Evidence for Fe-Ti Oxide Enrichment in the Lunar Subsurface from the LRO Mini-RF Observations

Abstract:

The Moon is thought to have formed from an enormous impact by a Mars-sized protoplanet with early Earth, later accreting from the resulting cloud of hot swirling debris, hence maintaining a similar bulk composition to the Earth. However, it remains a question whether the Earth fully differentiated before the impact, thereby leaving the Moon largely metal-poor, and how much the impactor, of unknown composition, would have contributed to compositional differences between the Moon and Earth. The above hypothesis of a differentiated early Earth is supported by the low Fe-Ti oxide content of the lunar highlands that represents the Moon’s anorthositic crust.

To address these questions, we investigate the vertical distribution of Fe-Ti oxide content in the lunar subsurface using Lunar Reconnaissance Orbiter’s (LRO) Miniature Radio Frequency (Mini-RF) monostatic radar observations. In particular we explore the differing dielectric properties of lunar fines on crater floors as deduced from polarimetric ratios of monostatic radar images. Since this thin layer of fines results from subsurface material excavated by the originatingimpact, we use their dielectric properties from different craters ranging in diameter from 5-20 km to infer Fe-Ti oxidecontent in the upper 2 km of the subsurface. Our initial study covers 21 equatorial and 15 north polar simple crater floors covered by smooth homogenous fines overlying rough regolith, satisfying the boundary conditions for the dielectric inversion model of Campbell et al. 2002.

For diameters of 2-5 km, the central part of craters floors appear to have a lower dielectric constant than in larger ones, implying a low Fe-Ti oxide concentration that increases with crater size and holds steady for craters between 5 and 20 km.After comparing the radar images of crater floors by LRO Mini-RF with Fe-Ti-oxide mapping by the LRO Wide-Angle Camera and JAXA Kaguya and subsurface [Fe] and [Ti] mapped by NASA Lunar Prospector, we conclude that thedielectric properties of lunar fines on larger crater floors coincide with increasing Fe-Ti abundance, implying that moremetallic oxides were excavated from 500 m to 2 km depth than from the upper 200-500 m of the lunar subsurface, confirming that the first few hundred meters of the lunar crust are iron-poor but that deeper down, more Fe-Ti oxides may be expected.