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Nitrate was recently detected in Gale Crater sediments on Mars at abundances up to similar to 600mg/kg, confirming predictions of its presence at abundances consistent with models based on impact-generated nitrate and other sources of fixed nitrogen. Terrestrial Mars analogs, Mars meteorites, and other solar system materials help establish a context for interpreting in situ nitrate measurements on Mars, particularly in relation to other cooccuring salts. We compare the relative abundance of nitrates to oxychlorine (chlorate and/or perchlorate, hereafter (per)chlorate) salts on Mars and Earth. The nitrate/(per)chlorate ratio on Mars is <1, significantly lower than on Earth (nitrate/(per)chlorate>10(3)), suggesting not only the absence of biological activity but also different (per)chlorate formation mechanisms on Mars than on Earth.