Spectral Properties and Heterogeneity of PHOBOS from Measurements by PHOBOS 2

Phobos 2observed parts of the surface of Phobos with three multispectral sensors covering the wavelength range 0.33-3.16 μm. These included a CCD camera (VSK) with two channels covering visible and near-infrared wavelengths, an ultraviolet-visible point spectrometer (KRFM) covering the 0.33-0.60 μm wavelength range, and a near-infrared imaging spectrometer (ISM) covering the 0.76-3.16 μm wavelength range. We have rederived the calibrations of all three data sets, using consistent methodologies in which known regions of Mars observed by the instruments serve as spectral standards. All three data sets yield mutually consistent results, and where thePhobos 2observations overlap previous measurements they are closely consistent.

The observed portion of Phobos is spectrally heterogeneous and consists of two fundamental spectral units. A “redder unit” with a visible/near-infrared color ratio of 0.6-0.85 covers most areas that were observed at high resolution by theVikingorbiters. Its visible-wavelength albedo is generally low, 6-7% in dark intercrater regions, but the rims on fresh craters are brightened by up to 40% without a change in color. Spectrally the redder unit exhibits a strong red slope at all wavelengths, with a steeper slope below an inflection at 0.5 μm. There is little or no absorption due to H₂O at 3 μm, indicating a nearly anhydrous surface composition. There is however a weak mafic mineral absorption at 1.0 μm, which increases in strength in progressively less red areas. The reddest material with the weakest 1-μm 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-infrared 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 wavelengths (albedo 5-6%) and it lacks the brightened crater rims typifying the redder unit. In contrast to the redder unit, in the bluer unit albedo and color are correlated; the brightest (albedo ∼8%), bluest materials occur on the walls and rim of Stickney. The bluer unit exhibits a shallower falloff of reflectance below 0.5 μm than the redder unit, but its near-infrared properties are unknown. Spatial distributions of the units show that the bluer unit originates at depth, and that the redder unit is a surficial layer.

Only the redder unit is well enough characterized spectrally for detailed analysis. Its spectral heterogeneities are inconsistent with simple particle size differences, and suggest the presence either of distinct lithologies or of a single lithology affected to different degrees by “space weathering.” It is distinct from the C and D asteroid classes with which Phobos has commonly been compared, and most closely resembles T-type asteroids or highly space-weathered mafic mineral assemblages. It is also distinct 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 affected 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/cm³), a high mafic mineral content would imply substantial internal porosity.