J. E. GUEST University of London Observatory, Mill Hill Park, London NW7 2QS England Stratigraphy of Ejecta from the Lunar Crater Aristarchus ABSTRACT associated with the hummocky rim unit are interbedded, arguing that the smooth flows Ejecta from lunar crater Aristarchus con- are part of the sequence of ejected rocks and sist of mappable units that have different sur- are not younger volcanic flows. face characteristics, lithologies, and geneses. Photogeologic mapping demonstrates that INTRODUCTION these units can be ordered into a stratigraphic succession representing stages in the emplace- The crater Aristarchus lies on the nearside ment of ejecta during a single impact event. hemisphere of the Moon at lat 23°40' N., long Four main ejecta units are recognized: (a) a 47°20 W. Its diameter is 42 km, and its depth highly fractured rim unit consisting of an over- is more than 3 km. The markedly uneroded turned flap of country rock, (b) a continuous aspect of the crater and the extensive, well- ejecta blanket, (c) a zone of bright discontin- defined, bright rays surrounding it (Figs. 1, 2) ous ejecta outside the continuous ejecta suggest that Aristarchus belongs to the upper blanket associated with numerous secondary part of the Copernican System (Wilhelms and impact craters, and (d) a group of ejecta McCauley, 1971). Apparently, Aristarchus deposits on the rim which are genetically and Tycho are two of the youngest of the large related to each other and include hummocky rayed craters on the nearside of the Moon. The material (the hummocky rim unit), blocky aim of this paper is to describe the stratigraphic lobes, and smooth flows. Other units, such as relations of the Aristarchus ejecta units and to the ridged and leveed flows and the dark explain these in terms of the sequence of events "lakes" or "playas," are relatively younger occurring during crater excavation. The than the ejecta and are only briefly discussed. information is derived from detailed photo- The continuous ejecta blanket lies strati- geologic mapping of Aristarchus. Interpreta- graphically above the rim unit. To the east tions are based on studies of cratering mech- and south, the ejecta blanket has been stripped anisms. off the overturned rim unit by outward Rayed craters are now generally accepted as flowage from the crater during ejecta produc- being of impact origin, produced by the col- tion, leaving parts of the rim eroded bare. lision with the Moon of either asteroid-sized Fall of large missiles to form the secondary meteoroids, or cometary nuclei. Shoemaker craters and bright ejecta preceded the emplace- (1962) presents a convincing case that Coper- ment of the continuous ejecta. The general nicus was formed in this way: his main argu- asymmetry of the continuous ejecta-blanket ment is based on the presence of large, second- distribution appears to be related to the ary impact craters clustered in a broad annulus Aristarchus Plateau boundary fault, which encircling Copernicus about one crater diam- may have controlled the way material was eter away from the rim. The similarity is excavated. The hummocky rim unit only striking between this array of secondary occurs on the northern and western rim and craters and those developed around shock- may be an overturned flap of premare Aristar- wave craters produced by nuclear devices, for chus Plateau bedrock not present near the example Sedan (Shoemaker, 1965). Rim surface on the southern and eastern rim of the morphology (Shoemaker, 1962; Baldwin, 1963; crater. Blocky lobes and smooth flows mainly Guest and Murray, 1969), circularity (Murray Geological Society of America Bulletin, v. 84, p. 2873-2894, 13 figs., September 1973 2873 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/9/2873/3443274/i0016-7606-84-9-2873.pdf by guest on 26 September 2021 2874 J. E. GUEST and Guest, 1970), and morphology of the preserved; also, there is very good coverage of ejecta blanket also support a single, high- this crater by high-resolution Orbiter V energy excavating event. An origin by vol- photography. canic explosion of the magnitude required The validity of the mapping approach to to form craters that may be as large as 200 km lunar problems has been emphasized by in diameter and have the other characteristics McCauley (1967), Mutch (1970), and Wil- of lunar rayed craters is not consistent with helms (1970). In the case of Aristarchus, the evidence, whereas the present under- individual ejecta units are recognized by sur- standing of impact mechanisms does explain face characteristics; these are quite different the observed features of these craters. for units of different composition, texture, or The formation of a large impact crater has mode of emplacement. Age relations are not been observed. Many large impact determined oy accepted photogeologic tech- craters have now been recognized on Earth, niques. Although stratigraphic studies