This image provided by NASA Thursday Nov. 4, 2008 shows layers of similar thickness repeating dozens to hundreds of times in rocks exposed inside four craters in the Arabia Terra region of Mars in this image from the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter. Three dimensional analysis based on stereo pairs of images confirmed the regularity of repetition in the thickness of the beds. In the left half of this image, some of the rhythm is apparent as a series of bundles of about 10 individual layers per bundle. By corresponding to a known 10-to-one pattern in changes in the tilt of Mars' rotation axis, this pattern suggests the periodicity in the rock layers results from cyclical changes in the planet's tilt. Sedimentary rocks appear pink.
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