LUNAR SOUTH POLE-AITKEN IMPACT BASIN: CLEMENTINE TOPOGRAPHY ANDIMPLICATIONS FOR THE INTERPRETATION OF BASIN STRUCTURE AND STRATIGRAPHY

H. Hiesinger, J.W. Head IIIDept. of Geological Sciences, Brown University, Providence, RI, 02912; Harald_Hiesinger@brown.edu

Introduction: Lunar impact basins show a variety of inter-nal and external structure [e.g., Wilhelms, 1987; Head etal., 1993, Spudis, 1993] and states of degradation and pres-ervation. Orientale, one of the youngest and best-preservedlarge basins, shows a configuration of internal topography,ring structure, and deposit morphology (Orientale-type;Figure 1) that is generally similar to other relatively unde-graded lunar basins. Other, more heavily degraded basinson the Moon (Tranquillitatis-type) have apparently under-gone significant viscous relaxation subsequent to their for-mation, reducing basin topography [Solomon et al., 1982].These older basins have also been subject to significantimpact degradation. Both of these morphologies are in con-trast to some impact basins on icy satellites characterizedby many more rings and little topographic expression (Val-halla-type) (e.g., Passey and Shoemaker, 1982; Greeley etal., 2000), a morphology thought to have been formed aspart of the very early modification stage of the impactevent, due primarily to sublithospheric flow into the cavityarea and attendant formation of many multiple rings[McKinnon and Melosh, 1980]. We discuss three basicscenarios for the formation of basin topography and facies:1) The Valhalla-type , formed by near instantaneous col-lapse of the cavity, sublithospheric flow due to the presenceof a ductile layer at shallow depths, and sublithosphericlateral shear leading to the formation of many multiplerings and little basin topography. 2) The Orientale-type ,formed by impact into a target that has sufficient targetrigidity to form a small number of multiple rings and topreserve them over geologic time. 3) The Tranquillitatis- type , formed in a manner likely similar to the Orientaletype but in a target thermal structure that subsequently al-lowed for long-term crustal flow and viscous relaxation ofthe major topographic elements to produce a very shallowbasin.

The lunar South Pole-Aitken (SPA) impact basin is oneof the largest and oldest impact basins and has thus beenheavily degraded by post-basin-formation primary impactsand their ejecta. Here we use Clementine altimetry data[e.g., Zuber et al., 1994] to delineate evidence for super-posed craters and basins, and for remaining evidence fortopographic internal structure. We summarize the evidencefor basin ring location and character, and use these data toassess which basin type the South Pole-Aitken Basin mostclosely resembles. Finally, we assess the implications forbasin origin, evolution and stratigraphy and suggest scenar-ios for future exploration.

Orientale Basin structure and cross-section interpreta-tion: The youngest large basin structure on the Moon, theOrientale basin, exhibits several clearly defined ring struc-

tures, the Cordillera ring, the Outer Rook ring, and the In-ner Rook ring. Head et al. [1993] proposed that the tran-sient crater penetrated to lower crustal depth and later col-lapsed along an outer rim fault (Cordillera ring) and rimmaterial rotated inwards and upwards in order to form theOuter Rook ring and the Inner Rook peak ring (Figure2).On the basis of morphologic and topographic comparisons,Head [1974, 1977] and Bratt et al. [1985] suggested thatthe Orientale Cordillera ring was equivalent to a scarpmarking the outer part of a collapsed megaterrace, theOuter Rook ring was the closest approximation to the tran-sient crater rim, and the Inner Rook Mountains wereequivalent to a peak ring. The innermost depression wasrelated to thermal contraction of the cooling central basinuplifted geotherms.

Fig.1 Lunar Orbiter IV image of the Orientale Basin

Fig.2 Cross-section through the Orientale Basin [Head etal., 1993]

South Pole-Aitken basin topography and structure:Clementine altimetry data indicate a complex structure ofthe SPA basin and support earlier findings based on Zond

Microsymposium 38, MS101, 2003(letter format)

TOPOGRAPHY AND RING STRUCTURES OF THE SOUTH POLE-AITKEN BASIN HIESINGER, H., HEAD III, J.W.

