Permanent Polar CO Caps on Early Mars
M. M. Joshi, R. M. Haberle (NASA Ames)
We have conducted simulations of the general circulation of the early atmosphere of Mars using a simplified global circulation model (SGCM) (1). The solar luminosity used is 75% of the present day value.
We find that at even at high orbital obliquities (<45 degrees), a pure CO atmosphere is unable to prevent permanent polar caps from forming. This is because when the surface pressure is below approx 100 mbar, heat transport to the polar regions is insufficient to allow removal of the caps during summer, whereas at pressures above this, the same result happens due to significant condensation of CO in the atmosphere during the winter.
The formation of permanent polar caps implies maximum surface pressures of less than O(10) mbar, which is far lower than previous modeling has suggested (2). These results are another indication that the warm conditions on early Mars suggested by geological evidence were associated with sporadic events such as impacts or the presence of other greenhouse gases.
(1) Joshi, M., et al., JGR Vol.100, p.5485-5500, 1995.
(2) Kasting, J., Icarus Vol.94, p.1-13, 1991.