Contemporary Liquid Water on Mars?

The martian surface preserves a record of aqueous fluids throughout the planet's history, but when, where, and even whether such fluids exist at the contemporary surface remains an area of ongoing research. Large water volumes remain on the planet today, but mostly bound in minerals or frozen in the subsurface, with limited direct evidence for aquifers. A role for water has been suggested to explain active surface processes monitored by orbital and landed spacecraft, such as gullies and slope streaks across a range of latitudes; however, dry mechanisms appear at least equally plausible for many active slopes. The low modern atmospheric density and cold surface temperatures challenge models for producing sufficient volumes of water to do the observed geomorphic work. The seeming ubiquity of salts in martian soils facilitates liquid stability but also has implications for the habitability of any such liquids. ▪ A thin modern atmosphere and low temperatures make pure liquid water unstable on the surface of modern Mars. ▪ Widespread salts could enhance liquid durability by lowering the freezing point and slowing evaporation. ▪ Dielectric measurements suggest active brines deep beneath the south pole and, in transient thin films, within shallow polar soils. ▪ Some characteristics of gullies, recurring slope lineae, and other active features challenge both current wet and dry formation models. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 49 is May 28, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.