24 site to be 100 m or less. According to Ivanov andComissarova [16], who devised a model of impact craterexcavation taking into account the particle motion dur�ing a cratering event, the estimated maximum depth forFahrenheit ejecta at the Luna 24 site was ~400 m. Settleet al. [14] concluded that the range of 100–400 m prob�ably brackets the maximum depth of excavation ofFahrenheit ejecta deposited at the Luna 24 site. Accord�ing to Ivanov and Comissarova [16] 90% of these ejectawere shocked in the Fahrenheit event to a level nothigher than 25 kb (only fracturing and fragmentation)and only about 2% to 100 kb and more.

The 100–400 depth of excavation is much greaterthan the depth of excavation by the craters composingthe observed population of small craters whose com�bined ejecta are responsible for formation of the major�ity of the regolith of this area. Florensky et al. [11] found

that in this area Dcr, the crater diameter value which isthe boundary between equilibrium and non�equilib�rium subpopulations of the craters [17–19] is about80 m. Applying this value and using the model of Basi�levsky [20] one can calculate the median and the maxi�mum depths of excavation corresponding to this Dcr.The median depth, Hmed = Dcr/25 = 3.2 m and the max�imum depth, Hmax = Dcr/5 = 16 m. The 100–400 mdepth of excavation implies that Fahrenheit ejecta at theLuna 24 site could be derived from the middle�lowerparts of thick lava flows and thus excavate more coarselycrystalline rocks, which, in turn, in the process of theirfragmentation should provide an increased amount ofmonomineralic particles.

The local geologic setting at the Luna 24 site isshown in Fig. 2. Its left part shows a mare surface withnumerous craters. Relatively small (tens of meters indiameter) craters showing fresh morphologic appear�ance are almost certainly secondaries of GiordanoBruno crater [4, 12]. Applying the technique of the esti�mation of the age of small impact craters based on cratermorphologic prominence and size [21], it was shown

20 km

Fahrenheit

С

Fig. 4. Surface of Mare Crisium and highland area (lower right) of the part of the Crisium basin rim as it is seen in the fragmentof LROC WAC mosaic. White star near the image center shows the location of the Luna 24 landing site. White circles—craterslarger than 1 km in diameter; white lines in the mare surface—wrinkle ridges; white lines in the highlands—rimcrests of hills;dotted lines on the mare surface outline circular features, probably craters buried by lavas.

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that they were formed sometime between 5 and 10 m.y.ago and that this is the time of formation of GiordanoBruno crater [13]. Most of subdued craters (amongwhich are both small and relatively large ones) predatedthe secondaries. Two examples of these, designated inFig. 2 (left) with black arrows, are 160 and 225 m indiameter and belong to the so�called morphologic classC. Applying the technique of Basilevsky [21], these cra�ters should be dated as ~200–400 and 450–900 m.y.old, far older than Giordano Bruno secondaries (5–10 m.y.), but probably significantly younger than thecrater Fahrenheit. Figure 2 (right) shows Lev crater andthe Luna 24 lander on the northwestern segment of thecrater rim. Using LROC image M119449091RE, with aresolution of 0.5 m, we measured the distance of thelander from the rimcrest of Lev crater (~7 m). The rimof this crater has a softened morphology with practicallyno visible rock fragments on it, while the crater floor hasa blocky texture with numerous rock fragments. Thissuggests that during its formation process, Lev craterpenetrated through the regolith layer and only margin�ally excavated into the sub�regolith bedrock. The craterdiameter (65 m) suggests that the penetration depth was~6 m, a value that is comparable to the regolith thick�ness in this area (4–6 m) estimated by Basilevsky andHead [13] based on changes of morphology of craters ofthis area along with their size change [22]. Robinsonet al. [4] produced a digital terrain model of the Luna 24landing site based on LROC NAC images and mea�sured the depth of crater Lev (6 m), supporting the sug�gestion that this is a secondary crater.

Insets in Fig. 2 (right) show the Luna 24 lander in theimages taken at different solar illuminations. The rightinset shows bright small features, probably fragments ofthe lander thermal insulation cover separated by theinteraction of the gas jet of the returned rocket during itsascent with the core sample on board. They are also rec�ognizable on the left inset but appear less bright. In bothinsets the shadow from the craft is seen adjacent to thespacecraft, and on the opposite side from it, slightlydarkened surface spots. The latter could be due to localsurface slopes and/or to the removal of upper part ofregolith material by gas jet action during the landerdescent and the returned rocket ascent.

Taking into account the size of Lev crater and theposition of the lander on the crater rim we estimated thethickness of the crater ejecta in the lander location. As itwas shown above, this crater formed mostly within aregolith layer. So for calculations we use the similarity ofejecta deposits for all gravity�dominated craters [15].

The Appendix to this paper presents the estimates ofthe depth where the returned Luna 24 drill core pene�trated the original pre�Lev surface. Starting with gravityscaling from laboratory experiments of Housen et al.[15], we test this simple estimate by comparison withother available data and a trial numerical model (seeAppendix). We conclude that the best available estimate

of Lev’s ejecta layer at the Luna 24 spot is 2% to 3% ofthe rim crater radius, or 0.6 to 0.8 m. Taking intoaccount deviations due to azimuthal variation of the rimheight and local mechanical properties of the regolithand basement, a factor of 2 accuracy is a good approxi�mation. So the original pre�Lev surface can be repre�sented in the Luna 24 core at a depth of samplingbetween ~0.5 and ~1 m. If the thickness of ejecta of Levcrater is 0.5 to 1 m then one may conclude that theejecta may be present only in the upper part of the 2�mlong Luna 24 core while the lower part of it should rep�resent a regolith formed before formation of Lev crater.

