Enlarge / Those dark streaks come and go with the seasons. We still don’t know what causes them. (credit: NASA/JPL-Caltech/Univ. of Arizona)
Mars clearly had extensive water in the past, and there’s still plenty of it locked up as ice in glaciers and the polar ice caps.

But the atmosphere is too thin and cold to allow liquid water to exist on the surface, which makes prospects for life on the red planet far less likely.
Back in 2011, however, researchers suggested that, contrary to our expectations, there might still be some water seeping out onto Mars’ surface.

Darkened features were identified on a variety of slopes, and they seemed to appear during warmer seasons and vanish as temperatures plunged again.

The Mars Reconnaissance Orbiter appeared to detect water at the site.

But other researchers proposed a physical mechanism that didn’t involve water that could account for the seasonal changes.

Now, a review of the evidence in Nature Geoscience argues that there are problems with almost all of the potential causes for these seasonal features.

And, in the absence of a compelling case for water, it’s best to assume that the harsh conditions mean what we typically thought they did: Mars is a dry planet.
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