A warmer and wetter solution for early Mars and the challenges with transient warming

摘要

The climate of early Mars has been hotly debated for decades. Although mostinvestigators believe that the geology indicates the presence of surface water,disagreement has persisted regarding how warm and wet the surface must havebeen and how long such conditions may have existed. Although the geologicevidence is most easily explained by a persistently warm climate, the perceiveddifficulty that climate models have in generating warm surface conditions hasseeded various models that assume a cold and glaciated early Mars punctuated bytransient warming episodes. However, I use a single-column radiative-convectiveclimate model to show that it is relatively more straightforward to satisfywarm and relatively unglaciated early Mars conditions, requiring only about 1percent 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 transientlywarm an initially frozen surface. Surface pressure thresholds required for warmconditions increase about 10 to 60 percent for transient warming models,depending on ice cover fraction. No warm solution is possible for ice coverfractions 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 20percent, respectively. If high temperatures (298 to 323 K) were required toproduce the observed surface clay amounts on a transiently warm early Mars(Bishop et al), I show that such temperatures would have required surfacepressures that exceed available paleopressure constraints for nearly all H2concentrations considered (1 to 20 percent). I then argue that a warm andsemi-arid climate remains the simplest and most logical solution to Marspaleoclimate.