Orcus Patera, an enigmatic elliptical depression near Mars's equator, in the eastern hemisphere of the planet, has baffled scientists for a long time now.
The term 'patera' is used for deep, complex or irregularly shaped volcanic craters such as the Hadriaca Patera and Tyrrhena Patera. However, despite its name and the fact that it is positioned near volcanoes, the actual origin of Orcus Patera remains unclear.
One theory is that originally a round impact crater; it may have been deformed by compression forces. Alternatively, it could have formed after the erosion of aligned impact craters.
However, the most likely explanation is that it was made in an oblique impact, when a small body struck the surface at a very shallow angle.
Numerous rift-valley-like structures called 'graben' cut across its rim. 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 deposition have taken place.
The occurrence of 'wrinkle ridges' within the depression proves that compressive forces as well as extensional forces would be needed to create graben.
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 a mystery.