201-1 The Power of Multiwavelength Measurement of Planetary Surface Composition

Session: The G.K. Gilbert Award Session: Geology of Mars, Mercury, Asteroids, and Icy Satellites in Honor of Scott Murchie

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Abstract:

Remote determinations of planetary surface compositions often suffer from ambiguity when using a single technique – for example limitations in sensitivity or knowledge of geologic context – that are resolved when combining two or more techniques to synthesize an improved understanding of target body geology. Two such examples from recent planetary missions, in which I had the privilege of participating, are the following. 

The Near-Earth Asteroid Rendezvous mission spent one year in close proximity to the S-type asteroid 433 Eros conducting VNIR, X-ray and gamma-ray spectroscopic investigations to ascertain the nature of space weathering on Eros and whether composition of the body is well represented by ordinary chondrite meteorites. Spectral and reflectance systematics at VNIR wavelengths revealed a style of space weathering distinct from that on the Moon [1] and X-ray spectroscopy – similarly sensitive to grain surfaces – indicates mobilization of moderately volatile elements including S [2]. Ambiguity in Eros's compositional affinity from these measurements was resolved by gamma-ray spectra sampling to 10s cm depth showing a close agreement with L and LL ordinary chondrites [3].

Exploration of Mercury by the MESSENGER mission showed that the oldest parts of the surface are covered by low-reflectance material (LRM) darker than basalt, despite depletion of the surface in Fe²⁺ [4]. Curvature of the UV-visible part of the reflectance spectrum suggested instead that elemental C as graphite might be the darkening agent, but a unique match was not possible [5]. Geochemical models suggest formation of a graphite flotation crust on early Mercury [6]. The global spectral mosaic was used to target LRM during low-altitude measurements by the neutron spectrometer; among common elements, C strongly backscatters (is bright) in neutrons. Correlation of backscattered neutrons with LRM is evidence for a carbon-rich primordial crust.

References: [1] Murchie et al. (2002) Icarus 155, 145-168. [2] Kracher & Sears (2005) Icarus 174: 36–45. [3] Peplowski et al. (2015) M&PS 50, 353-367. [4] Izenberg et al. (2014) Icarus, 228, 364–374. [5] Murchie et al. (2015) Icarus, 254, 287–305. [6] Vender Kaaden & McCubbin. (2015) J. Geophys. Res. Planets, 120, 195–209. [7] Peplowski et al. (2016) Nat. Geosci., 9, 273–276.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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