Geographic region of Mars
Orcus Patera is a region on the surface of the planet Mars first photographed by Mariner 4. Of unknown formation, whether by volcanic, tectonic, or cratering causes, the region includes a depression about long, wide, surrounded by a rim up to high.
Description.
Orcus Patera was first imaged by Mariner 4. It is a depression about long, wide, and about deep but with a relatively smooth floor. It has a rim up to high. 
It has experienced aeolian processes, and has some small craters and graben structures. However, it is not known how the patera originally formed. Theories include volcanic, tectonic, or cratering events. A study in 2000 that incorporated new results from Mars Global Surveyor along with older Viking data, did not come out clearly in favor of either volcanic or cratering processes.
Mars Express observed this region in 2005, yielding a digital terrain model and color pictures.
Location.
Orcus Patera is west of Olympus Mons and east of Elysium Mons. It is about halfway between those two volcanoes, and east and north of Gale crater.
References.
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