Carbon dioxide, an artist's conception:
Whoops. The top part of this table is in microns of mercury, but I read millimeters of mercury (mmHg) in my calculations yesterday, leading me to a temperature much colder than has been observed.
By using the correct numbers, the atmospheric pressure of Mars when the southern ice cap is at it's minimum (like now) is about 7.6 mmHg, one hundredth of earth's pressure of 760 mmHg. According to the above chart, carbon dioxide of about -122°C will have a vapor pressure of 7.5 mmHg, so the solid carbon dioxide that is vaporizing near the south pole should be at a temperature slightly greater than -122°C, not -159°C as previously stated.
This temperature is supported by data from the Viking Orbiter Infrared Thermal Mapper. For some reason, putting numbers to the images gives me a greater sense for the reality of Mars. I can imagine the Tundra region consisting of drifted and blowing dry ice 'snow'.
Can't you just see it:
Both images were taken this morning with the 7 inch f/15 Maksutov / 6 mm ortho, 1/8 second exposure. The left picture was made at 0730 UT with no filter, and the right one two hours later with a Wratten #23 red filter.
