Geological Context of Potential Landing Site of the Luna Glob Mission

ISSN 0038�0946, Solar System Research, 2014, Vol. 48, No. 6, pp. 391–402. © Pleiades Publishing, Inc., 2014.Original Russian Text © M.A. Ivanov, A.M. Abdrakhimov, A.T. Basilevsky, J.L. Dixon, J.W. Head, L. Chick, J. Vitten, M.T. Zuber, D.E. Simt, E. Mazarico, C.D. Neish, D.B.J. Bassey,2014, published in Astronomicheskii Vestnik, 2014, Vol. 48, No. 6, pp. 423–435.

391

INTRODUCTION

The main scientific goal of the Luna�Glob missionconsists in the investigation of volatile componentsincluding water in regolith of the southern polarregion. The theoretical considerations in favor of thepotential presence of volatiles including, primarily,water in polar regions of the Moon (Watson et al.,1961; Arnold, 1979) were confirmed by the dataobtained in the course of the Lunar Prospector (Feld�man et al., 1998), LCROSS (Colaprete et al., 2010),and Lunar Reconnaissance Orbiter (the LEND spec�trometer) (Mitrofanov et al., 2010) missions, whichobtained evidence in favor of the presence of water inboth polar domains of the Moon. The quantitative cal�culations show that the content of water in regolith ofthese regions may be as high as a few percents. TheLuna�Glob spacecraft is equipped with a set of toolstargeted at direct analyses of regolith for quantitativeinvestigations of volatiles with estimation of their

phase, chemical, and isotope compositions (e.g.,Zelenyi et al., 2010).

The ballistic parameters of the Luna�Glob missionmake it possible to investigate regolith in the southernpolar sector with coordinates 70°–85° S and 0°–60°E (Fig. 1). The new high�resolution topographic dataobtained by the LOLA laser altimeter during the LunarReconnaissance Orbiter (LRO) spacecraft mission(Smith et al., 2010) show that this region represents asegment of the external swell surrounding the largestand, probably, oldest impact structure on the Moonsurface: the South Pole–Aitken (SPA) basin (Stewart�Alexander, 1978; Wilhelms et al., 1979). Such a posi�tion of the landing sector allows the goals of the mis�sion to be widened: to investigate the oldest material ofthe Moon in addition to volatiles.

The knowledge of the regional/local geologicalsituation and altimetry of the surface is necessary forselecting the landing site of the descent module,understanding the type of material on the Moon, and

Geological Context of Potential Landing Site of the Luna�Glob Mission

M. A. Ivanova, b, c, A. M. Abdrakhimova, b, A. T. Basilevskya, b, c, J. L. Dixonb, J. W. Headb, L. Chickb,J. Vittenb, M. T. Zuberd, D. E. Simte, E. Mazaricoe, C. D. Neishf, and D. B. J. Basseyg

a Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, Russiab Brown University, Providence, USA

c Moscow State University of Geodesy and Cartography, Moscow, RussiadMassachusetts Institute of Technology, Boston, USA

e Goddard Center, Greenbelt, USAf Florida Institute of Technology, Melbourne, USA

g Laboratory of Applied Physics, John Hopkins University, Washington, USAe�mail: mishaivn@mtu�net.ru, [email protected]

