A Martian Land of Lakes

Another week, another leap in our understanding of water on Mars, but with many open questions.

The central peak within Gale Crater rises 18,000 feet above the valley floor.

Newly published results from the Curiosity rover reveal the presence of a lake system in Gale Crater  around 3.5 billion years ago that likely persisted for millions of years. Just as importantly, each lake would have been filled with liquid water for anywhere between 100 and 10,000 years at a time. The Curiosity rover team, led by John Grotzinger of CalTech, based their findings on a sequence of sedimentary rocks suggesting past environmental conditions with lots of water, including rivers, river deltas, and lakes. So-called “laminated mudstones,” in particular, indicate ancient deposits in lakes with a depth of at least a few meters.

It’s also intriguing what the team did not find: glaciated sediments or other markers associated with a cold and frigid climate, despite a variety of past studies suggesting that those were the conditions that prevailed on Mars 3.5 billion years ago. The climate in Gale Crater now appears to have been benign and pretty much Earthlike, at least locally.

How could that be? At that time the Sun was much weaker in its output than it is today, by about 20 to 30 percent, according to most estimates. Mars received less solar energy than it does today, and to offset the lack of heating, the planet’s atmosphere would have to have been much thicker than it is now. No modeling studies, however, can simulate climate conditions with the atmospheric composition and thickness that these new results seem to imply. Scientists call it the “faint young Sun paradox,” and no good solution is evident.

There is yet another paradox, if only on a local scale. How come Aeolis Mons, also known as Mount Sharp (where Curiosity is exploring today), rises with its central peak 5.5 kilometers (18,000 feet) above the valley floor? Marjorie Chan, who comments on the new Curiosity results in the same issue of Science where Grotzinger et al. report their findings, suggested that many layers of sediment were deposited and laid down at Gale Crater, which were eroded and later lifted up. How could that have happened with more than 5.5 km of sediment? And how could such a deep depression have formed between the central peak of Mount Sharp and the crater rim? The odd Martian landscape at Gale Crater seems to require a very special explanation.

One thing is clear, though. The depressions are the lowest topographic points in that region, and would have accumulated water in a time when the Martian atmosphere was thicker. And a moderate climate, with plenty of surface water, would have made Mars a very life-friendly place. It is difficult to imagine that these lakes were not inhabited by microbial life.

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