Researchers are still trying to figure out what caused these \"mystery\" features. One intriguing theory suggests they were sculpted by ancient glaciation. (credit:NASA/JPL/University of Arizona)
Raindrops of Sand in Copernicus Crater(02 of07)
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The dark features here look like raindrops, but are actually sand dunes. This spot was targeted an infrared spectrometer on the Mars Orbiter because the dunes are rich in the mineral olivine.\n\nOlivine-rich dunes are very rare on Earth, as olivine rapidly weathers to clays in a wet environment. There is also olivine-rich bedrock in the central peaks of Copernicus Crater on the Moon.\n\n(Caption: Alfred McEwen) (credit:NASA/JPL/University of Arizona)
Bright Tracks from Bouncing and Rolling Boulders(03 of07)
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This image shows a well-preserved impact crater. A closeup view highlights distinctive bright lines and spots on the steep slope on the north side.\n\nNo such pattern was visible when HiRISE imaged this crater 5 years ago (2.6 Martian years ago), in March 2008. The discontinuous bright spots indicate bouncing, thus these features are interpreted to be a result of boulders bouncing and rolling down the slope. \n\nWhere did the boulders come from? Maybe they fell from the crater\'s steep upper cliffs, although we don\'t see any new bright features there that point to the source. Maybe the rocks were ejected from a new impact event somewhere nearby.\n\nWhy are the trails bright? Perhaps the shallow subsurface soil here is generally brighter than the surface soil, as revealed by the Spirit rover in a part of Gusev Crater. It can\'t be bright from ice because this is a warm equator-facing slope seen in the summer.\n\n(Caption: Alfred McEwen) (credit:NASA/JPL/University of Arizona)
Ridges and Grooves That Wave and Buckle on a Valley Floor(04 of07)
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Long linear ridges and grooves curve, wave, and buckle across most of this image. Here, as elsewhere on Mars, these linear ridges and grooves fill a valley floor, hence their name, \"lineated valley fill.\"\n\nBecause these features are only found in valleys in the middle latitudes (30 to 60 degrees) of the Northern and Southern hemispheres, scientists had long suspected that they were associated with some ancient climate that had prevailed in that latitudinal band. Based on peering beneath the surface using radar, scientists now think that lineated valley fill is probably merely a rocky veneer atop a glacier of nearly pure ice! The rocks that make up the linear ridges and grooves were oriented by the ancient flow of the glacier underneath.\n\n(Caption: Ethan Schaefer) (credit:NASA/JPL/University of Arizona)
Megabreccia on the Floor of an Impact Crater(05 of07)
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\"\'Megabreccia\' is a term we use to describe jumbled, fragmented blocks of rock larger than 1 meter across, in a matrix of finer-grained materials,\" per the HiRISE website. \"It\'s the result of energetic processes, typically from an impact event.\" (credit:NASA/JPL/University of Arizona)
Defrosting of Dunes with Large Gullies(06 of07)
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The purpose of this observation is to image dunes where substantial \"gullies\" formed in the previous Martian winter. These features likely formed due to carbon dioxide defrosting or weight that caused the surface to slump. \n\nThe gullies at this site are particularly large, which is intriguing, suggesting that this site be monitored to see if stages of gully formation or details of activity can be observed. \n\n(Caption: HiRISE Science Team) (credit:NASA/JPL/University of Arizona)
Martian Honeycomb Hideout(07 of07)
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The most striking aspect of this image is the honeycomb-like pattern of the dunes.\n\nThis is a seasonal monitoring site, meaning HiRISE takes pictures across the seasons to view what changes occur and what causes them. The surface here is covered with seasonal carbon dioxide frost. In this case, we can compare locations of cracks in the frost to previous images. \n\n(Caption: HiRISE Science Team) (credit:NASA/JPL/University of Arizona)