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Schrodinger basin is a well-preserved peak-ring basin located on the lunar farside, along the rim of the much larger South Pole--Aitken (SPA) basin. The relatively young age (Lower Imbrian series, or 3.8 Ga) of this basin makes it an ideal site to study the geology of peak-ring basins in general, and the geological history of SPA specifically. Impact materials still recognizable include a well-defined crater rim, wall terraces, quasi-circular peak ring, and interior and exterior melt units. A small pyroclastic deposit fills a portion of the basin floor, along with several mare patches. This study uses Clementine multispectral ultraviolet-visible (UV-VIS) data, and a limited set of higher spectral resolution Chandrayaan-1 Moon Mineralogy Mapper ([M.sup.3]) data, as well as radar, camera, and topography data from the Lunar Reconnaissance Orbiter to better understand Schrodinger's geology. Sampled spectral profiles and linear unmixing models applied to the Clementine data indicate there is a heterogeneous distribution of both anorthositic and basaltic materials in the crater floor. [M.sup.3] data further validates this observation, and the high spectral resolution shows that most of the mafic content is dominated by pyroxene. These results challenge the traditional assumption that Schrodinger was formed in mostly highland terrain. Our assessment brings forth a new understanding regarding the placement of Schrodinger within SPA and the role SPA impact materials played in shaping the composition of Schrodinger. Le bassin de Schrodinger est un bassin d'impact d'anneau de crete bien preserve, situe sur la face cachee de la lune, le long de la bordure du bassin Pole Sud--Aitken (PSA), lequel est beaucoup plus grand. L'age relativement jeune (Imbrien inferieur ou 3,8 Ga) du bassin de Schrodinger en fait un site ideal pour etudier la geologie generale des bassins d'impact d'anneau de crete et plus specifiquement l'historique geologique du bassin PSA. Les materiaux d'impact encore reconnaissables comprennent une bordure de cratere bien definie, des terrasses murales, un anneau de crete quasi-circulaire et des unites interieures et exterieures de fusion. Un petit depot de roches pyroclastiques remplit une portion du plancher du bassin en plus de plusieurs parcelles de mers. La presente etude utilise des donnees multispectrales de la sonde UV-VIS Clementine et un ensemble limite de donnees a haute resolution spectrale de la sonde Chandrayaan-1 Moon Mineralogy Mapper ([M.sup.3]) ainsi que des donnees radar, de camera et de topographie de la sonde du Lunar Reconnaissance Orbiter afin de mieux comprendre la geologie de Schrodinger. Les profiles spectraux echantillonnes et les modeles d'ajustement lineaire appliques aux donnees Clementine indiquent que les materiaux anorthositiques et basaltiques du plancher du cratere sont distribues de maniere heterogene. Les donnees M3 valident cette observation et la haute resolution spectrale montre que le pyroxene constitue la plus grande partie du contenu mafique. Ces resultats remettent en question l'hypothese traditionnelle selon laquelle Schrodinger a ete forme surtout dans des hautes terres. Notre evaluation presente une nouvelle comprehension en ce qui concerne la place de Schrodinger dans le bassin PSA et le role que jouent les materiaux d'impact PSA dans la formation de la composition de Schrodinger. [Traduit par la Redaction], Introduction Data from the Lunar Reconnaissance Orbiter (LRO) and Chandrayaan-1 missions provide an opportunity to study lunar impact craters at previously unavailable resolutions and wavelengths (e.g., Bhandari 2004; Goswami et [...]

Spacecraft imaging data allow an accounting of the occurrence and characteristics of the largest craters on small asteroids and satellites. Data covering Phobos, Deimos, Gaspra, Ida, Mathilde, Vesta, Amalthea, Thebe, Janus, Epimetheus, Hyperion, and Proteus show that approximately 50% of rocky objects support craters with diameters of 1 object mean radius (R(sub m)), and the great majority have more than two craters with D > 0.5R(sub m). The morphology of these craters is consistent with gravity (g(super -1)) scaling from craters on larger objects. The amounts of visible peripheral damage, such as fracturing caused by these relatively large impacts, are usually small; the best possible example (Phobos) may have been assisted by tidal forces. The occurrence and characteristics of these craters supports modeling results that predict formation in gravity-controlled regimes with very large relative-sized craters formed before global fragmentation occurs.

