DEPARTMENT OF THE INTERIOR

UNITED STATES GEOLOGICAL )SURVEY TO ACCOMPANY MAPS 1-616-627

GEOLOGIC MAPS OF EARLY APOLLO LANDING SITES

By N. J. Trask, Coordinator

original surfaces of all lunar geologic units , exceptthe very youngest, have probably been greatly modi­fied by development of this debri s layer. The1: 1,OOO,OOO-scale reconnaissance maps attempt toshow the geologic units beneath the regolith. On thegeologic maps of the Apollo sites, many of the unitsrepresent crater material s which are contemporaneouswith and have contributed to the regolith; these areshown on a background of basic subdivisions of themare and terra material s that are interpreted to under­li e the regoli tho

Geologic units are arranged in the map explanationswith the youngest units at the top and the oldest unitsat the bottom. The distinguishing characteristics ofeach unit are given first, followed by an interpretationof the origin of the unit. The units are grouped insystems, as shown by (1) the brackets along theright-hand margin of each explanation, and (2) thenames of the systems in large capital letters. Thecapital letter in the symbol used for each unit standsfor the time-stratigraphic unit, the system, to whichthat unit is assigned; the srriallletters are descri ptiveabbreviations that indicate the lithogenetic natureof the unit (thus Ee, Eratosthenian crater material;1m, Imbrian mare material). All three systems arerepresented in some Apollo landing sites, only theEratosthenian and Copernican in others. Pre-Imbrianorpossiblepre-Imbrian materials occurin a few sites.

EVENTS

Formation of large ray craters(such as Copernicus)

Formation of large craterswhose rays are no longervisible (such as Eratos­thenes)

Deposition of most of the marematerial

Formation of pre-mare craters. (such as Archimedes)

Formation of the Imbri urn basin

Materials interpreted as predating the formationof the Imbrium basin (other mare basin materialsand crater materials) are designated pre-Irnbri anand are not assigned to systems.

Era t osth en ian

PERIOD (SYSTEM)

Copernican

1mbrian

Geologic units on 1: 1,OOO,OOO-scale reconnaissancemaps and Apollo . maps are assigned to time-strati­graphic divisions in the standard lunar stratigraphiccolumn (Shoemaker and Hackman, 1962; Shoemaker,1962; Wilhelms, 1966; McCauley, 1967), although thisbecomes more difficult at the larger scales of theApollo maps. Time-stratigraphic units are planet­wide groups of rocks that were formed during specifictime intervals. Development of a lunar stratigraphiccolumn beg an with the mapping of th e Mare Imbriumregion, where a series of well-defined rock units isvisible (Shoemaker and Hackman, 1962) . Some ofthese units can be correlated with similar-appearinguni ts in other parts of the Moon. The major time­stratigraphic divisions are called systems; eachsystem -corresponds to a period of time marked by amajor event in the Mare Imbrium region, as li sted inthe table below (top to bottom--most recent to oldest):

* The Geological Atlas of the Moon comprises severalcategories of maps all published as part of the Miscel­laneous Geologic Investigations of the U.S. GeologicalSurvey. In order of increasing map scale the categoriesare: (1) A. synoptic compilation of the Earthside ex­clusive of the limb regions incorporating the results oflarger scale maps; to be published at a scale of1:5,000,000. (2) Reconnaissance maps compiled largelyfrom Earth-based telescopic photographs and directtelescopic observations, and published on USAFAeronautical Chart and Information Center (ACIC) basesat a scale of 1: 1,000,000. (3) Ranger maps-compiledfrom photographs returned by the Ranger spacecraft andpublished on ACIC bases at scales ranging from1:5,000 to 1:250,000. (4) Apollo landing site maps.

