Figure 1

Alternating layers of more-resisant and less-resistant material make up this portion of a rock called "Tetl," on the west spur of the "Columbia Hills" on Mars. NASA's Mars Exploration Rover Spirit took this picture with its microscopic imager on the rover's 272nd martian day (Oct. 7, 2004). The view covers an area about 3 centimeters (1.2 inches) wide. The layers that are less resistant to eroding away recede in comparison to the more-resistant layers. Granules of apparently harder material can be seen eroding out of some layers. Several possible origins could account for this type of layering. One that scientists are considering for Tetl and similarly layered rocks is from multiple events of volcanic ash falling from the sky or flowing across the land. Another is from episodes of sediment precipitating from a body of water.