[e.g., Rodinov et al., 1971, Shevchenko, 1980, Shpekin,1983; Leikin and Sanovich, 1985] and Apollo laser altime-try data [e.g., Wollenhaupt and Sjogren, 1972; Kaula et al.,1973]. For example, Clementine data show that the SPAbasin is more than 8 km deep, hence having a morphologyvery different from the Valhalla-type multiringed impactbasins. Making use of Clementine topographic data, weidentified three, possibly four rings for the SPA basin. Thethree outer rings are best defined in the eastern and north-eastern sections of the SPA basin, but are indistinguishablefrom the highland terrain in the western parts of the basin,probably due to pre-SPA topography and post-SPA modifi-cations by younger basins. Our outermost ring (#1 in Figure3) is defined by a sharp drop in elevation of ~3-4 km andcan be traced for several hundreds of kilometers. Ring 2 ischaracterized by another drop in elevation of ~2-3 km andcan be traced for similar distances. Parts of this ring aremodified by the younger Apollo basin (Figure 3). Ring 3encloses the deepest parts of the basin, which are ~2-3 kmbelow the terrain between ring 2 and 3. These three ringstructures form a stairstep-like topography across thenortheastern quadrangle of the SPA basin (Figure 4).

Discussion: Pre-SPA Events: The general area around theSPA basin is characterized by numerous pre-Nectarianbasin structures, for example the Australe, Hertzsprung,Mendel-Rydberg and Korolev basins. Similar sized oldimpact basins in the South Pole-Aitken area would havecreated and modified the pre-SPA impact topography atlarge scales, thus influencing the final morphology andtopography of the SPA basin. This might be reflected insignificant differences in the present day topography ofterrain surrounding the South Pole-Aitken basin. In thenorth of the SPA basin the terrain is at elevations of ~3000-4000 m (peaks up to 7500 m) whereas areas to the west andeast appear to be significantly lower in elevation (~0 to–1000 m) and this has also been attributed to the depositionof SPA ejecta. We also identified a possible fragment of anancient basin ring (“A” in Figure3) that coincides with lowtopography within this inferred basin structure. Both fac-tors, pre-SPA basins and inherent topographic differences,contribute to the difficulties in ring placement and detailedcharacterization of the SPA structure in the western parts ofthe SPA basin.

Post-SPA Events: Since the South Pole-Aitken impact, theregion has been extensively modified by subsequent impactcratering at various scales. Multiple craters are superposedon the SPA basin and have further modified its structure.These impact craters include the large internal basins suchas the Apollo, Poincaré, Schrödinger and Planck, as well asexternal basins, e.g., the Orientale basin, which depositedtheir ejecta across the South Pole-Aitken basin (Figure 3)[e.g., Haskin, 2002].

Besides impact cratering, the South Pole-Aitken basinhas been modified by volcanism as evidenced by maredeposits and light plains. According to the geologic map ofWilhelms et al. [1979] and recent spectral studies [Yingst

and Head, 1997, 1999; Pieters et al., 2001], these maredeposits occur preferentially in post-SPA craters and basins(e.g., Mare Ingenii, Von Karman, Leibnitz, Apollo, andPoincaré) and are of upper Imbrian and Eratosthenian age.

