Mercury (Planet)
Mercury (Planet)
Mercury is the closest planet to the sun. It is a small world only slightly larger than Earth’s moon. Next to the dwarf planet Pluto, Mercury is one of the least explored planets within the solar system. Visited only once in all the years of solar system exploration (three brief fly-bys of Mariner 10 during 1974–1975), only about 45% of Mercury’s surface has been imaged from nearby. All that may change rather soon after NASA’s Messenger and Japan and the European Space Agency’s Beppi Columbo missions launched, in 2004, and launch, in 2013, respectively.
Basic properties
Mercury orbits the sun at a mean distance of 0.387 astronomical units (AU), where 1 AU is the mean distance from Earth to the sun, or about 92,956,000 mi (149,600,000 km). The high eccentricity of the planet’s orbit (e = 0.206), however, dictates that it can be as far as 0.467 AU away from the sun, and as close as 0.307 AU. The high eccentricity attributed to Mercury’s orbit is the second largest in the solar system, only the dwarf planet Pluto has a more eccentric orbit. (Eccentricity in astronomy indicates that an orbit is not absolutely circular. The value of e = 1 indicates an orbit is shaped as a parabola. An ellipse is less than one, and a circle has zero eccentricity.)
Constrained as it is in an orbit close to the sun, Mercury is not an easy planet for naked-eye observers to locate. The greatest separation between the planet and the sun, as seen from Earth, is 28° and consequently
the planet is never visible against a truly dark sky. Even at its greatest angular separation from the sun, Mercury will either set within two hours of sunset, or rise no earlier than two hours before the sun. Nonetheless, Mercury has been known since the most ancient of times, with observations of the planet being reported as far back as several centuries BC Greek philosopher Plato (c. 427–c. 347 BC) refers to the distinctive yellow color of Mercury in Book X of his Republic.
The sidereal period, or the time it takes Mercury to orbit the sun, is 87.969 days. The planet’s synodic period, which is the time required for Mercury to return to the same relative position with respect to the sun and Earth, is 116 days. As seen from Earth, Mercury undergoes a change of phase as it moves around the sun. These phase changes were first observed in the early seventeenth century by Polish astronomer Johannes Hewelcke (1611–1687), who is perhaps better known today through his latinized name, Hevelius. Zero phase occurs when Earth, the sun, and Mercury are directly in line, with Mercury on the opposite side of the sun to Earth. At this phase, Mercury is said to be at superior conjunction. Half phase occurs when Earth, the sun, and Mercury are once again in a line, but this time with Mercury being on the same side of the sun as Earth is. Mercury is said to be at inferior conjunction when it exhibits a half phase. While moving from inferior to superior conjunction Mercury passes through a quarter phase, during which the disk of the planet is half illuminated as seen from Earth. Mercury also passes through its greatest western elongation when moving from inferior to superior conjunction. Likewise, in moving from superior to inferior conjunction Mercury passes through greatest eastern elongation, and exhibits a second quarter phase, or half-disk illumination.
Because Mercury’s orbit is quite elongated, so that its distance from the sun varies significantly, the maximum angular separation between Mercury and the sun, as seen from Earth, can vary from a minimum of 18° to a maximum of 28° (Figure 1). The largest angular separation of 28° occurs when Mercury is at
either greatest western, or greatest eastern elongation and near its aphelion (its greatest distance from the sun). Irrespective of whether the planet is at aphelion or not, the best time to view Mercury in the evening is when the planet is near greatest eastern elongation. Because the synodic period of Mercury is about 116 days, the planet will be favorably placed for evening viewing three times each year. Similar conditions apply for viewing Mercury before sunrise.
The inclination of Mercury’s orbit to that of the ecliptic plane (the plane of Earth’s orbit about the sun) is 7.0°. This slight orbital tilt dictates that when Mercury is at inferior conjunction it is only rarely silhouetted against the sun’s disk as seen from Earth. On those rare occasions when Earth, Mercury, and the sun are in perfect alignment, however, a solar transit of Mercury can take place, and a terrestrial observer will see Mercury move in front of, and across the sun’s disk. A transit of Mercury can only occur when the planet is at inferior conjunction during the months of May and November. During these months, Earth is near the line along which the orbit of Mercury intersects the ecliptic plane—this is the line of nodes for Mercury’s orbit. Approximately a dozen solar transits of Mercury occur each century, and the final transit of the twentieth century occurred on 15 November 1999. The first solar transit of Mercury occurred in 2003, and the next one occured November 8, 2006.