Received July 18, 3014

Abstract—The region planned for performing the Luna�Glob mission is located in the southern part of theswell surrounding the largest South Pole–Aitken (SPA) basin. The photogeological analysis of the surfacetopography of this region using the LRO�WAC (resolution of 100 Mpxl) photomaps made it possible to definethe following groups of morphological units (area types): (1) related to the formation of relatively fresh impactcraters; (2) associated with larger (>100 km across) degraded craters including (2a) external and (2b) innerfacies; and (3) occupying intercrater spaces. The comparison of the geological map with the map illustratingthe distribution of the epithermal neutron flow (Mitrofanov et al., 2012) shows no correlation between them.Consequently, one should not expect development of rock complexes, which would be characterized by ele�vated concentrations of water in the region chosen for the Luna�Glob mission and, thus, considered amongthe first�priority targets. The comparison of the neutron flow distribution with the map of circular polariza�tion of the Mini�RF radar beam also shows no correlation. This means that high values of circular polariza�tion reflect elevated concentrations of rock fragments rather than water accumulations. Even though ice frag�ments are present, their sizes should only slightly be less as compared with the radar wavelength (12.6 cm). The regionplanned for investigations in the scope of the Luna�Glob mission corresponds to the swell of the largest (and, likely,oldest) preserved basin and offers a potential opportunity to analyze ancient material of this planet and introduceimportant constraints into the spectrum of models proposed for explaining the Moon’s origin.

DOI: 10.1134/S0038094614060021

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estimating the degree of risk connected with landing atthe selected point.

The photogeological analysis of the data obtainedby the Lunar Orbiter mission made it possible to com�pile a geological map of the region of the Moonlocated south of 45° S (Wilhelms et al., 1979). Becauseof restrictions related to the spatial resolution of map�ping, its geometry, and light conditions, this map illus�trates only general features of the regional geologicalstructure. New images of the surface and topographicdata characterized by high resolution and survey qual�ity offer an opportunity to specify and add substan�

tially to the available map and approach to the maintarget of the mission: selection of landing sites andestimate their priority with regard to scientific pur�poses of the mission and potential risks.

In this work, we characterize the morphologicalfeatures of surface areas in the region planned forinvestigations during the Lunar�Glob mission. Thenew geomorphological map of the region, which illus�trates the spatial distribution of these types of regionsis compared with the available geological map (Wil�helms et al., 1979) and calibrated in terms of their rel�ative ages. This procedure made it possible to impart

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Fig. 4а

Fig. 4bFig. 2а

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Fig. 3 Fig. 2b

Fig. 1. Photomap of the Luna�Glob spacecraft landing area. The mosaic of images obtained by the wide–angle camera (WAC)during the LRO spacecraft mission, resolution 100 Mpxl, polar stereographic projection. White squares indicate positions of fig�ures mentioned in the text.


GEOLOGICAL CONTEXT OF POTENTIAL LANDING SITE 393

the stratigraphic sense to mapped types of morpholog�ical units, correlate them with the global stratigraphicscale (e.g., Wilhelms, 1987), and carry out the pre�liminary analysis of features expected at potentiallanding sites.

MORPHOLOGICAL UNITS IN THE REGION OF POTENTIAL LUNA�GLOB LANDING

The first steps of our investigations were aimed atidentification and classification of morphologicalstructures and grouping them into separate morpho�logical units (area types) that form the surface of theLuna�Glob landing region. For characterizing thesemorphological units, we used mosaics of imagesobtained by the wide�angle camera (LRO�WAC) usedduring the LRO spacecraft mission. The spatial distri�bution of such images constitutes 100 m/element ofthe image (pixel). The units were determined by thetraditional photogeological method: when the surfaceof particular area is characterized by specific morpho�logy, the latter determines the morphological unit(e.g., Wilhelms, 1974). The topographic position andlocation relative to other notable relief forms serve asadditional important criteria for determining the mor�phological unit. For example, morphologicallysmooth plains between craters and at the bottom oflarge craters were attributed to different types of areas,which are likely characterized by different origins.

The photogeological analysis of mosaics of theLROC�WAC images revealed 11 types morphologicalunits in the region under consideration (Table 1).Practically all of them are related to impact craters.The relatively small (a few tens of kilometers in diam�eter) fresh impact craters are the most expressiveforms. The integrity of such craters may be dividedinto three groups. The first group includes craters withthe acute ridge of the swell and notable blanket ofejected material (cfpe type). There are only four suchcraters; their diameters vary from 10 to 30 km. Thelargest crater from this group (78.5° S, 23.5° E) over�

lies the southern part of the swell surrounding the largeSchomberger crater (Fig. 2a). Ejections from cratersof the cfpe type form a morphologically expressivehummocky and striated surface near the crater andaway from the latter, respectively (for example, at thebottom of the Schomberger crater). The diameter ofthe continuous zone of material ejected from the larg�est crater of this morphological type is approximately100 km. This material covers the underlying surfaceand smooths to some extent its large�block (hundredsof meters to a few kilometers) structure.

