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Science 6 December 2002:
Vol. 298. no. 5600, pp. 1977 - 1980
DOI: 10.1126/science.1073586
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Reports

Environmental Effects of Large Impacts on Mars

Teresa L. Segura,1*dagger Owen B. Toon,1 Anthony Colaprete,2 Kevin Zahnle2

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.

1 Program in Atmospheric and Oceanic Sciences, Laboratory for Atmospheric and Space Physics, University of Colorado, Campus Box 392, Boulder, CO 80309-0392, USA.
2 NASA-Ames Research Center, MS 245-3, Moffett Field, CA 94035, USA.
*   Present address: NASA-Ames Research Center, MS 245-3, Moffett Field, CA 94035, USA.

dagger    To whom correspondence should be addressed.

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