Color heterogeneity of the surface of Phobos: Relationships to geologic features and comparison to meteorite analogs

Abstract

Multispectral observations of Phobos by the VSK (Videospectrometric) TV cameras and KRFM (Combined Radiometer and Photometer for Mars) UV‐visible spectrometer on Phobos 2 have provided new determinations of the satellite's spectral reflectance properties, at greater spatial and spectral resolutions and over a greater geographic range than have previously been available. Images of the ratio of visible and NIR reflectances covering the longitude range 30°–250°W were constructed from 0.40–0.56 μm and 0.78–1.1 μm VSK images. Eight‐channel 0.3–0.6 μm spectra obtained by the KRFM instrument package were used to provide greater spectral resolution of parts of these images. The data were calibrated using instrumental parameters measured on‐ground and in‐flight, and the calibrations were refined and tested using previous spectral measurements of Phobos and telescopic spectra of Mars, which is visible in the background in both data sets. The average color ratio of Phobos was found to be ∼0.97±0.14, consistent with previously obtained measurements. However, the surface is heterogeneous, with at least four recognizable spectral units whose absolute color ratios were determined to within ∼10%: a “red” unit with a color ratio of 0.7–0.8, a “reddish gray” unit with a color ratio of 0.8–1.0, a “bluish gray” unit with a color ratio of 1.0–1.1, and a “blue” unit with a color ratio of 1.1–1.4. The “red” unit occurs in the interiors of several dark‐floored craters and as adjacent patches. The “blue” unit composes the interior of Stickney, as well as a lobate deposit superposed on the crater's rim and extending to the southwest. The “blue” lobe is surrounded by a broad “bluish gray” aureole that breaks up into patchy outliers in its distal portions. Intervening surfaces are “reddish gray.” The “red,” “reddish gray,” and “bluish gray” units were sampled by the KRFM spectrometer, and the “bluish gray” unit was found to have a distinct 0.3–0.6 μm spectrum. The spatial distributions of the color ratio units and their reflectance systematics are inconsistent with Mars shine or particle size differences alone being responsible for color variations, but lateral optical or compositional heterogeneity is supported by the units' different UV‐visible spectra. The redder and bluer color units are interpreted to have been excavated by impacts, from an optically and/or compositionally heterogeneous interior overlain by a “reddish gray” surficial layer. The location of the “blue” lobe emanating from Stickney correlates with the location of one of the morphologic classes of grooves, as predicted by ejecta reimpact models of groove origin. The large color ratio of “blue” material is inconsistent with a carbonaceous chondrite composition but is comparable to that of an assemblage of mafic minerals like that forming black chondrites. Qualitative and quantitative comparison of the color ratio and UV‐visible spectral properties of “bluish gray” material with those of meteorites indicates that black chondrites are this material's closest spectral analog. The UV‐visible spectra of “reddish gray” and “red” materials most resemble spectra of black chondrites but are also comparable to spectra of some carbonaceous chondrites.