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Mercury (Planet): Surface
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Mercury is the closest planet to the Sun. It has no satellites and only trace amounts of hydrogen and helium in its atmosphere (temporarily captured from the Solar wind). Its surface appears very similar to Earth's Moon, but the planet has a much larger iron core and is therefore much denser. Its most distinctive known surface feature is Caloris Basin, a colossal impact crater approximately 1350 km in diameter.
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Alternatively, Mercury may have formed from the solar nebula before the Sun's energy output had stabilized. The planet would initially have had twice its present mass. But as the protosun contracted, temperatures near Mercury could have been between 2500 and 3500K, and possibly even as high as 10000K. Much of Mercury's surface rock could have vaporized at such temperatures, forming an atmosphere of "rock vapor" which could have been carried away by the solar wind.
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The majority of Mercury's surface is covered by plains. Much of it is old and heavily cratered, but some of the plains are less heavily cratered. Scientists have classified these plains as intercrater plains and smooth plains. Intercrater plains are less saturated with craters and the craters are less than 15 kilometers in diameter. These plains were probably formed as lava flows buried the older terrain. The smooth plains are younger still with fewer craters.
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At certain points on Mercury’s surface, an observer would be able to see the Sun rise about halfway, then reverse and set before rising again, all within the same Mercurian day. This is because approximately four days prior to perihelion, Mercury’s angular orbital velocity exactly equals its angular rotational velocity so that the Sun’s apparent motion ceases; at perihelion, Mercury’s angular orbital velocity then exceeds the angular rotational velocity. Thus, the Sun appears to move in a retrograde direction. Four days after perihelion, the Sun’s normal apparent motion resumes at these points.
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Mercury’s high density indicates that the relatively dense and abundant element iron accounts for a large proportion of the planet’s composition. The surface of Mercury... contains little iron, suggesting that most of Mercury’s iron is now concentrated in a large iron core. Collisions with other protoplanets early in the history of the solar system may have stripped away much of Mercury’s low-density crust, leaving behind a dense, iron-rich core. Alternatively, Mercury could have formed from material enriched in iron close to the Sun early in solar system history.
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During and shortly following the formation of Mercury, it was heavily bombarded by comets and asteroids for a period that came to an end 3.8 billion years ago. During this period of intense crater formation, the planet received impacts over its entire surface, facilitated by the lack of any atmosphere to slow impactors down. During this time the planet was volcanically active; basins such as the Caloris Basin were filled by magma from within the planet, which produced smooth plains similar to the maria found on the Moon.
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