Near-infrared imaging of Mars from HST: Surface reflectance, photometric properties, and implications for MOLA data

We report initial results from Hubble Space Telescope (HST) images of Mars obtained at a near-IR wavelength of 1042 nm between February 1995 and June 1997, The images provide the highest spatial resolution global near-IR measurements of Mars obtained to date. The data have been calibrated to radiance factor (I/F) to an absolute accuracy of 2 to 5% (including new refinements to several HST calibration parameters) and now exist as a set of coregistered, map-projected images spanning a range of phase angles between 10 degrees and 41 degrees, The relatively high spatial resolution (20-30 km/pixel) global maps, when compared with Viking-era global albedo maps, reveal that substantial variations in Mars surface albedo patterns have occurred in the past 20 years. The most obvious changes include several classical dark regions that have become substantially brighter, and a few formerly bright regions that have darkened. The normal albedo of typical surface regions in the near-IR is derived from the multiple phase angle observations and is found to be bimodal with typical bright and dark values of 0.36 and 0.18, respectively. The surface near-LR phase coefficient ranges from 0.0 to 0.013 mag/deg, and there is no systematic correlation between phase coefficient and normal albedo. This observation is consistent with the diffusely scattering nature of the martian atmosphere even during times of low dust opacity. The data presented here may facilitate the planning and optimization of measurements to be obtained by the Mars Global Surveyor (MGS) Mars Orbiter Laser Altimeter (MOLA), which will begin mapping the planet in 1999. These HST data also provide a basis for comparison with future MOLA and Mars Orbiter Camera (MOC) data in searches for seasonal and interannual variations in surface and polar cap albedo. (C) 1999 Academic Press.