Mars Global Surveyor and Mars Odyssey accelerometer measurements of martian upper atmospheric densities reveal the large-scale and small-scale structure of the thermosphere in unprecedented detail. Meridional density gradients, which are seasonally dependent, are greater on the dayside than the nightside, indicating stronger meridional winds on the dayside. Density scale heights do not vary greatly with latitude. Densities and density scale heights are lower on the nightside than on the dayside, but the day-night contrast is not as great as at Venus. Thus atmospheric circulation plays a greater role in determining the state of the thermosphere on Mars than on Venus. Oscillations in density along individual density profiles, probably due to gravity waves, are common and their amplitudes are tens of percent. Step-like changes in density are also observed, but their origin is less clear. Data coverage is extensive enough to permit studies of weather, not just climate.