1. Behavior of Carbon Dioxide and Other Volatiles on Mars
2. The predicted composition of the seasonal cap was confirmed to be CO 2 by infrared measurement of its temperature in 1969 (24). The predicted pressure variation cycle was confirmed by in situ measurement by the Viking 1 and Viking 2 landers between 1976 and 1982 (25–27).
3. Leighton and Murray (1) posited that the permanent deposit of CO 2 would be found in the north but this view was shown to arise from an oversimplification in the inputs to their thermal model (28). Other studies (29 30) suggested that the remnant nonseasonal frost caps were both water ice although Murray and Malin (28) argued that the north polar region was favored over the south for retention of CO 2 because at more than a 3 km lower altitude (31) (now known to be more than 6 km) (32) (and hence higher pressure) the CO 2 frost point was at an appreciably higher temperature (almost 6 K higher) (33). Calculations show that deposits of CO 2 arbitrarily introduced at the south pole migrate north rapidly (14). Viking orbiter infrared observations demonstrated that the north polar residual cap was indeed water ice but that the south polar residual cap was CO 2 (34–36). The presence of the CO 2 cap in the south despite the argument raised by Murray and Malin (14 28 33) was explained by the extreme sensitivity of the equilibrium temperature to albedo; the south polar cap is more than 30% brighter in the spring than is the comparable northern cap (37) an effect attributed to northern winter deposition of dust raised each year during the period of most intense dust storm activity (southern summer) (38) which leads to “dirtier” frost and hence lower albedo in the north.
4. Although there is broad consensus that the southern residual cap is CO 2 the general impression from the literature is that the material is thin and occasionally may completely sublime. The only evidence put forth for this variability is the ground-based detection of abundant water vapor during the 1969 southern summer (39) an observation that would be at odds with the presence of CO 2 ice upon which the atmospheric water vapor would tend to deposit. The Viking orbiters observed only trace amounts of water vapor in 1977 (40) as would be expected in the presence of year-round CO 2 ice and an analysis of Mariner 9 infrared measurements indicated that the southern residual cap in 1971 and 1972 also retained CO 2 frost throughout the summer (41). These inconsistent observations have been taken as evidence of an interannual instability (42) and have been used to argue that Leighton and Murray's prediction of a large surface reservoir is wrong (42) or that as yet unknown feedback processes between the other CO 2 reservoirs (atmosphere polar cap carbonate rocks and gas adsorbed onto fine-grained regolith materials) maintain the near-zero mass of the surface frost (41).
5. North–south geological differences between the residual polar caps on Mars