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. 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.
Updated at 2:00 p.m. ET
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