Environmental effects of large impacts on Mars
Science, 298 (5600):
1977--1980 (2002)Abstract
The martian valley networks formed near the end of the period of heavy
bombardment of the inner solar system, about 3.5 billion years ago.
The largest impacts produced global blankets of very hot ejecta,
ranging in thickness from meters to hundreds of meters. Our simulations
indicated that the ejecta warmed the surface, keeping it above the
freezing point of water for periods ranging from decades to millennia,
depending on impactor size, and caused shallow subsurface or polar
ice to evaporate or melt. Large impacts also injected steam into
the atmosphere from the craters or from water innate to the impactors.
From all sources, a typical 100-, 200-, or 250-kilometers asteroid
injected about 2, 9, or 16 meters, respectively, of precipitable
water into the atmosphere, which eventually rained out at a rate
of about 2 meters per year. The rains from a large impact formed
rivers and contributed to recharging aquifers.