So summarizing the above discussion, we may saythat the analysis and model estimates suggest the fol�lowing: Luna 24 landed on the rim of 65 m Lev craterabout 7 m northwest of its rimcrest. Lev crater is one ofthe secondaries formed by ejecta from the distant craterGiordano Bruno at some time between 5 and 10 m.yago. Prior to this time at this location, we should expectarrival of ejecta from the 6.5 km crater Fahrenheit ofEratosthenian age located about 20 km northwest of theLuna 24 site. These ejecta were derived from a depth of~100–400 m and landed at the Luna 24 site in anamount equivalent to a layer ~0.5 m thick. The ratherlarge depth from which the ejecta were excavated sug�gests that at least part of the ejecta could derive from thecentral to lower parts of mare�forming lava flows andthus the rocks initially could be relatively coarse�grained. At the landing point of Luna 24 the estimatedthickness of Lev crater ejecta is 0.5 to 1 m. During thelanding event an unknown upper part of the surfacematerial could be blown away by the lander engine jet.

ANALYSIS OF THE CHARACTERISTICS OF THE LUNA 24 CORE

As it was mentioned above, the Luna 24 lander wasequipped with a drilling device that took a core, sam�pling the local regolith. The core was emplaced inside aflexible plastic tube which after the end of the drillingsession was extracted from the bore�hole, wound on thecoil which was put into the returned capsule, deliveredto Earth and finally was brought to the lunar curatorialfacility at Vernadsky Institute (Fig. 5).

The lunar soil sampling was accomplished by themethod of impact�rotary drilling. The drilling wasmostly conducted in the rotary regime, but the impact�rotary regime was automatically turned on when theresistance increased. Increase in resistence happenedtwice: 1—for a relatively long time during drilling of thegabbro fragment in the interval of 169–171 cm, and2—for a very short time, before the final halting of thedrilling process. The drilling was conducted at an angleof 30° to the local lunar vertical. The internal diameterof the drill head was 8 mm. The outer diameter of theelastic sample holder was 12 mm and its total length was260 cm. In the plastic tube (which was the externalsheath of the sampler) were emplaced eight en–eche�



Fig. 5. The sample taker of Luna 24 in the transportation position in the receiving chamber in the operator hands.

lon–arranged tapes, which were in immediate contactwith the sampling material. The tube sheath thicknesswas on average ~0.085 mm. The tape width was~7.5 mm and the thickness 0.35 mm. So the tapes cov�ered the external sheath from inside practically twice.The total thickness of the sample holder (thickness ofthe external tube sheath plus thickness of two tapes) was0.77 mm. So the calculated internal diameter of thesample holder was 10.4 mm, that is more than 2 mmlarger than the internal diameter of the drill head(8 mm). Simple calculations show that with such inter�nal diameters of the drill head and the sample holder,the emplacement of the material inside the latter theheight of the material column decreases by factor of 1.7.

Figure 6 shows a model of the regolith in the place ofthe Luna 24 sampling site based on estimates of ejectathickness of Lev crater and a schematic representationof the sample core separated into lithologic zonesdescribed by Florensky et al. [10] and Rode et al. [23].The Luna 24 regolith core is characterized by a notice�able clear heterogeneity of material with increasingdepth (Fig. 7). Based on visual study of the core mate�rial and the results of analyses of selected samples, thecore was divided into four basic zones, in which a num�ber of secondary layers were distinguished [10, 23, 25].

Dustiness of the soil sampler tapes marking the placeof the drill contact with lunar soil and actually deter�mining the beginning of the soil sampling process, wasrecorded at a distance of 37 cm from the upper edge ofthe soil sampler. This place is considered as the begin�ning of the count of the soil core depth. Thus, a maxi�mum depth of a drill head is 225 cm, and this corre�

sponds to 200 cm of depth on vertical. Dust traces oflunar material on the soil sampler were seen down to alevel of 47 cm. At greater depths, the amount of sampledmaterial increased. A complete filling of the sampleholder started from a level of 58 cm. The real length ofthe column of material in the sample holder was 160 cmand its mass was 170.1 g [26].

Zone I is considered as starting at a depth of 47 cm,where a small amount of fine material was found, andcontinuing down the level of 73 cm. The total thicknessof the zone is 26 cm. The real core started from the58 cm level, where relatively large (from 1 to 6–8 mm indiameter) fragments of different types were observed.This assemblage seems not to be natural, but is an arti�fact (see below). The thickness of this layer is more than10 cm.

Zone II, with a total thickness of 60 cm (levels of73–133 cm from the beginning of the count), is repre�sented by a visually homogeneous regolith of dark�graycolor, with a small enrichment of relatively morecoarse�grained material in some places. A concentra�tion of agglutinates are typical for the material of thiszone, and these are the highest agglutinate concen�trations among all the zones of the core [25] (Fig. 7).For the < 0.2 mm fraction, a regular decrease in theagglutinate concentration downward in the zone wasobserved. The regolith breccia concentrations arerather constant in fractions of < 0.2 mm, and monot�onously decreased with depth in the fraction of< 0.37 mm. The concentration of metal, both totaland finely�dispersed, in the material of fraction of< 0.2 mm, systematically decreased with depth [26],

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in concert with the regular downward decrease ofmagnetic susceptibility and concentration of aggluti�nates. The characteristics of the zone II regolith ingeneral—the homogeneous character of the materialin the zone, together with a regular decrease of mag�netic susceptibility and the amount of agglutinates(being indicators of maturity of regolith down fromthe top)—provide the possibility to consider materialof this zone to be a single layer (ejecta?), that later wasaffected by exogenous reworking in situ [10, 23].