Mercury’s rotation rate
When, in the mid-1880s, Italian astronomer Giovanni Schiaparelli (1835–1910) attempted to construct a map of Mercurian features, he found that the shape and relative position of the fuzzy surface features that his telescope could reveal did not change greatly with time. Schiaparelli subsequently reasoned that his observations could be best explained if Mercury kept the same face pointed toward the sun at all times.
If a planet or a satellite spins on its axis at exactly the same rate that it moves around in its orbit, then it is said to be in synchronous rotation. Earth’s moon, for example, is in synchronous rotation about Earth, and consequently is always seen with the same lunar features. Synchronous rotation arises through gravitational interactions, and mathematicians have been able to show that once the spin of an object has been synchronized it remains so in a stable fashion. An alternative way of saying that an object spins in a synchronous manner is to say that is satisfies a 1-to-1 spin-orbit coupling.
Astronomers believed that Mercury was in a 1-to-1 spin-orbit coupling with the sun until the mid-1960s. The first hint that Mercury might not be in synchronous rotation about the sun was revealed through
Earth-based radar measurements. By analyzing the Doppler shift in the returned radar signals, astronomers were able to show that Mercury did not rotate fast enough to be in a 1-to-1 spin-orbit coupling with the sun. Rather they found that Mercury’s rotation rate is 58.646 days. Since Mercury orbits the sun once every 87.969 days, the radar measurements indicated that the planet is in a 3-to-2 spin-orbit coupling with the sun. That is, Mercury spins three times about its axis for every two orbits that it completes about the sun. The Mercurian day, that is the time from sunrise to sunset is therefore 88 terrestrial days long. From Earth its greatest angular diameter is just 4/1000th of a degree. This angular size translates to a physical diameter of 3,030 mi (4,879 km), making Mercury about 1/3 the size of Earth, or about 1.5 times larger than the moon.
Surface features
Mercury is a small planet that is quite hot (approximately 800°F [427°C] during a Mercurian day) when the sun shines on its surface. It has a very thin atmosphere of oxygen, potassium, and sodium vapors. The surface pressure of atmosphere is too low to have wind. Without wind, running water, and flowing ice, the range of surface processes is limited to physical weathering effects of heating and cooling and meteoritic impact.
Mariner 10 is the only spacecraft to have photographed the surface of Mercury. Completing a total of three close encounters with the planet, in March and September 1974, and March 1975, the space probe was able to record details over about 45% of Mercury’s crater strewn surface. The remaining half of Mercury’s surface has never been photographed.
Mercury’s surface is very similar to that of the moon. There are, however, some important differences in features. Mercury has, for example, relatively fewer craters larger than 15.5 to 31 mi (25 to 50 km) in diameter. There are no extensive highland regions on Mercury, and the surface is subdivided simply into cratered terrain and intercrater plains based upon differences in crater density and size. Resurfaced regions on Mercury are rare (15% of the surface), and are referred to as smooth plains. Unlike the moon, Mercury exhibits many scalloped cliffs, or lobate scarps that can run for
several hundred kilometers and be as much as 0.6 mi (1 km) high. Only one lunar-like mare, the Caloris Basin, has been discovered on Mercury, however, many large multi-ring crater basins (124 to 373 mi [200 to 600 km] in diameter) are filled with flood basalts like the lunar mare. Sporting a relatively flat but wrinkled floor, and being surrounded by a ring of 1.24 mi (2 km) high mountains, the Caloris Basin, with a diameter of about 807.5 mi (1,300 km), is the largest Mercurian feature.
