At such speeds, the projecticle explodes on impact and carves out a round bowl-shaped depression on the surface. How can you distinguish an impact crater from a volcanic crater?Volcano craters are above the surrounding area on mountaintops while the craters from impacts are below the surrounding area with raised rims.Some of them have orbits that cross the orbits of the planets and moons.When they get close enough to a planet or moon, they will be pulled in by the large body's gravity and strike the surface at a speed of at least the escape velocity of the planet or moon, i.e., faster than a bullet.Astronomers had hoped that secondaries could be identified, thereby alleviating the confusion.Not so; a new paper in indicates that many secondaries are very difficult to distinguish from primaries, because debris lofted up may go into orbit for years, falling down far away from the initial impact (distant secondaries).Worries about the crater count dating method, widely relied upon to infer ages of planetary surfaces, began emerging in 2005.Those worries have not subsided; they have only grown worse. We’ve kept track of the crater count crisis since 2005, when the problem of secondary craters was brought to light (10/20/2005, 6/08/2006, 9/25/2007, 3/25/2008, 7/25/2010).
"I basically analyzed maps and drew crater rim circles for four years," Robbins said. Select the photographs to display the original source in another window.There are still small chunks of rock orbiting the Sun left over from the formation of the solar system.The flake came from Zagami, an 18-kilogram meteorite named after the Nigerian village where it was found in 1962.It is one of the rarest and most sought-after types of meteorite — a piece of Mars that was blasted into space by an asteroid impact and eventually landed on Earth.
The authors tested dating by counting small craters in a variety of presumed “old” and “young” regions of the moon, and got widely divergent results despite using standard methods and software.