The craters with less notable ejections (cfse type)constitute the second group of relatively fresh craters.Many such craters are barren of hummocky and stri�ated ejection facies, although they are surrounded bychains of secondary craters (Fig. 2b). Such craters aremost widespread and characterized by acute edges ofswells, a smooth bottom (when diameters of cratersexceed 10 km), a conical shape of the crater depres�sion and, locally, terraced walls. The largest craterfrom the second group is approximately 25 km indiameter and 2.5 km deep.

The third group of youngest craters (cfne morpho�logical type) includes the varieties without the distinctejection zone, although they are surrounded by a dis�tinct swell (Fig. 2c). The largest crater from this groupis approximately 16 km in diameter and 2 km deep.

The most significant morphostructures in theregion under consideration are represented by large(approximately 100 km in diameter) craters. The areasof such a type marked by these craters are subdividedinto the outer, inner, and bottom facies.

Material constituting swells of large craters formsthe outer facies (lcr morphological unit) (Fig. 3). Theswells of craters represent significant topographic fea�tures towering above the surrounding surface for 3–5 km.The surface is characterized by the block structure.Some blocks tower above the surrounding surface fortens to a few hundreds of meters being 1–1.5 kmacross and characterized by rounded edges. None

Table 1. Morphological units in the planned landing area

Association Facies Unit Index

Small craters Craters with distinct ejections cpe

Craters with vague ejections cse

Craters with acute ridges, but barren of ejected material cne

Large craters Smooth plains at the bottom fps

Bottom Smooth–hummocky plains at the bottom fpsh

Hummocky plains at the bottom fph

External Swells of large craters lcr

Inner Walls and central massifs lcwm

Intercrater material Smooth intercrater plains icps

Hummocky intercrater plains icph

Elevated massifs icmr

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of the craters have notable covers of ejected material.Material constituting the walls and central mounds oflarge craters form the inner facies (lcwm morphologi�cal unit) (Fig. 3). The edges of the swells rimming largecraters are usually scalloped and their walls are ter�raced, being as high as 8 km above the crater bottom(for example, Demonax crater); the walls are usually4–6 km high. The central massifs are missing fromsome craters: Bossingauls (~140 km across), Scott

(~130 km), Boguslawsky (~100 km), and Simpelius(~70 km). In some craters, where central massifs aredistinguishable, they are several kilometers across anda few hundreds of meters high above the bottom.

The group of morphological types forming the bot�tom facies consists of smooth (fps unit, Fig. 3),smooth hummocky (fpsh unit, Fig. 3), and hum�mocky (fph unit, Fig. 3) plains. All the plains of thesetypes are smooth and subhorizontal: topography vari�ations in them never exceed a few hundreds of meters

NN

N

(a) (b)

(c)

20 km 10 km

10 km

Fig. 2. Examples of youngest craters in the region of planned investigations. (a) Crater with a clearly seen blanket of ejected mate�rial (fcpe morphological unit). White arrow indicates the striated structure of the surface, black arrow shows ejected material cov�ering the bottom of the large Schomberger crater. (b) Crater with an indistinct blanket of ejected material (fcse morphologicalunit). The crater is characterized by the swell with the acute ridge and terraced walls. (c) Craters with acute ridges of swells,although without a distinct blanket of ejected material (fcne morphological unit). All images are obtained by the WAC camera,resolution 100 Mpxl, polar stereographic projection.