The prominent scarp features recorded on Mercury by Mariner 10 are unique to the planet. The scarps are interesting from a geological standpoint, because they run cross other surface markings, such as craters. This suggests that the scarps were formed through the shrinkage of the planet’s outer mantle or some other stresses perhaps due to change in planet shape upon attainment of its current 2:3 spin orbit couple with the sun. The rises of the observed scarp features suggests that since Mercury formed it has shrunk by about 2 mi (3 km) in radius.
The geological history of Mercury starts with accretion and differentiation about 4.6 billion years ago. Now long after the planet’s mass came together there was a global melting episode for the crust and mantle and planetary differentiation so that an iron core formed and a mantle and crust developed. Mercury, as the other planets, went through the heavy bombardment period in which much of the impact structure of the old cratered terrains of Mercury were formed. The Caloris Basin impact was a punctuation mark in Mercury’s history. The impact dispersed a huge amount of ejecta over large areas of the planet and the passage of shock waves directly through Mercury caused an antipodal (opposite side) effect of crustal shattering and disruption. Since Caloris, flood basalts filled the large impact basins, including Caloris, and the global system of scarp features formed due to planetary contraction. Light impact cratering followed the scarp formation event and continued for the last three billion years of Mercury’s history.
The International Astronomical Union (IAU) has established a guide to the naming of planetary features, and for Mercury the convention is that craters are named after artists, musicians, painters, and authors; plains are given names corresponding to Mercury in various languages; scarps are named after famous ships of scientific discovery, and valleys are named after radio telescopes.
Polar ice
Perhaps one of the most unexpected Mercurian features to be discovered in recent times was that of water—ice at the planet’s poles. The discovery of water ice on Mercury was made in 1991 by bouncing powerful radar signals off the planet’s surface. The discovery of water ice on Mercury was surprising, since it was believed that the high daytime temperatures caused by the proximity of the planet to the sun would lead to the rapidly evaporation of any ices that might chance to form.
The polar regions of Mercury can be seen from Earth with powerful radars, because of the relatively large inclination (7°) that the planet’s orbit presents to the ecliptic. These same polar regions, however, are never fully illuminated by the sun, and it appears that water ice has managed to collect in the permanently-shadowed regions of many polar crater rims. It is not clear where the ice seen at Mercury’s poles comes from, but it has been suggested that comet crashes may be one source.
Internal structure
Mercury has no natural satellites and consequently it is not an easy task to determine the planet’s mass. By carefully recording the acceleration of the Mariner 10 space probe during its close encounters with planet, however, NASA scientists were able to determine a Mercurian mass equivalent to 1/6,023,600 that of the sun. This mass, of about 3.3 × 1023 kg, is some 6% that of Earth’s mass.
Some idea of Mercury’s internal structure can be gained from the knowledge of its mass and radius. These two terms indicate that Mercury has a bulk density of 5,430 g/cm3. This density is only slightly smaller than that of Earth, suggesting by analogy that Mercury has a large nickel-iron alloy core, and a thin rocky mantle. The nickel-iron core probably accounts for about 40% Mercury’s volume (Figure 2).
Mercury’s relatively large nickel-iron core and thin crustal mantle suggests that the planet may have undergone a catastrophic collision during its final stages of formation. A glancing blow from a large planetesimal may have caused most of the planet’s initial mantle to be ejected into space, leaving behind a planet with a relatively large core.
Instruments carried on-board Mariner 10 detected a weak Mercurian magnetic field. The magnetic field strength is about 1% that of Earth’s. Even though Mercury’s magnetic field is very weak, it was a surprise to scientists that it displayed one at all. It is presently believed that planetary magnetic fields are created by the so-called dynamo effect. It is thought that the dynamo effect should operate in those planets that have hot, electrically conducting, liquid inner cores, and that rotate reasonably quickly. Mercury is not thought to satisfy any of the conditions necessary for a planetary dynamo to operate, and consequently the observed magnetic field suggests that the standard picture of Mercury’s internal structure needs revising, or that another, presently unknown, mechanism exists through which planets can generate magnetic fields.
Mercury’s weak magnetic field is strong enough to force charged particles in the solar wind to flow around the planet. The cavity that consequently exists about the planet is called a magnetosphere, and its existence prevents solar wind material (mainly protons and electrons) impacting directly on the planet’s surface.
