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Summary
Mars is covered by channels that were likely formed by liquid water sometime in its past. Among the very few available clues to a wetter and warmer ancient climate on Mars are the isotopic signatures left by processes that have modified its atmosphere over time. In their Research Commentary, Yung and Kass discuss two sets of results in the same issue: Krasnopolsky et al . have used the Hubble Space Telescope to detect the presence of deuterium in the upper atmosphere of Mars, and Farquhar et al . report measurements of the Martian meteorite ALH84001 showing that the oxygen isotopes in the carbonate are fractionated with respect to silicate minerals. The findings suggest the presence of chemical processes operating in Mars' atmosphere that have yet to be fully understood.