All five successful Surveyors provided evidence of alayer of surficial debris--termed the lunar regolith(Shoemaker and others, 1967, p . 41)-in the immediatevicinity of the landing sites. Indirect evidence thatthe regolith occurs everywher...e on the Moon has beenprovided by Lunar Orbiter photographs. Thus , the

GEOLOGIC MAPPING OF APOLLO SITES

The U.S. Geological Survey is publishing geologicmaps of potential early Apollo landing sites on theMoon at scales of 1: 100,000 and 1:25,000 as part ofits Geologic Atlas of the Moon. The 1: 100,000-scalemaps show the regional setting of each landing siteand can be compared to smaller scale reconnaissancegeologic maps which cover most of the earthsidehemisphere of the Moon.* The 1: 25,000-scale map sshow the details of each landing site and its imme­diate vicinity. The maps were constructed primarilyby study and annotation of Lunar Orbiter photographs;stereoscopic coverage was available for some partsof each one . The Langley Research Center of theNational Aeronautics and Space Administration pro­vided the photographs. Base materials for the mapsare photomosaic s prepared by the Army Map Servic eand the Aeronautical Chart and Information Center ofthe U. S. Air Force.

Geologic maps of the Moon at relatively large scalessuch as 1: 100,000 and 1: 25,000, like those at smallscales, are based on the same fundamental principlesemployed in mapping terrestrial geology. In general,material s that apparently formed under the same con­ditions and at approximately the same time are groupedinto units , .and the relative ages of the units areestimated as closely as practical. The surface of theMoon is heterogeneous. It features were apparentlyshaped both by impact of objects from space andby effusion of material brought up from the Moon'sinterior by some form of. volcanism.

The fundamental geologic units of the Moon areportrayed most easily at the smallest map scales . The

. differences between the mari a and terrae and betweenrayed and unrayed areas show up readily on photo­graphs of the entire disk. Basic subdivisions of themaria and terrae are on the order of tens of kilometersacross and are shown conveniently on the reconnais­sance geologic maps at a scale of 1: 1,000,000. Addi­tional subdivi sicns, 1-10 km acros s. can be shown on1: 100,OOO-scale maps , but the boundaries of most ofthese are indefinite. Except for a few minor units , noadditional basic subdivisions of the surface materialscan be made on the 1:25,000-scale maps . The in­creased difficulty in subdividing lunar surface mate­rials at larger scales probably is caused by the impactof meteoritic particles and, possibly , of high-energysolar particles and radiation, all of which tend toblur the differences between surface materials thatare thin 0 r limited in extent.

Materials of most .small features on the lunarsurface (largely craters less than 100 meters indiameter) belong to the Copernican System. Numbershave therefore been added to the symbols used formost Copernican units in order to provide a finerbreakdown by relative age. The highest numbers arefor the youngest units (thus, CC5 is material of a rela­ti vely young Copernican crater, CCl of a relatively oldCopernican crater).

CRATER MATERIALSLike most of the Moon, every site is densely

covered with craters whose sizes extend downwardto the limit of photographic resolution. The cratersform a continuum from sharp and fresh-appearing tohighly subdued. In order to estimate, the relative agesof craters, the following fundamental assumptionsmust be made: (1) when first formed, mos t cratersappear fresh; (2) crater forms are progressivelydegraded, or subdued, with time, the smaller cratersat a more rapid rate than the larger; (3) the rate ofcrater degradation is approximately the same every­where on the Moon. These fundamental assumptionswere used to construct the graph in figure 1. Theoverall slopes of the lines on the graph were fixedby the types of craters occurring on geologic units ofwell-established age; for example, craters having theform of gentle depressions are as much as 1 kmacros s on the Imbrium basin ej ecta bla nket, whichmarks the base of the Imbrian System. The graph wasalso made to fit the 1:1,OOO,OOO-scale reconnaissancemaps on which rayed craters as small as 3 to 5 kmare assigned to the Copernican and unrayed cratersof the same s iz e are as signed to the Eratosthenian.The six subdivisions within the Copernican Systemwere chosen solely for convenience.

