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Researchers at the University of Calgary, led by Jason Nyra, have announced the discovery of a new type of crater at the Meteoritical Society Meeting in Munster, Germany. The new craters were recognised on the planet Mars and have unusual ring-like ridges within them. Although ring-peak craters, with circular mountains , have been known to exist for many years, the newly recognised craters often have both a central peak and a circular inner ridge, that has not been described before.
Nyra explains that the craters, which he has described as peripheral ring craters, are smaller than ring-peak craters and probably form in a quite different way. Ring peak craters form like the rings on the surface of a pond. The rocks, which are highly fluid at the enormous pressures generated in collisions, rebound and fall back, just like the rings on the surface of the water. "Peripheral ring craters are different", says Nyra, "they're only 10 to 90 km in diameter, which is too small for ring-peaks, and their concentric ridge is always close to their external crater rim."
Nyra thinks that peripheral-ring craters form by collapse of high external crater wall immediately after the impact. It is the intense shaking, generated by the impact-induced earthquakes, that causes the rocks from the crater rim to flow into the crater, as if they were a fluid, and stop at a certain distance inwards.
The researchers believe they have found at least one peripheral-ring crater on Venus, and say they may also have formed on the Earth and have yet to be recognised. The new discovery again illustrates there is much to learn about exactly what happens when an asteroid or comet collides with the planets.
More info: The Meteoritical Society
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