Identification of minerals on the surface of Mars is critical to understanding the geological history of our neighbor planet. In this issue of American Mineralogist, Ehlmann et al. report their discovery of alunite [KAl3(SO4)2(OH)6] in Cross Crater on Mars. Because terrestrial alunite forms from Al-rich acid sulfate waters, these results strongly suggest the past presence of Al-rich acid saline martian waters.

You do not currently have access to this article.