Mercury is the closest planet to the sun and the eighth largest
The surface of mercury is very similar to the surface of the Earth's moon. It is heavily cratered and very old. But it is much denser than the moon being the second most dense planet in the solar system after Earth. There is a thin layer of dust covering the planet and it has huge cliffs that tower two miles above the landscape. No plate tectonics occur on Mercury.
Mercury has a large iron (iron/nickel) core that takes up the majority of the planet, some of which is molten. A relatively thin silicate mantle and crust lie over this.
The atmosphere of mercury is very thin and is made up of atoms that have been blasted off the surface by solar wind. The atmosphere is very hot so atoms escape quickly into space, allowing the atmosphere to be constantly replenished. The planet has little atmosphere, no water and nothing can grow on the surface.
The temperature varies from 700 degrees centigrade at noon to -300 degrees centigrade at night, due a lack of atmosphere to retain the heat.
Mercury spins on its axis, which allows the iron core to generate a very small magnetic field (about 1% of the Earths).
Surprisingly, small ice caps exist at Mercury's north and south poles. They exist in deep craters and are in perpetual shadow so the sun cannot melt the ice.
Mercury fact sheet
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