REGOLITH PETROLOGY, REGOLITH BRECCIAS, AND REMOTE SENSING

A goal of remote sensing is to infer planetary interiors from inferring rocks and minerals in planetary crusts from interpretations of spectra of surface regolith. A correspondence between regolith petrology and bedrock is assumed. Impact comminution of bedrock, however, imparts some fractionation and the regolith so produced is not identical to its parent. Older regolith, if impact indurated, becomes regolith breccias that upon newer impacts produce another fractionated regolith that incorporates additional fresh bedrock ejecta. For example, mare basalt fragments have decreased from 38% to 22% and highland rock fragments have increased from 0.3% to 22% in the present day Apollo 11 soil 10084 (38% agglutinates) from it's the main precursor the 1 Ga old Apollo 11 soil breccia 10068 (12% agglutinates). Present-day regolith on any planetary surface is thus a strongly fractionated product of its crust. Mature regolith breccias are absent in the collection of meteorites. Hence, correspondence between asteroidal regolith and meteoritic breccias are harder to establish. Note that compositions of Martian soils are different from that of SNC meteorites. Our current petrographic studies of six Antarctic howardites with suspected regolith components show that their texture is more akin to ancient lunar breccias than to agglutinate-bearing lunar regolith breccias. It is not clear how the petrologic compositions of howardites would relate to the regolith on the HED parent body. Similar arguments extend to chondritic breccias. Hence, large uncertainties remain in inferences about the early history of the solar system only from spectral data of present-day regolith on asteroids.