[P-PS05] Mercury Science and Exploration

[PPS05-03] Temperature and composition dependent space-weathering on Mercury

★Invited Papers

*Oceane Barraud¹、Mario D'Amore¹、Sebastien Besse²、Jörn Helbert¹ (1.German Aerospace Center (DLR), Institute of Planetary Research, Rutherfordstrasse 2, 12489 Berlin, Germany、2.European Space Agency (ESA), European Space Astronomy Centre (ESAC), Villanueva de la Cañada, Spain)

キーワード：Mercury, Spectroscopy, Space-weathering

Spectroscopy is a key tool for investigating surface composition and space-weathering of planetary bodies. On the Moon, space weathering alters spectral signatures at visible (VIS) to near-infrared (NIR) wavelengths by darkening, reddening (increasing the slope of the continuum reflectance with increasing wavelength) and decreasing the strength of absorption features of the spectrum. Because of its proximity to the Sun, Mercury’s surface is exposed to extreme conditions and environment (e.g. extreme temperature variation, high rate of impact). Among the youngest features at the surface of Mercury are the hollows and fresh craters which exhibits a higher spectral albedo and less red continuum slope. Multispectral observations by the Mercury Dual Imaging System (MDIS) wide angle camera (WAC) onboard MESSENGER (Mercury Surface, Space Environment, Geochemistry and Ranging) revealed that hollows exhibit a reflectance twice higher than the average surface of Mercury and a weak absorption feature centered around 630 nm. The unique reflectance of the hollows is commonly attributed to the presence of sulfides, chlorides or mix of pyroxenes and sulfides or graphite and sulfides. Spectroscopic observations by the Visible and InfraRed Spectrograph (VIRS), the surface component of the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) onboard MESSENGER lack such absorption features even for the hollows materials observed with MDIS. However, MASCS spectra exhibit a strong concave curvature between 300 and 600 nm, unique to hollows. Here, we present an analysis of the spectral curvature on the entire surface of Mercury using the whole MASCS/VIRS dataset. We show that the spectral curvature is mainly linked to the youngest geological terrains of Mercury: hollows, bright spots and very bright craters. This first result suggests a relationship between spectral curvature in the visible and the age of the surface features. In addition, we found that the highest curvature values are distributed around Mercury's cold poles. This second result suggests that the thermal component of the space-weathering could play a major role in the behavior of the spectral curvature between 300 and 600 nm. Space-weathering due to extreme thermal regime of Mercury’s surface seems to strongly reduce the spectral curvature in the visible domain previously reported as the effect of sulfur-bearing minerals.