Zone III, with a thickness of 30 cm (133–165 cmfrom the beginning of the count), has a visual similaritywith the upper zone but is notably different from it inboth lithology and grain�size distribution. A peculiarityof this zone is its enrichment in relatively coarse�grained material represented mostly by homogeneousregolith breccias with porphyroclastic structure (Fig. 7).The concentration of such breccias in the material ofthis zone is seen to sharply increase in comparison withthe upper zone. We interpret the distribution here ofparticles of different lithological types, and the chang�ing of magnetic sensitivity in this zone, to indicatecyclicity in regolith formation [3, 22].

The deep part of the core, united into zone IV, witha thickness of 50 cm (below 165 cm), represents thelightest and most stratified part of the core. The upperboundary of the zone is recorded by a sharp decrease ofthe concentration of regolith breccias (Fig. 7). At adepth of ~169–171 cm a layer of light coarse�grainedmaterial of ~2 cm thickness is seen, representing obvi�ously a fragment of gabbro broken by the drilling pro�

cess. During the drilling through this interval theimpact�rotary regime was turned on automatically. Atthe depth of 180 cm, a sharp boundary between theupper layer of dark�grey fine�grained regolith and alight�grey layer 7–8 cm of thickness, which is charac�terized by high concentration of gabbro and monomin�eralic particles, is observed. The lower boundary of it isrecorded by an increase in exogenous reworking prod�ucts. At a depth of 197 cm, an about 11 cm thick partincompletely filled with material is observed; this couldbe due to partial subsidence of the material during thelifting of the sample taker after the end of drilling. In thelowest part of the core, several fragments of melano�cratic gabbro occur; drilling of the latter in the impact�rotary regime represented the end of sampling. Thechaotic change of observed parameters of the corematerial in zone IV indicates obviously very intensivereworking, mixing of regolith while the material was inthe near�surface environment.

As mentioned above, the upper part of the coreshows some peculiarities: 1—the presence of soil mate�rial only as traces of dust in the interval of 37–47 cm,2—partial filling in the interval of 47–58 cm, and 3—concentration of large fragments in the interval of 58–73 cm. Probably these features, and some peculiaritiesof the core on the whole, are artifacts resulting from oneor several reasons: (1) blowing out the upper part ofregolith by a gas jet during the spacecraft landing on theMoon with preferential removal of the fine�grainedfraction; (2) reducing of the core length because of thesignificantly larger internal diameter of the sample takerin comparison with the diameter of the drill head; (3)compaction of material in the sample taker during thedrilling in the impact�rotary regime.

It is difficult to judge the relative efficiency of thethree factors discussed. The first factor could be respon�sible for formation of the assemblage of large fragmentsin the uppermost part of the core. Reduction of the corelength caused by difference between internal diametersof the drill head and the sample holder obviouslyoccurred during the whole process of soil sampling.Compaction of material during the drilling in theimpact�rotary regime should happen to a significantdegree for levels above ~170 cm, because exactly hereimpacts accompanied by vibration of the whole systemtook place for a rather long time [3].

A very important characteristic of regolith is itsmaturity, that is the degree of alteration of the materialcaused by exogenous factors—meteorite and microme�teorite bombardment, and solar and cosmic irradiation.For estimation of the degree of maturity, various param�eters of regolith are used: concentration and composi�tions of noble gases, density of tracks, concentration ofagglutinates and the Is/FeO ratio determined as a ratioof intensity of ferromagnetic resonance Is to the con�centration of total iron, expressed as FeO. The latterparameter is usually considered as the most accurate

0

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Filled

Void

Youn

g cr

ater

ejec

taO

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reg

olih

0

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Fig. 6. Left—model of a regolith in the place of Luna 24based on estimates of ejecta thickness of crater Lev.Right—a schematic presentation of the sample coreaccording to Figure 1 of Bogard and Hirsh [23] separatedinto lithologic zones described by Florensky et al. [3] andRode et al. [22] (right).



representation of maturity. The Is/FeO value was mea�sured for the fraction <0.25 mm from the six levels of thethree low zones of the regolith core of Luna�24 [27].Based on this parameter the material of zone II, indexesof which are 39.0 and 31.0, is determined to be subma�ture, while the material from zones III and IV withindexes 21.0–27.0 and 19.0 respectively—are consid�ered immature. The concentration of agglutinatesagrees with such a characterization of the maturity ofthe material [28].

A peculiarity of the Luna�24 regolith is indicated bya number of parameters suggesting the two�component

character of its composition that is not typical for otherstudied samples of lunar regolith. For example, thegranulometric composition of six samples of materialfrom zones II, III and IV has a bimodal distribution,especially clear for the sample 24077 of zone II [28].The material bimodality is observed also based on thedistribution of metal iron between grain size fractions,especially for the material of the lower zone IV [29]. Thedegree of maturity, determined by magnetic character�istics and concentration of agglutinates, is not in agree�ment with the maturity degree determined by the effectof cosmic radiation on the material—rare gas contents

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11.71.51.31.10.9 2 3 4 5 10 30 0 20 40 30 50 70

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IV

Co

re d

epth

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Magnetic properties Content of particle types, %

Agglutunates Breccias Monomin

0 100

Magnetic susceptibility Intensity of

Fig. 7. A diagram of the Luna 24 core. Changing with depth of magnetic properties and concentrations of the basic types of theparticles is shown [3, 25].

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Age of crystallization, Ga Age of exposition, Ma0

I

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Fig. 8. Age characteristics of fragments of rocks from the Luna 24 core. Values of crystallization age and the exposition age areshown. The data are from [38, 40–46].

n 103 SGS units/g stand. units/gEPR signal,

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[30] and track density [31, 32]. Such a situation is pos�sibly connected with mixing of materials with differentradiation histories [23]. On the whole, the bimodality ofthe material parameters of the Luna�24 regolith coreagrees well with the model of its formation due to mix�ing of more mature fine�grained material with the lessreworked coarse�grained material [26, 28]. Accordingto the evidence outlined in section 2, it could be themixing of the “local” regolith material with the materialof ejecta from Fahrenheit crater.

