The High Resolution Imaging Science Experiment (HiRISE) on board NASA’s Mars Reconnaissance Orbiter (MRO) has done it again. When the instrument caught a Mars avalanche in action in March, we were able to witness a fairly common terrestrial event on a different planet. The impact was huge; we were all fascinated by the slide of rock and ice for weeks. Now looking on the very small-scale, HiRISE has picked up a seemingly mundane terrestrial occurrence: a rock rolling down a hill. But this rock rolled down a crater side on Mars, leaving a track in the Martian regolith big enough to be spotted by the MRO…
These new pictures were observed by the HiRISE instrument onboard the MRO currently orbiting the Red Planet. Since its orbital insertion in 2006, the orbiter, a multi-purpose satellite, has returned some of the highest resolution images ever seen of the surface of Mars. Back in March, the HiRISE instrument took pictures of an escarpment in the north polar region of the planet. Along this scarp, HiRISE captured four separate avalanches occurring hundreds of kilometres apart. Never before had such a geologically dynamic event been captured by a Mars orbiter.
And now for the lowly rock. Looking at these new HiRISE images (taken on March 6th), it appears that rocks roll on Mars too. It’s not that we didn’t already know this, but this is the first time we’ve been able to resolve recently disturbed surface debris after it has rolled some distance down a slope on Mars (objects measuring ~167 cm across are resolved). What is really special are the very clear tracks from the rolling rocks imprinted in the regolith. In one example (pictured top) a large boulder (about 4 meters in diameter) had rolled down the crater side, picked up speed, hit a mini crater, skipped and bounced down the slope until coming to a stop. Taking a rough estimate, the rock in the image possibly rolled for a few hundred meters. These images were taken around the southern branch of Shalbatana Vallis, where it links with Chryse Planitia.
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It is thought that the boulders were disturbed in some way, breaking them loose from the crater edge (possibly a meteorite impact or other tremor) as there are several tracks in the regolith pointing in two directions. It also seems possible that they might be the ejecta from another meteorite impact in the area.
Either way, it’s great to see the small-scale geological activity in action as well as huge Mars avalanches…