Mercury’s atmosphere
The mid-day surface temperature on Mercury rises to about 700 K (803°F; [428°C]), while the mid-nighttime temperature falls to 100 K (–279.4°F; [–173°C]). This temperature variation, the largest experienced by any planet in the solar system, is due to the fact that Mercury has essentially no insulating atmosphere.
The main reason that Mercury does not have a distinctive atmosphere is that it is small and because it is close to the sun. Mercury’s small radius indicates that it has a low escape velocity, just 2.5 mi (4.2 km)/sec. Mariner 10 did detect a very thin atmosphere of hydrogen and helium on Mercury. It is believed, however, that Mercury’s wispy atmosphere is composed of atoms that have been temporarily captured from the solar wind. Ground-based observations have found that a sodium and potassium atmosphere exists on the daylight side of Mercury. These atoms are probably released through the interaction of ultraviolet radiation with surface rocks.
NASA spacecraft MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) was launched on August 4, 2004, for its trip to Mercury. When it arrives at the planet it will make three close approaches to Mercury in 2008 and 2009. After these approaches, the spacecraft will enter the orbit around Mercury in the early part of 2011. The mission of MESSENGER will be to learn more about Mercury’s geological history, magnetic field, atmosphere, core structure, and poles (specifically, whether any ice is contained on the areas). MESSENGER contains spectrometers (to determine element composition) and magnetometers (to measure charged particles).
KEY TERMS
Astronomical unit— The average distance between the sun and Earth. One astronomical unit, symbol AU, is equivalent to 92.9 million mi (149.6 million km).
Doppler effect— The apparent change in the wavelength of a signal due to the relative motion between the source and observer.
Dynamo effect— A model for the generation of planetary magnetic fields: The circulation of hot, conducting fluids within a planet’s liquid core leads to the generation of a magnetic field.
Escape velocity— The speed that an object must have in order to escape the gravitational pull of another body.
Lobate scarp— A long, near vertical wall of rock running across a flat plain.
Mantle— The outer layers of a planets interior core, usually composed of silicate rock.
Planetesimals— Asteroid-sized bodies that accumulated to form protoplanets.
Solar wind— A stream of charged and neutral particles that emanates from the sun and moves into the solar system.
Synchronous rotation— Any object that spins on its axis at the same rate that it moves along in its orbit is said to be in synchronous rotation. Also called to 1-to-1 spin-orbit coupling.
Japan and the European Space Agency (ESA) are planning a mission to Mercury called BepiColombo. The spacecraft was named after Italian scientist and mathematician Giuseppe (Bepi) Columo (1920–1984), who determined Mercury’s orbital resonance with the sun. With a lift-off from Earth in 2013, Japanese/ESA scientists hope to orbit the planet in 2019 with two probes onboard for the express purpose of mapping the planet and to study its magnetosphere.
See also Doppler effect.
Resources
BOOKS
de Pater, Imke, and Jack J. Lissauer. Planetary Sciences Cambridge, UK: Cambridge University Press, 2001.
Morrison, D., and Tobias Owen. The Planetary System. 3rd ed. Addison-Wesley Publishing, 2002.
Sobel, Dava. The Planets. New York: Viking, 2005.
Taylor, F.W. The Cambridge Photographic Guide to the Planets. Cambridge University Press, 2002.
OTHER
Jet Propulsion Laboratory, National Aeronautics and Space Administration. “Welcome to the Planets: Mercury.” <http://pds.jpl.nasa.gov/planets/choices/mercury1.htm> (accessed October 16, 2006).
SpaceKids, National Aeronautics and Space Administration. “Tour the Solar System and Beyond.” <http://spacekids.hq.nasa.gov/osskids/animate/mac.html> (accessed October 14, 2006).
TeachNet-lab.org. “A Tour of the Planets.” <http://www.teachnet-lab.org/miami/2001/salidoi2/a_tour_of_the_planets.htm> (accessed October 14, 2006).
Martin Beech
David T. King, Jr.