Although the system outlined by the graph offigure 1 gives a general idea of the relative ages ofcraters, it cannot be used to assign precise crater

ages. Therefore, categories Cc 1 through Cc 6 areinformal designations and are not proposed ae formalsubdivisions or series within the Copernican System.One difficulty is that all newly formed craters in thesame size class probably do not have the same mor­phologies and depth/diameter ratios . Craters ofinternal origin and secondary impact craters may well beinitially shallower relative to their diameter thanprimary impact craters of the same diameter, andtheir rim crests may be initiallymbre subdued thanthose of primary impact craters. Attempts have beenmade on the geologic maps to show those cratersinterpreted most confidently as secondary impactcraters and craters of internal origin-- such as clearlyalined groups of craters of the same apparent age­separately from the rest of the crater population. Inestimating the ages of these special types, accountis taken of the possibility that their initial forms mayhave differed signi ficantly from those of youngprimary impact craters. Most si ng l e round craters areplaced in the main sequence of crater classes shownin the right-hand column of each explanation; how­ever, some of these craters may also have formed bysecondary impact or by internal processes and thusmay have been assigned to the wrong age categorybecause their original configurations differed fromthose of fresh primary impact craters.

Another difficulty inherent in the crater-age assign­ment system is that physical properties of the lunarmaterials may also influence the initial morphologyof a crater. In some units, a clear deficiency ofcraters with sharp, blocky rims suggests that thematerials of these units may be less cohesive thanother lunar materials. Craters in these less cohesivematerials will appear to be subdued ·even when firstformed.

Topography also exerts an influence on the rateof crater aging. On sloping surfaces, such as mareridges, downslope movement of material appears to

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10km5kmlam 50m 100m 500m 1kmCRATER DIAMETER (RIM CREST TO RIM CREST)

Figure i.--Graphs showing assumed relations among diameters , properties , and ages ofcraters: Categories are Intergradational. Horizontal lines are isochrons indicatingvariations in crater populations on surfaces of increasing age.

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destroy craters faster than they would be destroyedon level or gently rolling terrain. Craters on ridges,therefore, are probably younger than the maps indicate.Locally, the rate of crater degradation may be tempo­r.arily augmented owing to the presence of a blanketof volcanic material. Thus, a crater degraded slowlyby continuous bombardment of small meteoriticparticles will be indistinguishable from one degradedrapidly by a vol canic blanket.

Despite these uncertainties, craters assigned to theImbrian, Erato stheni an, and oldest Copernican (Cel)are almost certainly relatively old members of thetotal population because they are dotted with manysmall craters (3 to 20 m in diam.). Similarly, cratersas signed to the middle and upper Copernican (CC2 ­

CC6) must represent a sequence of decreasing agesinc~ they have progressively fewer such smallsuperposed craters. In any local area on the samegeologic unit, the crater designations are probablya r-easonably accurate indication of relative age, butcorrelations between areas are Iess certain.

Only the largest craters in each map area are out­lined by geologic contacts, intermediate-sizecraters are identified by numbers or letters only, and.the smallest craters are unmapped. Some of theintermediate craters are surrounded by mappable rimdeposits, but outlines have been omitted to ensurelegibility. Many of the older craters, including someoutlined by geologic contacts and some shown only bynumbers and letters, lack mappable rim deposits onOrbiter photographs, possibly because such depositsare finer grained than the limit of re solution.

The craters of greatest scientific interest are thoselarge enough to have penetrated bedrock beneath theregolith and young enough so that the resultingejecta deposits can be clearly differentiated. Theremay be little to distinguish crater deposits fromadjacent regolith around craters too small to havepenetrated the regolith and around very old craters.Samples that display shock phases or volcanictextures and compositions will be of value for deci­phering crater origins only if their parent craters canbe definitely identified. Also, of all the fragmentalmaterial on the surface, blocks ejected from youngcraters have most clearly been derived from theunderlying bedrock unit. The se bedrock samplesshould be less contaminated by debris derived frommore distant parts of the Moon than samples fromintercrater areas, which have been recycled throughthe regolith one or more times. If the exposure agesof crater material s are determinable, samples fromcraters of various ages could be used to assess thehi story of radiation on the lunar surface and would behelpful for assigning absolute ages to the lunargeologic time scale. In all likelihood, the cratersinvestigated by astronauts on early Apollo missionswill be smaller than those shown on the 1:25,000- scalemap s, but figure 1 can be used to estimate the age ofany crater that has penetrated bedrock beneath thelunar regolith.

