Mars's mysterious elongated crater

Aug 27, 2010
Orcus Patera on Mars. Credits: ESA/DLR/FU Berlin (G. Neukum)

(PhysOrg.com) -- Orcus Patera is an enigmatic elliptical depression near Mars's equator, in the eastern hemisphere of the planet. Located between the volcanoes of Elysium Mons and Olympus Mons, its formation remains a mystery.

Often overlooked, this well-defined depression extends approximately 380 km by 140 km in a NNE-SSW direction. It has a rim 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’ is used for deep, complex or irregularly shaped craters such as the Hadriaca Patera and Tyrrhena Patera at the north-eastern 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 , 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 very shallow angle, perhaps less than five degrees from the horizontal.

Credits: ESA/DLR/FU Berlin (G. Neukum)

The existence of 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 deposition have taken place.

The occurrence of ‘wrinkle ridges’ within the depression proves that not only extensional forces, as would be needed to create graben, 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 excavated 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 . The true origin of Orcus Patera remains an enigma.

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User comments : 6

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Shootist
2.7 / 5 (6) Aug 27, 2010
Earth has one as well.

lat 70.0914
lon -159.430

It would be a mighty large "small" object to create a crater 380km x 140km.
Blicker
3.6 / 5 (5) Aug 27, 2010
The enigma for me is why most impact craters are round. I can only guess that so much energy is released that the craters are vaporized too rapidly for the impacting material to gouge out its direction; like a snapshot of a moving bullet with a super high speed camera looks motionless. That would suggest this crater was formed by a slower object as well as a shallow angle.
yyz
5 / 5 (3) Aug 27, 2010
A good view of this area is available using Google Mars or WWT Mars. The ESA release gives the coordinates as: 14deg N 177deg W. It's easy to spot the graben mentioned in the article on the left side of the crater, stopping at the rim. In one case, a very faint extension of a graben on the other side of the crater is visible! More images and info here: http://www.esa.in...subhead3
Caliban
3.7 / 5 (3) Aug 27, 2010
At the left side of the top image, you can see what appear to be crescent, or curved areas on opposite sides of the crater rim. My best guess is that this patera represents a String, or series of impacts, from a single, mutliple-impact event, later eroded and deformed tectonically.
Parsec
5 / 5 (3) Aug 27, 2010
The enigma for me is why most impact craters are round. I can only guess that so much energy is released that the craters are vaporized too rapidly for the impacting material to gouge out its direction; like a snapshot of a moving bullet with a super high speed camera looks motionless. That would suggest this crater was formed by a slower object as well as a shallow angle.


The analogy is sound. Consider what happens when a surface is impacted with a bullet. The bullet has to hit at a very shallow angle in order for the elongation of the entry wound. The relative velocities of most orbiting objects relative to each other are orders of magnitude more than of any bullet to a fixed surface.
kuro
5 / 5 (2) Aug 28, 2010
The enigma for me is why most impact craters are round.


when an object hits the surface, the kinetic energy it delivers causes a massive explosion. it is the effects of this explosion that create a crater many times larger than the size of the impacting object.

since the effects of the explosion - the heat and the shock wave - expand in about the same way in all directions, the resulting crater is more or less spherical.

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