Fig. 3. Images and morphological map of facies of the large Schomberger impact crater. The image is obtained by the WAC device,resolution 100 Mpxl, polar stereographic projection.

over distances of tens of kilometers. The plains differfrom each other in the share of smooth (plain�form�ing) material, which surrounds either central massifsor blocks originating from the slopes of craters. Withinthe smooth plains (fps), such blocks and relicts of cen�tral massifs are missing and the thickness of plain�forming material in these areas is probably maximal.In the transitional zone between smooth–hummocky(fpsh) and hummocky (fph) plains, the share ofexposed blocks increases and the cover of plain�form�ing material becomes correspondingly thinner.

The areas located beyond the external facies oflarge craters form intercrater morphostructures ofthree types (Table 1). They are dominated by inter�crater hummocky plains (icph morphological unit,Fig. 4a). These plains are widespread in the northwest�ern part of the region in question. Their surface exhib�its the rough hummocky structure, although beingsmoothed with variations in the topography neverexceeding 1 km. Despite the fact that plains are sur�rounded by swells of large craters, chronological rela�tions between these two morphological types areunclear.

N

20 km

lcr

lcwm

fphfps

fpsh

lcr

lcr

lcwm

fphfps

fpsh

lcrlcr

lcwm

fph

fpsh

fps

Material ejected from the young crater

Bottom facies:

smooth plains

smooth–hummocky plains

hummocky plains

Inner facies:walls, central massif

External facies:

swell

Ridge of the swell

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The high massifs, which are largely observable inthe southeastern sector of the region under consider�ation, represent the second type of intercrater plains(icmr morphological unit, Fig. 4a). The surface ofthese plains is very uneven consisting of blocks tens ofkilometers across and 2–3 km high.

The third type of intracrater areas is represented bysmooth plains (icps morphological unit, Fig. 4b). Theplains occupy several percent of the region forming

small (tens of kilometers across) fields between craters.The surface of these plains is morphologically smooth(except for impact craters) and subhorizontal. Varia�tions in the relief within their limits never exceed a fewhundreds of meters.

CORRELATION OF MORPHOLOGICAL UNITS WITH THE GLOBAL STRATIGRAPHIC SCALE

The geomorphological map compiled for theregion planned for investigations during the Luna�Glob mission demonstrates the spatial distribution ofmorphological units (area types). At the same time,geochronological relations between these units remainvague. The freshest craters (cfpe, cfse, and cfne units)look as being the younger structures since they disturbthe surface of all other morphological units. The innerand external facies of large impact craters were formedduring the parental impact event or soon after them.The bottom facies are likely younger being formed byeither impact melt or products of post�crater volcan�ism. The hummocky intercrater plains grade intoswells of large craters and are composed (at leastpartly) of material ejected from them and entirelytransformed by subsequent impact events. The smoothintercrater plains are superposed on hummocky plainsbeing, thus, younger as compared with them. The ele�vated massifs are most likely composed of the oldestmaterial complexes, the surface of which is disturbedby large and small impact structures.

Such approximate geochronological relationsbetween defined morphological units provide nogrounds for a correlation between them and with theglobal stratigraphic scale. To overcome this complica�tion, we have compared our morphological map withthe available geological map compiled previouslyusing the low�resolution data (100–500 Mpxl) (Wil�helms et al., 1979).

In this geological map (Fig. 5), the area planned forthe Luna�Glob mission comprises the following strati�graphic units of the global scale (Table 2). The pre�Nectarian System is represented by the pNt, pNbr,pNbm, and pNc units reflecting the formation ofancient impact basins. The pNt unit composed of con�tinental material forms highly deformed areas, whichinclude relicts of craters. The small fragments of thisstratigraphic unit were mapped in the western part ofthe region planned for the Luna�Glob mission, wherethey correspond to intercrater hummocky plains (icphmorphological unit, Fig. 4a). The next stratigraphicunit (pNbm) composed of material of swells surround�ing impact basins forms large arcuate mountains. Onlythe insignificant part of this stratigraphic unit isobservable in the region under consideration, where itcorresponds to the icmr morphological unit (Fig. 4a),which characterizes Mount Malapert. The pNbrstratigraphic unit (material constituting swells ofimpact basins with differentiated topography) corre�sponding to the icmr morphological unit is most wide�

