GEOLOGIC STUDIES OF THE LUNAR FAR SIDE CRATER TSIOLKOVSKY. J. L. Whitford- Stark, Dept. of Geology, Univ. of Missouri, Columbia, MO 65211; B.R. Hawke, Hawaii Inst. of Geophysics, Univ. of Hawaii, Honolulu, HI 96822. Tsiolkovksy is a 192 km diameter impact crater located on the far side of the Moon at 129 E, 20 S. Both the crater and the basaltic floor materials appear to be of Imbrian age (1). The asymmetry of the crater and its ejecta blanket indicate formation via an oblique impact from the NhW (2). Additional factors controlling the morphology of the crater included the topography of the pre-impact site and post-impact faulting (3). The crater can be subdivided into four morphologic units; the central peak, the basalt floor, the interior walls, and the exterior ejecta blanket. The central peak has an area of 881 km2 and forms approximately 8% of the crater floor. The peak rises to a height of over 3 km above the floor and "layering is visible on the south and west exposed scarps of the peak, dipping to the north at about 30 degrees" (4). The basaltic floor covering has an area of 11,080 km2 and is extremely flat, varying by only about 350 m from the lowest point in the southeast to the highqst in the northwest. The floor has an average altitude of 1,735,600 m above the lunar center of mass, comparable to the near side maria. The interior wall varies between 10 and 60 km in width and 2.2 to 5.5 km in height. The average wall slope is 7 degrees although individual wall units are much steeper. For example, the outer scarp has an average slope of 18 degrees and, in places, slopes at 27.5 degrees. The asymmetric continuous ejecta blanket extends to a saximum distance af about 200 km from the rim crest and has an average thickness of 650 m near the crater rim and 50 m at a distance of 120 km from the rim (5). Recent studies have focused attention on the importance of impact melt depositsasso- ciated rjith lunar craters and have provided strong evidence that these deposits are the products of shock melting and not post-impact or impact induced volcanism (2,141. Abun- dant melt deposits can be recognized in and around Tsiolkovsky and exhibit a number of distinctive morphologies including flow lobes and channels, hard rock veneer, and com- plexly fractured ponds. Although much of the crater floor is covered by mare basalt, unflooded portions of the floor exhibit both ponded material and hummocky areas which are draped with a thin hard rock veneer. Bumerous melt ponds can be seen on the crater walls and are particularly abundant on the eastern walls. While veneer, flows, and ponds occur on the crater rim, ponds are the dominant mor- phology and contain the bulk of the recognizable melt volume. Flows of impact melt are not common around Tsiolkovsky but are well-developed on the north wall of Waterman crater (4,5) and within Chenier crater (15). Numerous inter-pond areas on the southeastern por- tion of the crater rim exhibit a subdued appearance associated with melt-draped regions and this veneer material exhibits gradational contacts with pond material. Veneers are particularly well developed in the region of Waterman crater. Exterior ponds are located around over 180' of the crater perimeter but are most extensive in the southeastern quad- rant of the rim. Here, the melt ponds occur at a maximum distance of 30 to 55 km from the rim crest. At least two factors have been identified as being important in control- ling exterior melt distribution. First, the most extensive melt deposits are often inthe inferred downrange direction of an oblique impact (2,14). Second, melt deposits are often found concentrated on crater rins adjacent to topographic lows in the rim crest and com- monly occur on the rim opposite zones of maximum wall slumping (14). Oblique impact appears to have exercised greater control over the distribution of TsioV~ovskyexterior melt, however, there is some correspondence between local rim crest lows and extensive exterior melt