Mercury (Planet)
Mercury (planet)
Mercury is the closest planet to the Sun . It is a small world only slightly larger than Earth's Moon . Next to the planet Pluto , Mercury is one of the least explored planets within our solar system . Visited only once in all the years of solar system exploration (three brief fly-bys of Mariner 10 during 1974–75), only about 45% of Mercury's surface has been imaged from nearby. All that may change rather soon when NASA's Messenger and the European Space Agency's Beppi Columbo missions launch in 2004 and 2009, respectively.
Basic properties
Mercury orbits the Sun at a mean distance of 0.387 astronomical units (AU). The high eccentricity of the planet's orbit (e = 0.206), however, dictates that it can be as far as 0.467 AU away from the Sun, and as close as 0.307 AU. The high eccentricity attributed to Mercury's orbit is the second largest in the solar system, only the planet Pluto has a more eccentric orbit. (Eccentricity in astronomy indicates that an orbit is not absolutely circular. The value of e = 1 indicates an orbit shaped as a parabola . An ellipse is less than one, and a circle has zero eccentricity.)
Constrained as it is in an orbit close to the Sun, Mercury is not an easy planet for naked-eye observers to locate. The greatest separation between the planet and the Sun, as seen from Earth , is 28° and consequently the planet is never visible against a truly dark sky. Even at its greatest angular separation from the Sun, Mercury will either set within two hours of sunset, or rise no earlier than two hours before the Sun. Nonetheless, Mercury has been known since the most ancient of times, with observations of the planet being reported as far back as several centuries b.c. The Greek philosopher Plato refers to the distinctive yellow color of Mercury in Book X of his Republic.
The sidereal period, or the time it takes Mercury to orbit the Sun, is 87.969 days. The planet's synodic period, which is the time required for Mercury to return to the same relative position with respect to the Sun and Earth, is 116 days. As seen from Earth, Mercury undergoes a change of phase as it moves around the Sun. These phase changes were first observed in the early seventeenth century by the Polish astronomer Johannes Hewelcke (1611-1687), who is perhaps better known today through his latinized name, Hevelius. Zero phase occurs when Earth, the Sun, and Mercury are directly in line, with Mercury on the opposite side of the Sun to Earth. At this phase, Mercury is said to be at superior conjunction. Half phase occurs when Earth, the Sun, and Mercury are once again in a line, but this time with Mercury being on the same side of the Sun as the Earth. Mercury is said to be at inferior conjunction when it exhibits a half phase. While moving from inferior to superior conjunction Mercury passes through a quarter phase, during which the disk of the planet is half illuminated as seen from the Earth. Mercury also passes through its greatest western elongation when moving from inferior to superior conjunction. Likewise, in moving from superior to inferior conjunction Mercury passes through greatest eastern elongation, and exhibits a second quarter phase, or half-disk illumination.
Because Mercury's orbit is quite elongated, so that its distance from the sun varies significantly, the maximum angular separation between Mercury and the Sun, as seen from the Earth, can vary from a minimum of 18 to maximum of 28°. The largest angular separation of 28° occurs when Mercury is at either greatest western, or greatest eastern elongation and near its aphelion (its greatest distance from the Sun). Irrespective of whether the planet is at aphelion or not, the best time to view Mercury in the evening is when the planet is near greatest eastern elongation. Because the synodic period of Mercury is about 116 days, the planet will be favorably placed for evening viewing three times each year. Similar conditions apply for viewing Mercury before sunrise.
The inclination of Mercury's orbit to that of the ecliptic plane (the plane of Earth's orbit about the Sun) is 7.0°. This slight orbital tilt dictates that when Mercury is at inferior conjunction it is only rarely silhouetted against the Sun's disk as seen from Earth. On those rare occasions when Earth, Mercury, and the Sun are in perfect alignment, however, a solar transit of Mercury can take place, and a terrestrial observer will see Mercury move in front of, and across the Sun's disk. A transit of Mercury can only occur when the planet is at inferior conjunction during the months of May and November. During these months Earth is near the line along which the orbit of Mercury intersects the ecliptic plane—this is the line of nodes for Mercury's orbit. Approximately a dozen solar transits of Mercury occur each century, and the final transit of the twentieth century occurred on 15 November 1999.