N

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Fig. 4. Morphological units located between large craters.(a) Intercrater hummocky plains (icph) and elevated mas�sifs (icmr); (b) intercrater smooth plains (icps). All imagesare obtained by the WAC device, resolution 100 Mpxl,polar stereographic projection.



spread in the southern part of the region planned forthe Luna�Glob mission. In this region, the strati�graphic unit of the pre�Nectarian craters includesexternal and inner facies, as well as the bottom faciesof the Boussingault, Boguslawsky, Scott, Nobile, andSimpelius craters.

The Nectarian System in the region in questionincludes the Ntp, Nbc, and Nc units. The first twounits were defined in (Wilhelms et al., 1979) as charac�terizing the morphostructures (crater clusters, chains,furrows, and others) arranged in the radial mode rela�tive to Nectarian basins. In the region under consider�ation, the Nbc unit (secondary craters of Nectarianbasins) corresponds to two relatively small (30–35 kmin diameter) craters in its northern part. The fragmentsof the Ntp unit (blanket deposits) correspond to thebottom facies of the Demonax, Boussingault, andBoguslawsky craters. The stratigraphic unit of theNectarian craters (Nc) was defined in (Wilhelms et al.,1979) as including craters which are younger as com�pared with the Nectarian basins, but older than theSea of Showers (Mare Imbrium) basin. These cratersin the study area are Demonax and Boussingault. Cra�ter Boguslawsky we refer to the pre�Nektarian.

The Imbrian System in the region in questionincludes the following stratigraphic units: Ic1, Ic2, andIsc. All of them are represented by material of impactcraters of either Early Imbrian (Ic1) or Late Imbrian(Ic2) ages or secondary craters (Isc) corresponding tofacies of the large (95 km in diameter) Schombergercrater (Ic2) and two relatively small (35–40 km) craterslocated southeast of the latter.

The Eratosthenian System is characterized bymaterial ejected from the Moretus crater (Ec strati�graphic unit) and two small (15–20 km) craters corre�sponding to the cfse morphological unit. The Coper�nican System is represented by the Cc stratigraphicunit, which corresponds in the region under consider�ation to the cfpe morphological unit. The crater

located on the southern swell of the Schomberger cra�ter is the most notable structure of this age.

The comparison of the map illustrating the distri�bution of morphological units with the geological mapallows them to be stratigraphically calibrated and cor�related with the global stratigraphic scale of the Moon.The considerably higher resolution of the LRO�WACimages, which were used for mapping morphologicalunits, provides the substantially higher resolution ofthe geological map.

Most of mapped units are related in their origin tothe impact crater formation and subsequent transfor�mation of craters. Such units are, for example, theinner and external facies of craters. It is conceivablethat these plain complexes represent accumulations ofthe impact melt, although their volcanic origin withinlarger craters cannot be ruled out as well. The inter�crater plains grading into swells of craters are mostlikely composed of alternating layers of ejected mate�rial transformed by subsequent impact events. The ele�vated massifs, which are distributed in the southernpart of the region under consideration, may representrelicts of the swell surrounding the largest SPA lunarbasin (e.g., Wilhelms et al., 1979). Despite their loca�tion in depressions, the smooth intercrater plains arehardly related to any impact events; their materialunderfloods the surrounding areas (Fig. 4b). Such fea�tures of these plains are poorly consistent with theirinterpretation as representing relicts of the impactmelt and indicate, most likely, their volcanic origin orcharacterize smooth ejection blankets related toremote impact basins (Oberbeck et al., 1975).

