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Making an impact

Craters come in all sizes; they come in all shapes, too. These fascinating geological surface features, seen on some planets, moons, and even smaller heavenly objects, can be linked to either volcanic-related or impact-related events.

Here on Earth, we're all familiar with six-mile-wide Crater Lake in Oregon (a volcanic caldera) and 4,000-feet-wide Barringer Meteor Crater in Arizona (formed by a 300,000-ton meteor). Some larger terrestrial features, such as the Gulf of Mexico and Hudson Bay, may be massive eroded asteroid impact basins. The nearest impact feature to Vermont is the 6.2-mile-wide inverted-crater, known as Panther Mountain, in the Catskill Mountains of New York.

In our solar system, we can observe craters on the Moon through a backyard telescope, on asteroids, other moons, and even some of the other planets such as Mercury and Mars. But recent analysis of Mars-using the remote-sensing capabilities of robot spacecraft-now reveals what appears to be, by far, the mother of all craters. This is the largest impact crater discovered in the solar system so far.

NASA's Mars Reconnaissance Orbiter and Mars Global Surveyor satellites have revealed an ancient, long-eroded impact crater called the Borealis Basin. This giant surface feature spans 40 percent of the martian surface. But unlike typical craters, the Borealis feature is elliptical in shape rather than circular.

Using fresh gravity data and surface-elevation measurements, collected by NASA's armada of orbiting martian spacecraft, the researchers were able to reconstruct a "prehistoric" map of Mars before geological and meteorological forces obliterated the sharp features of Borealis-thus, the expected crater bowl shape was revealed.

Over 5,300 miles in diameter, Borealis is four times wider than the older Hellas Basin, located to its south. Hellas was once considered the grandpappy of the solar system's impact craters. The basin could contain the United States and then some.

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