Mercury's rotation rate
When, in the mid-1880s, the Italian astronomer Giovanni Schiaparelli (1835-1910) attempted to construct a map of Mercurian features, he found that the shape and relative position of the fuzzy surface features that his telescope could reveal did not change greatly with time. Schiaparelli subsequently reasoned that his observations could be best explained if Mercury kept the same face pointed toward the Sun at all times.
If a planet or a satellite spins on its axis at exactly the same rate that it moves around in its orbit, then it is said to be in synchronous rotation . Our Moon, for example, is in synchronous rotation about Earth, and consequently we always see the same lunar features. Synchronous rotation arises through gravitational interactions, and mathematicians have been able to show that once the spin of an object has been synchronized it remains so in a stable fashion. An alternative way of saying that an object spins in a synchronous manner is to say that is satisfies a 1-to-1 spin-orbit coupling.
Astronomers believed that Mercury was in a 1-to-1 spin-orbit coupling with the Sun until the mid-1960s. The first hint that Mercury might not be in synchronous rotation about the Sun was revealed through Earth-based radar measurements. By analyzing the Doppler shift in the returned radar signals, astronomers were able to show that Mercury did not rotate fast enough to be in a 1-to-1 spin-orbit coupling with the Sun. Rather they found that Mercury's rotation rate is 58.646 days. Since Mercury orbits the Sun once every 87.969 days, the radar measurements indicated that the planet is in a 3-to-2 spin-orbit coupling with the Sun. That is, Mercury spins three times about its axis for every two orbits that it completes about the Sun. The Mercurian day, that is the time from sunrise to sunset is therefore 88 terrestrial days long. From Earth its greatest angular diameter is just 4/1000th of a degree. This angular size translates to a physical diameter of 3,030 mi (4,879 km), making Mercury about 1/3 the size of Earth, or about 1.5 times larger than the Moon.
Surface features
Mercury is a small planet that is quite hot (approximately 800°F [427°C] during a Mercurian day) when the Sun shines on its surface. It has a very thin atmosphere of oxygen , potassium, and sodium vapors. The surface pressure of atmosphere is too low to have wind . Without wind, running water , and flowing ice , the range of surface processes is limited to physical weathering effects of heating and cooling and meteoritic impact.
Mariner 10 is the only spacecraft to have photographed the surface of Mercury. Completing a total of three close encounters with the planet, in March and September 1974, and March 1975, the space probe was able to record details over about 45% of Mercury's crater strewn surface. The remaining half of Mercury's surface has never been photographed.
Mercury's surface is very similar to that of the Moon. There are, however, some important differences in features. Mercury has, for example, relatively fewer craters larger than 15.5-31 mi (25-50 km) in diameter. There are no extensive highland regions on Mercury, and the surface is subdivided simply into "cratered terrain" and "intercrater plains" based upon differences in crater density and size. Resurfaced regions on Mercury are rare (<15% of the surface), and are referred to as "smooth plains." Unlike the Moon, Mercury exhibits many scalloped cliffs, or lobate scarps that can run for several hundred kilometers and be as much as 0.6 mi (1 km) high. Only one lunar-like mare, the Caloris Basin, has been discovered on Mercury, however, many large multi-ring crater basins (124-373 mi [200-600 km] in diameter) are filled with flood basalts like the lunar mare. Sporting a relatively flat but wrinkled floor, and being surrounded by a ring of 1.24 mi-(2 km) high mountains , the Caloris Basin, with a diameter of about 807.5 mi (1,300 km), is the largest Mercurian feature.
The prominent scarp features recorded on Mercury by Mariner 10 are unique to the planet. The scarps are interesting from a geological standpoint, because they run cross other surface markings, such as craters. This suggests that the scarps were formed through the shrinkage of the planet's outer mantle or some other stresses perhaps due to change in planet shape upon attainment of its current 2:3 spin orbit couple with the Sun. The rises of the observed scarp features suggests that since Mercury formed it has shrunk by about 2 mi (3 km) in radius.
