Mercury, along with Venus, Earth, and Mars, is a rocky planet. It, like our Moon, has a solid surface covered with craters. But unlike the Moon, which has no magnetic field, Mercury has a very weak one that comes from its iron core. This means that it should experience some of the same geologic processes as Earth, including volcanism and erosion.
However, because of its distance from the Sun (87 million km or 54 million miles), only the far side of Mercury is not burned by solar radiation. Most of the planet is scorched by heat reaching 500°C (930°F) or more. Cratering evidence indicates that much of this surface was once liquid. As on Earth, this may have been oceanic water, but possibly also an alkali lake or even steam.
Any water that did exist would have been lost to space over time through evaporation. However, some scientists think there might be frozen reservoirs beneath the surface.
It is thought that most of Mercury's original water disappeared about 4 billion years ago, when its environment was different to what it is today. The cause of this change is still unknown, but it may have had something to do with the formation of the Solar System during the early stages of planetary development.
Mercury's surface resembles that of the moon, and the planet is most likely composed of the same kind of rocks and dust. Both worlds have impact craters on their surfaces, but Mercury's Caloris Basin is one of the largest in the solar system. It measures 5500 kilometers wide and was created when a large object (possibly a giant comet) passed directly over the center of the planet.
However, the only solid material we know for certain to exist on Mercury is iron. The rest of the planet is made up of metal ores such as gold, silver, zinc, copper, and platinum. Although scientists have found evidence of other elements on the planet, they can't explain how these materials got there unless they were put there by human activity.
For example, scientists discovered organic molecules in the atmosphere using the Hubble Space Telescope. These molecules are composed of carbon and hydrogen and could be the building blocks of life as we know it. However, because no living things exist on Mercury, this carbon must have been brought from elsewhere in the solar system.
Scientists also found evidence of silicon on the planet, which suggests that mercury might be able to support life in some form or another. However, because silicon is not an element used in making chemicals that would be necessary for simple forms of life, this silicon must have been introduced into the planet's atmosphere through volcanic eruptions or meteor impacts.
Mercury, like Earth, has lava flows. Mercury's surface has dips, much as Earth's has hills and valleys, and both are rocky planets. Mercury's hollows, on the other hand, have been named "hollows" to distinguish them from impact craters and other depressions on the small, hot orb nearest to the Sun. These are formed when the pressure inside the planet causes rocks beneath the surface to collapse, forming deep holes that sometimes reach down more than 1000 feet (300 m).
Like Earth, however, most of these are too deep to be able to support life as we know it. The deepest known hole on Mercury is Mariner 10's Deep Freeze, which is about 3282 feet (1000 m) deep. It lies in a region of relatively smooth terrain called the Caloris Basin, which is itself about 23 miles (37 km) across. This is larger than most countries on Earth!
The depth of this crater indicates that the rock underneath must be quite strong to hold up so much weight. Indeed, much of Mercury appears to be made up of hard, metallic elements, including iron, nickel, and cobalt. A few places, such as the Tharsis Bulge, are mostly composed of lighter materials that have been pushed up into peaks, but for the most part, Mercury is a very dense planet with high levels of gravity. Its average density is almost 3 times that of Earth's.