Evidence in Tissint for recent subsurface water on Mars

- Near-surface and magmatic volatiles were sampled by the impact melts in Tissint.
- The near-surface source was rich in water with δD value of ∼4600±600‰.
- The near-surface source contained abundant carbon and halogens.
- The near-surface source was formed by water activity on Mars after ∼600 Ma.
- The magmatic water was in nominally anhydrous minerals with δD value of 0‰.

We report unambiguous chemical evidence for subsurface water activity in the martian crust at <600 Ma based on the data from Tissint, a fresh martian meteorite fall with minimal terrestrial weathering. The impact-melt pockets in Tissint contain abundant volatiles (H2O, CO2, F, and Cl), and their concentrations are positively correlated with each other. Higher H2O concentrations also accompany higher deuterium contents. These correlations suggest mixing between two volatile sources. The first source is H2O in the precursor basalt inherited from martian magma. Magmatic H2O in the basalt had low deuterium concentration and was likely stored in the nominally anhydrous minerals. This source contributed little CO2 or halogens to the impact melts. The second source is inferred to be aqueous-alteration products introduced to the basalt by water activity after the basalt erupted. These alteration materials contributed more volatiles to the impact melts than the magmatic source, and had high deuterium abundance, reflecting isotope equilibrium with recent martian atmosphere. The water activities occurred beneath the martian surface after ∼600 Ma (crystallization age), but before ∼1 Ma (ejection age). The chemical and isotopic signatures of the alteration products in Tissint resemble previously known martian samples associated with old water activities on Mars, which can be traced back to ∼4.2 billion years ago (e.g., the mudstone at Gale Crater). This similarity in chemistry and the wide age-span indicate that such water activities were common on Mars throughout its history, which had the potential to form habitable environment. However, the rarity of the volatile-rich zone in Tissint suggests that Martian crustal aqueous processes, where they have occurred are generally limited in their extent of water-rock reaction.