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|Before the spacecraft exploration of Mars, the surface
pressure had been deduced to be less than about 10 mbar. The thermal structure
of the Martian atmosphere was investigated at four points by the Mariner 6 and 7
radio occultation, and Earth-based high resolution IR spectroscopy of the
dayside provided mean rotational temperatures, averaged in a complex way over
altitude, latitude, and time of day, that were known to reach as high as about 0
During the Viking missions, thermal IR mapping of both the daytime and the nighttime surface by the Viking orbiters was combined with radio occultation data, probe entry deceleration measurements, and in situ surface temperature measurements by the lander meteorology experiment package to permit synoptic study of the thermal structure and motions of the atmosphere. The radiometry results shows a range of surface brightness temperatures from as low as 130 K to as high as 290 K. It appears that there is no simple argument by which the very low winter polar temperatures can be explained away. Not low emissivity, high-altitude H20 clouds, or high surface altitude can fully resolve the problem posed by such low apparent temperatures. Even at 140 K the vapor pressure of CO2 is below 2 mbar, and it is very likely that the near-surface atmosphere near the winter pole has a composition that reflects the thermodynamic necessities; the CO2 partial pressure must be very severely depressed by condensation, whereas the total atmospheric pressure must be roughly the same.
At temperature low enough to condense CO2 (about 148 K at typical polar surface elevations), the vapor pressures of a number of potential atmospheric constituents are high enough so that they would remain wholly in the gas phase. At this temperature, the vapor pressures of some of these species are as follows: Xe, 240 mbar, and SO2, 10 m bar. Ar, CO, N2, and O2 would be completely evaporated. It is thus clear that any retention of "inert" gases such as N2 or Ar in the polar regions would have to be due to adsorption or trapping, not condensation.