Limits on the CO2 content of the Martian polar deposits

It has recently been suggested that large quantities of CO2 could be sequestered within the martian polar deposits as CO2 ice or CO2 clathrate hydrate, stabilized against sublimation by a non-porous overburden of water ice. I investigate the phase stability of CO2-bearing polar ices and place limits on the quantity of CO? that can be sequestered. The magnitude of this limitation follows from CO2-bearing ices having thermal conductivities on the order of 5 to 6 times smaller than that of water ice, producing a higher geothermal gradient. Results indicate that only under favorable, and probably unrealistic, conditions an upper limit of 112 mbar of CO2 can be sequestered as CO2 clathrate or 254 mbar as CO2 ice in the north polar deposit. More realistic conditions will limit CO2 to no more than a few 10s of mbar. In either case, less CO2 is also allowed and none is required by existing observations. The south polar deposit probably consists of a similar composition, but thinner deposits make placing limits difficult. Limited quantities of CO2 in the polar regions primarily implies that large amounts of CO2 are not released at high obliquity, reducing the magnitude of periodic climate change. In addition, climate-evolution models that predict large CO2-bearing caps at present should be rejected, favoring models that do not predict extensive CO2-bearing ice deposits. (C) 1996 Academic Press, Inc.