Geologic Implications of the Low Albedo of South Pole-Aitken Basin

C. M. Pieters (Brown University)

Galileo digital images of the Moon's farside returned in 1991 showed the interior of the huge South Pole-Aitken (SPA) Basin to be significantly lower in albedo than typical highland crust and to exhibit a stronger ferrous absorption [Belton et al., 1992, Science]. These two characteristics were also observed with more extensive Clementine data [Lucey et al., 1994, Science] and were used to derive estimates of SPA FeO abundance [Lucey et al., 1995, Science]. Since SPA basin is quite ancient crust (Nectarian or older), survival of the unusually dark character of SPA soils is particularly noteworthy. Most major lunar basins formed after SPA [Wilhelms, 1987, USGS PP 1348] redistributing crustal material and contributing deposits to the megaregolith predicted to typically be several hundred meters thick [McGetchin et al., 1973, EPSL 20]. Several high albedo smooth plains, for example, are believed to be early basaltic maria (cryptomaria) masked by higher albedo feldspathic deposits. Assuming the current SPA low albedo is a remnant feature due to the presence of local mafic material, some mechanism must protect this characteristic from being masked by subsequent deposits from surrounding feldspathic highlands. Two possibilities are suggested: 1) Local geography prevented feldspathic highland material from being deposited across the basin (perhaps governed by the size of the basin). If true, this would put substantial constraints on the extent of lateral mixing even during the period of basin formation and heavy bombardment. 2) Alternatively, cumulative vertical mixing may have been exceptionally deep and thorough within SPA. Any foreign feldspathic breccia deposited across SPA by exterior basin-forming events could be darkened by becoming increasingly diluted with the more mafic in situ SPA material. For a feature as ancient as SPA to maintain a low albedo, the depth of regolith or megaregolith mixing during or since deposition must be greater than the ejecta deposit thickness. This implies that the original surface within SPA would have to have been reworked to a depth of at least several hundred meters since its formation.

NASA research support (NAG5-4366) is gratefully acknowledged.