The title "Visible Mercury" is somewhat misleading, since the planet is exceptionally hard to see from Earth. Mercury is only visible in the sky close to dawn and dusk because it is always close to the Sun from the Earth's point of view. Scientists have better images of distant planets such as Saturn than they have of Mercury. Some of the highestresolution images of Mercury can only discriminate features larger than 75 miles (120 km)! Interpreting anything about the planet's history is difficult with such limited data.
In general, from what can be seen, Mercury has an ancient, heavily cratered surface, much like the Moon's. The image of Mercury on page 98 is from Mariner 10 mission on March 29, 1974.This picture is assembled from a number of separate images that were taken while the spacecraft was 3,340,000 miles (5,380,000 km) away.
Mercury's surface, as photographed by Mariner 10, seems to consist of four major terrain types: ancient, heavily cratered areas; intercrater plains; smooth, young plains; and lastly, a chaotic region that lies on the opposite side of the planet from an immense impact crater named Caloris Basin. Mercury's surface is known to be ancient because of the high cratering rate it records. Scientists are fairly sure that there was more material bombarding the planets early in the solar system. If the planets accreted from a nebula, as is widely accepted, then it stands to reason that there were more stray bodies orbiting near the terrestrial planets early in the solar system and that their population has been gradually depleted as they fall into the Sun or crash into planets. Over time, the surfaces of the terrestrial planets collect more impact craters, though the rate of cratering was highest early in solar system development. Mercury's heavily cratered surface indicates that no later processes,
This surface mosaic of Mercury, assembled from photos taken by the Mariner 10 mission craft, shows the ancient cratered surface of the planet. (NASA/Mariner/JPL)
such as volcanism, have obliterated its cratering record.Though some of Mercury's surface is designated as young plains, they are only young in comparison with the rest of the planet.
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