The geological history of Mercury starts with accretion and differentiation about 4.6 billion years ago. Now long after the planet's mass came together there was a global melting episode for the crust and mantle and planetary differentiation so that an iron core formed and a mantle and crust developed. Mercury, as the other planets, went through the "heavy bombardment" period in which much of the impact structure of the old cratered terrains of Mercury were formed. The Caloris Basin impact was a punctuation mark in Mercury's history. The impact dispersed a huge amount of ejecta over large areas of the planet and the passage of shock waves directly through Mercury caused an "antipodal" (opposite side) effect of crustal shattering and disruption. Since Caloris, flood basalts filled the large impact basins, including Caloris, and the global system of scarp features formed due to planetary contraction. Light impact cratering followed the scarp formation event and continued for the last three billion years of Mercury's history.
The International Astronomical Union has established a guide to the naming of planetary features, and for Mercury the convention is that craters are named after artists, musicians, painters, and authors; plains are given names corresponding to Mercury in various languages; scarps are named after famous ships of scientific discovery, and valleys are named after radio telescopes.
Polar ice
Perhaps one of the most unexpected Mercurian features to be discovered in recent times was that of water—ice at the planet's poles. The discovery of water ice on Mercury was made in 1991 by bouncing powerful radar signals off the planet's surface. The discovery of water ice on Mercury was surprising, since it was believed that the high daytime temperatures caused by the proximity of the planet to the Sun would lead to the rapidly evaporation of any ices that might chance to form.
The polar regions of Mercury can be seen from Earth with powerful radars, because of the relatively large inclination (7°) that the planet's orbit presents to the ecliptic. These same polar regions, however, are never fully illuminated by the Sun, and it appears that water ice has managed to collect in the permanently-shadowed regions of many polar crater rims. It is not clear where the ice seen at Mercury's poles comes from, but it has been suggested that comet crashes may be one source.
Internal structure
Mercury has no natural satellites and consequently it is not an easy task to determine the planet's mass. By carefully recording the acceleration of the Mariner 10 space probe during its close encounters with planet, however, NASA scientists were able to determine a Mercurian mass equivalent to 1/6,023,600 that of the Sun. This mass, of about 3.3x1023 kg, is some 6% that of the Earth's mass.
Some idea of Mercury's internal structure can be gained from the knowledge of its mass and radius. These two terms indicate that Mercury has a bulk density of 5,430 g/cm3. This density is only slightly smaller than that of Earth, suggesting by analogy that Mercury has a large nickel-ironalloy core, and a thin rocky mantle. The nickel-iron core probably accounts for about 40% Mercury's volume .
Mercury's relatively large nickel-iron core and thin crustal mantle suggests that the planet may have undergone a catastrophic collision during its final stages of formation. A glancing blow from a large planetesimal may have caused most of the planet's initial mantle to be ejected into space, leaving behind a planet with a relatively large core.
Instruments carried on-board Mariner 10 detected a weak Mercurian magnetic field. The magnetic field strength is about 1% that of Earth's. Even though Mercury's magnetic field is very weak, it was a surprise to scientists that it displayed one at all. It is presently believed that planetary magnetic fields are created by the so-called dynamo effect. It is thought that the dynamo effect should operate in those planets that have hot, electrically conducting, liquid inner cores, and that rotate reasonably quickly. Mercury is not thought to satisfy any of the conditions necessary for a planetary dynamo to operate, and consequently the observed magnetic field suggests that the standard picture of Mercury's internal structure needs revising, or that another, presently unknown, mechanism exists through which planets can generate magnetic fields.
Mercury's weak magnetic field is strong enough to force charged particles in the solar wind to flow around the planet. The cavity that consequently exists about the planet is called a magnetosphere , and its existence prevents solar wind material (mainly protons and electrons) impacting directly on the planet's surface.
Mercury's atmosphere
The mid-day surface temperature on Mercury rises to about 700K (803°F; 428°C), while the mid-nighttime temperature falls to 100K (-279.4°F; -173°C). This temperature variation, the largest experienced by any planet in the solar system, is due to the fact that Mercury has essentially no insulating atmosphere.
