Orcus Patera is an enigmatic（神秘的） elliptical（椭圆的） depression near Mars's equator（赤道） , in the eastern hemisphere of the planet. Located between the volcanoes of Elysium Mons and Olympus Mons, its formation remains² a mystery. Often overlooked, this well-defined depression extends approximately 380 km by 140 km in a NNE–SSW direction. It has a rim³ that rises up to 1800 m above the surrounding plains, while the floor of the depression lies 400-600 m below the surroundings.

The term 'patera（插座，圆盘饰） ' is used for deep, complex or irregularly shaped volcanic⁴ craters⁶ such as the Hadriaca Patera and Tyrrhena Patera at the north-eastern margin⁷ of the Hellas impact basin. However, despite its name and the fact that it is positioned near volcanoes, the actual origin of Orcus Patera remains unclear.

Aside from volcanism, there are a number of other possible origins. Orcus Patera may be a large and originally round impact crater⁵（陨石坑） , subsequently deformed（使……残缺） by compressional forces. Alternatively, it could have formed after the erosion of aligned⁸（对齐的，均衡的） impact craters. However, the most likely explanation is that it was made in an oblique⁹（斜的） impact, when a small body struck the surface at a very shallow angle, perhaps less than five degrees from the horizontal.

The existence of tectonic（构造的，建筑的） forces at Orcus Patera is evident from the presence of the numerous 'graben', rift-valley-like structures that cut across its rim. Up to 2.5 km wide, these graben are oriented roughly east–west and are only visible on the rim and the nearby surroundings.

Within the Orcus Patera depression itself, the large graben are not visible, probably having been covered by later deposits. But smaller graben are present, indicating that several tectonic events have occurred in this region and also suggesting that multiple episodes of deposition¹⁰ have taken place.

The occurrence of 'wrinkle ridges¹¹（纹脊） ' within the depression proves that not only extensional forces, as would be needed to create graben（地堑） , but also compressive forces shaped this region. The dark shapes near the centre of the depression were probably formed by wind-driven processes, where dark material excavated¹² by small impact events in the depression has been redistributed.

However, the presence of graben and wrinkle-ridges has no bearing on the origin of Orcus Patera, as both can be found all over Mars. The true origin of Orcus Patera remains an enigma¹（谜） .