Liquid water inferred on present-day Mars

Morphological evidence abounds for the flow of liquid water on ancient Mars; see, for example, the article by Bruce Jakosky and Michael Mellon, Physics Today, April 2004, page 71 . However, little definitive evidence for it exists on today’s surface. What surface water Mars holds has long been thought to reside as ice at its poles. In 2010 the Mars Reconnaissance Orbiter (MRO) spotted features known as recurring slope lineae that are consistent with transient flow: The dark, narrow streaks—the ones in the figure are hundreds of meters long—elongate on warm slopes in the spring and summer when their temperatures exceed about 250 K, fade and vanish in colder seasons, and reappear annually. Georgia Institute of Technology doctoral student Lujendra Ojha and his colleagues have now analyzed absorption spectra taken by MRO of four separate regions containing the streaks. They found signatures of a family of hydrated salts known as metal perchlorates at all of the sites during the warm seasons when the streaks are most extensive. Perchlorates can depress the freezing point of water by up to 70 K, and even in Mars’s scant relative humidity, they may be able to adsorb moisture from the atmosphere (or from seeps underground) and dissolve; the hydrated salts whose MRO spectra the researchers analyzed are thought to be the dried precipitates. Ojha and colleagues conclude that the spectra support the hypothesis that the seasonal streaks form as a result of the activity of liquid, albeit very briny, water on contemporary Mars. Even so, the proof is indirect; the spectral lines are too narrow to come from liquid water itself. (L. Ojha et al., Nat. Geosci. 8, 829, 2015, doi:10.1038/ngeo2546 ; image courtesy of NASA/JPL/University of Arizona.)