Photos Show Strange Elongated Martian Crater

Photosjust released by the European Space Agency give an up-close look at thestrange, elongated Martian crater known as Orcus Patera.

Thecrater lies near the equator in Mars' eastern hemisphere, between the volcanoesElysium Mons and OlympusMons. It's big, measuring 380 km (236 miles) long, 140 km (87miles) wide and about 2.3 km (1.5 miles) from rim to floor. [Newphoto of the crater.]

Astronomershave known about this odd crater for decades, but they still aren't sure howOrcus Patera formed. The word "patera" generally refers to deep,complex or irregularly shaped volcanic craters. But despite its name and itslocation near some big volcanoes, Orcus Patera's origin may not be volcanic.

Theoval structure may instead be an impact crater, scientists think. It may oncehave been an ordinary ? though large ? round hole in the ground that wassubsequently deformed by tectonic forces. Such forces are evident at OrcusPatera: Long rifts in the Martian surface called"graben" cut across the crater's rim, and "wrinkle ridges"are found throughout the depression itself.

Butgraben and wrinkle ridges can be found all over the Martian surface, so theirpresence at Orcus Patera is no smoking gun.

It'salso possible that erosion wore away the boundaries between several smallimpact craters, creating what looks like a single oval crater. But scientiststhink the most likely origin of Orcus Patera may be a single, oblique impact: Asmall body struck the Martian surface at a very shallow angle, perhaps lessthan five degrees from the horizontal.

On thispath, the impactor would have dug through a long stretch of Mars'red dirt, creating the bottle-shaped structure we see today.

But thejury's still out, so the true origin of Orcus Patera remains one of manyMartian mysteries.

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