There is abundant geological evidence for large volumes of liquid water having formed fluvial features and hydrated minerals early in Mars’ history, approximating a ~100–1500 global equivalent layer (GEL) in meters. Today, most water is stored in the polar cap or subsurface ice and amounts to only 20-40 m GEL of water. Previous studies have suggested that Mars may have experienced significant water loss due to exospheric escape processes. As the atmosphere at 4.1 Ga had a D/H composition of 2-4 x SMOW (standard mean ocean water) and now has a composition of 5-10 x SMOW, it is clear that the Martian atmosphere has been fractionated significantly. However, measured and modeled current H atmospheric escape fluxes alone cannot fractionate a large water reservoir to present-day D/H >5 × SMOW.
To resolve these discrepancies, we developed a comprehensive water budget and D/H model that integrates all major water sinks and sources, including crustal hydration and volcanic degassing, which had not been considered by previous studies. We modelled possible past escape fluxes independently using KINETICS that permitted a range of ~1025–5×1029 H/s, 0.1–1000× the present-day escape flux. The D/H of the water inventory increases rapidly through the Noachian in all possible simulations due to the combination of quickly diminished reservoir size through crustal hydration and fractionation by atmospheric escape. By contrast, the D/H evolution during the Hesperian is relatively unconstrained because models with low total volcanic outgassing (10-20 m GEL) result in D/H increases while models with high outgassing (60-120 m GEL) result in D/H decreasing or staying approximately constant.
Simulations from this model reproduce early Noachian water reservoir volumes of 100-1500 m global layers, simultaneously compatible with geologic estimates of water, protoatmospheric water volumes, estimates of loss rates, and observed D/H evolution. Water availability decreased 40-95% over the Noachian (~4.1–3.7 Ga), while Amazonian (<1.5-3.0 Ga) water availability was similar to today. The majority of simulations are compatible with a 100-150 m GEL Hesperian ocean, while only endmember simulations are compatible with a 550 m GEL Noachian ocean. Between 30-99% of Martian water was to lost not to space, but to crustal hydration, highlighting irreversible chemical weathering’s crucial role in increasing aridity and reducing long-term habitability of terrestrial planets.