THE ALBEDO OF MARS: SIX MARS YEARS OF OBSERVATIONS FROM PANCAM ON THE MARS EXPLORATION ROVERS AND COMPARISONS TO HIRISE

The Mars Exploration Rovers (MER) Spirit and Opportunity have systematically estimated the Lambert albedo of the surface across their traverses in Gusev Crater and Meridiani Planum. Each rover is equipped with a Panoramic Camera (Pancam) instrument which acquires 360° “albedo pan” observations through the broadband (400-1080 nm) L1 filter, allowing quantitative albedo estimates. As of September of 2016, Spirit acquired 20 albedo pans over 7,730 m traversed, and Opportunity acquired 117 albedo pans over 42,368 m traversed. The Pancam-derived albedo values at Gusev Crater (0.14-0.24) are slightly higher than those viewed at Meridiani Planum (0.11-0.22, with one anomalously high measurement of 0.27 after a global dust storm in July 2007), and measurements from both regions are consistent with orbital observations by the Viking Orbiter (Infrared Thermal Mapper), the Mars Global Surveyor (MGS) Thermal Emission Spectrometer and MGS Mars Orbiter Camera, and the Mars Reconnaissance Orbiter (MRO) Context Camera and Mars Color Imager [Bell et al., 2008; 2009; 2013]. Comparing atmospheric opacity to temporal albedo variations provides insights into interactions between the Martian surface and atmosphere. A general increase in albedo was observed by Pancam after atmospheric events due to dust deposition on the surface. Pancam albedo observations also provide a “ground truth” for orbital imagers to validate radiometric calibration, and we present comparisons across full rover traverses to MRO High Resolution Imaging Science Experiment (HiRISE) data. The Pancam and HiRISE albedo values generally agree to within +/-15%, and the few instances that exceed this level of uncertainty can be attributed to differences in viewing geometries and/or atmospheric effects.