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 meas urements of Mars obtained to date. The data have been calibrated to radianc e factor (I/F) to an absolute accuracy of 2 to 5% (including new refinement s to several HST calibration parameters) and now exist as a set of coregist ered, map-projected images spanning a range of phase angles between 10 degr ees 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 th e past 20 years. The most obvious changes include several classical dark re gions that have become substantially brighter, and a few formerly bright re gions that have darkened. The normal albedo of typical surface regions in t he near-IR is derived from the multiple phase angle observations and is fou nd to be bimodal with typical bright and dark values of 0.36 and 0.18, resp ectively. The surface near-LR phase coefficient ranges from 0.0 to 0.013 ma g/deg, and there is no systematic correlation between phase coefficient and normal albedo. This observation is consistent with the diffusely scatterin g nature of the martian atmosphere even during times of low dust opacity. T he data presented here may facilitate the planning and optimization of meas urements to be obtained by the Mars Global Surveyor (MGS) Mars Orbiter Lase r 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 su rface and polar cap albedo. (C) 1999 Academic Press.