Mercury is the planet closest to the sun, and as such, it has many interesting and unique features. It has been considered the smallest planet ever since Pluto lost its status as a planet. Mercury is very dense. Because it is so close to the sun, it has lost almost all of its atmosphere, and the Mercury surface is more like that of Earth's moon than that of the other rocky planets. What scientists know about Mercury is mostly based on data from spacecraft such as Mariner 10 and the robotic probe MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging). Additional information has been obtained by analyzing the light reflected from the planet and examining its magnetic field. Until a space mission lands on Mercury and gathers rock samples, scientists will not be completely certain about the composition of its crust.

Because no spacecraft has ever landed on Mercury and retrieved rock samples, scientists can't be sure of the planet's exact composition. Mariner 10 flew by the planet three times in 1973 and 1974 and photographed the surface. The robotic probe MESSENGER orbited the planet from 2011 to 2015, measuring its magnetic field and gathering data. Based on this information and data from other measurements of Mercury's magnetic field and reflected light, scientists have developed theories about the planet's core and surface.

Mercury's core is unusually large and makes up about 70 percent of the planet. It is probably composed of molten iron and nickel and is responsible for the planet's magnetic field. Above the metallic core is a rocky mantle about 500 kilometers thick. Finally, there is a thin surface layer of rocks and dust that has been pitted and cratered by the impact of many meteors and other stray celestial objects.

Mercury has almost no atmosphere, partly because its gravity is so low that it can't keep gases close to its surface. In addition, the planet is so close to the sun that the solar wind blows away any gases that accumulate near the surface. The trace atmosphere of the planet includes small amounts of oxygen, hydrogen and helium. The combination of a large iron magnetic core with a loose surface layer and an almost complete lack of atmosphere differentiates Mercury from all the other planets of the solar system.

Mercury rotates on its axis very slowly so that half the surface is facing the sun for an extended period. This means that the hot side of mercury can reach 800 degrees Fahrenheit while the cold side is at -300 degrees Fahrenheit. Scientists used to think that one side of Mercury was always facing the sun, but more accurate observations have shown that the planet rotates three times in two Mercury years, meaning it rotates once about every 60 Earth days while it orbits around the sun every 90 Earth days.

When compared to Earth, Mercury is about 0.4 times the Earth's diameter, which makes it a little bit bigger than our moon. The planet also has a gravity of about 0.4 times that of Earth, and its distance from the sun is on average about 0.4 times the Earth's distance. While the Earth's orbit is almost circular (technically it is elliptical, but by a relatively minor amount), Mercury's is much more elliptical.

Mercury's surface looks similar to that of the moon, and the planet is probably made up of the same kind of rocks and dust. Impact craters cover the surfaces of both bodies, but Mercury's Caloris Basin is one of the biggest in the solar system. Scientists believe a large asteroid hit the planet after it was first formed and created the basin. The impact was so powerful that it produced the 1,300 kilometer multi-ring impact crater on one side of the planet, as well as an impact wave that traveled through the center of the planet, forming a 500-kilometer area of large hills and valleys on the other side.

With its extreme surface temperatures and its evident inability to support life, Mercury is unlikely to be the target of a probe landing in the near future. However, in-orbit observation attempts continue. In October 2018, the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) launched BepiColombo, a joint mission in which two spacecraft were launched as a package, each carrying an orbiter that will observe more about the planet. Meanwhile, scientists are still analyzing the data from the MESSENGER probe and assembling a more complete picture of the planet and its composition.