DISCUSSION

The region planned for investigations during theLuna�Glob mission represents part of the ancientcontinental domain devoid of any significant accumu�lations of marine material. It means that regolith

Table 2. Stratigraphic units in the planned landing area (Wilhelms et al., 1979)

Period Stratigraphic unit Index

Copernican Copernican primary craters Cc

Eratosthenian Eratosthenian primary craters Ec

Eratosthenian secondary craters Esc

Imbrian Imbrian primary craters (after the Eastern Sea) Ic2

Imbrian primary craters (prior to the Eastern Sea) Ic1

Imbrian secondary craters Isc

Nectarian Nectarian primary craters Nc

Material of Nectarian basins Nbc

Material of assumed Nectarian basins Ntp

Pre�Nectarian Pre�Nectarian primary craters pNc

Material of pre�Nectarian basins pNbm

Material of assumed pre�Nectarian basins pNt

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Fig. 5. Geological map of the planned landing area of the Luna�Glob spacecraft compiled using the mosaic of images obtainedby the WAC device. Polar stereographic projection.

should be dominated by continental material. Thewhole region is located within the spacious domaincharacterized by the lowered neutron flow, which wasestablished by the Lunar Prospector mission (Feld�man et al., 1998; 2000; 2001) and represents a peculiarsouthern “polar” cap of the Moon.

Since 2000, the LEND neutron spectrometermounted on the LRO spacecraft has gathered datawhich would allow the spatial distribution patterns ofhydrogen established during the Lunar Prospectormission (Mitrofanov et al., 2010; 2012; Sanin et al.,2010; 2012; Boynton et al., 2012) to be structured. TheLEND data show that the areas with the lowered neu�

tron albedo correspond to both continuously and dis�cretely shadowed regions (Boynton et al., 2012) andthe spatial distribution of the neutron flow at the sur�face is characterized by spotty rather than banded orzonal patterns (Mitrofanov et al., 2012). Conse�quently, the areas with the lowered neutron flow areisolated from each other, if such patterns do not resultfrom the smoothing and interpolation procedureapplied for the map compilation (Mitrofanov et al.,2012).

The comparison of the map of the neutron flow dis�tribution (Mitrofanov et al., 2012) with the geologi�cal–morphological map reveals a distinct absence of

Simpelius

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SchombergerBoguslawsky

Demonax

ScottMalapert

Nobile

Smooth plainsSmooth–hummocky

Hummocky plains

Inner walls of

External part of swell

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tari

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Craters

Fresh craters

Degraded craters

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Geological period

Copernican

EratosthenianMo

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Ec2

Ec1

lcps

lcphs

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lco

Ncps

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Ncph

Nci

Nco

pNcps

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pNcph

pNci

pNco

pNips

pNiph

pNtr

mat

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pNci

pNcphs

pNtr

Ncps

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pNtrpNcphs

pNci

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pNiph

pNtr

pNci

pNcphs

pNcpspNci

pNcphpNcps

pNco

pNcph

pNiph

pNcphpNcph

pNiph

Ici

IciIci

pNiph

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pNcph

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correlation between them. Both the lowered and ele�vated neutron flow values associate with morphostruc�tures of different ages. The flow anomalies (both posi�tive and negative) are recorded in the bottoms of cra�ters, above their swells, and in the intercrater plains.One of the areas of a smooth intercrater plain exhibitsa lowered neutron flow, while other fields of plains arecharacterized by relatively elevated neutron flow val�ues. No correlation is also observed between variationsin the neutron flow and relief: the lowered neutronflow is documented in bottoms of large craters locatedin depressions, above elevated swells of caters, andover intecrater plains.

