Wednesday, June 17,
2009
A University of Colorado at Boulder research team has discovered
the first definitive evidence of shorelines on Mars, an indication
of a deep, ancient lake there and a finding with implications for
the discovery of past life on the Red Planet.
Estimated to be more than 3 billion years old, the lake appears to
have covered as much as 80 square miles and was up to 1,500 feet
deep -- roughly the equivalent of Lake Champlain bordering the
United States and Canada, said CU-Boulder Research Associate
Gaetano Di Achille, who led the study. The shoreline evidence,
found along a broad delta, included a series of alternating ridges
and troughs thought to be surviving remnants of beach
deposits.
"This is the first unambiguous evidence of shorelines on the
surface of Mars," said Di Achille. "The identification of the
shorelines and accompanying geological evidence allows us to
calculate the size and volume of the lake, which appears to have
formed about 3.4 billion years ago."
A paper on the subject by Di Achille, CU-Boulder Assistant
Professor Brian Hynek and CU-Boulder Research Associate Mindi
Searls, all of the Laboratory for Atmospheric and Space Physics,
has been published online in Geophysical Research Letters, a
publication of the American Geophysical Union.
Images used for the study were taken by a high-powered camera known
as the High Resolution Imaging Science Experiment, or HiRISE.
Riding on NASA's Mars Reconnaissance Orbiter, HiRISE can resolve
features on the surface down to one meter in size from its orbit
200 miles above Mars.
An analysis of the HiRISE images indicate that water carved a
30-mile-long canyon that opened up into a valley, depositing
sediment that formed a large delta. This delta and others
surrounding the basin imply the existence of a large, long-lived
lake, said Hynek, also an assistant professor in CU-Boulder's
geological sciences department. The lake bed is located within a
much larger valley known as the Shalbatana Vallis.
"Finding shorelines is a Holy Grail of sorts to us," said
Hynek.
In addition, the evidence shows the lake existed during a time when
Mars is generally believed to have been cold and dry, which is at
odds with current theories proposed by many planetary scientists,
he said. "Not only does this research prove there was a long-lived
lake system on Mars, but we can see that the lake formed after the
warm, wet period is thought to have dissipated."
Planetary scientists think the oldest surfaces on Mars formed
during the wet and warm Noachan epoch from about 4.1 billion to 3.7
billion years ago that featured a bombardment of large meteors and
extensive flooding. The newly discovered lake is believed to have
formed during the Hesperian epoch and postdates the end of the warm
and wet period on Mars by 300 million years, according to the
study.
The deltas adjacent to the lake are of high interest to planetary
scientists because deltas on Earth rapidly bury organic carbon and
other biomarkers of life, according to Hynek. Most astrobiologists
believe any present indications of life on Mars will be discovered
in the form of subterranean microorganisms.
But in the past, lakes on Mars would have provided cozy surface
habitats rich in nutrients for such microbes, Hynek said.
The retreat of the lake apparently was rapid enough to prevent the
formation of additional, lower shorelines, said Di Achille. The
lake probably either evaporated or froze over with the ice slowly
turning to water vapor and disappearing during a period of abrupt
climate change, according to the study.
Di Achille said the newly discovered pristine lake bed and delta
deposits would be would be a prime target for a future landing
mission to Mars in search of evidence of past life.
"On Earth, deltas and lakes are excellent collectors and preservers
of signs of past life," said Di Achille. "If life ever arose on
Mars, deltas may be the key to unlocking Mars' biological
past."
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University of Colorado at Boulder
Thanks to University of Colorado
at Boulder for this article.