SPECTRAL PROPERTIES AND HETEROGENEITY OF PHOBOS FROM MEASUREMENTS BY PHOBOS-2

Phobos 2 observed parts of the surface of Phobos with three multispect ral sensors covering the wavelength range 0.33-3.16 mu m. These includ ed a CCD camera (VSK) with two channels covering visible and near-infr ared wavelengths, an ultraviolet-visible point spectrometer (KRFM) cov ering the 0.33-0.60 Ccm wavelength range, and a near-infrared imaging spectrometer (ISM) covering the 0.76-3.16 mu m wavelength range. We ha ve rederived the calibrations of all three data sets, using consistent methodologies in which known regions of Mars observed by the instrume nts serve as spectral standards. All three data sets yield mutually co nsistent results, and where the Phobos 2 observations overlap previous measurements they are closely consistent. The observed portion of Pho bos is spectrally heterogeneous and consists of two fundamental spectr al units. A ''redder unit'' with a visible/near-infrared color ratio o f 0.6-0.85 covers most areas that were observed at high resolution by the Viking orbiters. Its visible-wavelength albedo is generally low, 6 -7% in dark intercrater regions, but the rims on fresh craters are bri ghtened by up to 40% without a change in color. Spectrally the redder unit exhibits a strong red slope at all wavelengths, with a steeper sl ope below an inflection at 0.5 mu m. There is little or no absorption due to H2O at 3 mu m, indicating a nearly anhydrous surface compositio n. There is however a weak mafic mineral absorption at 1.0 Crm, which increases in strength in progressively less red areas. The reddest mat erial with the weakest 1-mu m absorption is exposed in several small, dark-floored craters, which previously have been interpreted as local concentrations of impact melt. A ''bluer'' unit with a visible/near-in frared color ratio of 0.85-1.2 makes up the interior and ejecta of the large crater Stickney. This unit is typically darker at visible wavel engths (albedo 5-6%) and it lacks the brightened crater rims typifying the redder unit. In contrast to the redder unit, in the bluer unit al bedo and color are correlated; the brightest (albedo similar to 8%), b luest materials occur on the walls and rim of Stickney. The bluer unit exhibits a shallower falloff of reflectance below 0.5 mu m than the r edder unit, but its near-infrared properties are unknown. Spatial dist ributions of the units show that the bluer unit originates at depth, a nd that the redder unit is a surficial layer. Only the redder unit is well enough characterized spectrally for detailed analysis. Its spectr al heterogeneities are inconsistent with simple particle size differen ces, and suggest the presence either of distinct lithologies or of a s ingle lithology affected to different degrees by ''space weathering.'' It is distinct from the C and D asteroid classes with which Phobos ha s commonly been compared, and most closely resembles T-type asteroids or highly space-weathered mafic mineral assemblages. It is also distin ct from most previously proposed meteorite analogs. Several models can explain the properties and spatial distributions of the two spectral units. Phobos's surface material may be rich in mafic minerals, and af fected to different degrees by space weathering. Alternatively, Phobos 's surface could consist of a mixture of mafic-poor material (possibly resembling D asteroids) and mafic-rich material. In view of the moon' s low density (1.9 +/- 0.1 g/cm(3)), a high mafic mineral content woul d imply substantial internal porosity. (C) 1996 Academic Press, Inc.