One more unusual characteristic of the Luna�24regolith is a uniquely high concentration of monomin�eralic grains [2, 23]. For the six samples from the threelower zones of the core this concentration for the frac�tion 20–250 microns varies from 47.5 to 58.2 wt %, onaverage—52 wt %, [2]. A similar situation is observedfor the 250–500 micron fraction where the concentra�tion of monomineralic grains reaches almost 50% [33],although, naturally, the concentration of monominer�alic grains in this grain size fraction is decreased due tothe increase of the rock fragment concentration, espe�cially for zone II where concentrations of the regolithbreccias fragments are close to 50%. Among themonomineralic grains, pyroxenes predominate, whoseconcentrations usually are above 50% for all the grains,followed by plagioclases and olivines. Accessory andrare minerals are represented by chromite, ulvospinel,ilmenite, phases of silica, troilite and iron metal. Themajority of these mineral phases are also identified inthe rock fragments. Taking into account what wasdescribed in the section 2, one can infer that an admix�ture of Fahrenheit crater ejecta is responsible for thehigh content of monomineralic grains.

The crystalline rocks of the Luna�24 regolith arerepresented by mare basalt and gabbro characterized bya very low content of TiO2 (VLT) and a high Fe/Mgratio [34]. Several research groups studied the fragmentsof crystalline rocks. The description of these rocks givenbelow is based mostly on Taylor et al. [33], because inthis work a significant amount of relatively large (250–500 microns) fragments from six samples from the threelower zones of the core were investigated. It should benoted that the results obtained agree with the results ofother groups doing similar studies [10, 35, 36].

Mare basalts are most abundant among the crystal�line fragments, and the fragments with ophitic texture,also named ferrobasalts, predominate. The fine�grainedbasalts with granulitic texture, described as metabasalts,are less abundant. Both groups have identical chemicalcompositions and average compositions of minerals. Inboth groups of fragments the grain size of mineral doesnot exceed 100 microns.

The best representative of the coarse�grained gab�bro�like rock is the gabbro 24170 fragment broken bythe drilling process, with a mineral grain size more than1 mm. Based on its chemical composition, this rockbelongs to VLT mare basalt. It should be noted that frag�

ment 24170 is not the only representative of this type ofrock. Fragments consisting of crystals >250 microns arefound in the 250–500 microns size fraction. Crystals ofsuch a size are also typical enough among monominer�alic grains such that they provide a reason to link themto the gabbro�type rocks.

Pyroxenes of the crystalline fragments of 250–500 microns in size and monomineralic pyroxenes ofthis fraction have a notably wider range of compositionsin comparison with pyroxene composition of gabbro24170; this leads to the interpretation of the existence ofa second type of gabbro, differing from the sample24170 in its significantly higher value of magnesiumcontent [33]. It was shown that the contents of MgO inpyroxenes and olivines of the second type of gabbro aremore than twice higher than those of the 24170 gabbrotype.

In the early stages of investigation of the Luna�24regolith, some contradiction between MgO content inthe fine�grained fractions of the regolith (~10 wt %) [forexample, 34, 37] and MgO content in the majority ofthe crystalline fragments (6–7 wt %) was noted. Identi�fication of two types of VLT gabbro—iron�rich (whichthe best, although not the only representative, is sample24170), and the magnesium�rich one represented bymonomineralic grains of the fine�grained fractions ofthe regolith, removes this contradiction.

On the whole, crystalline rocks of the Luna�24 coreare VLT basalts (and rocks related to them). Taking thisinto account that both rocks of the local bedrock andobvious ejecta from Fahrenheit crater derived from thedepth of several hundred meters are present in the core,one can conclude that this type of basaltic volcanismwas rather widely represented in the southeastern part ofMare Crisium.

For understanding the geologic history of the Luna24 landing site, the results of isotopic dating of frag�ments of igneous rocks (the age of crystallization andthe exposure age) are important. Dated samples aremostly fragments of basalts and gabbro and one sampleis a fragment of cataclastic anorthosite with traces ofmelting (sample 2460.3–005.1); the latter was consid�ered by Fugzan et al. [38] as a highland rock. The crys�tallization ages were mostly determined by the 40Ar–39Ar technique. All 40Ar–39Ar dates considered by uswere obtained taking into account a new constant of 40Кdecay [39]. Some age determinations were done usingRb–Sr and Sm–Nd techniques. Results obtained bythe different techniques agree rather well. The exposureages were calculated from concentrations of cos�mogenic 38Ar. The dating results are presented in Fig. 8,where age values are shown according to the sampledepths in the core and thus reflect age variations or theirabsence along the stratigraphic sequence. It is seen fromFig. 8 that fragments of basalts and gabbro cluster in theinterval of 3.2–3.4 Ga sometimes extending beyond.This is close to the estimate of Stoffler et al. [47] (3.22 ±



0.07 Ga) based on dating results considered by theseauthors. Fugzan et al. [38] distinguished two groupsamong the seven samples of basalts and gabbro dated bythem: 3 samples with ages of 3.43–3.52 Ga from a depthof 60–142 cm (Zones I, II and the upper part of ZoneIII) and 4 samples with ages 3.25–3.39 Ga from a depthof 170–218 cm (Zone IV). However dating results ofother researchers do not completely agree with such astratified age distribution. So it seems that 3.2–3.4 Ga isa more correct estimate and taking into account theerror bars it should be considered as the time ofemplacement of low�titanium basalts that formed thesoutheastern part of Mare Crisium [5, 8].

The fragment of cataclastic anorthosite 2460.3–005.1 mentioned above showed an age of 3.75 ±0.07 Ga. This value appears too low for highland rocks,which are usually older than 3.9 Ga [47]. This is proba�bly the result of a superposed impact that led to theobserved cataclasis and partial melting of this rockand naturally to the age decrease. It is very possiblethat this specific impact brought this fragment to theLuna 24 site.

