Today's Mars is a frigid desert, a place where water—the key to life as we know it—has gone into hiding. Whatever water may have once existed on Mars in rivers, lakes or even oceans is now frozen into ice caps, locked up in hydrated minerals or buried in debris-coated glaciers.
But last year compelling evidence emerged that when conditions are right, salty brines may persist to this day in liquid form at midlatitude regions on Mars. Alfred McEwen of the University of Arizona and his colleagues found tracks in high-resolution imagery that looked like liquid flowing downhill. The tracks appeared annually during the warmer Martian months on equator-facing slopes, extended downhill and then faded as temperatures dropped once again. One tantalizing interpretation was that the streaks were caused by briny water melting and seeping downhill through the soil.
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