Facing south, a perspective view looking up Ma'adim Vallis. The channel flowed into Gusev Crater (foreground), which is currently being considered as a landing site for one of the 2003 Mars Exploration Rovers. A view across part of the large former lake (background) shows its deep bowl-shaped floor. Credit: R. P. Irwin III and G. A. Franz, National Air and Space Museum, Smithsonian Institution

Geologists at the Smithsonian's National Air and Space Museum have discovered a large former lake in the highlands of Mars that would cover an area the size of Texas and New Mexico combined, and which overflowed to carve one of that planet's largest valleys. The findings appear in the June 21 issue of the journal Science.

The flood channel, Ma'adim Vallis, is more than 550 miles long and up to 6,900 feet deep, making it larger than Earth's Grand Canyon.

"Imagine more than five times the volume of water in the Great Lakes being released in a single flood, and you'll have a sense of the scale of this event," said Ross Irwin, a geologist in the museum's Center for Earth and Planetary Studies (CEPS) and the paper's lead author.

Mars is now a cold desert planet but its many dry valleys could indicate that water once flowed on its surface. Recent results from the Mars Odyssey spacecraft have found evidence of water trapped in the near surface of the polar regions.

"The size of this lake-1,400 miles long-suggests Mars was warmer and wetter than previously thought," said Robert Craddock, a CEPS geologist and co-author of the paper.

Former lakes are considered the most likely places to preserve the record of any past Martian life. Calm water would allow sediments to be deposited slowly, preventing small organisms from being destroyed.

The source of water to carve the flood channel had long been a mystery to scientists, who had known very little about Mars' topography prior to the Mars Global Surveyor mission, which has been orbiting Mars since 1997.

False-color topographic map of the recently discovered former lakes in the cratered highlands of Mars. Lighter colors denote higher elevations in both images. The largest of the three lakes overtopped its basin rim and the resulting outflow toward the north (arrow) carved Ma'adim Vallis, which is larger than the Earth's Grand Canyon. The basin and valleys are currently dry, but evidence of the former lake shorelines has been preserved. North is at the top. Credit: R. P. Irwin III and G. A. Franz, National Air and Space Museum, Smithsonian Institution

Detailed elevation data from the Mars Global Surveyor shows the large valley originated nearly full-size at a ridge, much like the spillway of a dam. Late in the lake's history, rising water levels overflowed the lake basin rim, releasing the huge flood as the river cut into this former dividing ridge. What remained was "some of the best geological evidence for a lake found to date on Mars, including clear indications of the former shoreline," Irwin says.

Two other smaller lake basins were identified in the region by paper co-author Alan Howard, a geologist at the University of Virginia. All three lakes shared the same water level prior to the flood, indicating the possibility of an ancient water table and suggesting the locations of other dry lake basins on Mars. Such information could be important in determining where to land robotic probes in coming years.

CEPS is the scientific research unit within the Collections and Research Department of the National Air and Space Museum. CEPS performs original research and outreach activities on topics covering planetary science, terrestrial geophysics, and the remote sensing of environmental change.