The small and rocky planet Mercury is the closest planet to the Sun; it speeds around the Sun in a wildly elliptical (non-circular) orbit that takes it as close as 47 million km and as far as 70 million km from the Sun. Mercury completes a trip around the Sun every 88 days, speeding through space at nearly 50 km per second, faster than any other planet. Because it is so close to the Sun, temperatures on its surface can reach a scorching 467 degrees Celsius. But because the planet has hardly any atmosphere to keep it warm, nighttime temperatures can drop to a frigid -183 degrees Celsius. Because Mercury is so close to the Sun, it is hard to see from Earth except during twilight. Until 1965, scientists thought that the same side of Mercury always faced the Sun. Then, astronomers discovered that Mercury completes three rotations for every two orbits around the Sun. If you wanted to stay up for a Mercury day, you'd have to stay up for 176 Earth days!
Like our Moon, Mercury has almost no atmosphere. What little atmosphere exists is made up of atoms blasted off its surface by the solar wind and has less than a million-billionths the pressure of Earth's atmosphere at sea level. It is composed chiefly of oxygen, sodium, and helium. Because of Mercury's extreme surface temperature, these atoms quickly escape into space and are constantly replenished. With no atmosphere to protect the surface, there has been no erosion from wind or water, and meteorites do not burn up due to friction as they do in other planetary atmospheres. Mercury's surface very much resembles Earth's Moon, scarred by thousands of impact craters resulting from collisions with meteors. While there are areas of smooth terrain, there are also cliffs, some soaring up to a mile high, formed by ancient impacts.
Mercury is the second smallest planet in the solar system, larger only than Pluto, the most distant planet in our solar system. If Earth were the size of a baseball, Mercury would be the size of a golf ball. Viewed from Mercury, the Sun would look almost three times as large as it does from Earth. Mercury is the second densest body in the solar system after Earth, with an interior made of a large iron core with a radius of 1,800 to 1,900 km, nearly 75 percent of the planet's diameter and nearly the size of Earth's Moon. Mercury's outer shell, comparable to Earth's outer shell (called the mantle) is only 500 to 600 km thick. Only one spacecraft has ever visited Mercury: Mariner 10 in 1974-75. In 1991, astronomers using radar observations showed that Mercury may have water ice at its north and south poles. The ice exists inside deep craters. The floors of these craters remain in perpetual shadow, so the Sun cannot melt the ice.