Hypothetically, material from any mapped unitscould be characterized by elevated water content asso�ciated, for example, with outcrops of deep material, asprobably occurs in the central massif of the Ballialduscrater (Klima et al., 2012). The apparent absence ofcorrelation between the neutron flow distribution andthe geological structure of the region under consider�ation indicates that none of the stratigraphic units isresponsible for its documented anomalies. Conse�quently, none of the lithological complexes that deter�mine the geological structure of the region planned forinvestigations during the Luna�Glob mission repre�sents material “enriched” with hydrogen/water up to adegree which could be detected by the LEND spec�trometer. For example, none of the central massifs inlarge craters is marked by the lowered neutron flow. Ifthese structures host water in any form, its contentshould be below the detection limit of the spectrome�ter: approximately 100 ppm (Mitrofanov et al., 2010).

The LEND neutron spectrometer is designed forthe immediate acquisition of data on concentrationsof hydrogen (water) in the upper layer of regolith(Mitrofanov et al., 2010). In addition, the indirectdata on the content of ice may be obtained using theMini�RF radiolocation sounding device also installedon board the LRO spacecraft (Bussey et al., 2010;2011; Nozette et al., 2010). Both sensors obtain infor�mation on the layer of approximately uniform thick�ness (1–1.5 m), but differ significantly in spatial reso�lution of the survey: the resolution of the LEND spec�trometer in polar areas is approximately 10 km/pxl(Mitrofanov et al., 2010), while that of the radar is150 m/pxl (Nozette et al., 2010). In addition, theLEND spectrometer is sensitive only to the content ofhydrogen (water) regardless of its origin (hydrogen�bearing minerals or protons of the solar wind) or phase(for example, nanophase or coarse�detrital accumula�tions of water ice). At any rate, hydrogen accumula�tions exceeding the detection limit of the deviceshould be recorded as areas with reduced neutron flow.

For the radar survey, the form of ice in regolith is ofimportance. Indeed, the circular polarization ratio(CPR) is the main parameter of surface radiolocation,which may indicate the presence of ice in regolith. Itrepresents the ratio between the power of the reflectionpolarized in a direction similar to that of the transmit�

ted beam and the power of the reflection polarized inthe opposite direction (Spudis et al., 2009; 2010). Thebeam is polarized during its multiple reflections in thesampled layer and, thus, depends on the presence oflarge fragments in the latter, the size of which is com�parable with the length of the radar wave (12.6 cm).Thus, the elevated CPR values may be explained bythe presence of both ice and rock fragments (Nozetteet al., 2010). Such an important feature of theobtained signal makes it possible to test the hypothesisof the presence of large ice fragments in regolith incomparison with the maps of the reduced neutron flowand CPR values.

As follows from this comparison, the CPR and sur�face neutron flow values show no correlation betweeneach other (Fig. 6). The elevated CPR values are asso�ciated with young impact craters and their ejectedmaterial, the walls of large craters and areas where thepresence of coarse�detrital material is expected. Thereduced neutron flow exhibits no association withsuch morphostructures and is not reflected in the radarsounding results (Fig. 6). This means that, first, highCPR values indicate the elevated contents of rockfragments in regolith rather than ice accumulationsand, second, if ice is present in regolith, the sizes of itsaccumulations should be significantly less as com�pared with the wavelength of the radar wave.

Consequently, the areas marked by the loweredneutron flow may host hydrogen either in the form offine�grained ice inclusions originating from juvenileand meteoric/comet water redeposited in cold traps(Basilevsky et al., 2012), or as hydrogen compounds,the formation of which is related to activity of the solarwind and magnetosphere of the Earth (Starukhina andShkuratov, 2000; Crider and Vondrak, 2000), or inboth these forms.

The radar survey provides lower resolution in areaswith accumulations of coarse�detrital material and theneutron flow data are obtained from large objects, thestructure of which cannot be understood with such aresolution of the neutron survey. They may be bothdispersed and concentrated accumulations, whichbecome fuzzy because of the low spatial resolution andthe methodology used for neutron flow map compila�tion (Mitrofanov et al., 2012). In any event, theseaccumulations do not modulate the reflected radarsignal.

