Image of Mercury. Credit: Universe Today
Image of Mercury. Credit: Universe Today

Mercury is the smallest and closest to the Sun of the eight planets in the Solar System, with an orbital period of about 88 Earth days. Seen from the Earth, it appears to move around its orbit in about 116 days, which is much faster than any other planet. This rapid motion may have led to it being named after the Roman deity Mercury, the fast-flying messenger to the gods. Because it has almost no atmosphere to retain heat, Mercury’s surface experiences the greatest temperature variation of all the planets, ranging from 100 K (−173 °C; −280 °F) at night to 700 K (427 °C; 800 °F) during the day at some equatorial regions. The poles are constantly below 180 K (−93 °C; −136 °F). Mercury’s axis has the smallest tilt of any of the Solar System’s planets (about 1⁄30 of a degree), but it has the largest orbital eccentricity. At aphelion, Mercury is about 1.5 times as far from the Sun as it is at perihelion. Mercury’s surface is heavily cratered and similar in appearance to the Moon, indicating that it has been geologically inactive for billions of years.

Mercury does not experience seasons in the same way as most other planets, such as the Earth. It is locked so it rotates in a way that is unique in the Solar System. As seen relative to the fixed stars, it rotates exactly three times for every two revolutions it makes around its orbit. As seen from the Sun, in a frame of reference that rotates with the orbital motion, it appears to rotate only once every two Mercurian years. An observer on Mercury would therefore see only one day every two years.

Because Mercury’s orbit lies within Earth’s orbit (as does Venus’s), it can appear in Earth’s sky in the morning or the evening, but not in the middle of the night. Also, like Venus and the Moon, it displays a complete range of phases as it moves around its orbit relative to the Earth. Although Mercury can appear as a very bright object when viewed from Earth, its proximity to the Sun makes it more difficult to see than Venus.

mt-mercury-interior-normalInternal Structure:

Mercury is one of four terrestrial planets in the Solar System, and is a rocky body like the Earth. It is the smallest planet in the Solar System, with an equatorial radius of 2,439.7 km. Mercury is even smaller—albeit more massive—than the largest natural satellites in the Solar System, Ganymede and Titan. Mercury consists of approximately 70% metallic and 30% silicate material. Mercury’s density is the second highest in the Solar System at 5.427 g/cm3, only slightly less than Earth’s density of 5.515 g/cm3. If the effect of gravitational compression were to be factored out, the materials of which Mercury is made would be denser, with an uncompressed density of 5.3 g/cm3 versus Earth’s 4.4 g/cm3.

Mercury’s density can be used to infer details of its inner structure. Although Earth’s high density results appreciably from gravitational compression, particularly at the core, Mercury is much smaller and its inner regions are not as compressed. Therefore, for it to have such a high density, its core must be large and rich in iron.

Geologists estimate that Mercury’s core occupies about 42% of its volume; for Earth this proportion is 17%. Recent research suggests that Mercury has a molten core. Surrounding the core is a 500–700 km mantle consisting of silicates. Based on data from the Mariner 10 mission and Earth-based observation, Mercury’s crust is believed to be 100–300 km thick. One distinctive feature of Mercury’s surface is the presence of numerous narrow ridges, extending up to several hundred kilometers in length. It is believed that these were formed as Mercury’s core and mantle cooled and contracted at a time when the crust had already solidified. Mercury’s core has a higher iron content than that of any other major planet in the Solar System, and several theories have been proposed to explain this. The most widely accepted theory is that Mercury originally had a metal-silicate ratio similar to common chondrite meteorites, thought to be typical of the Solar System’s rocky matter, and a mass approximately 2.25 times its current mass. Early in the Solar System’s history, Mercury may have been struck by a planetesimal of approximately 1/6 that mass and several hundred kilometers across. The impact would have stripped away much of the original crust and mantle, leaving the core behind as a relatively major component. A similar process, known as the giant impact hypothesis, has been proposed to explain the formation of the Moon.

