Dating recent aqueous activity on Mars

Amazonian-age Martian meteorites contain products of indigenous aqueous alteration; yet, establishing when this alteration occurred, and therefore when liquid water was available in the planet's crust, has proven challenging. New Ar-40/Ar-39 dates for iddingsite within the Martian meteorite Lafayette show these minerals precipitated from liquid water at 742 +/- 15 Ma (2 sigma). This age is the most precise constraint to date on water-rock interaction on Mars, and postdates formation of the host igneous rock by similar to 580 Myr. We infer that magmatic activity most likely induced melting of local permafrost and led to alteration of the nakhlites, suggesting that activation of localised hydrological cycles on Amazonian Mars by magmatism was infrequent and transient, but not unusual.