Insights into Martian water reservoirs from analyses of Martian meteorite QUE94201

The martian atmospheric D/H value of 5.2 times terrestrial is significantly higher than any found on Earth, and has been ascribed to preferential loss of H relative to D from the atmosphere through Jeans escape over time. Here, based on ion microprobe analyses of apatite grains from martian meteorite QUE94201, it is shown that the pre‐Jeans escape martian water reservoir has a D/H value ∼twice that of terrestrial water, rather than the “terrestrial” value that has been assumed in prior work. The data support a two‐stage history for martian volatiles in which early hydrodynamic escape enriched martian water to ∼2x terrestrial D/H values. Subsequent Jeans escape to produce the current atmospheric values has thus been responsible for less D‐enrichment than previously thought. A martian crust containing 2–3 times more water than previously proposed is implied by the results.