Implications of MAVEN's planetographic coordinate system for comparisons to other recent Mars orbital missions

The Mars Atmosphere and Volatile EvolutioN (MAVEN) mission uses a planetographic coordinate system to report altitude, latitude, and longitude on Mars. By contrast, Mars Global Surveyor, Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter generally used a planetocentric coordinate system. These two coordinate systems are different: latitudes differ by up to 0.34 degrees and altitudes differ by up to 2km. These differences are large enough to affect the scientific results of comparisons between MAVEN and other orbital data sets. This is illustrated with three examples. (a) Comparisons of neutral density inferred from ionospheric peak altitude could contain errors of 25%. (b) Comparisons of mesopause altitude found from ultraviolet stellar occultations could contain errors of 2km. (c) Comparisons of zonal variations in thermospheric density found from accelerometer observations could contain errors of 12%. Scientists who compare MAVEN data to other data sets, or to models derived from other data sets, should be aware of these differences in the coordinate systems and make appropriate adjustments.