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Mercury: Quick Facts

Larry Slawson received his master's degree at UNC Charlotte. He specializes in Russian and Ukrainian history.

The planet Mercury (Image taken by MARINER).

The planet Mercury (Image taken by MARINER).

Scientific Properties of the Planet Mercury

  • Orbital Semimajor Axis: 0.39 Astronomical Units (57.9 Million Kilometers)
  • Orbital Eccentricity: 0.206
  • Perihelion: 0.31 Astronomical Units (46 Million Kilometers)
  • Aphelion: 0.47 Astronomical Units (69.8 Million Kilometers)
  • Mean Orbital Speed: 47.9 Kilometers Per Second
  • Sidereal Oribital Period: 88 Solar Days (0.241 Tropical Years)
  • Synodic Orbital Period: 115.9 Days (Solar)
  • Orbital Inclination to the Ecliptic: 7.00 Degrees
  • Greatest Angular Diameter (As Viewed From Earth): 13”
  • Overall Mass: 3.30 x 1023 Kilograms (0.055 of Earth’s Mass)
  • Equatorial Radius: 2,440 Kilometers (0.38 the Equatorial Radius of Earth)
  • Mean/Average Density: 5,430 Kilograms per Meters Cubed (0.98 of Earth’s Mean Density)
  • Surface Gravity: 3.70 Meters Per Second Squared (0.38 of Earth’s Surface Gravity)
  • Escape Speed/Velocity: 4.2 Kilometers Per Second
  • Sidereal Rotation Period: 58.6 Days (Solar)
  • Axial Tilt: 0.0 Degrees
  • Surface Magnetic Field: 0.011 of Earth’s Magnetic Field
  • Magnetic Axis Tilt Relative to Rotation Axis: <10 Degrees
  • Mean/Average Surface Temperature: 100-700 Kelvins (-279.67 Degrees Fahrenheit to 800.33 Degrees Fahrenheit)
  • Total Number of Moons/Satellites: N/A
Internal structure of the planet Mercury. Notice its exceptionally large core that dominates much of its overall structure.

Internal structure of the planet Mercury. Notice its exceptionally large core that dominates much of its overall structure.

Quick Facts About Mercury

  • Due to its close proximity to the Sun, one year on Mercury is equal to eighty-eight days. Despite this short amount of time, Mercury’s days are quite long due to its slow rotation rate (an effect of the Sun’s gravitational pull). One solar day (noon to noon) is equivalent to 176 days on Earth, whereas the sidereal day is equivalent to 59 days on Earth.
  • Despite the planet’s small size, Mercury is one of the densest planets (second to Earth). Scientists believe that this is because the planet is composed primarily of heavy rocks and metals. It is also believed that Mercury possesses a molten core. Although scientists previously believed the core to be composed primarily of iron, they now believe that it is composed of sulfur instead. Altogether, Mercury’s core makes up around forty-two percent of its total volume/density (compared to Earth’s core, which comprises only seventeen percent of its volume).
  • Despite its close proximity to the Sun, Mercury is not the hottest planet. Venus (the second planet in our solar system) is actually the hottest planet due to its intense atmosphere. Mercury, in contrast, possesses no atmosphere to trap and regulate heat. This helps to explain the wide temperature fluctuations on Mercury which range from 427 Degrees Celsius (on the side facing the Sun), to temperatures as low as -173 Degrees Celsius (on the dark side of the planet).
  • Mercury has often been compared to the Moon due to its geographical similarities; particularly its “cratered” look. Craters dominate much of Mercury’s surface, indicating a violent history of collisions with asteroids, meteors, and comets in its past. The biggest crater on Mercury is known as the Caloris Basin and is approximately 1,550 kilometers in diameter. This crater was first discovered by Mariner 10 in 1974.
  • Due to the planet’s close proximity to the Sun, Mercury is a difficult planet to explore. As a result, only two spacecraft have been able to scout the planet. Mariner 10 conducted three flybys between 1974 and 1975, and helped scientists to map out a considerable amount of the planet’s surface. More recently, NASA launched the “Messenger” probe (3 August 2004) to return to the planet for additional study.
  • It is unknown who (or when) the planet Mercury was first discovered. However, many scholars and scientists, alike, believe that the Sumerians (around 3,000 B.C.) may have been the first people to record the planet. It wasn’t until 1543, however, that astronomers recognized Mercury as a planet (rather than a star). The planet garnered its name from the Roman messenger to the gods (also known as Hermes in Greek mythology).

