The smallest planet in the solar system, Mercury bears a strong resemblance to Earth's moon. Like the other three terrestrial planet, Mercury contains a core surrounded by a mantle and a crust. But Mercury's core makes up a larger portion of the planet than others in the solar system, hinting at a chaotic beginning.
The surface of Mercury
The first images of Mercury revealed a cratered, rocky planet that closely resembled Earth's moon. The early days of the solar system, soon after the rocky planet formed, were violent, with constant collisions, and conditions on Mercury preserved evidence of many of these impacts.
Not all of the craters came from the beginning. The planet also shows evidence for volcanism, with a number of plains that smoothed over some of the first craters. But craters on top of these plains show that the volcanism occurred early in the planet's history, and that Mercury still felt the brunt of the violence of the young solar system.
Although temperatures on the planet can reach as high as 801 degrees Fahrenheit (427 degrees Celsius), it is possible that the planet could boast water-ice on its surface in the shaded portions of some of its craters.
In addition to testifying as to the planet's early volcanism, the smooth plains also show evidence of wrinkle ridges, created as the planet squeezed together. This coming together most likely happened as the interior cooled. Although some compression is common among bodies in the solar system, the compression of Mercury as it pulled more tightly in on itself is the most significant yet seen. Scientists estimate that the radius of the planet shrank by 0.6 to 1.2 miles (1 to 2 kilometers) as it temperatures deep within dropped.
A small body like Mercury would have a difficult time holding on to an atmosphere in the best of circumstances. Because of the close distance between Mercury and the sun, Mercury also feels the brunt of the solar wind, which constantly sweeps away the thin atmosphere the planet does manage to gather. With only the most negligible of atmospheres, the temperatures on the night and day side differ dramatically.
The crust of Mercury is likely very thin, thinner than Earth's. The outer shell is only about 300 to 400 miles (500 to 600 km) thick.
The planet has no plate tectonics, which is part of the reason the cratered surface has been preserved for billions of years.
The core of the matter
Although it's the smallest planet, Mercury is the second densest. Only Earth is denser. Scientists used the calculated density to determine that Mercury holds a large metallic core. With a radius of 1,100 to 1,200 mile (1,800 to 1,900 km), the core makes up 75 percent of the planet's radius. Radar images taken from Earth revealed that the core is molten liquid, rather than solid.
Mercury's core has more iron than any other planet in the solar system. Scientists think this had to do with its formation and early life. If the planet formed quickly, increasing temperatures of the evolving sun could have vaporized much of the existing surface, leaving only a thin shell.
Another alternative is that a larger Mercury was struck in its early life, during the violent, chaotic beginnings of the solar system. Such an impact could have stripped away much of its outer shell, leaving a core too big for remaining planet.
Mercury's iron core generates a magnetic field about one percent as strong as Earth's. The field is quite active, frequently interacting with the solar wind and funneling plasma from the sun to the planet's surface. The hydrogen and helium captured from the solar wind help create part of Mercury's thin atmosphere.
— Nola Taylor Redd, SPACE.com Contributor