A lake that might once have been habitable may have filled a crater for a long time on early Mars, new spacecraft images reveal.
NASA's Mars Reconnaissance Orbiter (MRO) captured the images that suggest the debris-strewn Holden Crater once held a calm body of water that could have harbored life.
There is so far no convincing evidence life does or ever did exist on Mars, however.
The crater debris includes a mix of broken boulders and smaller particles called megabreccia.
"Holden crater has some of the best-exposed lake deposits and ancient megabreccia known on Mars," said Alfred McEwen, principal scientist for MRO's HiRISE camera.
"Both contain minerals that formed in the presence of water and mark potentially habitable environments," he added. "This would be an excellent place to send a rover or sample-return mission to make major advances in understanding if Mars supported life."
That mission could be NASA's Mars Science Laboratory, set for launch next year. The Holden Crater is one of six landing sites under consideration.
The Holden impact crater formed inside a larger impact basin that was crisscrossed by large, natural channels.
Blocks as large as 164 feet (50 meters) were blasted from the basin by the impact, before falling back to the surface to form the megabreccia layer.
Water later settled a layer of fine-grained sediment on top of the megabreccia, including clay that could preserve any signs of life that might have existed.
"If we were looking on Earth for an environment that preserves signatures related to habitability, this is one of the kinds of environments we would look at," said John Grant, Hi-RISE scientist.
That clay may have remained hidden except for a stroke of luck, when the Holden crater rim crumbled under the force of nearly 960 cubic miles of water (4,000 cubic kilometers) that it was holding back.
The resulting flood tore up blocks the size of football fields and left boulder-filled debris, according to Grant, but also revealed parts of the clay layer.
The first long watery period at Holden Crater probably lasted thousands of years, while the second lake that formed after the crater rim was breached may have lasted just hundreds of years, Grant added.
Most evidence for long periods of wet conditions on Mars rests in the planet's earliest history, according to HiRISE scientists. Water may have only flowed later on during catastrophic events such as impacts on the planet surface.
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