Role of Impact-Induced, Vapor-Phase Deposition in the Lunar Regolith Formation: Clues from the New Mineral, Hapkeite

Details Role of Impact-Induced, Vapor-Phase Deposition in the Lunar Regolith Formation: Clues from the New Mineral, Hapkeite Role of Impact-Induced, Vapor-Phase Deposition in the Lunar Regolith Formation: Clues from the New Mineral, Hapkeite

Dec 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufm.p11a..04a&link_type=abstract

American Geophysical Union, Fall Meeting 2003, abstract #P11A-04

Physics

5420 Impact Phenomena (Includes Cratering), 5464 Remote Sensing, 5470 Surface Materials And Properties, 6240 Meteorites And Tektites, 6250 Moon (1221)

Scientific paper

To correctly interpret the spectral properties of airless planetary bodies such as the Moon, Vesta, etc., it is important to understand the complex space-weathering processes responsible for their soil formation. In the absence of water and atmosphere, micrometeorite impacts play the dominant role in soil development on such airless bodies. Kinetics of such transient impacts can result in ultra-high-temperature events and cause melting and vaporization of even less-volatile elements in the soil. A regolith-breccia clast in lunar meteorite Dh-280 contains small (e.g. 10-20μ m) opaque mineral grains consisting of three distinct new lunar mineral phases- FeSi, Fe2Si, FeSi2 [Anand et al., 2002, 2003, LPSC]. We have named the Fe2Si mineral, HAPKEITE, in honor of Prof. Bruce Hapke. We are using a new X-ray microdiffraction technique with beamline 7.3.3 of ALS to refine and determine the crystal structures of these Fe-Si phases. Preliminary results indicate the possibility of superlattice structures in some cases. The presence of Fe-Si phases in a lunar soil fragment in Dh-280 indicates extreme reducing conditions. The two most plausible reduction mechanisms include impact-induced a) melting, evaporation, and vapor deposition and b) solar-wind hydrogen reduction. Our preferred scenario for the formation of these phases involves the melting and vaporization of lunar soil by micrometeorite impact. The transient, ultra-high-temperature impact could cause vaporization and thermal dissociation of FeO and SiO2 into their constituent atoms. The ubiquitous presence of np-Fe in silica-rich glass on the surface of most mature, lunar-soil grains has already been confirmed (Keller & McKay, 1993, Science). We propose that with further dissociation, the Si in the vapor phase could readily combine, in various proportions, with Fe, and condense as Fe-Si grains. These observations necessitate further careful investigation of lunar soils for the presence of Fe-Si phases, as these can significantly contribute towards the spectral reflectance of lunar soils, the actual material observed remotely.

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