Just as NASA’s Curiosity Mars rover completed a Martian year—687 Earth days—at Gale Crater this week, an international team led by Alberto Fairén of Cornell University and the Centro de Astrobiología in Spain presented evidence that the crater was covered by glaciers 3.5 billion years ago.
The researchers published this new result after analyzing images taken not by Curiosity on the surface, but by the HiRISE and CTX cameras on NASA’s Mars Reconnaissance Orbiter and the High/Super Resolution Stereo Color Imager onboard Europe’s Mars Express. Analysis of the orbital images showed that the glaciers were particularly extensive on a mountain called Aeolis Mons (also called Mount Sharp).
This new insight is consistent with the idea that Mars was once a cold, wet planet. That hypothesis has to reconcile two apparently contradictory findings: widespread evidence of flowing and standing water in the Martian past, and climate models indicating that temperatures on early Mars would have been below freezing (0oC, 32oF). Not only was the Sun weaker in the Solar System’s early history, there wouldn’t have been enough greenhouse gases to warm the Martian surface above the freezing point.
So what accounts for the flowing water? The Martian landscape is very similar to what we see today in Iceland and Antarctica, with many landscapes carved and processed by water and ice. The answer lies in salts that would have kept pools of liquid water well below the normal freezing point of water. The salt content in aqueous environments on early Mars must have been as high as in current brine environments on Earth; NASA rovers have found huge salt deposits on the Martian plains to back up this hypothesis. Could life have existed in these cold brines? Probably yes, as extremophilic organisms on Earth can thrive under very similar conditions.
Environmental conditions would have been fairly hospitable in Gale Crater, which appears to have been excavated by a huge asteroid impact. The crater would have filled with liquid water right after the impact, and would have been covered by glaciers shortly thereafter. The energy released by the impact is likely to have created hydrothermal conditions (think of Yellowstone’s hot springs) for thousands of years, maybe longer, which could have made it a local habitat and a haven of life 3.5 billion years ago. If so, the Curiosity rover might still find “freeze-dried” life underneath patches of ice remaining in Gale Crater today.