"I had rather be Mercury, the smallest among seven [planets], revolving around the Sun than the first among five [moons] revolving around Saturn."

— Johann Wolfgang von Goethe

Fun Facts about Mercury

  • Mercury’s orbital speed is extremely fast (compared to other planets). As a result, early civilizations were convinced that Mercury was actually a second star in our solar system.
  • Mercury continues to be the smallest planet in the solar system. Its diameter is only 4,879 kilometers. Despite its small size, Mercury remains one of five planets that can be observed with the naked eye.
  • Mercury’s gravity is only thirty-eight percent of Earth’s gravitational pull. As a result, the planet is incapable of maintaining a stable atmosphere and possesses no seasonal variations. This also helps explain why the planet possesses no rings or moons.
  • While most of the planets in our solar system maintain relatively circular orbits around the Sun, Mercury’s orbit follows an elliptical and elongated pattern as it rotates around the Sun. At times, the planet reaches as close as 29 million miles (47 million kilometers) from the Sun, and as far as 43 million miles (70 million kilometers) at other times of its orbit.
  • The outer shell (crust) of Mercury is relatively thin. Altogether the crust is believed to be only 500 to 600 kilometers thick (approximately 310 to 375 miles). This is in stark contrast to Earth’s outer crust and mantle which is 2,930 kilometers (or 1,819 miles) thick.
  • Mercury maintains an extremely weak magnetic field. In contrast to Earth, Mercury’s magnetic field is only one percent the strength of Earth’s.
  • Many scientists now believe that Mercury’s craters contain ice; particularly on the planet’s north and south poles which are cold and relatively shadowy. It is believed that this ice may have formed from water vapor beneath the ground, or was delivered by comets and meteorites after impacting the planet’s surface.
  • In addition to the possibility of ice, some scientists believe that Mercury may have been covered in volcanoes at one point in its history. According to images delivered by the “Messenger” probe, the northern plains of the planet appear smooth across its surface; indicating the possible presence of dried lava beds. This, according to scientists, would help explain many of the other smooth locations across Mercury’s surface, as well as the smooth appearance of several of its craters.
  • On 20 October 2018, the ESA launched two additional orbiters to study the planet Mercury. The BepiColombo contains both the ESA’s Mercury Planetary Orbiter, as well as Japan’s Mercury Magnetosphere Orbiter. BepiColombo is scheduled to enter Mercury’s orbit in 2025 after completing two Venus flybys. The spacecraft will conduct six flyby missions across Mercury’s orbit.
  • Although it remains unclear how the planet Mercury formed, many scientists believe that the planet developed around 4.5 billion years ago. Scientists and astronomers alike argue that the planet appears to have formed as a result of gravity pulling hot gas and dust together.
Up-close view of Mercury's surface. Notice how the planet is dotted with meteor and asteroid craters.

Up-close view of Mercury's surface. Notice how the planet is dotted with meteor and asteroid craters.

Concluding Thoughts

Although small, Mercury continues to play a large role in the scientific community as more and more information is gleaned from space probes about its inner and outer structures and origins. As additional spacecraft, such as the BepiColombo, make additional flybys around the planet, it will be interesting to see what new information can be learned about this fascinating member of our solar system, and if this planet holds additional clues to understanding the galaxy and universe at large.

Works Cited


Wikipedia contributors, "Mercury (planet)," Wikipedia, The Free Encyclopedia, (accessed January 3, 2019).

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2019 Larry Slawson


Larry Slawson (author) from North Carolina on January 09, 2019:

Thank you Liz! I'm glad you enjoyed!

Liz Westwood from UK on January 09, 2019:

This is a great reference article, packed with information.

Larry Slawson (author) from North Carolina on January 04, 2019:

I’m glad both of you enjoyed. I’ve been reading more astronomy related material these past few weeks. Really fascinating stuff.

Alexander James Guckenberger from Maryland, United States of America on January 04, 2019:

I love astronomy.

Pamela Oglesby from Sunny Florida on January 04, 2019:

This is such an interesting article about Mercury. I never thought any ice would be anywhere on Mercury. Each of the facts was interesting.