In addition to the main formulated task of theLuna�Glob mission consisting in the investigation ofvolatiles in regolith, the selection of the landing areaoffers an opportunity to accomplish another potentialtask of the mission: the investigation of old material.The point is that the southern part of the plannedlanding region corresponds to the swell of the largestlunar structure: the South Pole–Aitken (SPA) basin(Stewart�Alexander, 1978; Wilhelms et al., 1979).

The analysis of craters in the basin shows that it isolder than 4 Ga (Hiesinger et al., 2012). This estimate

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corresponds to the upper age limit of the basin. Itslower limit is poorly known since the curve for cratercounting for structures exceeding 70 km in size, whichserved as objects for age assessment, is close to thecurve for crater saturation. Nevertheless, the size ofthe basin is approximately 2500 km across (Leikin andSanovich, 1985; Wilhelms, 1987; Hiesinger and Head,2004; Shevchenko et al., 2007; Garrick�Bethell andZuber, 2009) provides grounds for considering it asboth the largest and oldest basin. Thus, the landing ofthe spacecraft on the swell of the SPA basin offers anunique opportunity to investigate very old material ofthe lunar crust and, probably, mantle, which could

have been ejected due to such a powerful, althoughlikely oblique, impact event (Garrick�Bethell andZuber, 2009).

CONCLUSIONS

The region selected for landing site of the Luna�Glob spacecraft is located in the southern part of theswell surrounding the largest SPA basin. The photo�geological analysis of its surface using the LRO�WACphotoimages (resolution of 100 Mpxl) made it possibleto define the following groups of morphological units(area types): (1) related to the formation of relatively

0–80

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270

240

210

180

–80

150

120

9060

–85

–85

Planned Luna�Glob

spacecraft landing sector

Fig. 6. Comparison of the map illustrating values of the radar signal circular polarization and the spatial position of statisticallymost stable areas with the reduced neutron flow (Mitrofanov et al., 2012). For explaining, see the text.



small (a few tens of kilometers in diameter) and rela�tively fresh impact craters (cfpe, cfse, and cfne units);(2) associated with larger (>100 km) degraded cratersincluding (2a) external facies (lcr unit) and (2b) innerfacies (lcwm, fps, fpsh, and fph units); (3) occupyingintercrater spaces (kph, icmr, and icps units).

The comparison of the map illustrating the distri�bution of morphological units with the geological mapof the southern hemisphere of the Moon compiledwith the low spatial resolution of 100–500 Mpxl (Wil�helms et al., 1979) shows good consistency betweenmorphological and stratigraphic units in the areaselected for the spacecraft landing during the Luna�Glob mission. This made it possible to calibrate themap of morphological units in terms of model geo�chronology and correlate them with the global strati�graphic scale, thus, imparting a geological sense to it.

The comparison of this geological map with themap of the epithermal neutron flow distribution(Mitrofanov et al., 2012) reveals no correlationbetween them. It means that the region planned for theLuna�Glob investigations is devoid of complexes con�taining elevated water concentrations and, thus, can�not be considered as the first�priority object.

The comparison of the map illustrating the distri�bution of the surface neutron flow with the map of theradar beam (Mini�RF device) CPR values demon�strates no correlation between them as well. Thismeans that high CPR values reflect the elevated con�tent of rock fragments in the upper layer of regolithrather than ice accumulations. In addition, eventhough ice fragments are present, their sizes should besignificantly less as compared with the wavelength ofthe radar wave.

The region planned for investigations during theLuna�Glob mission is located on the swell surround�ing the largest and, likely, oldest preserved lunar basinproviding the possibility of analyzing very old materialof this planet, which may introduce important con�straints into models of its origin.

ACKNOWLEDGMENTS

Photogeological analysis of the potential region oflanding of Luna�Glob expedition made byM.A. Ivanov and A.T. Basilevsky supported by thegrant of the Russian Science Foundation (projectno. 14�22�00197).

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Translated by I. Basov

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Geological Context of Potential Landing Site of the Luna Glob Mission