The High Resolution Imaging Science Experiment (HiRISE) continues to churn up stunning images as NASA’s Mars Reconnaissance Orbiter passes over the Martian surface. However, today’s example probably creates more questions than answers. Close to the Mars equator, south of Elysium Planitia, exists a crater and inside are some strange mounds that have so far eluded formal explanation. There are a few possibilities how these mounds may have formed and there may also be some examples on Earth too…
These features resemble mesas being stripped by Martian winds, or a build-up of sand/sediment dropped after a sand storm. Actually, these “mystery” features are not formed by sand and may not have been carved out by the wind. This image was commissioned by the HiRISE team to investigate a previous Mars Orbital Camera (MOC, on the Mars Global Surveyor) image of the region showed an ancient filled-in crater with some strange undulations in the bottom. Using the full 25 cm/pixel resolving power of HiRISE, these features can be seen in great detail.
The largest mounds appear to be around 200 meters wide and vary in shape. Between the mounds appear to be wind-blown sand features, but scientists cannot explain the formation of the mounds at present. Attention is being paid to the rough surface texture of the mounds which suggests they may be outcrops of tough bedrock where loose sand or sedimentary rock has been eroded away, leaving the mounds behind. But how did this erosion occur and why is the bedrock so hardy?
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The mounds could be ancient lava flows, fluvial sediment (indicating a plentiful supply of water in the past) or impact ejecta (i.e. hot material kicked into the old crater after another impact). Any one of these factors may have produced these hardened features. The strange thing is that there is a huge plain of these mounds, they aren’t isolated features. To be able to determine the origin of these mounds, further analysis needs to be carried out. The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on board the Mars Reconnaissance Orbiter will now be used to derive the mineral content of the region so a better understanding can be attained. But until then, these mounds will remain a true Martian mystery…