Hydrogen isotope evidence for loss of water from Mars through time

Abstract

The high D/H of the Martian atmosphere (∼5-6 × terrestrial) is considered strong evidence for the loss of a Martian water to space. The timing and magnitude of the losss of water from Mars can be constrained by measurements of D/H in Martian meteorites. Previous studies of Martian meteorites have shown a large range in D/H, from terrestrial values to as high as the current Martian atmosphere. Here we show that the ancient (∼4 Ga) Mars meteorite ALH84001 has a D/H 4 × terrestrial and that the young (∼0.17 Ga) Shergotty meteorite has a D/H 5.6 × terrestrial. We also find that the young Los Angeles shergottite has zoning in D/H that can be correlated to igneous growth zoning, strongly suggesting assimilation of D-enriched waterir during igneous crystallization near the Martian surface. In contrast to previous studies, we find higher and less variable D/H ratios in these three meteorites. Our results suggest a two-stage evolution for Martian water - a significant early loss of water to space (prior to 3.9 Ga) followed by only modest loss to space in the last 4 billion years. The current Martian atmospheric D/H has remained essentially unchanged for the last 165 Ma. Copyright 2008 by the American Geophysical Union.