[meteorite-list] Some Ancient Mars Lakes Came Long After Others

[Ron Baalke]

http://www.jpl.nasa.gov/news/news.php?feature=6619

Some Ancient Mars Lakes Came Long After Others
Jet Propulsion Laboratory
September 15, 2016

Lakes and snowmelt-fed streams on Mars formed much later than previously 
thought possible, according to new findings using data primarily from 
NASA's Mars Reconnaissance Orbiter.

The recently discovered lakes and streams appeared roughly a billion years 
after a well-documented, earlier era of wet conditions on ancient Mars. 
These results provide insight into the climate history of the Red Planet 
and suggest the surface conditions at this later time may also have been 
suitable for microbial life.

"We discovered valleys that carried water into lake basins," said Sharon 
Wilson of the Smithsonian Institution, Washington, and the University 
of Virginia, Charlottesville. "Several lake basins filled and overflowed, 
indicating there was a considerable amount of water on the landscape during 
this time."

Wilson and colleagues found evidence of these features in Mars' northern 
Arabia Terra region by analyzing images from the Context Camera and High 
Resolution Imaging Science Experiment camera on the Mars Reconnaissance 
Orbiter and additional data from NASA's Mars Global Surveyor and the European 
Space Agency's Mars Express.

"One of the lakes in this region was comparable in volume to Lake Tahoe," 
Wilson said, referring to a California-Nevada lake that holds about 45 
cubic miles (188 cubic kilometers) of water. "This particular Martian 
lake was fed by an inlet valley on its southern edge and overflowed along 
its northern margin, carrying water downstream into a very large, water-filled 
basin we nicknamed 'Heart Lake.'"

The chain of lakes and valleys that are part of the Heart Lake valley 
system extends about 90 miles (about 150 kilometers). Researchers calculate 
Heart Lake held about 670 cubic miles of water (2,790 cubic kilometers), 
more than in Lake Ontario of North America's Great Lakes.

Wilson and co-authors of the report in the Journal of Geophysical Research, 
Planets, map the extent of stream-flow in "fresh shallow valleys" and 
their associated former lakes. They suggest that the runoff that formed 
the valleys may have been seasonal.

To bracket the time period when the fresh shallow valleys in Arabia Terra 
formed, scientists started with age estimates for 22 impact craters in 
the area. They assessed whether or not the valleys carved into the blankets 
of surrounding debris ejected from the craters, as an indicator of whether 
the valleys are older or younger than the craters. They concluded that 
this fairly wet period on Mars likely occurred between two and three billion 
years ago, long after it is generally thought that most of Mars' original 
atmosphere had been lost and most of the remaining water on the planet 
had frozen.

The characteristics of the valleys support the interpretation that the 
climate was cold: "The rate at which water flowed through these valleys 
is consistent with runoff from melting snow," Wilson said, "These weren't 
rushing rivers. They have simple drainage patterns and did not form deep 
or complex systems like the ancient valley networks from early Mars."

They note that similar valleys occur elsewhere on Mars between about 35 
and 42 degrees latitude, both north and south of the equator. The similar 
appearance and widespread nature of these fresh, shallow valleys on Mars 
suggest they formed on a global scale rather than a local or regional 
scale.

"A key goal for Mars exploration is to understand when and where liquid 
water was present in sufficient volume to alter the Martian surface and 
perhaps provide habitable environments," said Mars Reconnaissance Orbiter 
Project Scientist Rich Zurek of NASA's Jet Propulsion Laboratory, Pasadena, 
California. "This paper presents evidence for episodes of water modifying 
the surface on early Mars for possibly several hundred million years later 
than previously thought, with some implication that the water was emplaced 
by snow, not rain."

The findings will likely prompt more studies to understand how conditions 
warmed enough on the frozen planet to allow an interval with flowing water. 
One possibility could be an extreme change in the planet's tilt, with 
more direct illumination of polar ice.

Wilson's co-authors are Alan Howard of the University of Virginia; Jeffrey 
Moore of the NASA Ames Research Center, Moffett Field, California; and 
John Grant of the Smithsonian.

NASA's Mars orbiter missions are advancing understanding about the Red 
Planet that serves in preparation for human-crew missions to Mars beginning 
in the 2030s. For more about NASA's Journey to Mars, visit:

http://www.nasa.gov/content/nasas-journey-to-mars

News Media Contact
Guy Webster
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6278
guy.webster at jpl.nasa.gov

Dwayne Brown / Laurie Cantillo
NASA Headquarters, Washington
202-358-1726 / 202-358-1077
dwayne.c.brown at nasa.gov / laura.l.cantillo at nasa.gov

2016-241