nor- and some of these are in a state of preservation mally apply to a sequence of rock laid down sufficient to glean information on the detailed over a long period of time, the techniques mechanics of their formation. Much of our may be applied equally well to the ejecta of knowledge on this subject is also based on an impact crater where indi vidual units were theory and on observation of nuclear and emplaced to form a thick sequence during the chemical explosions. In the study of impact- relatively short time period of just a few cratering phenomena, the large scale and the minutes. By analyzing the results of the fresh, uneroded nature of many lunar craters photogeologic mapping, it is possible to compensate for their inaccessibility. Aristarchus reconstruct the events that occurred during the was chosen for study because it is so fresh and excavation of the crater. most of the original features apparently are Photographs used in this study were Figure 1. Telescopic view of Aristarchus and the raphy. (Photograph from Consolidated Lunar Atlas). Aristarchus Plateau at low sun angle to show topog- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/9/2873/3443274/i0016-7606-84-9-2873.pdf by guest on 26 September 2021 STRATIGRAPHY OF EJECTA FROM LUNAR CRATER ARISTARCHUS 2875 Orbiter IV-150-H3 and Orbiter V 194 (Fig. 1). On the basis of surface appearance through 201 (medium and high resolution). and the results of the Apollo 11, 12, and 15 The Orbiter V medium-resolution frames have missions, the dark mare material is interpreted sufficient overlap to allow relative heights to as extensive lava flows of "basaltic" composi- be determined qualitatively. Detailed mapping tion (Lunar Sample Preliminary Examination on the high-resolution frames was carried out Team, 1969, 1970, 1972). The Aristarchus at a scale of 1:32,000. Mapping was accom- Plateau, on the other hand, may have varied plished by overlaying the photographs with compositions and origins; the northern part clear acetate film, which served as a base for of the plateau appears to consist of ejecta from plotting the geology. The Consolidated Lunar the Imbrium basin; Herodotus is a crater of Atlas (Kuiper and others, 1967) was used for middle Imbrium age and is overlain by a group comparison of the appearance of the area of volcanic rocks at least in part related to under different lighting conditions. Schroter's Valley (the Vallis Schroteri and Cobra Head Formations of Moore, 1965, GENERAL GEOLOGY 1967). The thickness of the mare material below Geologic Setting Aristarchus is unknown, but because Aristar- Aristarchus lies in Oceanus Procellarum on chus has been emplaced against a fault scarp the southeast margin of the Aristarchus bordering the plateau, rather than on a Plateau, and is the youngest major feature in gently inclined slope, the mare material may the area; the ejecta blanket from Aristarchus be thick, even close to the edge of the plateau. overlies dark mare material on the south- Thus, although the crater Aristarchus prob- eastern half and premare material of the ably cuts quite deeply into premare material on Aristarchus Plateau on the northwestern half the northwest, a considerable volume of mare Figure 2. Same area as Figure 1 at high sun angle bright rays around Aristarchus. (Photograph from to show variations in albedo. Note the dark halo and Consolidated Lunar Atlas). Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/9/2873/3443274/i0016-7606-84-9-2873.pdf by guest on 26 September 2021 2874 J. E. GUEST material must have been excavated on the as the most likely cause. Certainly, study of southeastern side. As will be suggested later, volcanic processes indicates that such a large the position of Aristarchus on this major crater could not have been caused by volcanic- geologic boundary may have affected the gas explosion (Roddy, 1968). One piece of character of the ejected materials on either positive evidence that tends to confirm an side of the crater. impact rather than a volcanic-explosion origin is the presence of central peaks in many Geology of Aristarchus craters, including Aristarchus. These are Examination of Orbiter and telescopic characteristic of the larger terrestrial impact photographs shows that Aristarchus is made up craters (Roddy, 1968; Dence, 1968), which of a number of geologic units (Fig. 3), each have central peaks consisting of highly de- with different surface characteristics. There formed, often shock-metamorphosea rocks are also variations in albedo, but these do not (Howard and others, 1972). They are also always appear to correspond with geologic formed in artificial shock-wave craters pro- boundaries. duced by a surface charge and thus closely simulating an impact. Seme authors have Origin. Craters of the type represented by claimed a volcanic origin for peaks in the Aristarchus are generally considered to have floors of large lunar craters.