Exposure ages of the fragments studied vary from14.5 to 1370 Ma. The highest variations are typical fordepths of 143 cm and less (Zones I, II and the upperpart of Zone III), while at greater depths (Zone IV), therange of values decreases to 158–308 Ma. The mostinteresting value appears to be 14.5 Ma (error bar±1.5 million years). This is the result of analysis of afragment of cataclastic anorthosite 2460.3–005.1 andthis age is close to the age of Lev crater and correspond�ingly to the age of the crater Giordano Bruno [13, 48].It appears plausible that this fragment of cataclastic,and partly melted, anorthosite is a part of the projectilethat formed Lev crater, and is thus a fragment of the far�side highlands.

In order to understand the history of formation ofthe regolith at the Luna 24 landing site, it is worthwhileto involve model considerations of regolith formationthat have been described in a number of works, forexample in Gault [49] and Basilevsky [19]. Accordingto these models, in the process of reworking of the lunarsurface by meteorite impacts, the number of episodes ofoverturn and the depths of such overturn events, are to afirst approximation proportional to the time duringwhich the regolith was being formed in this specificplace. For the Luna 24 site this is equivalent to the timethat has passed since the emplacement of lava flows thatformed the south�eastern part of Mare Crisium, that is~3.2–3.4 Ga. The maximum depth of overturn in thisgiven place is the regolith thickness; a model value canbe conveniently determined through the value of thecritical diameter Dcr mentioned in section 2. The valueof Dcr is a boundary between the equilibrium and non�equilibrium subpopulations of craters in this given area[16–18]. In this analysis, minimum, median and maxi�mum thicknesses of regolith are calculated as Dcr/50,

Dcr/25 and Dcr/5, correspondingly [20, formulae 11–13]. For the Luna 24 landing site, the critical diameter,Dcr, was determined by Florensky et al. [11] to be 80 m.Correspondingly, the minimum, median and maximumthicknesses of regolith here are 1.6 m, 3.2 m 16 m. It isobvious that in the process of formation of lunarregolith the mechanical overturns are accompanied byall the range of effects of impact�involved transforma�tions: fragmentation, crystalline structure damage, par�tial or complete melting and partial or complete vapor�ization and so on. So, an increase in the number of over�turns should lead to an increase in regolith maturity.

This approach provides the possibility to try tounderstand which part of the Luna 24 core representsthe ejecta from Lev crater. If the latter was formed 5–10 Ma ago, that is, three orders of magnitude less thanthe total time duration of regolith formation in the Luna24 site (3.2–3.4 Ga), then the depth of impact rework�ing of its ejecta for the time passed since this craterformed, should be smaller than a centimeter. This per�mits us to exclude Zone II from the candidates for theejecta of crater Lev because of the typical signatures ofgradual “maturation” of its material along its 60�cmlength. Such a length implies impact reworking during aperiod more than hundreds of millions to one billionyears. In this case practically the only possible candidateon the role of ejecta of Lev crater is Zone I, where, bythe way, was found the fragment of cataclasticanorthosite 2460.3–005.1 having the exposure age closeto the Lev crater age estimates and possibly representingpart of the impactor that formed this crater as ejectafrom the farside crater Giordano Bruno.

DISCUSSION AND CONCLUSIONS

The completed analysis of the geologic framework ofthe landing site, the estimation of the ejecta thickness ofLev crater expected at the place on the rim that thespacecraft landed, and the analysis of the characteristicsof the lunar material core returned by Luna 24, togetherallow us to distinguish two concrete episodes of historyin the context of the background of the multiphase (butwithout association with any concrete events) process ofregolith formation: 1—arrival of ejecta from the 6.5�kmcrater Fahrenheit, located 20 km to northwest from thelanding site, and 2—formation of the 65�m crater Lev.

The presence in Zone IV (165–223 cm) of a frag�ment of gabbro and significant amounts of relativelylarge monomineralic grains may suggest that they orig�inated from deeper parts of lava flows, that in turn canindicate the presence of ejecta from crater Fahrenheit inthe material of this zone. The ejecta, arriving at thelanding site with high enough velocity (165–230 m/s,[14]), should form secondary craters and be mixed upwith the material of local regolith in which the materialof the local bedrock had to dominate. This informationcan be used for additional consideration of Zone IVmaterial, attempting to distinguish igneous rock frag�

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ments of local origin and those brought from Fahren�heit, or to find that there is no difference. The latterwould mean that at the distance of the order of 20 km,and in the interval of depths down to 100–400 m belowthe surface, we are dealing with the same volcanic asso�ciation.

Zone III (133–165 cm) and II (73–133 cm), above,seem to represent separate episodes of excavation fromsmall craters. In particular Zone II (73–133 cm), judg�ing by the continuity and direction of changes of char�acteristics reflecting the degree of maturity, probablyrepresents the relatively recent stage of reworking oflocal regolith by small and very small impacts. Thematerial participating in this reworking is essentially thematerial of the underlying Zones III and IV,with theaddition of what was being brought by small impactsfrom some distance from the landing site.

Zone I represents an area of special interest for addi�tional study. As described above, Luna 24 landed on therim of the 65�m crater Lev, which seems to be one of thetypical secondary craters formed by ejecta of the distantand very young crater Giordano Bruno. The age of thelatter and secondary craters formed by impacts of itsejecta is about 5–10 Ma [13, 18]. These estimates agreewith the results of recently published work [50], inwhich based on analysis of publications on age andgeochemical characteristics of lunar meteorites, data onremote sensing of vicinities of Giordano Bruno crater,distribution of He3 in young sediments of Earth andother information it is concluded that lunar meteoritesYamato 82193/821983/86032 ejected from the Moon8.5 ± 1.5 m.y. were delivered to Earth by impact formedthis crater.