Among the special types of craters in each site,those interpreted as produced by secondary impactare probably of the greatest significance. Included inthis category are most craters mapped under theheading of crater cluster. Some of these clustersappear to be clearly related to a primary impactcrater: they are concentrated along lines roughlyradial to a larger, primary crater, or they lie alongbright rays (visible on telescopic full-Moon photo­graphs) that emanate from a primary crater. Bedrocksamples quarried from considerable depth by theimpact that formed the primary may be present in andaround these clusters. Clusters that apparently wereformed by the impact of fragments ej ected duringformation of the crater Tycho occur in several of thesites.

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Craters interpreted on the maps as being of internalorigin include irregularly shaped craters, those whoseshapes appear to be strongly controlled by lineaments,chain craters, craters at the summits of low domes,and dimple craters (those with walls strongly convexupward and very low rims). Most of these cratersappear to have been subdued by the superposition ofyounger smaller craters.

'MARE AND TERRA MATERIALS

The properties which distinguish the various mareand terra units on the Apollo site maps are mainlydiffering crater populations which in turn appear tobe related to age, mechanical properties of the ma­terials, or mode of emplacement. Whether the chemicalor mineralogical properties of the units al so differcannot be determined until manned or unmannedtraverses are made. A limited amount of informationon' the chemical properties of these materials isavailable from the Surveyor alpha particle scatteringexperiments (Turkevich and others, 1968). Analysesat two points in typical mare material indicate achemi stry similar to that of iron-rich basalt; analysesat a single point on the ejecta blanket of the craterTycho on the southern lunar highlands also indicatea bas al tic chemi stry , thoug h with somewhat l e s siron than the mare material. These analyses, togetherwith substantial photographic evidence 'of volcaniclandforms, indicatethat volcanic rocks are widespreadon the Moon; the mare material s are probably made upmostly of volcanic materials. These materials un­doubtably vary both in age .and composition fromplace to place, although the contacts between con­trasting material shave been blurred by the develop­ment of th e lunar regolith.

The oldest craters that have formed on the marematerial in the three early Apollo landing sites inOceanus Procellarum (Maestlin G, Wichmann CA,and Flamsteed K regions) are Eratosthenian; and inthe Maestlin G region and near the Flamsteed Kregion, craters mapped as Eratosthenian are partiallyburied by mare material. These mare material s aretherefore assigned to the Eratosthenian System. Fresh~

appearing domes and mare ridges, suggestive ofrelative youth, are also present in these three sitesin Oceanus Procellarum. The oldest craters in thesite in Sinus Medii (Oppolzer A region) are alsoEratosthenian, but subdued craters and gentle depres­sions are larger than in sites to the west and thereforeare older (fig. 1). The mare material in the OppolzerA region is therefore estimated to be latest Imbrian inage. Large Imbrian craters are present on the marematerial in the Sabine D region in southwestern MareTranquillitatis, which is accordingly mapped asImbrian, older than that in the Oppol zer A region.These surfaces of increasing age can be thought ofas represented by successively lower horizontallines on the .graph of figure 1. On an older surface,crater degradation has operated for a longer periodthan on a younger one, and gentle depressions aretherefore larger. Note that the ages assigned to themare material sapply only in and around the landingsites and not to entire maria. The Eratosthenian marematerials in Oceanus Procellarum appear to be con­fined to small patches which were chosen as potentiallanding sites because they appeared dark and smoothon t el esc opi c photographs.

