WATER ABUNDANCE OF THE MARTIAN INTERIOR: CLUES FROM THE MINERAL APATITE

Mars has supplied the surface of Earth with over 100 meteorites, consisting of basalts and related mafic and ultramafic intrusive igneous rocks that help us unlock Mars’ geologic history. Mars has been the focal point in the search for extraterrestrial life in our Solar System. Finding habitable environments that could have once harbored life on Mars has become an important scientific goal. Through many observations from orbiter and lander missions, there is now a substantial amount of evidence pointing towards the past presence of liquid water that once flowed across the martian surface. However, the source of this H₂O has remained a mystery. Early analyses of potentially hydrous phases in martian meteorites, including apatite, indicated low abundances of H₂O, and it was concluded that martian magmas, and by inference the martian interior, were very dry. Estimates of martian mantle H₂O contents were as low as a few ppm H₂O (Filiberto and Treiman, 2009; Patiño Douce and Roden, 2006; Patiño Douce et al., 2011). However, developments in analytical instrumentation for the analysis of volatiles, similar to those developed for studying apatite in lunar rocks, has facilitated a complete reconsideration of hydrous phases within martian meteorites (Greenwood et al., 2008; McCubbin et al., 2010b, 2012). Since apatite is the only hydrous phase that occurs among all martian meteorite types, it has been the focal point of this reassessment. A number of recent studies have found that apatites in martian meteorites are enriched in H₂O and Cl in comparison to apatites from typical terrestrial volcanic rocks, which has led to ideas that the martian interior is rich in both Cl (Filiberto and Treiman, 2009; McCubbin et al., 2013) and H₂O (Gross et al., 2013; McCubbin et al., 2012). Current estimates of H₂O abundances in the martian mantle, based on apatite analyses, are approximately 100 ppm H₂O, which is comparable to the mantle source of terrestrial mid-ocean ridge basalts (MORB).