Early in its history, Mars underwent fluvial erosion that has been interpreted as evidence for a warmer, wetter climate. However, no atmosphere composed of only CO2 and H2O appears capable of producing mean planetary temperatures even close to 0 degrees C. Rather than by precipitation, aquifer recharge and ground water seepage may have been enabled by hydrothermal convection driven by geothermal heat and heat associated with impacts. Some climatic warming was probably necessary to allow water to flow for long distances across the surface. Modest warming could be provided by even a low-pressure CO2 atmosphere if it was supplemented with small amounts of CH4, NH3, or SO2. Episodic excursions to high obliquities may also have raised temperatures over some portions of the planet’s surface.