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Moons - Orbital and Rotational Detail - Atmosphere - Surface and Geology - Interior - Magnetic Field - Perihelion Advance - Evolution of Mercury


Mercury. Combind and reprocessed Mariner 10 data.

Mercury is the closest planet to the sun, named after the Roman messenger god for its swift movement in the sky. Its proximity to the sun, coupled with a modest mass of only 5.5% that of the Earth, creates a barren, baked surface with no atmosphere . Surface temperature ranges from 100 Kelvin in the night up to 700 K in daytime. The low surface gravity and high temperatures have permitted most of the gases of this planet to escape into the interplanetary medium.

Mercury is the second smallest of the nine planets (only Pluto is smaller) and both Jupiter and Saturn have larger satellites. The only space probe to visit Mercury (as of 2006) was Mariner 10 in 1974, where it mapped one side of the planet with a resolution of 1 km. Radar observations of the remaining surface were made by the Arecibo Observatory with resolutions of up to 1 km. Mercury's surface is heavily cratered, suggesting that is not geologically active.


Mercury has no moons.

Orbital and Rotational Information

As seen from Earth, Mercury always appears very close to the sun. That's why we never see it at midnight and why it is hard to detect with the naked eye, in addition to the fact that it is very small and reflects little sunlight.

Orbital data are the quantities that tell us how Mercury moves in its orbit around the Sun. Mercury's orbit is the second most eccentric of all the planets (after Pluto). Mercury's distance from the Sun at perihelion is about 66% of the distance at aphelion. For comparison, the Earth's distance at perihelion is 97% of the distance at aphelion. The sunlight striking Mercury at perihelion is 2.3 times more intense than at aphelion.

Average distance from the Sun 57.9×106 km (36 million miles, 0.387 astronomical units)
Distance at perihelion(= minimum distance from the Sun) 46×106 km (29 million miles)
Distance at aphelion (= maximum distance from the Sun) 70×106 km (44 million miles)
Orbital eccentricity 0.206 (second most eccentric orbit of all planets)
Mean orbital speed 47.89 km/s (29.8 miles/s)
Maximum orbital speed (at perihelion) 59 km/s (37 miles/s)
Minimum orbital speed (at aphelion) 39 km/s (24 miles/s)
Sidereal period 87.97 Earth days
Synodic period 115.88 days
Rotation period 58.646 Earth days = 2/3
Solar rotation period (noontime to noontime, or one Mercurian day) 175.84 Earth days = 2 Mercurian years
Inclination of orbit to the ecliptic
Elongation (average) 23°
Greatest elongation (maximum) 28°
Inclination of equator to orbital plane 2° ?


Mercury's orbital plane is the second most tilted, after Pluto's, with respect to the ecliptic (the plane of Earth's orbit around the Sun). Mercury takes 88 days to orbit the Sun. The rotational data are the quantities that tell us how Mercury spins about its own axis. Because Mercury's rotation is a result of tidal interactions with the Sun, its rotational and orbital period are coupled to each other. The rotation of Mercury was first measured in 1965 with radar and was found to be exactly 2/3 of its orbital period: about 59 days. This means that it turns once on its axis when it has gone 2/3 of its way around the Sun (position 1 to position 3 in the diagram). After another half rotation, it completes its orbit around the Sun.

Imagine that at the beginning of a rotation, a feature is pointing directly to the Sun (this position is the equivalent of noon—position 1 in the diagram). When Mercury has completed one revolution around the Sun, this feature will end up pointing directly away from the Sun, or at midnight (position 4). It will take another full revolution before the feature points directly to the Sun again, ending up at noon. Therefore, the solar day in Mercury (noon to noon) is exactly twice its orbital period. If you lived on Mercury, you would be two years older every solar day!

Physical Data

Mercury is the body with the largest known difference in daytime-nighttime temperatures in the whole solar system. This is because Mercury doesn't have a permanent atmosphere to shield it from sunlight during the day or hold the heat during the night. You might have noticed during winters on Earth that it is colder when the sky is cloudless, especially after a snowstorm. In this case all the heat can escape freely as the snow is melting. In addition, Mercury rotates very slowly, so it has a long time to get very hot in the sunlit side and very cold in the dark side. Notice that the temperature difference is 600 degrees Kelvin (1080°F); in contrast, Earth's day/night temperature variation rarely exceeds 20 K.

Mass 3.30×1023 kg (0.055 Earth masses)
Radius (equatorial = polar) 2,439 km (0.38 Earth's radius, 1,516 miles)
Mean density 5.43 g/cm3
Equatorial acceleration of gravity 370 cm/s2
Escape velocity 4.23 km/s
Surface temperature 120 to 720 K
Albedo 0.11


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