|Image Credit: NASA/JPL-Caltech/Cornell/MSSS|
Two Different Environments
This set of images compares rocks seen by NASA's Opportunity rover and Curiosity rover at two different parts of Mars. On the left is " Wopmay" rock, in Endurance Crater, Meridiani Planum, as studied by the Opportunity rover. On the right are the rocks of the "Sheepbed" unit in Yellowknife Bay, in Gale Crater, as seen by Curiosity.
Both color images have been white–balanced using the same technique to show roughly what they would look like if they were on Earth. The rock on the left is formed from sulfate-rich sandstone. Scientists think the particles were in part formed and cemented in the presence of water. They also think the concretions (spherical bumps distributed across rock face) were formed in the presence of water. The Meridiani rocks record an ancient aqueous environment that likely was not habitable due the extremely high acidity of the water, the very limited chemical gradients that would have restricted energy available, and the extreme salinity that would have impeded microbial metabolism -- if microrganisms had ever been present.
In the Sheepbed image on the right, fine-grained sediments represent the record of an ancient habitable environment. The Sheepbed sediments were likely deposited under water. Scientists think the water cemented the sediments, and also formed the concretions. The rock was then fractured and filled with sulfate minerals when water flowed through subsurface fracture networks (white lines running through rock). Data from several instruments on Curiosity all support these interpretations. They indicate a habitable environment characterized by neutral pH, chemical gradients that would have created energy for microbes, and a distinctly low salinity, which would have helped metabolism if microorganisms had ever been present.
Michael Meyer, lead scientist for NASA's Mars Exploration Program at the agency's headquarters in Washington, said "A fundamental question for this mission is whether Mars could have supported a habitable environment. From what we know now, the answer is yes."
Ancient Network of Stream Channels
The patch of bedrock where Curiosity drilled for its first sample lies in an ancient network of stream channels descending from the rim of Gale Crater. The bedrock also is fine-grained mudstone and shows evidence of multiple periods of wet conditions, including nodules and veins.
Curiosity's drill collected the sample at a site just a few hundred yards away from where the rover earlier found an ancient streambed in September 2012. These clay minerals are a product of the reaction of relatively fresh water with igneous minerals, such as olivine, also present in the sediment. The reaction could have taken place within the sedimentary deposit, during transport of the sediment, or in the source region of the sediment. The presence of calcium sulfate along with the clay suggests the soil is neutral or mildly alkaline.
Scientists were surprised to find a mixture of oxidized, less-oxidized, and even non-oxidized chemicals providing an energy gradient of the sort many microbes on Earth exploit to live.