36th DPS Meeting, 8-12 November 2004
Session 38 Mars Surface and Water
Poster II, Thursday, November 11, 2004, 4:15-7:00pm, Exhibition Hall 1A

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[38.01] Martian Phase Function: Modeling the Visible to Near IR Surface Photometric Function using HST-WFPC2 Data.

J. M. Soderblom, J.F. Bell III, M. YH Hubbard (Cornell), M.J. Wolff (SSI)

Images of Mars in the visible to near-IR acquired from 1994 to 2003 using the Hubble Space Telescope WFPC2 have been used to model the martian surface photometric function at 410, 502, 673, 953, and 1042 nm. These data range in spatial resolution from 12 to 70 km/pixel at the sub-Earth point, and in phase angle coverage from 2.7 to 40.6. The WFPC2 images have been calibrated to radiance factor or I/F and projected to a cylindrical map for coregistration and comparison to similarly-mapped spacecraft data sets of albedo, topography, thermal inertia, composition, and geology. We modeled the observed I/F as a function of phase angle using Hapke, Minneart, Lambert and lunar-Lambert photometric functions for numerous regions of interest binned into low, moderate and high albedo units defined by Viking and TES albedo maps, and low, moderate, and high thermal-inertia units defined by TES thermal-inertia maps. Visibly-opaque dust and/or water ice clouds as well as regions with incidence or emission angles greater than 60 were excluded from the analysis. Here we present the results from this study. For example, the average I/F of the high albedo regions at 402 nm is observed to increase from 0.05 to 0.09 between phase angles of 40 to 3, with most of the increase occurring within a linear regime below a phase angle of 6, whereas the average I/F at 673 nm is observed to increase from 0.23 to 0.40, with only a weakly linear opposition surge observed near the 2.7 lower phase angle limit. We also present a summary of the goodness-of-fit of the different photometric models for the different regions of interest, a brief discussion of the correlations of albedo and thermal-inertia units with the photometric properties of the surface, and details on the wavelength dependence of the surface photometric function.

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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.