[meteorite-list] Mars' Mysterious Elongated Crater (Mars Express)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 24 Sep 2010 15:22:19 -0700 (PDT)
Message-ID: <201009242222.o8OMMJDb027667_at_zagami.jpl.nasa.gov>

http://www.esa.int/export/SPECIALS/Mars_Express/SEMDV9BO3DG_0.html

Mars' mysterious elongated crater
European Space Agency
27 August 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.
 
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
volcanic 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 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 very shallow angle, perhaps less than
five degrees 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 2.5 km wide, these graben are oriented roughly
easti-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 Mars. The true
origin of Orcus Patera remains an enigma.
 
Received on Fri 24 Sep 2010 06:22:19 PM PDT


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