Polar Volatiles on Mars—Theory versus Observation

Abstract

The residual frost caps of Mars are probably water-ice. They may be the source of the water vapor associated with seasonal polar hoods. A permanent reservoir of solid CO₂ is also probably present within the north residual cap and may comprise a mass of CO₂ some two to five times that of the present atmosphere of Mars. The martian atmospheric pressure is probably regulated by the temperature of the reservoir and not by the annual heat balance of exposed solid CO₂ (37). The present reservoir temperature presumably reflects a long-term average of the polar heat balance. The question of a large permanent north polar cap is reexamined in light of the Mariner 9 data. The lower general elevation of the north polar region compared to the south and the resulting occurrence in the north of a permanent CO₂ deposit are probably responsible for the differences in size and shape of the two residual caps. The details of the processes involved are less apparent, however. It might be argued that the stability of water-ice deposits depends on both insolation and altitude. The present north and south residual caps should be symmetrically located with respect to such a hypothetical stability field. However, the offset of the south cap from the geometrical pole, the non-symmetrical outline of the north cap, and the apparently uniform thickness of the thin, widespread water-ice all argue against control by simple solid-vapor equilibrium of water under present environmental conditions. We think that the present location of the water-ice may reflect, in part, the past location of the permanent CO₂ reservoir. The extreme stability of polar water-ice deposits increases the likelihood that past environmental conditions may be recorded there. Detailed information on elevations in the vicinity of the residual caps is needed before we can further elucidate the nature and history of the residual caps. This, along with measurements of polar infrared emission, should be given high priority in future missions to Mars. Two conclusions follow from the limitation of the mass of solid CO₂ on Mars at present to two to five times the mass of CO₂ in the atmosphere. If all of this CO₂ was entirely sublimated into the atmosphere as a result of hypothetical astronomical or geophysical effects, the average surface pressure would increase to 15 to 30 mbar. Although such a change would have considerable significance for eolian erosion and transportation, there seems to be little possibility that a sufficiently earthlike atmosphere could result for liquid water to become an active erosional agent, as postulated by Milton (38). The pressure broadening required for a green-house effect requires at least 10 to 20 times more pressure (39). If liquid water was ever active in modifying the martian surface, it must have been at an earlier epoch, before the present, very stable CO₂/H₂O system developed. There can be no intermittent earthlike episodes now. Furthermore, the present abundance of CO₂ on Mars may be an indicator of the cumulative evolution of volatiles to the surface of the planet (40). Thus, even the possibility of an earlier earth-like episode is dimmed. On Mars, the total CO₂ definitely outgassed has evidently been about 60 ± 20 g/cm². On the earth, about 70 ± 30 kg/cm² of CO² have been released to the surface (41). Hence, the total CO₂ devolved by Mars per unit area is about 0.1 percent of that evolved by the earth. Thus, the observational limits we place on solid CO₂ presently located under the north residual cap also may constitute considerable constraints on the total differentiation and devolatilization of the planet. If they are valid, it would seem unlikely that Mars has devolatilized at all like the earth, or ever experienced an earthlike environment on its surface.