Mars 6: Improved analysis of the descent module measurements

Abstract

More exact Doppler data, obtained by digital processing of the radio signals received from the Mars 6 descent module, provide, by direct conversion, estimates of the parameters of the Martian atmosphere at 12–90 km: the mass density at altitude 12 km is 4.5 × 10 −3 kg m −3; at 29 km, 0.98 × 10 −3 kg m −3; and at 82 km, 1.4 × 10 −6 kg m −3. The estimate of temperature at 12 km is 215°K and of the mean temperature at 29–82 km is 155°K. The estimate of the temperature lapse rate at 12–29 km is −3 to −4°K km −1. Mars 6 descended iin Pyrrhae Regio at −24° latitude and 19° longitude at 15:30 local time in early spring. The combined statistical analysis of the total set of Mars 6 measurements led to estimates of the mean local atmospheric parameters which made more exact the results of the preliminary analysis. These parameters define the trajectory of the descent module which is in statistical agreement with the more direct Mars 6 observations. The residuals of the measured quantities for this trajectory are practically within the real uncertainties of the measurements. The combined analysis yielded estimates to the mass density, pressure, and temperature near the planet's surface of 0.0117 ± 0.0005 kg m −3, 5.45 ± 0.3 mbar, and 246 ± 8°K, respectively; an estimate of the mean zonal component of wind speed at 0–8 km of 8–12 m sec −1; and an estimate of the local distance of the surface from the center of mass of 3388.9 ± 0.4 km. The mean values of these parameters together with estimates of the lower boundary of the isothermal zone of 33 km and the temperature of the isothermal zone 149°K, which expands at least up to 75–90 km, form the set of parameters with which to define the local model of the atmosphere. The data of the remote observations of Mariner and Mars orbiting spaceprobes are in agreement with these estimates. A close coincidence of the estimates of mass density near the planet's surface obtained both by these Mars 6 measurements and by the Mariner 9 occultation experiment limits the concentration of argon in the atmosphere to between 0 and 25 or 30%.