Initial results from radio occultation measurements with Mars Global Surveyor

A series of radio occultation experiments conducted with Mars Global Survey or in early 1998 has yielded 88 vertical profiles of the neutral atmosphere . The measurements cover latitudes of 29 degrees N to 64 degrees S and loca l times from 0600 through midnight to 1800 during early summer in the south ern hemisphere (L-s = 264 degrees-308 degrees). Retrieved profiles of press ure and temperature versus radius and geopotential extend from the surface to the 10-Pa pressure level. Near-surface uncertainties in temperature and pressure are about 1 K and 2 Pa, respectively, far smaller than in previous radio occultation measurements at Mars. The profiles resolve the radiative -convective boundary layer adjacent to the surface and also reveal gravity waves, particularly at northern and equatorial latitudes, which appear to b e breaking in some cases. Distinctive meridional gradients of pressure and temperature indicate the presence of a low-altitude westerly jet st latitud es of 15 degrees-30 degrees S at southern summer solstice. This jet appears in predictions of general circulation models in connection with the strong , seasonal, cross-equatorial Hadley circulation. The pressure gradient at s imilar to 2 km altitude implies a wind speed of 33 m s(-1), stronger than p redicted, which may help explain the occurrence of numerous local dust stor ms within this latitude band in late southern spring. These measurements al so characterize the response of the atmosphere to stationary thermal forcin g at midsouthern latitudes, where high terrain south of Tharsis and low ter rain in Hellas Planitia produce large, zonal temperature variations in the lowest scale height above the surface. Pressure measured at constant geopot ential decreases at an average rate of 0.13% per degree L-s, due primarily to condensation of CO2 at the North Pole.