Altimetric profiles from the Clementine LIDAR are used to calculate the depths of 29 large craters and basins on the Moon. Plotting the depths of the best preserved structures together with values for simple and complex craters measured in pre-Clementine studies reveals an inflection in the depth/diameter (d/D) curve in addition to the one revealed by pre-Clementine data. This inflection occurs in the diameter range that corresponds to the morphologic transition from complex crater to basin. The best empirical power law fit for basin depths is [log.sub.10]0(d) = 0.41 x [[[log.sub.10](D)].sup.0.57]. This relationship is characterized by a lower slope than that for complex craters, demonstrating that this morphologic transition corresponds to a further decrease in the depth of an impact structure relative to its diameter with increasing size. Qualitative consideration of possible causes for the second inflection leads to the conclusion that it is most likely a consequence of a short-term modification mechanism that influences fundamental crater morphology, such as the increasing influence of gravity with diameter. Thicknesses of maria in the major basins are calculated by assuming that their unfilled depths would follow the d/d relation. Results are compared with previous estimates and yield thicknesses that are generally greater than those determined by studies of flooded craters and less than those obtained from analysis of gravity. Key Words: moon; moon surface; cratering; impact processes; collisional physics.

The ultraviolet-visible camera on the Clementine spacecraft obtained high-spatial resolution images of the moon in five spectral channels. impact craters mapped with these multispectral images show a scale of lithologic diversity that varies with crater size and target stratigraphy. Prominent lithologic variations feldspathic versus basaltic) occur within the south wall of Copemicus (93 kilometers in diameter) on the scale of 1 to 2 kilometers. Lithologic diversity at Tycho (85 kilometers in diameter) is less apparent at this scale, although the impact melt of these two large craters is remarkably similar in this spectral range. The lunar surface within and around the smaller crater Giordano Bruno (22 kilometers in diameter) is largely dominated by the mixing of freshly excavated material with surrounding older soils derived from a generally similar feldspathic lithology., The Clementine spacecraft acquired digital images of the lunar surface using four co-aligned framing cameras sensitive to radiation from the visible to mid-infrared (1). Preliminary results derived from processed data [...]

Analysis of laser altimetry data from Clementine has confirmed and extended our knowledge of nearly obliterated multiring basins on the moon. These basins were formed during the early bombardment phase of lunar history, have been filled to varying degrees by mare lavas and regional ejecta blankets, and have been degraded by the superposition of large impact craters. The Mendel-Rydberg Basin, a degraded three-ring feature over 600 kilometers in diameter on lunar western limb, is about 6 kilometers deep from rim to floor, only slightly less deep than the nearby younger and much better preserved Orientale Basin (8 kilometers deep). The South Pole-Aitken Basin, the oldest discernible impact feature on the moon, is revealed as a basin 2500 kilometers in diameter with an average depth of more than 13 kilometers, rim crest to floor. This feature is the largest, deepest impact crater yet discovered in the solar system. Several additional depressions seen in the data may represent previously unmapped ancient impact basins., Multiring basins make up the basic structural, geological, and topographic framework of the lunar crust 1, 2). Over 40 basins have been recognized and mapped on the moon [see, for [...]

The application of a thermal model to the high-latitude impact craters on the Moon and Mercury reveals that water-ice could have existed throughout the geological period in some of the craters. The Chao Meng Fu crater in Mercury, the floor of which is permanently devoid of sunlight, is capable of supporting water ice inside the crater walls. The unnamed bowl-shaped crater in Mercury is not capable of having water-ice, even though its floor is shadowed from direct sunlight because of its high temperature. Water ice could have existed in the Peary crater on the Moon due to the obliquity of the Moon.

Lunar surface features such as the crater Bullialdus are studied by a spectral mixture modeling through the application of multispectral images. The method requires charge coupled device data and enables accurate mapping of the compositional boundaries. The Bullialdus crater's stratigraphic data and lithologic map indicate the presence of anorthositic norite and noritic anorthosite 6 kilometers below the surface. From this information the Nubium basin's formation can be obtained.

Objects which caused the Tunguska event on the Earth and a meteoroid storm on the moon in June 1975 have the same place of origin. Although the direction of arrival of the meteorite which caused the crater Giordano Bruno is not known, laser ranging of the moon may reveal new information. Meteoroids of the beta-Taurid meteor stream, the comet, Encke, and the asteroids 2201, Olijata and DS 1978 cross the earth's orbit at the same time of the year in the same direction. An analysis of secondary fragments from impacts of primary objects indicates that the Corvid meteors of June 1937 are secondaries from the Giordano Bruno impact of June 1178. The Taurid complex is constituted by the remains of an earlier collision.

[sup.39.Ar-.sup.40.Ar] analyses of three pertrographically distinct, shocked Apollo 15 KREEP (i.e., high K, rare earth element, P, and other trace element contents) basalt samples demonstrate that a major impact event affected all three samples at about 2.1 Ga. The Copernican System craters Aristillus and Autolycus are to the north; a ray from one of them crosses the landing site and had been suggested prior to the Apollo 15 mission as a source of material that could be used to date these craters. Autolycus, the older of the two, is in a particularly appropriate terrain and is the most likely source of the 2.1 Ga heating and delivery event. With this calibration point, and if Autolycus really is a Copernican crater, the Copernican System lasted twice as long as has previously been suggested. Furthermore, the Moon was not subjected to a constant cratering rate over the past 3 b.y.; the average rate in the preceding Eratosthenian must have been twice that in the Copernican.