Alternatively, Mercury may have formed from the solar nebula before the Sun’s energy output had stabilized. The planet would initially have had twice its present mass, but as the protosun contracted, temperatures near Mercury could have been between 2,500 and 3,500 K and possibly even as high as 10,000 K. Much of Mercury’s surface rock could have been vaporized at such temperatures, forming an atmosphere of “rock vapor” that could have been carried away by the solar wind.

A third hypothesis proposes that the solar nebula caused drag on the particles from which Mercury was accreting, which meant that lighter particles were lost from the accreting material and not gathered by Mercury. Each hypothesis predicts a different surface composition, and two upcoming space missions, MESSENGER and BepiColombo, both will make observations to test them. MESSENGER has found higher-than-expected potassium and sulfur levels on the surface, suggesting that the giant impact hypothesis and vaporization of the crust and mantle did not occur because potassium and sulfur would have been driven off by the extreme heat of these events. The findings would seem to favor the third hypothesis, however further analysis of the data is needed.

Discovered By
Known by the Ancients
Date of Discovery
Orbit Size Around Sun (semi-major axis)
Metric: 57,909,227 km
English: 35,983,125 miles
Scientific Notation: 5.7909227 x 107 km (0.38709927 A.U.)
By Comparison: Earth is 1 A.U. (Astronomical Unit) from the sun.
Perihelion (closest)
Metric: 46,001,009 km
English: 28,583,702 miles
Scientific Notation: 4.600 x 107 km (3.075 x 10-1 A.U.)
By Comparison: 0.313 x Earth
Aphelion (farthest)
Metric: 69,817,445 km
English: 43,382,549 miles
Scientific Notation: 6.982 x 107 km (0.4667 A.U.)
By Comparison: 0.459 x Earth
Sidereal Orbit Period (Length of Year)
0.2408467 Earth years
87.97 Earth days
By Comparison: 0.241
Orbit Circumference
Metric: 359,976,856 km
English: 223,679,248 miles
Scientific Notation: 3.600 x 108 km
By Comparison: 0.383 x Earth
Average Orbit Velocity
Metric: 170,503 km/h
English: 105,946 mph
Scientific Notation: 4.7362 x 104 m/s
By Comparison: 1.590 x Earth
Orbit Eccentricity
By Comparison: 12.305 x Earth
Orbit Inclination
7.0 degrees
Equatorial Inclination to Orbit
0 degrees
By Comparison: Earth’s equatorial inclination to orbit is 23.45 degrees.
Mean Radius
Metric: 2,439.7 km
English: 1,516.0 miles
Scientific Notation: 2.4397 x 103 km
By Comparison: 0.3829 x Earth
Equatorial Circumference
Metric: 15,329.1 km
English: 9,525.1 miles
Scientific Notation: 1.53291 x 104 km
Metric: 60,827,208,742 km3
English: 14,593,223,446 mi3
Scientific Notation: 6.08272 x 1010 km3
By Comparison: 0.056 x Earth’s
Metric: 330,104,000,000,000,000,000,000 kg
Scientific Notation: 3.3010 x 1023 kg
By Comparison: 0.055 x Earth’s
Metric: 5.427 g/cm3
By Comparison: 0.984 x Earth
Surface Area
Metric: 74,797,000 km2
English: 28,879,000 square miles
Scientific Notation: 7.4797 x 107 km2
By Comparison: 0.147 x Earth
Surface Gravity
Metric: 3.7 m/s2
English: 12.1 ft/s2
By Comparison: If you weigh 100 pounds on Earth, you would weigh 38 pounds on Mercury.
Escape Velocity
Metric: 15,300 km/h
English: 9,507 mph
Scientific Notation: 4.25 x 103 m/s
By Comparison: Escape Velocity of Earth is 25,030 mph
Sidereal Rotation Period (Length of Day)
58.646 Earth days
1407.5 hours
By Comparison: 58.81 x Earth
Minimum/Maximum Surface Temperature
Metric: -173/427 °C
English: -279/801 °F
Scientific Notation: 100/700 K
By Comparison: Earth’s temperature range is ~ 185/331 K.
Atmospheric Constituents
By Comparison: Earth’s atmosphere consists mostly of N2, O2