deposits. The x-ray spectrometer data permits the identification of four chemically distinct units within and around Tsiolkovsky; the crater floor, the walls, the ejecta blanket, and the surrounding highlands (Table 1). Also listed in Table 1 are geochemical data for returned lunar samples. None of the orbital data corresponds to specific rock types, a. result not unexpected in the light of the wide field of view of the instrumentation. ' Better agreement between the orbital and sample data can be obtained by using simple mix- ing models. For example, the basaltic floor orbital data is fairly well replicated by a mixture of Apollo 15 mare basalt and 16% anorthosite. Neglecting horizontal transfer of material at the lunar surface, such a situation could arise from the incorporation of 8% of the central peak and 8% of wall material to the basalt response. Such a mixture does not replicate the apparently low iron values obtained for Tsiolkovsky hut the ana- lytical uncertainty of the gamma-ray data for this area is extremely large (6) and a small amount of highland terrain was included in the region for which the gamma-ray data were obtained. The x-ray data appears to be more consistent with a low-titanium basalt. Addi- tionally, the x-ray data does not appear to indicate a high-magnesium basaltic fill within Tsiolkovsky similar to that within the eastern maria (7). O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System GEOLOGIC STUDIES OF TSIOLKOVSKY Whitford-Stark, J.L. and Hawke, B.R. The Tsiolkovsky ejecta blanket is apparently more aluminous and less magnesian than the crater walls and average far side highlands. The x-ray data for the crater walls can also be fairly well replicated by a mixture of 84% ejecta materials and 16% Apollo 15 basalt (Table 1). The ejecta would therefore appear to be slightly more feldspathic than the walls. Gravity data (8) indicate that the basaltic fill within Tsiolkovsky is not very thick. This is supported by modeling based on crater dimensions (9) which gives an aver- age fill thickness of about 250 m. This leads to an estimate of approximately 2,750 km 3 for the volume of basalt within the crater. This value is only about a fifth of that calculated for the volume of impact melt based on available models (10). Floor Walls Ejecta Highlands Ap. 15 basalt 0.29$ 0.23$ 0.90$. 14.9$ Ap. 17 basalt 0.25$ 0.19$ 5.8$ 14.9$ Ap. 15 anorthosite 0.90$ 0.02$ 0.01$ 0.18$ Ap. 16 breccia 0.73$ 0.11s 0.29$ 2.82$ 84% Ap. 15 basalt + 0.39 0.20 0.76 12.54 16% anorthosite 84% Ap. 17 basalt + 0.35 0.16 4.97 12.54 16% anorthosite 84% ejecta + 0.57 0.16 16% Ap. 15 basalt Table 1: Al/Si and Mg/Si concentration ratios and Ti and Fe wt % 'for the Tsiolkovsky crater, various returned samples, and calculated mixtures. The data labeled *are from (ll), #are from (12), @arefrom (6), and $ are from (13) . References: 1) Wilhelms, D. and El-Baz, F. (1977) U.S. Geol. Survey Mis. Inv. Ser. Map. 1-948. 2) Howard, K.A. and Wilshire, H.G. (1975) J. Res. U.S. Geol. Survey 3, 237-251. 3) Whitford-Stark, J.L. (1981) fn Multi-Ring Basins, p. 113-124. 4) El-Baz, F. and Worden, A.M. (1972) Apollo 15 Preliminary Science Report p. 25-1 to 25-27. 5) Whitford- Stark, J.L. and Hawke, B.R. (1979) Reports of Planetary Geology Program, 1978-1979. 6) Davis, P.A. (1980) J. Geophys. Res. 3209-3224. 7) Andre, C.G. et al. (1979) e. Lunar Planet. Sci. Conf. 10th p. 1739-1751. 8) Frontispiece (1975) Lunar Sci. Conf. 6th. 9) Whitford-Stark, J.L. (1979) Proc. Lunar Planet. Sci. Conf. 10th p. 2975-2994. 10) Lange, M.A. and Ahrens, T. f. 10th p. 2707-2725. 11) Adler, I.A.,etal. (1973) Naturwissenschaften 60, 231-242. 12) Pletzger, A.E. and Parker, R.E. (1979) Earth Planet. Sci. Letters 45, 155-171. 13) Wanke, H., et al. (1975) Lunar Science VI, p. 844-846. 14) Hawke, B.R. and Head, J.W. (1977) Impact and Explosion Cratering, p. 815-841. 15) Hawke, B.R. and Whitford-Stark, J.L. (1982) This volume. O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System .