The martian water cycle, a key for the habitability of Mars, largely relies on the abundance of water in martian magmas and their mantle sources, yet martian (SNC: shergottite, nakhlite, and chassigny classes) meteorites contain minimal water. However, some experimental studies have suggested that martian parental magmas contained as much as 2% H2O. Here we integrate mineral-chemical, experimental, and cosmochemical constraints to show that martian magmas contained little water but abundant chlorine. Apatite and amphibole in martian meteorites are chlorine rich and water poor; this constrains the chlorine contents in their parental magmas to >0.3 wt% and water contents to <0.3 wt%. Our experimental work has shown that large amounts of water are not needed to explain the mineralogy of the martian meteorites because chlorine has effects similar to those of water on crystallization. Such chlorine-rich, water-poor martian magmas are consistent with Mars being chlorine rich (~2.5 ×) compared with the Earth. Furthermore, the bulk Cl composition of martian meteorites shows that they have not preferentially lost Cl by degassing of an H2O-rich vapor. Together, these results show that chlorine, not water, was the dominant volatile species in martian basalts, and that these basalts contributed little H2O to Mars' surface environment.

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