Space Weathering Effects on Mercurian Surface Analogs: Insights From Coordinated Spectral, Microstructural, and Chemical Analyses

Nanophase iron (npFe) is known to be the cause of increasing optical maturity (spectral reddening, an overall darkening, and decreasing prominence of spectral features) on the Moon and S-type asteroids, but what happens when Fe is in short supply? How do highly reduced surfaces behave in a harsh space weathering environment? MESSENGER observations of Mercury found no features due to Fe2+ in visible-to-near-infrared (VNIR) spectra of the surface, and other instruments confirmed that the surface contained less than 1% FeO. If npFe is an unlikely space weathering product on reduced bodies, could carbon play a role in the optical and microstructural maturation of these regoliths?

Carbon is hypothesized to be present as a darkening agent on the surface of Mercury and on carbonaceous asteroids. On Mercury, this carbon may be exogenic material introduced by impactors or endogenic material excavated from a theorized graphite floatation crust. Carbon may exist in concentrations of up to 5 wt. % both within Mercury’s low reflectance material and throughout carbonaceous asteroids. To better understand the role carbon may play in space weathering on Mercury and C-type asteroids, we utilized pulsed laser irradiation to simulate aspects of micrometeorite bombardment on powders of low- to no-iron silicates mixed with C-bearing opaques.

We present VNIR reflectance spectra from 0.3-2.5 μm, transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS) analyses of two select samples from our sample suite.