EARLY MARS - HOW WARM AND HOW WET

Early in its history, Mars underwent fluvial erosion that has been int erpreted as evidence for a warmer, wetter climate. However, no atmosph ere composed of only CO2 and H2O appears capable of producing mean pla netary temperatures even close to 0 degrees C. Rather than by precipit ation, aquifer recharge and ground water seepage may have been enabled by hydrothermal convection driven by geothermal heat and heat associa ted with impacts. Some climatic warming was probably necessary to allo w water to flow for long distances across the surface. Modest warming could be provided by even a low-pressure CO2 atmosphere if it was supp lemented with small amounts of CH4, NH3, or SO2. Episodic excursions t o high obliquities may also have raised temperatures over some portion s of the planet's surface.