Mercury is the planet closest to the Sun in our Solar System. This small, rocky planet has almost no atmosphere. Mercury has a very elliptical orbit and a huge range in temperature. During the long daytime (which lasts 58.65 Earth days or almost an entire Mercurian year, which is 88 days long), the temperature is hotter than an oven; during the long night (the same length), the temperature is colder than a freezer.
Mercury is so close to the Sun that you can only see it near sunrise or sunset.
Mercury is a heavily cratered planet; its surface is similar to the surface of our Moon. Cratering on Mercury triggered volcanic eruptions that filled much of the surrounding area. Mercury does have a magnetic field (probably generated by a partly-liquid iron core).
Mercury is about 3,031 miles (4,878 km) in diameter. It is the smallest planet in our solar system (it used to be considered the second-smallest planet, when Pluto was still considered to be a planet). Mercury is a bit over one third of the diameter of the Earth. Mercury is only slightly larger than the Earth’s moon.
Mass and Gravity
Mercury’s mass is about 3.3 x 1023 kg. This is about 1/20th of the mass of the Earth.
The gravity on Mercury is 38% of the gravity on Earth. A 100 pound person would weigh only 38 pounds on Mercury. To calculate your weight on Mercury, just multiply your weight by 0.38 (or go the planetary weight calculator)
Mercury’s thin atmosphere consist of trace amounts of hydrogen and helium. The atmospheric pressure is only about 1 x 10-9 millibars; this is a tiny fraction (about 2 trillionths) of the atmospheric pressure on Earth.
Since the atmosphere is so slight, the sky would appear pitch black (except for the sun, stars, and other planets, when visible), even during the day. Also, there is no “greenhouse effect” on Mercury. When the sun sets, the temperature drops very quickly since the atmosphere does not help retain the heat.
Mercury’s Orbit and Distance from the Sun
Mercury is closest planet to our Sun and the fastest moving planet in our Solar System.
Mercury is just over a third as far from the sun as the Earth is; it is 0.387 A.U. from the sun (on average). Mercury’s orbit is very eccentric; at aphelion (the point in the orbit farthest from the sun) Mercury is 70 million km from the sun, at perihelion Mercury is 46 million km from the sun.
There are no seasons on Mercury. Seasons are caused by the tilt of the axis relative to the planet’s orbit. Since Mercury’s axis is directly perpendicular to its motion (not tilted), it has no seasons.
If you were on the surface of Mercury, the Sun would look almost three times as big as it does from Earth!
Mercury has a huge range in temperatures. Its surface ranges in temperature from -270°F to 800°F (-168°C to 427°C). During the very long daytime (88 Earth-days long), the temperatures are very high (the second-highest in the Solar System - only Venus is hotter); during the long night, the thin atmosphere lets the heat dissipate, and the temperature drops quickly.
Mercury has no moons.
Mercury was visited by NASA’s Mariner 10 in 1973 and 1974. Under half of Mercury’s surface was mapped by this spacecraft.
Mercury’s Name and Symbols
Mercury was named after Mercury, the mythical Roman winged messenger and escort of dead souls to the underworld. It was named for the speedy Mercury because it is the fastest-moving planet.