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Mercury (Planet): Craters
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Craters on Mercury range in diameter from a few meters to hundreds of kilometers across. The largest known crater is the enormous Caloris Basin, with a diameter of 1300km. The impact which created the Caloris Basin was so powerful that it caused lava eruptions and left a concentric ring over 2km tall surrounding the impact crater. At the antipode of the Caloris Basin is a large region of unusual, hilly terrain known as the "Weird Terrain". It is believed that shock waves from the impact traveled around the planet, and when they converged on the antipodal point of the impact caused extensive fracturing of the surface there.
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A complex history of geological evolution is recorded in this frame from the Narrow Angle Camera (NAC), part of the Mercury Dual Imaging System (MDIS) instrument, taken during MESSENGER's close flyby of Mercury on January 14, 2008. Part of an old, large crater occupies most of the lower left portion of the frame. An arrangement of ridges and cliffs in the shape of a "Y" crosses the crater's floor. The shadows defining the ridges are cast on the floor of the crater by the Sun shining from the right, indicating a descending stair-step of plains. The main, right-hand branch of the "Y" crosses the crater floor, the crater rim, and continues off the top edge of the picture; it appears to be a classic "lobate scarp" (irregularly shaped cliff) common in all areas of Mercury imaged so far. These lobate scarps were formed during a period when Mercury's crust was contracting as the planet cooled.
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The polar regions of Mercury are never fully illuminated by the Sun, and it appears that ice managed to collect in the permanently shadowed regions of many polar crater rims. It is not clear where the ice came from, but scientists believe comet crashes may be one source.
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Despite the generally extremely high temperature of its surface, observations strongly suggest that ice exists on Mercury. The floors of some deep craters near the poles are never exposed to direct sunlight, and temperatures there remain far lower than the global average. Water ice strongly reflects radar, and observations reveal that there are patches of very high radar reflection near the poles. While ice is not the only possible cause of these reflective regions, astronomers believe it is the most likely.
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The European Space Agency ... has ambitious plans to explore Mercury. At some future date, it proposes to send a trio of spacecraft called BepiColombo that, like MESSENGER, will study the planet's atmosphere and search for water ice in polar craters. BepiColombo will include two satellites and a vehicle that will land on the surface, deploying a tiny, tethered rover to gather information.
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This image shows two prominent craters (upper right) with bright halos on Mercury. The craters are about 40 kilometers (25 miles) in diameter. The halos and rays cover other features on the surface indicating that they are some of the youngest on Mercury. (Copyright Calvin J. Hamilton; FDS 275)
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