Formation of Rivers From the Effects of Large Impacts on Mars

The Martian valley networks formed near the end of the period of heavy bombardment, ~3.5 billion years ago. The largest impacts produced global blankets of hot ejecta meters to hundreds of meters thick; much of this initially >2000 K. The hot ejecta warm the surface, keeping it above the freezing point of water for decades or millenia, depending on impactor size, and cause shallow subsurface or polar ice to evaporate or melt. Large impacts also inject steam directly into the atmosphere from the crater or from water innate to the impactor. From all sources, a typical 100 (200, 250) km asteroid injects ~2 (9, 16) m precipitable water into the atmosphere, which eventually rain out at 2 m/yr. The rains from a large impact would form rivers and contribute to recharging aquifers. The rarity and brevity of these events thwart the development of mature drainage systems, which are rare on Mars