Images captured by the Mars Reconnaissance Orbiter (MRO) of 152 kilometre-wide Gale crater provide a detailed window into the past environmental changes on the red planet.

Each layer of rock on any planet records the local conditions at the time the rocks were laid down; the hundreds of exposed layers in Gale crater's four kilometre tall central mound tracks the region's environmental history over billions of years.

“Looking at the layers from the bottom to the top, from the oldest to the youngest, you see a sequence of changing rocks that resulted from changes in environmental conditions through time,” explains Ralph Milliken of NASA’s Jet Propulsion Laboratory. “This thick sequence of rocks appears to be showing different steps in the drying-out of Mars.”

Miliken and colleagues report that clay minerals, which form under very wet conditions, are concentrated in layers near the bottom of Gale crater's central stack. Layers above those contain sulphate minerals – which are often deposited when the water in which they are dissolved evaporates – as well as clays. Progressing further up the stack sees the concentration of clays gradually decline, while at the very top a thick formation of regularly spaced layers bears no evidence of any water-related minerals.

Rock exposures with compositions matching various individual layers of the Gale stack have been found elsewhere on Mars, but Gale crater is the first location where a single series of layers has been found to contain these clues in a clearly defined sequence from older rocks to younger rocks. The findings suggest that initial clay-producing conditions were followed by sulphate-producing conditions and then dry conditions as Mars 'dried out'.

“If you could stand there, you would see this beautiful formation of Martian sediments laid down in the past, a stratigraphic section that’s more than twice the height of the Grand Canyon, though not as steep,” says Bradley Thomson of the Johns Hopkins University Applied Physics Laboratory.

The findings were made possible by combing data from three MRO instruments. The High Resolution Imaging Science Experiment camera captured stereo images of the crater, which the US Geological Survey used to create three-dimensional models to discern elevation differences as small as a metre. Observations by the Compact Reconnaissance Imaging Spectrometer for Mars provided information about the chemistry of the minerals within the crater and the Context Camera provided context by seeing how the layers relate to geological features in the surroundings.

“This work demonstrates the synergy of the instruments on the Mars Reconnaissance Orbiter,” adds Thomson. “We wouldn’t have as complete a picture if we were missing any of the components.”

Gale Crater is one of four finalist landing sites for the Mars Science Laboratory rover, Curiosity, which is scheduled to launch in 2011. All locations have evidence for past water activity and the new report clearly demonstrates the importance of evaluating potential landing sites for their scientific merit in great detail prior to the mission.

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