NASA’s Mars Odyssey orbiter has revealed evidence of salt deposits, the tell-tale signs that water was once abundant in these locations, which may preserve evidence of past life on Mars.
Using Mars Odyssey’s Thermal Emission Imaging System (THEMIS), which takes images in a range of visible light and infrared wavelengths to identify different minerals and rock types, a team lead by Mikko Osterloo of the University of Hawaii have found two hundred locations in the southern latitudes of Mars that show spectral characteristics consistent with salt deposits. The sizes of the locations range from 1 to 25 square kilometres, and appear in individual locations. “The sites are disconnected, so they are unlikely to be the remnants of a global ocean,” says Osterloo. "They could come from groundwater reaching the surface in low spots. The water would evaporate and leave mineral deposits, which build up over years."
This false-colour THEMIS image highlights mineral compositional differences - bright blue marks a deposit of chloride (salt). The black rectangle shows the outline of the close up view shown below. Image: NASA/JPL/Arizona State University/University of Hawaii.
In this view the colours are closer to their natural appearance, the salt deposits look bright in tone, like salt pans on Earth. The deposit seems to be emerging as the overlying material erodes away. Inset boxes show two areas in greater detail, revealing cracks that formed as the salt deposit dried. Image: NASA/JPL/Arizona State University/University of Hawaii.
Plotted on a Mars map, the salt sites appear in the middle to low latitudes, in the heavily cratered southern highlands, representing the most ancient rocks on Mars. "Many of the deposits lie in basins with channels leading into them," says Philip Christensen, principal investigator for the THEMIS camera. "This is the kind of feature, like salt-pan deposits on Earth, that is consistent with water flowing over a long time. And because the deposits lie in what are probably sedimentary basins, if you look upstream you might only find a trace of organic materials because they’re thinly dispersed, but the flowing water can concentrate the organic materials which could be preserved in the salt.”
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. Previously, scientists looking for evidence of past life on Mars have focused on a handful of places where clay, which indicates weathering by water, or sulfate minerals, which could have been formed by water evaporation, exist. The new research, however, suggests an alternative mineral target to explore as part of the quest to find life on Mars.
NASA is currently studying potential landing sites for its Mars Science Laboratory (MSL), a new-generation rover due for launch in late 2009. Sites featuring clay deposits number heavily in the short-list of candidate places to send the rover.