Structure and Atmosphere
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Mercury has a radius of 2439 km (1524 mi), less than half of Earth. Its cratered surface gives it an appearance similar to our moon's. It is believed to have an iron-nickel core that takes up about 75% of its diameter. Measurement's of the planet's magnetic field taken from Mariner 10 as it flew by, indicate that the core is likely hot and fluid. In contrast with the other terrestrial planets, it has a solid rocky silicate mantle and a rocky silicate crust. Cliffs on the surface of Mercury measure as much as 3 km (2 mi) high and run for as long as 500 km (310 mi). They are of later origin than the craters whose ridges are dissected by these fractures that were formed when the planet cooled and the crust shrank. This shrinkage would account for 0.1% of the planet's original diameter or about 1km.
The larger Mercurial craters are surrounded by rays of material ejected during impact, and secondary craters caused by debris from the main impact lie close to these. They are closer than similar features on our moon because of Mercury's greater gravity. Also similar to our moon, Mercury is host to smooth intercrater plains that date back much older than the crater speckled areas. An area called the Caloris Basin measures 1300 km (800 mi) across. It was formed early in the evolution of our solar system when a meteorite impacted that was large enough to penetrate the crust allowing lava to flow and harden to form a smooth plain.
So far, observations of Mercury from ground-based observers and the Mariner 10 spacecraft have not shown any evidence of a significant atmosphere. Mariner 10 did observe a tiny amount of helium (exerting a pressure 1/1,000,000,000,000 of the Earth's atmosphere) 1000 km above the surface, but this is probably produced by capturing particles from the solar wind and radioactive decay of Mercury's crust. Temperatures at the surface range between -180°C and 450°C (-290°F to 840°F - hot enough for lead to melt). This large range in surface temperature is possible because Mercury is so close to the Sun, a day 176 Earth days long, and does not have an atmosphere present to moderate the range in surface temperature.
Although Mercury is, for the most part without water, shaded areas near the poles are permanently frozen and water ice may exist there.