Our calculations showed that the expected thicknessof ejecta from Lev crater at the landing point is 0.5–1 m.However, analysis of the characteristics of the corematerial led to the conclusion that the ejecta from thiscrater is probably represented only by Zone I, the totallength of which is 26 cm while the complete filling of thesoil tube occurred only in the lower 15 cm of this zone.This appears to mean that the essential part of ejectafrom Lev crater at the landing point was lost, possibly,due to blowing out by the engine jet at the landing; thisagrees with the increased concentration of relativelycoarse fragments, and possibly due to other factors.

If Zone I is indeed material of ejecta from Lev crater,it was derived obviously from the lowest horizons of theregolith in this place because, as a rule, material of thedeepest horizons of the target excavated by the crateringevent is deposited on the crater rim (see for example,[14, 15, 51]. Grains of basalts from Zone I probably rep�resent fragments of lava flows underlying the regolith inthis place. The Zone I material deserves special atten�tion with the goal of searching for remnants of the pro�jectile that formed Lev crater (very likely to be materialfrom the farside highlands ejected from the crater Gior�dano Bruno). Obviously these should be impact�modi�

fied fragments of the highland rocks. The latter, in prin�ciple, could be also brought from the nearest segment ofthe Crisium basin rim south�east of the Luna 24 landingsite, but the very young age of Lev crater makes this pos�sibility to be of very low probability.

In conclusion we would like to note that joint con�sideration of the high�resolution images of the Luna 24landing site, including estimations of expected thick�ness of ejecta from Lev crater, on whose rim the space�craft landed, and characteristics of the core of regolithdelivered to Earth, allowed us to understand the detailsof the geologic structure, stratigrphy and geologic his�tory for the immediate vicinity of the landing site. This,in turn, allows us to suggest what events could beresponsible for formation of different parts of the Luna24 core: 1—arrival of ejecta from the 6.5�km craterFahrenheit (Zone IV), 2—reworking of the localregolith by small impacts (Zones II and III), and 3—emplacement of ejecta of the 65–m Lev crater, which isinterpreted to be a secondary from the distant 22�kmfarside crater Giordano Bruno (Zone I).

ACKNOWLEDGMENTS

We acknowledge helpful discussions with J. B. Ples�cia, G. A. Burba, V. V. Vysochkin, C. A. Lorenz andM. M. Fugzan. Financial assistance for JWH and ATBwas provided by the Brown�MIT NASA Lunar ScienceInstitute (NNA09DB34A).

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APPENDIX

RELEVANT CRATERRIM STRUCTURE INFORMATION

The rim formation at the edge of an impact crater isa complicated process. The main features of the craterrim are the structural uplift of the pre�impact target sur�face, and the overturned flap of material deposited atopof this uplifted surface. At distances more than ~1 craterradii the overturned flap gradually converts to the con�tinuous ejecta layer. The structural uplift amplitude andthe ejecta thickness decrease with the distance from the

crater center. The amplitude and thickness depends onthe mass of material displaced from the crater cavity tothe given distance and the change of its density duringthis displacement. Loose materials may compact to adensity above the initial pre�impact density, while densematerials (like rock and clays) or densely packed granu�lar materials (like well tapped sand) experience volumebulking. Hence simple models for quantitative predic�tions of the rim structure for arbitrary initial conditionsare not yet available. The situation is improved with theuse of analogues—small scale laboratory impact crater�ing and terrestrial explosion craters well studied in thefield.

Below we review: (a) simplest estimates for impactand explosion cratering in uniform granular media, (b)terrestrial decameter�scale explosion craters, (c) somespeculations about layered targets (mostly—less com�petent material above more competent basement).

The general estimates for uniform targets made ofgranular materials are useful as a starting point due tothe fact that craters in cohesionless media with dry fric�tion are formed in the so�called gravity regime [Al]. Inthis regime rims for these craters are self�similar. Fig�ures presented in [Al] are overplotted with numerousexperimental points, so one can draw outlines boundingthe individual data clouds. Housen et al. [Al] use theapparent crater radius (the radius measured at the levelof the pre�impact target surface), Ra, to normalize thedistance from the crater center and the vertical coordi�nates (hereafter x and y, correspondingly). The usage ofRa is easy for experimental conditions where the geom�etry is controlled by experimentors. On other planetsthe rim crest crater radius, Rrim is the simplest parameterto measure. For impact craters in granular mediaRrim/Ra ratio is close to 1.25 [A2], but in individualexperiments this value can vary from ~1.2 to ~1.3. Forsimplicity we re�plot data from [Al] using the visible rimcrest radius for normalization of distances and thick�nesses assuming that Rrim/Ra = 1.25 (Fig. A1). The nor�malized distance of the Luna 24 landing site from therim crest of Lev crater (see the main text) is about x =1.3 Rrim where for the crater Lev Rrim = 64/2 =32. Thisnormalized distance from the crater center (x =1.3 Rrim) is outlined in many figures below.

Laboratory impacts in dry sand used in Fig. A1(Fig. 7c in [Al]) are Boeing laboratory scale impacts inthe velocity range from 1.8 to 7.2 kms–1 at the gravityacceleration of 1, 100, and 400 G (1G = 9.81 ms–2).The target of dry sand had a density of 1.8 g/cm3.No data about preimpact surface uplift at the crater rimis presented by [Al].

Another set of relevant data is available for shallowburied explosions for so called “tangent below” geome�try (a spherical HE charge is buried at 1 charge radius).This explosion geometry is the best proxy for impacts,but some deviations could not be excluded [A3].



0.1

0.01

0.0011.81.61.4 2.01.21.0

x/Ra

Housen et al. (1983), impacts

y/R

a

Fig. A1. The “envelope” of experimental points from [A1] (thick grey curves), re�plotted using Rrim as the unit length.

