Mars: The case against permanent CO₂frost caps

Abstract

Leighton and Murray have argued that there is a polar reservoir of solid CO: on Mars that lasts throughout the year and whose vapor pressure determines the mean partial pressure of CO: in the atmosphere. This model is discussed in the light of recent data, and several difficulties emerge. First, such a system might be unstable, owing to the tendency of poleward heat transport to increase with atmospheric pressure. Second, the annual retreat of the CO: frost cover would be slower according to the model than that observed. Moreover, the observations seem to indicate that the residual polar cap that lasts throughout the year is composed of water ice rather than CO:. Finally, observations of water vapor in the atmosphere appear to be inconsistent with a permanent CO: cold trap in continuous existence for many years. These difficulties hold also for a CO: reservoir buried by water ice and for a hydrated CO:

A Sublimation Model for Martian South Polar Ice Features

“…However, there are other problems with the Leighton and Murray model. First, the rapid springtime retreat of the seasonal ice implies a rate of mass loss that is incompatible with the long-term survival of CO 2 ice (8,9). Second, in 1969 there was an unusual amount of H 2 O vapor over the south pole in summer, suggesting that the covering of CO 2 ice had partially disappeared and exposed H 2 O ice underneath (10).…”

“…These observations favor the model of a substrate of H 2 O ice beneath a thin (ϳ10 m) slab of CO 2 ice. We consider it unlikely that there is a buried CO 2 ice reservoir beneath the H 2 O ice, because heat conducted down through the H 2 O ice will cause buried CO 2 to sublime (8).…”

“…A central tenet of the Leighton and Murray model (1,8,18) is that the atmospheric CO 2 is buffered by a much larger reservoir of solid CO 2 . However, a CO 2 residual ice cap with an area of 88,000 km 2 , a thickness of 10 m, and a density of 1.6 g cm Ϫ3 , would contribute only 0.36 mbar of additional atmospheric pressure if it completely sublimed.…”

Distribution of the ices exposed near the south pole of Mars using Thermal Emission Imaging System (THEMIS) temperature measurements

“…The south polar layered deposits (SPLD) of Mars and the surrounding mantled terrains are of high interest because they contain the largest body of water ice near the surface of Mars [ Zuber et al , 2007a], they have recorded the past climate and global catastrophic events modifying the surface [ Clifford et al , 2000], and because they concentrate some of the most dynamic processes shaping the surface within a single region. These dynamic processes include the waning and waxing of the seasonal cap, basal sublimation and venting of the seasonal CO 2 frost leading to the formation of spiders and etched polygons [ Kieffer et al , 2006; Kieffer , 2007; Piqueux and Christensen , 2008b], expansion and contraction of the perennial cap over many years [ Piqueux and Christensen , 2008a], possible initiation of local dust storms due to high atmospheric thermal gradients [ Toigo et al , 2002 and references therein], and trapping and release of volatiles in the atmosphere during climate shifts [ Ingersoll , 1974; Toon et al , 1980; Fanale and Jakosky , 1982; Jakosky and Carr , 1985; Mellon and Jakosky , 1995; Mellon et al , 2004]. The ices exposed at the surface form the interface between the solid water and the atmospheric reservoirs.…”

The atmosphere of Mars

“…There are some observational arguments which partially confirm this proposal (see Section 1). Ingersoll (1974) collected many objections, although no one of them can be accepted as final. His model of polar caps supposed a very thin layer of frozen CO2 and H20.…”