We have used density measurements obtained during aerobraking (AB) on Mars Global Surveyor and Mars Odyssey to examine mean densities and the characteristics and variability of gravity waves (GWs) in the upper atmosphere of Mars. Mean densities exhibit variability with latitude and season and in response to variable forcing and filtering from below. These data also reveal significant variability of GWs, both spatially and temporally. In general, GW amplitudes increase, while apparent horizontal scales remain the same, with increasing altitude. The dominant spatial scales at the lower AB altitudes are typically ~20--200 km and density fluctuations vary from ~5 to 50%, though even larger values are occasionally observed. GW amplitudes also vary significantly with season, being generally larger in winter and at middle and high latitudes, and apparently reflecting mean source and filtering conditions. Amplitudes also appear to vary with longitude and time and may provide clues to variability of, and interactions with, larger-scale motions.