August 27, 2010
Orcus Patera is an enigmatic elliptical depression near Mars' equator in the eastern hemisphere of the planet. Located between the volcanoes of Elysium Mons and Olympus Mons, its formation remains a mystery. A new image from the High Resolution Stereo Camera (HRSC) aboard the European Space Agency's (ESA) Mars Express orbiter shows the unusual feature with unprecedented clarity.
Often overlooked, this well-defined depression extends approximately 240 miles by 90 miles (380 kilometers by 140 kilometers) in a NNE-SSW direction. It has a rim that rises up to 5,900 feet (1,800 meters) above the surrounding plains, while the floor of the depression lies 1,300 to 2,000 feet (400 to 600 meters) below the surroundings.
The term patera is used for deep, complex, or irregularly shaped volcanic craters such as the Hadriaca Patera and Tyrrhena Patera at the northeastern margin 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 crater, subsequently deformed by compressional 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 shallow angle, perhaps less than 5° from the horizontal.
The existence of tectonic forces at Orcus Patera is evident from the presence of the numerous "graben," rift-valley-like structures that cut across its rim. Up to 1.5 miles (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 deposition have taken place.
The occurrence of "wrinkle ridges" within the depression proves that not only extensional forces, but also compressive forces shaped this region. Wind-driven processes probably formed the dark shapes near the center of the depression where dark material excavated by small impact events in the depression has been redistributed.
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 enigma.