  The climate of early Mars has been hotly debated for decades. Although most
investigators believe that the geology indicates the presence of surface water,
disagreement has persisted regarding how warm and wet the surface must have
been and how long such conditions may have existed. Although the geologic
evidence is most easily explained by a persistently warm climate, the perceived
difficulty that climate models have in generating warm surface conditions has
seeded various models that assume a cold and glaciated early Mars punctuated by
transient warming episodes. However, I use a single-column radiative-convective
climate model to show that it is relatively more straightforward to satisfy
warm and relatively unglaciated early Mars conditions, requiring only about 1
percent H2 and 3 bar CO2 or about 20 percent H2 and 0.55 bar CO2. In contrast,
the reflectivity of surface ice greatly increases the difficulty to transiently
warm an initially frozen surface. Surface pressure thresholds required for warm
conditions increase about 10 to 60 percent for transient warming models,
depending on ice cover fraction. No warm solution is possible for ice cover
fractions exceeding 40, 70, and 85 percent for mixed snow and ice and 25, 35,
and 49 percent for fresher snow and ice at H2 concentrations of 3, 10, and 20
percent, respectively. If high temperatures (298 to 323 K) were required to
produce the observed surface clay amounts on a transiently warm early Mars
(Bishop et al), I show that such temperatures would have required surface
pressures that exceed available paleopressure constraints for nearly all H2
concentrations considered (1 to 20 percent). I then argue that a warm and
semi-arid climate remains the simplest and most logical solution to Mars
paleoclimate.