The crater population in southern Mare Tranquillitatishas relatively fewer intermediate size craters (50 to125 m in diameter) than the population on the marematerial mapped asyounger in Sinus Medii and OceanusProcell arum in the vicinity of the landing sites. Thisseeming paradox must mean that the rates of craterproduction and (or) degradation have not been exactlythe same on all surfaces. The excess craters on theyounger mare surfaces may be of internal origin or maybe secondary impact craters; the craters may be miss­ing in southern Mare Tranquillitatis because of burial

by a layer of younger material or because of especiallyrapid degradation; or a combination of all of thesefactors may have produced the crater populationspresently observed,

Compared with the dark, level plains of the lunarmaria, most of the lunar terrae are light and rugged.Isolated patches of terra material s occur in several ofthe sites, but the Maskelyne DA region in south­western Mare Tranquillitatis is the only one withextensive terra materials. Much of this material isas signed to the Copernican System. It occurs on agently undulating surface with more relati ve reliefthan the typical mare material in the other five site sbut generally is smoother and less cratered than anyother materials in the early Apollo landing sites. Thecrater population of the Copernican terra materialincludes several large subdued craters of apparentImbrian and Eratosthenian age which are interpretedas buried. There is a very marked deficiency of cratersof intermediate sizes (50-200 m) and ages (Ec andCc j ). This material appears to be a relatively young,thin layer of volcanic rocks or erosional debris whichoverlies an older, more heavily cratered surface. Thus,samples collected from the Maskelyne DA region maybe more heterogeneous than those collected else­where.

OTHER GEOLOGIC UNITS AND STRUCTURALAND TOPOGRAPHIC FEATURES

Among the other geologic features that have beenmapped, ridges, scarps, and lineaments are the mostcommon. ' Some of the mare ridges may consist ofdi screte bodies of extruded volcanic material whichcould provide important data on lunar volcanism.Scarps and lineaments are more difficult to map thanthe ridges because of thedi scontinuitie s in theOrbiter photographs at the framelet boundaries andbecause of ' spurious relief along the framelets intro-

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duced by the TV transmission system. Sinuous scarpsare of special interest because they may resembleterrestrial lava flow fronts and might give evidence oflunar volcanism. Other features of special interest aresmall (~200 m di am.) .domes and peculiar ring struc­tures in the three sites in Oceanus Procellarum.

REFERENCES

McCauley, J.F., 1967, The nature of the lunar surfaceas determined by systematic geologic mapping, ,inRuncorn, S.K., ed., Mantles of the Earth andterrestrial planets: London, Interscience Pub.,p. 431-460.

Shoemaker, E. M.,1962, Interpretation of lunar craters,in Kopal , Zdenek, ed., Physics and astronomy of theMoon: London, Academic Press, p. 283-359 ~

Shoemaker, E.M., and Hackman, R.J., 1962, Strati­graphic basis f'or a lunar time scale, in Kopal,Zdenek, and Mikhailov, Z.K., eds., The 'Moon­Internat. Astron. Union Symposium 14: London,Academic Press, p. 289-300.

Shoemaker, E.M., Batson, R.M., Holt, H. E., Morris,E.C., Rennilson, J.J., and Whitaker, E.A., 1967,Television observations from Surveyor III, inSurveyor III-A preliminary report: Natl , Aeronauticsand Space Adm. Spec. Pub. 146, p. 9-59.

Turkevich, A.L., Franzgrote, E.J., arid Patterson,J.H., 1968, Chemical analysis of the Moon at theSurveyor VII landing site--Preliminary results:Science, v. 162, p. 117-118.

Wilhelms, D. E., 1966, Summary of , tel e scopi c lunarstratigraphy, Sec. 4 of Astrogeol. Studies Ann.Prog. Rept., July 1, 1965 to July 1, 1966, pt. A:U. S.' Geol. Survey .open-file report, p. 237-305[ 1968].