Named after the Roman messenger God, Mercury is the closest planet to the Sun and the smallest planet in our solar system. Mercury is a rocky planet, also known as a terrestrial planet. Mercury’s surface resembles that of Earth’s moon as it’s covered with impact craters from meteor strikes.

Because of its closeness to the Sun, temperatures on Mercury’s surface can reach 800 degrees Fahrenheit. Since the planet has almost no atmosphere to retain that heat, nighttime temperatures on the surface can drop to -280 degrees Fahrenheit. The little atmosphere that Mercury does have is composed mostly of oxygen (O2), sodium (Na), hydrogen (H2), helium (He), and potassium (K).

Mercury has no moons and is the second densest planet after Earth. It has a large iron core with a  radius around 1,100 to 1,200 miles which generates a magnetic field about 1 percent as strong as that of the Earth. Mercury’s outer shell, comparable to Earth’s mantle, is only 300 to 400 miles thick.

Recorded observations of Mercury date back to at least the first millennium BC. Before the 4th century BC, Greek astronomers believed the planet to be two separate objects: one visible only at sunrise, which they called Apollo; the other visible only at sunset, which they called Hermes. Timocharis made the first recorded observation of Mercury in 265 BC. Because it is so difficult to make out features on the surface of the planet from Earth, it was not until the 1960s that scientists determined the correct day length rate (59 Earth days) of the planet on its axis. This also showed that Mercury’s day length and year length are the same.

Mankind’s first visit to Mercury was the Mariner 10 spacecraft, which passed by the planet three times in 1974 and 1975. Images taken by Mariner 10 were the only close up images we had of the planet’s surface until NASA’s Messenger spacecraft flew by Mercury in 2008 and 2009. In March 2011, it began to orbit Mercury and will study parts of Mercury never before seen.

Mercury Figures

mean distance from Sun 57,909,227 km

(0.39 AU)

eccentricity of orbit 0.2056
inclination of orbit to ecliptic 7.0°
Mercurian year (sidereal period of revolution) 87.97 Earth days
maximum visual magnitude −1.9
mean synodic period* 116 Earth days
mean orbital velocity 47.36 km/sec
radius (mean) 2,439.7 km
surface area 74,797,000 km2
mass 3.30 × 1023 kg
mean density 5.43 g/cm3
mean surface gravity 370 cm/sec2
escape velocity 4.25 km/sec
rotation period (Mercurian sidereal day) 58.646 Earth days
Mercurian mean solar day 175.9 Earth days
inclination of equator to orbit
magnetic field strength 0.003 gauss
mean surface temperature 440 K (332 °F, 167 °C)
surface temperature extremes 700 K (800 °F, 430 °C);

90 K (−300 °F, −180 °C)

typical surface pressure about 10−15 bar
number of known moons none

Mercury Facts

  • Even though Mercury is the closest planet to the sun, it is not the hottest.
  • If you weigh 100 pounds on Earth, you would only weigh 37 pounds on Mercury.
  • Because of the lack of atmosphere, Mercury’s sky is black and the stars probably can be seen during the day.
  • The sun appears 2 and a half times larger in Mercury’s sky compared to the Earth’s.
  • Mercury can only be seen just after the Sun has risen and just before the sun sets.