Craters come in all sizes; they come in all shapes, too. These fascinating geological surface features, seen on some planets, moons, and smaller heavenly objects, can be linked to either volcanic-related and impact-related events. Here on Earth, were 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 huge, eroded asteroid impact basins. The nearest impact feature to Vermont is a 6.2-mile-wide inverted-crater, known as Panther Mountain, in the Catskill Mountains of New York. In our solar system, we see 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 Marsusing the remote-sensing capabilities of robot spacecraftnow reveals what appears to be, by far, the mother of all craters. This is the largest impact crater discovered in the solar system. NASAs 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 rather than circular. Using fresh gravity data and surface-elevation measurements, collected by NASAs 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 Borealisthus, 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 systems impact craters. What created such a titanic scar? Probably a dwarf planet. NASA astronomers now calculate that the Borealis impacting body was 1,200 miles acrossthat's bigger than Pluto, the so-called ninth planet. Imagine a crater wider than the continental United States the mind boggles at the magnitude of the explosion that must have created it! Detonating all the nuclear weapons stockpiled on Earth and those exploded since the Trinity Test in 1945 all at once, and within a compact ground zero area, would be like a few sputtering Fourth of July firecrackers compared to the ancient Borealis explosion on Mars. When did the ultra-violent Borealis event occur? NASA researchers think it must have taken place around 3.9 billion years ago. Lets hope a big whack like the Borealis event doesnt occur on Mars or Earth anytime soon. Whats in the Sky: On the weekend of July 11-12, Mars and Saturn are low in the western sky at sunset. Mars is to the left of Saturn. Lou Varricchio, M.Sc., lives in Vermont. He is a former NASA senior science writer and is currently involved with the NASA/JPL Solar System Ambassador program in Vermont.