Initial results from radio occultation measurements with Mars Global Surveyor

A series of radio occultation experiments conducted with Mars Global Surveyor in early 1998 has yielded 88 vertical profiles of the neutral atmosphere. The measurements cover latitudes of 29°N to 64°S and local times from 0600 through midnight to 1800 during early summer in the southern hemisphere ( L s = 264°–308°). Retrieved profiles of pressure 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 be breaking in some cases. Distinctive meridional gradients of pressure and temperature indicate the presence of a low‐altitude westerly jet at latitudes of 15°–30°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 ∼2 km altitude implies a wind speed of 33 m s −1 , stronger than predicted, which may help explain the occurrence of numerous local dust storms within this latitude band in late southern spring. These measurements also characterize the response of the atmosphere to stationary thermal forcing at midsouthern latitudes, where high terrain south of Tharsis and low terrain in Hellas Planitia produce large, zonal temperature variations in the lowest scale height above the surface. Pressure measured at constant geopotential decreases at an average rate of 0.13% per degree L s , due primarily to condensation of CO 2 at the North Pole.