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Mercury Moons - Orbital and Rotational Detail - Atmosphere - Surface and Geology - Interior - Magnetic Field - Perihelion Advance - Evolution of Mercury Atmosphere Mercury's weak gravity and high surface temperature (700 K) cannot retain a permanent atmosphere. However, a non-permanent atmosphere is detected around the planet; consisting of different types of gases coming from two sources: 1. Hydrogen and helium gases come primarily from the solar wind. 2. Sodium and potassium gases escape from the rocks inside the planet (a process called outgassing), or are ejected from the surface by solar wind particles. Surface and Geology Radar image of the crater Chao Meng-Fu at Mercury's south pole. Radar data gives strong evidence for ice beneath the surface. (Image from Arecibo Observatory). Mercury's surface was photographed by Mariner 10 at a resolution of 1 km. Only one hemisphere was seen by the spacecraft. Mercury is covered by impactcraters which are named after artists, composers, architects, writers, and other cultural figures of the world. Some regions are observed to have less craters. These moderately cratered plains appear to have formed on top of older craters that perhaps were erased by volcanism, although at present it cannot be proven that Mercury had volcanic eruptions in the past. A major feature on Mercury is the Caloris Basin, the largest impact basin of all. Its diameter is 1300 km and it shows rings typical or large impacts. The interior floor of the basin is not smooth, but crinkled, probably resulting from the rapid cooling of lava that resulted from the melting of material at the time of impact. Mercury's surface shows cliffs, called scarps, that were formed by compression when the planet cooled and shrank. Models demonstrate that the planet shrunk about 2 km. The scarps' length vary from 20 to 500 km, and the heights, from a few hundred meters to about one kilometer. Since they cut through craters, they formed after these craters. Radar images of Mercury have revealed polar caps, which were unexpected. Interior Mercury's interior is deduced from its small size and high density (5.43 g/cm3, almost as high as Earth's density), which together require a large metallic core. It is estimated that 60% of Mercury's interior is an iron core (with nickel and/or silicates), it is not known whether it is molten or solid. Surrounding the core is a layer of rock, and finally a crust of unknown thickness. Magnetic Field Mariner 10 measured a magnetic field on Mercury that is about 1% as strong as Earth's, nearly aligned with the planet's spin axis. Mercury was not expected to have a magnetic field, since planetary magnetic fields are generated by electric currents circulating in the planet's liquid core. Mercury rotates very slowly. Perhaps the magnetic field indicates that the core is molten, but a small planet like Mercury would have had time to cool down since its formation; or perhaps it indicates a different source for the field. Future missions to Mercury will help answer many unknowns about this planet. Perihelion Advance Observations have revealed that the major axis of Mercury's orbit rotates in space with respect to the stars, covering an angle of about 16° every 10,000 years (5.74 seconds of arc per year). When the gravitational forces from the Sun and other planets are taken into account, the orbit should turn 5.31 arc seconds per year. The residual 0.43 arc seconds per year are accounted for by Einstein's theory of general relativity, which predicts a strong curvature of space time near a massive object. The value predicted by general relativity is the same as the observed value. Evolution of Mercury Not enough is known about Mercury to have a detailed understanding of its evolution. One theory is that it formed by accretion and, during this early stage, the planet material differentiated into its current core and crustal components. During the early Solar System, it probably underwent intense bombardment. As it cooled, large meteor impacts caused the multiringed basins. A second theory proposes that Mercury was originally about 10 times as massive than it is now, and that early in its history it was hit by a Mars-sized body, ejecting much of the mantle and leaving behind a core that is large compared to the rest of the planet's mass. Bibliography Moons and Planets, W. Hartmann, 1993, 3rd edition, Wadsworth Publishing Co. Astrophysical Data: Planets and Stars, K.R. Lang, 1992, Springer-Verlag The Planetary System, D. Morrison and T. Owen, 1996, 2nd edition, Addison-Wesley Publishing Co., Inc. Sun-Scorched Mercury, The Universe in the Classroom, Teachers' Newsletter # 22, Winter 1992/93, The Astronomical Society of the Pacific (http://www.aspsky.org/html//22/22.html) <<< Previous \| Next >>> Home

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Back to the MIRA Website | Field Trips to the Stars Home > Introduction to the Solar System > Mercury

Mercury

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

Atmosphere

Mercury's weak gravity and high surface temperature (700 K) cannot retain a permanent atmosphere. However, a non-permanent atmosphere is detected around the planet; consisting of different types of gases coming from two sources: 1. Hydrogen and helium gases come primarily from the solar wind. 2. Sodium and potassium gases escape from the rocks inside the planet (a process called outgassing), or are ejected from the surface by solar wind particles.

