Phobos Disk-Integrated Photometry: 1994–1997 HST Observations

Abstract

A program of synoptic monitoring of Mars was carried out with the Hubble Space Telescope from August 8, 1994, through October 9, 1997, using the Wide Field Planetary Camera 2 (WFPC2). With the improved optics of the WFPC2 and the high angular resolution of the Planetary Camera (0.0442–0.0455 arcsec/pixel) we have been able to use these images to study the properties of the martian satellite Phobos. Phobos has been observed with the Planetary Camera from the ultraviolet (255 nm) to the near-infrared (1042 nm) in the phase range from 10.6° to 40.5°. Phase curves obtained with HST are consistent with results from a model (Simonelli et al. 1998, Icarus 131, 52–77) using a global-average Hapke function and a normal albedo of 0.0689. The beginning of an opposition surge along with variations in surface reflectance across Phobos were observed by HST. Disk-integrated photometry encompassing the sub-Mars point suggests the detection of Marsshine at wavelengths longward of 502 nm. Possible weak spectral absorption features seen at 953 and 1042 nm hint at the existence of pyroxene on the surface. However, spectral comparison to different meteorite types resulted in finding no conclusive analogs to the surface reflectance spectrum of Phobos.