Washington, March 21 : NASA's Mars Odyssey orbiter has found evidence of salt deposits on the surface of the Red Planet, which point to places where water once was abundant and where evidence might exist of possible Martian life from the planet's past.
Discovered by a team led by Mikki Osterloo of the University of Hawaii, Honolulu, there are approximately 200 places on southern Mars that show spectral characteristics consistent with chloride minerals.
Chloride is part of many types of salt, such as sodium chloride or table salt.
Scientists used Odyssey's Thermal Emission Imaging System, a camera designed and operated by Arizona State University, Tempe, to take images in a range of visible light and infrared wavelengths.
Thermal infrared wavelengths are useful for identifying different mineral and rock types on the Martian surface. Osterloo found the sites by looking through thousands of images processed to reveal, in false colors, compositional differences on the Martian surface.
Plotted on a Mars map, the chloride sites appear only in the southern highlands, the most ancient rocks on Mars.
The sites range from about half of a square mile to 25 times that size.
"They could come from groundwater reaching the surface in low spots," said Osterloo. "The water would evaporate and leave mineral deposits, which build up over years. The sites are disconnected, so they are unlikely to be the remnants of a global ocean," he added.
"Many of the deposits lie in basins with channels leading into them," said Philip Christensen, co-author and principal investigator for the camera at Arizona State University. "This is the kind of feature, like salt-pan deposits on Earth, that's consistent with water flowing in over a long time," he added.
Scientists think the salt deposits formed approximately 3.5 to 3.9 billion years ago.
Several lines of evidence suggest Mars then had intermittent periods with substantially wetter and warmer conditions than today's dry, frigid climate.
Scientists looking for evidence of past life on Mars have focused mainly on a handful of places that show evidence of clay or sulfate minerals. Clays indicate weathering by water, and sulfates may have formed by water evaporation.
The new research, however, suggests an alternative mineral target to explore for biological remains.
"By their nature, salt deposits point to a lot of water, which potentially could remain standing in pools as it evaporates." said Christensen. "That's crucial. For life, it's all about a habitat that endures for some time," he added.