The surface of ancient Mars may have been dotted by vast lakes that lasted long enough to give microbial life a chance to emerge on the Red Planet, NASA scientists said Monday.
Geological evidence discovered by NASA's Curiosity rover suggests that a lake could have spanned the 96-mile-wide Gale Crater for perhaps millions of years. If so, that would contradict the idea that liquid water made only transient appearances on the Martian surface.
These findings are based on an analysis of rocks at the base of Mt. Sharp, a 3-mile-high mound in the middle of the crater. Shapes and patterns in the rock point to a lake that filled and drained over tens of millions of years, according to scientists at the Jet Propulsion Laboratory in La Cañada Flintridge.
In fact, the sediments deposited in this lake by snowmelt running down from the crater's rim could be what helped form Mt. Sharp in the first place.
"The puzzle pieces are coming together," said Michael Meyer, lead scientist for NASA's Mars Exploration Program.
The Curiosity scientists examined rocks that seem to have formed at strange slanting angles. On Earth, this kind of buildup occurs when fast-moving river water suddenly hits a lake and has to decelerate.
Curiosity's Mast Camera spotted these deposits at many different elevations along Mt. Sharp. This is a sign that the water cycle on Mars was active enough to produce steady amounts of runoff several times in Gale Crater's history, said Curiosity participating scientist Sanjeev Gupta of Imperial College in London.
The delta-like deposits, with their characteristic slants, are in some ways even more telling than the finely layered sedimentary rock that most likely indicates a lake bed, said Curiosity project scientist John Grotzinger.
"This was a complete surprise," said Grotzinger, a Caltech geologist. "There was no way to have recognized this from orbit."
Scientists don't believe there's much water left on Mars today, but that wasn't always the case.
"Mars was once a planet shaped by water," said JPL's Ashwin Vasavada, deputy project scientist for the Curiosity mission.
Even in the past, the thinking goes, much of the planet's water was locked up in ice or held underground. But if there were large liquid lakes on the surface, then Mars must have had a thick atmosphere capable of preventing that water from escaping into space.
"The climate system must have been loaded with water," Vasavada said. "To sustain a lake at Gale Crater for millions of years, Mars would need a vigorous hydrological cycle to keep the atmosphere humid."
The problem is, it's very difficult to generate a realistic model of the Martian atmosphere that explains how it could have been warm enough, not just thick enough, for these conditions to arise, Vasavada said.
Some of those atmospheric mysteries may yet be addressed by NASA's MAVEN mission, which is studying what's left of the Martian upper atmosphere to gain insight into the Red Planet's past.
Curiosity, known formally as the Mars Science Laboratory, is in its third year of exploration on the Red Planet. The $2.5-billion laboratory on wheels has already drilled up rocks in a spot called Yellowknife Bay that revealed signs of a past, water-rich, life-friendly environment on Mars.
The sedimentary layers at the base of Mt. Sharp could hold the key to whether Mars was a much more habitable planet than it appears today.