Basin Structure: Previous attempts to define ring structuresof the South Pole-Aitken basin suffered from the fact thatthe basin was located at high southern farside latitudes andthus had limited image coverage as well as from the lack oftopographic data that covered the entire basin. Conse-quently there are large differences in the location of ringstructures defined in previous studies, e.g., Stuart-Alexander [1978], Wilhelms et al. [1979], and Leikin andSanovich [1985]. As described earlier, we used Clementinealtimetry data to defined at least three ring structures of theSPA basin. On the basis of our investigation, we think thatour ring 1 might be related to the initial collapse of the SPAtransient crater, inward slumping of the rim along an outerrim fault, and the formation of a megaterrace. Ring 2 and 3might have formed by inward and upward movement ofcrustal blocks in response to the transient crater collapse. Inthis scenario, ring 2 would be the closest approximation ofthe transient crater. If our interpretation is correct, thismakes ring 1 similar to the Orientale Cordillera ring, ring 2might be the analog to the Outer Rook ring, and ring 3 theanalog of the Inner Montes Rook ring of the Orientale ba-sin. Thus we interpret the original dimension and structureof the basin as follows: ring 1 is approximately 2400 km indiameter, ring 2 is ~2100 km in diameter, and ring 3 is~1500 km in diameter. On the basis of our ring structures,we find the basin center at roughly 174˚W and 55˚S.

This is different from previous interpretations in the fol-lowing manner. Stuart-Alexander [1978] proposed that thebasin is 2000 km in diameter, centered at 180˚W and 50˚S.Wilhelms et al. [1979] mapped a basin 2500 km in diameterand centered at 180˚W and 56˚S and a possible inner ring1800-2000 km in diameter. Wood and Gifford [1980] esti-mated a diameter of ~2600 km with a center at 180˚W and60˚S, whereas Leikin and Sanovich [1985] find the diame-ter of the SPA basin to be on the order of 2200 km and itscenter at 183.5˚E and 41.5˚S.

Conclusions: On the basis of our investigation we con-clude that the South Pole-Aitken basin is not the Valhalla-type, and that it is also not the Tranquillitatis type in thatthere has not been significant viscous relaxation. Thus, theimplication is that the impact was originally into a targetthat was not sufficiently ductile at depth to produce initialValhalla-type basin structure, and also not to producelonger-term viscous relaxation.

One goal for future sample return missions is to establishthe age of the SPA basin. Impact melt, which dates the SPAevent most reliably, is likely to be found in high concentra-tions within the inner ring (#3). In addition the innermostand deepest parts of the SPA basin likely tapped lowercrustal or upper mantle material, making the basin center agood candidate for returning such samples to Earth.

ReferencesBratt et al., (1985), JGR 90, 3049-3064; Greeley et al.,(2000), Planet. Space Sci. 48, 829-853; Haskin, (2002),Lunar Planet. Sci. Conf. 33, #1364; Head, (1974), Moon11, 324-356; Head, (1977), Pergamon Press (Roddy, Pepin,Merrill, eds.), 563-573; Head et al., (1993), JGR 98, 17149-17181; Kaula et al., (1973), Proc. Lunar planet. Sci. Conf.4, 2811-2819; Leikin and Sanovich, (1985), Astron. Vest-nik 19, 113-119; McKinnon and Melosh, (1980), Icarus 44,454-471; Passey and Shoemaker, (1982), Arizona Press,379-434; Pieters et al., (2001), JGR 106, 28001-28022;Rodinov et al., (1971), Kosm. Issled. 9, 450-458;

Shevchenko, (1980), Moscow Science, 284 pp.; Shpekin,(1983), Kazan Univ. Thesis, 142 pp.; Solomon et al. (1982)JGR 87, 3975-3992; Spudis, (1993), Cambridge Univ.Press, 263 pp.; Stuart-Alexander, (1978), U.S. Geol. Surv.Map I-1047; Wilhelms, (1987), USGS Prof. Paper 1348;Wilhelms et al. (1979) U.S. Geol. Surv. Map I-1162; Wol-lenhaupt and Sjogren, (1972), NASA SP-315, 30-1 to 30-5;Wood and Gifford, (1980), Lunar Planet. Inst. Rep. 414,121-123; Yingst and Head, (1997), JGR 102, 10909-10931;Yingst and Head, (1999), JGR 104, 18957-18979; Zuber etal., (1994), Science 266, 1839-1843.

Fig. 3 Clementine Topography of the South Pole-Aitken Basin and interpretative sketch map of its ring structures

Fig. 4 Profile across the SPA basin (along the black line in Figure 3) showing the approximate position of three possible ringstructures and a stairstep-like topography