0.1

0.01

0.0011.61.41.2 1.81.00.8

x/Rrim

Housen et al. (1983), tange

y/R

rim 0.023 to 0.028

НЕ�2

Fig. A2. The “envelope” of experimental points from [A1] (thick grey curves) in comparison with the HE�2 explosion crater rimprofile digitized from [A4, fig. 5]: black curve with black dots is for the rim profile, lower black curve with dots is for the pre�impactsurface uplift.

Figure A2 illustrates envelopes for experimental data insand.

For such geometry of explosions there are experi�mental data for the craters up to 40 m radius in allu�vium. The rim structure for HE�2 experiment (Opera�tion Jangle) is published by [A4]. For HE�2, craterradius is about 12 m. These data are shown in Fig. 2.They were also used in the work [A5], cited in the recentpaper [A6]. In some respects dry alluvium (density of1.6 g/cm3) is a good available proxy to a meter—fewmeters deep regolith where density reaches values of~1.8 g/cm3 (see Chapter 7 in Lunar Sourcebook [A7]).

The rim thickness estimate both for the experimentsof Housen et al. and for the HE–2 explosion are closeat x/Rrim =1.3 (y/Rrim = 2.3% to 2.8%, or 0.7 to 0.9 mfor the gravity scaled Lev crater). It is possible however,that the structural uplift is about 1/4 to 1/3 of the rimprofile (~0.8% Rrim or ~0.25 m in the Lev case) due topre�impact surface uplift. If so, the overturned flap

sequence at the Luna 24 spot is ~0.5 m below the sur�face.

To test how close the HE�2 event is to the case ofpure gravity scaling, Fig. A3 compares dimensionlesscrater radius and volume (πRa and πVa) for three Jangleevents and the Boeing laboratory experiments with“tangent below” explosions in dry sand and the modelalluvium.

Of interest us is that the HE�2 event (scaled yieldpi_2 = 5.2 × 10–6 ) is in the transitional zone betweenranges of pure strength (the lower pi_2) and gravity (thehigher pi_2) scaling [A8]. Hence, a simple gravity scal�ing of HE�2 rim profile to the lunar environment shouldbe taken with a certain caution.

For comparison we review a few cases relevant to theestimates in Figs. A1 and A2.

Low Velocity Laboratory Impact. Oberbeck [A10]published (his Fig. 9) the photo image of a profile ofa crater in sand formed by impact at 1.04 km/s–1.

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Digitized images allow us to estimate the rim profilefor this low�velocity impact (Fig. A4). The compari�son does not show any dramatic deviation forimpacts > 1.7 km/s–1, and estimates the structuraluplift (0.6% to 0.7% of Rrim) close to the HE�2explosion crater (Fig. A2).

Layered Targets. The presence of less competentregolith layer over the more competent fractured rockbasement on the Moon results in the evolution of cratermorphology from bowl�shaped to flat�floor and furtherto concentric craters while the ratio of crater diameterto regolith thickness, D/hreg, grows from ~4 to ~10 [A9].Lev crater diameter (D = 64 m) is in the upper range ofthis interval assuming a regolith thickness of 4 to 6 m(D/hreg of 10 to 16). Note that the real boundary valuesof D/hreg depend on the “competence” of the base�ment—fractured basement may shift the transition to

concentric craters toward larger D/hreg. Later below wereturn to this point.

To begin we consider available data concerning therim height variation in the case of double�layer targets.The oldest data on the explosion crater rim in the lay�ered target are presented as a single figure (Fig. 7) in[A10]. The quality of the figure is not good enough forquantitative analysis. However the qualitative tendencyis obvious—as the D/hreg ratio grows from 3.8 to 12.6the relative rim height and the rim crest/apparent craterdiameter ratio decrease (Fig. A5).

A number of explosion experiments in layered tar�gets are discussed in [A12]. The best image (Fig. 17 in[A12]) for a crater in a layer of dense dry sand (zeroDOB explosion semiburied spherical charge) was usedto draw profiles of the crater rim in four perpendicularazimuths (Fig. A6). Taking the rim crest radii alongeach of these directions, we see that rim crest/apparent

10000

1000

100

101.0E–041.0E–051.0E–061.0E–071.0E–08

Scaled yield, π2

Sca

led

volu

me,

πV

a100

10

11.0E–041.0E–051.0E–061.0E–071.0E–08

Scaled yield, π2

Sca

led

radi

us, π

Ra

sand

alluvium

Jangle series

Jangle HE�2

MG�1

Fig. A3. Scaled apparent crater volumes (left panel) and radii (right panel) for “tangent below” Boeing laboratory experiments insand and model alluvium in comparison with three Jangle events v.s. scaled explosion yield π2. The black square designates theHE–2 event. See definitions and scaling law discussions in [A8].

0.1

0.01

0.0011.61.41.2 1.81.00.8

x/Rrim

y/R

rim

0.020 to 0.026

Fig. A4. The same as in Fig. Al with the addition of data for 1.04 km/s impact, digitized from [A3, Fig. 9]: Black curve is for therim profile, broken line with open circles is for the structural uplift.



0.1

0.01

0.0011.81.61.4 2.01.21.0

x/Ra

y/R

a

1

Housen et al.,1983, impacts

D/h_reg = 3.8

D/h_reg = 8.3

D/h_reg = 12.6

Fig. A5. Crater rim profiles around model craters in sand layer over competent basement, [A10] citing [A11], normalized with theapparent crater radius. Grey curves outline experimental data for impact in the uniform dry sand target [A1].

0.1

0.01

0.0011.61.41.0 1.80.80.6

x/Rrim

y/R

rim

1.2

Fig. A6. Four azimuthal (0°, 90°, 180°, and 270°) rim profiles for the explosion crater in a dry dense sand over a competent base�ment (D/hreg = 5) [A12] in comparison with rim profiles for impact craters in the uniform dry sand.