Life in the early solar system was probably like a food fight at the end of a long week at summer camp--all of the leftovers were thrown around and produced [...]

Spray and Thompson have investigated the Sudbury impact crater in Canada to understand the formation of impact craters. The 1.85-billion-year-old Sudbury crater is a mountain ring type crater. The investigation reveals the process of deep-seated faulting, which occurs due to the impact of a celestial object and helps understand the formation of such mountain ring craters.

The compensation state of 49 intermediate size (120 to 600 km diameter) lunar craters are investigated using the most recent spherical harmonic models of the lunar topography and gravity, truncated at degree n = 110. The total mass anomalies per unit area (i.e., the lateral variations of the vertically integrated density perturbations per unit area) within an otherwise uniform crust of 60 km thickness are determined such that, together with the surface topography, give rise to the model gravity anomalies. Crustal thicknesses of 40 and 80 km are also considered, but the general results of this study are not significantly affected. Excess mass anomalies are obtained by subtracting from the total mass anomalies the mass anomalies that are required for the isostatic compensation of the surface topography. The excess mass anomaly of a crater denotes its particular state of compensation. Dependencies of the excess mass anomalies on crater location, size, and age are investigated, but in general few discernable trends are evident. Although the vast majority of craters indicate some compensation, no correlation exists between age or size and the state of compensation. Roughly 16% of the craters show no compensation, and in some cases have mass deficiencies most likely due to the shock fractured bedrock: the breccia lens of lower density. The crust in these regions was likely cold and rigid enough at the time of impact to rigidly support the stress caused by crater excavation. These features are seen throughout different geological periods, demonstrating that the lunar crust cooled quickly and strengthened soon after formation. A comparison of the compensation state of craters Apollo, Korolev, and Hertzsprung suggests that the thermal and mechanical properties of the crust prior to impact had an appreciable effect on the compensation, and that crustal thickness may be the single most important factor controlling the compensation of intermediate size craters. The characteristics of the excess mass anomaly profiles of the eight well-known near side mascon basins are used to identify new mascon-like craters. Ten newly found mascons are confirmed: Humboldtianum, Moscoviense, Mendel-Rydberg, Lorentz, Hertzsprung, Korolev, Schrodinger, Freundlich-Sharonov, Coulomb-Sarton, and Schiller-Zucchius, while two more, Deslandres and Dirichlet-Jackson, are very plausible. These results show that mare flow is not necessarily required to produce mascon-like characteristics. Key Words: Moon; crater compensation; lunar mascons; lunar crust.

Research into the possible origins of large craters on the Moon is discussed. Studies published in Science journal, based on dating of impact melts in lunar meteorites, suggest a sudden, specific cataclysmic event 3.9 billion years ago.

Numerous studies of the lunar gravity field have concluded that the lunar Moho is substantially uplifted beneath the young multiring basins. This uplift is presumably due to the excavation of large quantities of crustal material during the cratering process and subsequent rebound of the impact basin floor. Using a new dual-layered crustal thickness model of the Moon, the excavation cavities of some nearside multiring basins (Grimaldi and larger, and younger than Tranquillitatis) were reconstructed by restoring the uplifted Moho to its preimpact location. The farside South Pole-Aitken (SPA) basin was also considered due to its importance in deciphering lunar evolution. Restoring the Moho to its preimpact position beneath these basins resulted in a roughly parabolic depression from which the depth and diameter of the excavation cavity could be determined. Using these reconstructed excavation cavities, the basin-forming process was investigated. Excavation cavity diameters were generally found to be on the small side of most previous estimates (for Orientale the modeled excavation cavity lies within the Inner Rook Ring). Additionally, with the exception of the three largest basins (Serenitatis, Imbrium, and South Pole-Aitken) the depth/diameter ratios of the excavation cavities were found to be 0.115 [+ or -] 0.005, a value consistent with theoretical and experimental results for impact craters orders of magnitude smaller in size. The three largest basins, however, appear to have significantly shallower depths of excavation compared to this trend. It is possible that this may reflect a different physical process of crater formation (e.g., nonproportional scaling), special impact conditions, or postimpact modification processes. The crustal thickness model also shows that each basin is surrounded by an annulus of thickened crust. We interpret this thickened crust as representing thick basin ejecta deposits, and we show that the radial variation in the thickness of these deposits is consistent with scaling laws obtained from small-scale experimental studies. If multiring basins ever possessed a terraced main crater rim, this terraced zone may be presently unrecognizable at the surface due to the emplacement of ejecta deposits that exceed a few kilometers in thickness exterior to the excavation cavity rim. We also show that the interiors of many basins were superisostatic before mare volcanism commenced. Those basins that were closest to approaching a premare isostatic state lie close to or within an anomalous geochemical province rich in heat-producing elements. Key Words: impact processes; cratering; Moon.