Surface and Geology

Radar image of the crater Chao Meng-Fu at Mercury's south pole. Radar data gives strong evidence for ice beneath the surface. (Image from Arecibo Observatory).

Mercury's surface was photographed by Mariner 10 at a resolution of 1 km. Only one hemisphere was seen by the spacecraft. Mercury is covered by impactcraters which are named after artists, composers, architects, writers, and other cultural figures of the world. Some regions are observed to have less craters. These moderately cratered plains appear to have formed on top of older craters that perhaps were erased by volcanism, although at present it cannot be proven that Mercury had volcanic eruptions in the past.

A major feature on Mercury is the Caloris Basin, the largest impact basin of all. Its diameter is 1300 km and it shows rings typical or large impacts. The interior floor of the basin is not smooth, but crinkled, probably resulting from the rapid cooling of lava that resulted from the melting of material at the time of impact.

Mercury's surface shows cliffs, called scarps, that were formed by compression when the planet cooled and shrank. Models demonstrate that the planet shrunk about 2 km. The scarps' length vary from 20 to 500 km, and the heights, from a few hundred meters to about one kilometer. Since they cut through craters, they formed after these craters.

Radar images of Mercury have revealed polar caps, which were unexpected.

Interior

Mercury's interior is deduced from its small size and high density (5.43 g/cm3, almost as high as Earth's density), which together require a large metallic core. It is estimated that 60% of Mercury's interior is an iron core (with nickel and/or silicates), it is not known whether it is molten or solid. Surrounding the core is a layer of rock, and finally a crust of unknown thickness.

Magnetic Field

Mariner 10 measured a magnetic field on Mercury that is about 1% as strong as Earth's, nearly aligned with the planet's spin axis. Mercury was not expected to have a magnetic field, since planetary magnetic fields are generated by electric currents circulating in the planet's liquid core. Mercury rotates very slowly. Perhaps the magnetic field indicates that the core is molten, but a small planet like Mercury would have had time to cool down since its formation; or perhaps it indicates a different source for the field. Future missions to Mercury will help answer many unknowns about this planet.

Perihelion Advance

Observations have revealed that the major axis of Mercury's orbit rotates in space with respect to the stars, covering an angle of about 16° every 10,000 years (5.74 seconds of arc per year). When the gravitational forces from the Sun and other planets are taken into account, the orbit should turn 5.31 arc seconds per year. The residual 0.43 arc seconds per year are accounted for by Einstein's theory of general relativity, which predicts a strong curvature of space time near a massive object. The value predicted by general relativity is the same as the observed value.

Evolution of Mercury

Not enough is known about Mercury to have a detailed understanding of its evolution. One theory is that it formed by accretion and, during this early stage, the planet material differentiated into its current core and crustal components. During the early Solar System, it probably underwent intense bombardment. As it cooled, large meteor impacts caused the multiringed basins. A second theory proposes that Mercury was originally about 10 times as massive than it is now, and that early in its history it was hit by a Mars-sized body, ejecting much of the mantle and leaving behind a core that is large compared to the rest of the planet's mass.

Bibliography

Moons and Planets, W. Hartmann, 1993, 3rd edition, Wadsworth Publishing Co.
Astrophysical Data: Planets and Stars, K.R. Lang, 1992, Springer-Verlag
The Planetary System, D. Morrison and T. Owen, 1996, 2nd edition, Addison-Wesley Publishing Co., Inc.
Sun-Scorched Mercury, The Universe in the Classroom, Teachers' Newsletter # 22, Winter 1992/93, The Astronomical Society of the Pacific (http://www.aspsky.org/html//22/22.html)

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