0.1

0.01

0.0011.61.41.0 1.80.80.6

x/Rrim

y/R

rim

1.2

Fig. A7. The MG–1 explosion crater lips after Fig. 1 in [A13]. Black labeled curves are for the rim profile, open signs connectedwith dashed curves are for the original surface structural uplift. Two opposite radial crossection are shown.

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crater diameter ratio is about 1.41, what is significantlylarger than the reference value of 1.25. The rim height isalso slightly larger than for impact experiments in uni�form targets. The dimensionless rim heights at a dis�tance of 1.3 Rrim vary from ~2% Rrim (that is typical forimpact craters in uniform dry sand) to ~3.5 % Rrim). Thereasons for this deviation are not obvious as both largerbulking of dense sand in the rim and the presence of thebasement (D/hreg = 5) may contribute to the change ofrim profile. We present these data to illustrate a level ofcaution that should accompany the scaling of variousexperimental data to the lunar Lev crater case.

Large (for gravity scaling) scale explosion craterswith well documented rim structures are rare in the lit�erature. The largest crater formed by zero DOB explo�sion with the known rim structure was created by theexplosion of 20 tons of TNT. This is the MG–1 explo�sion (Middle Gust series) on the surface of wet claylying over wet weathered shale [A13, A14]. The craterwas explored with a trench that revealed barelydeformed basement below ~5 m depth. Taking the rimcrater diameter of 35 m one may estimate the relativethickness of the “soft” layer as D/hreg = ~7. In contrastto previous laboratory example, the rim of the craterformed by the MG–1 explosion is more narrow(crest/apparent crater radius ratio is as small as ~1.13),while the rim height decreases faster with a distancebeyond the rim height (Fig. A7). Without a guarantee ofgravity scaling applicability one can say that at a dis�tance of 1.3 Rrim the ejecta layer thickness is of the.orderof 0.7% to 1% of the rim crest radius. The useful pieceof information is that for such a crater the structuraluplift seems to be less than ~20% of the total rim thick�ness at the distance of interest ~1.3 Rrim.

We conclude, that the best currently available esti�mate of Lev’s ejecta layer at the Luna 24 spot is 2% to3% of the rim crater radius, that is 0.6 to 0.8 m. It is nec�essary to take into account that the presence of a rockybasement beneath the regolith layer, and variations oflocal mechanical properties of regolith and base�ment, suggest that a factor of 2 accuracy is a goodguess. So the available experimental data show thatthe drilling device of Luna 24 should cross the origi�nal slightly uplifted pre�Lev crater surface at a depthbetween ~0.5 and ~1 m.

REFERENCES QUOTED IN THE APPENDIX

A1. K. R. Housen, R. M. Schmidt, and K. A. Holsapple,“Crater ejecta scaling laws—Fundamental formsbased on dimensional analysis,” J. Geophys. Res. 88,2485–2499 (1983).

A2. D. E. Gault and J. A. Wedekind,”Experimental hyper�velocity impact into quartz sand. II—Effects of grav�itational acceleration,” in Impact and Explosion Cra�tering” (Pergamon Press, New York, 1977), pp. 1231–1244.

A3. V. R. Oberbeck, ”Laboratory simulation of impact cra�tering with high explosives,” J. Geophys. Res. 1971.V. 76. P. 5732–5749.

A4. R. H. Carlson and G. D. Jones, “Distribution of ejectafrom cratering explosions in soils,” J. Geophys. Res.70, 1897–1910 (1965).

A5. R. Arvidson, R. J. Drozd, C. M. Hohenberg,C. J. Morgan, and G. Poupeau, “Horizontal transportof the regolith, modification of features, and erosionrates on the lunar surface,” Moon 13, 67–79 (1975).

A6. M. S. Robinson, J. B. Plescia, B. L. Jolliff, andS. J. Lawrence, “Soviet lunar sample return missions:Landing site identification and geologic context,”Planet. Space Sci., 2012. (in press).

A7. Lunar Sourcebook—A User’s Guide to the Moon(Cambridge University Press. Cambridge, 1991).

A8. K. A. Holsapple and R. M. Schmidt, A material�strength model for apparent crater volume,” Proc.Lunar Planet. Sci. Conf. 10, 2757–2777 (1979).

A9. W. L. Quaide and V. R. Oberbeck, “Thickness deter�minations of the lunar surface layer from lunar impactcraters,” J. Geophys. Res. 73, 5247 (1968).

A10. V. R. Oberbeck, “Application of high explosion crater�ing data to planetary problems,” in Impact and Explo�sion Cratering: Planetary and Terrestrial Implications(Pergamon Press, New York, 1977), pp. 45–65.

A11. E. P. Forston and F. R. Brown, “Effects of soil�rockinterface on cratering morphology,” Tech. Rep.No. 29487 U.S. Army. Eng. St., Corp. of Eng., Vicks�burg, Miss., 28 (1958).

A12. A. J. Piekutowski, “Cratering mechanisms observed inlaboratory�scale high�explosive experiments,” inImpact and Explosion Cratering: Planetary and Terres�trial Implications, (Pergamon Press, New York, 1977),pp. 67–102.

A13. D. J. Roddy, “High�explosive cratering analogs forbowl�shaped, central uplift, and multiring impact cra�ters,” Proc. Lunar Planet. Sci. Conf. 7, 3027–3056(1976).

A14. Nuclear Geoplosics Sourcebook. Volume IV. Part II.Empirical Analysis of Nuclear and High�Explosive Cra�tering and Ejecta (General Electric Company�TEMPO, Santa Barbara, 1979).

clarification of sources of material returned by luna 24 ...lev tarasov. photo taken in 1970s. in...

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