The main reason that Mercury does not have a distinctive atmosphere is that it is small and because it is close to the Sun. Mercury's small radius indicates that it has a low escape velocity , just 2.5 mi (4.2 km)/sec. Mariner 10 did detect a very thin atmosphere of hydrogen and helium on Mercury. It is believed, however, that Mercury's wispy atmosphere is composed of atoms that have been temporarily captured from the solar wind. Ground-based observations have found that a sodium and potassium atmosphere exists on the daylight side of Mercury. These atoms are probably released through the interaction of ultraviolet radiation with surface rocks .
See also Doppler effect.
Resources
books
Beatty, J. Kelly, Carolyn Collins Petersen, and Andrew L. Chaikin. The New Solar System. Cambridge: Cambridge Univ. Press, 1999.
de Pater, Imke, and Jack J. Lissauer. Planetary Sciences Cambridge, UK: Cambridge University Press, 2001.
Morrison, D., and Tobias Owen. The Planetary System. 3rd ed. Addison-Wesley Publishing, 2002.
Taylor, F.W. The Cambridge Photographic Guide to the Planets. Cambridge University Press, 2002.
periodicals
Strom, Robert. Mercury: The Elusive Planet. Washington, DC: Smithsonian Institution Press, 1987.
Strom, Robert. "Mercury: The Forgotten Planet." Sky & Telescope (September 1990): 256-60.
other
Arnett, B. SEDS, University of Arizona. "The Nine Planets, a Multimedia Tour of the Solar System." November 6, 2002 [cited February 8, 2003]. <http://seds.lpl.arizona.edu/nineplanets/nineplanets/nineplanets.html>.
Martin Beech David T. King, Jr.
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Astronomical unit
—The average distance between the Sun and Earth. One astronomical unit, symbol AU, is equivalent to 92.9 million mi (149.6 million km).
- Doppler effect
—The apparent change in the wavelength of a signal due to the relative motion between the source and observer.
- Dynamo effect
—A model for the generation of planetary magnetic fields: The circulation of hot, conducting fluids within a planet's liquid core leads to the generation of a magnetic field.
- Escape velocity
—The speed that an object must have in order to escape the gravitational pull of another body.
- Lobate scarp
—A long, near vertical wall of rock running across a flat plain.
- Mantle
—The outer layers of a planets interiorcore, usually composed of silicate rock.
- Planetesimals
—Asteroid-sized bodies that accumulated to form protoplanets.
- Solar wind
—A stream of charged and neutral particles that emanates from the Sun and moves into the solar system.
- Synchronous rotation
—Any object that spins on its axis at the same rate that it moves along in its orbit is said to be in synchronous rotation. Also called to 1-to-1 spin-orbit coupling.
Mercury
Mercury
Mercury is the innermost and second smallest planet (4,878 kilometers [3,024 miles] in diameter) in the solar system (Pluto is the smallest). It has no known moons. As of the beginning of the twenty-first century, Mariner 10 had been the only spacecraft to explore the planet. It flew past Mercury on March 29 and September 21, 1974, and on March 16, 1975. Mariner 10 imaged only about 45 percent of the surface and only in moderate detail. As a consequence, there are still many questions concerning the history and evolution of Mercury. Two new missions to Mercury will be launched this decade. An American mission called MESSENGER will be launched in March 2004. It will make two flybys of Venus and two of Mercury before going into Mercury orbit in April 2009. A European mission called Bepi Colombo, after a famous Italian celestial dynamicist, is scheduled for launch in 2009.
Motion and Temperature
Mercury has the most elliptical and inclined (7 degrees) orbit of any planet except Pluto. Its average distance from the Sun is only 0.38 astronomical unit (AU). Because of its elliptical orbit, however, the distance varies from 0.3 AU when it is closest to the Sun to 0.46 AU when it is farthest away. Mercury's orbital velocity is the greatest in the solar system and averages 47.6 kilometers per second (29.5 miles per second). When it is closest to the Sun, however, it travels 56.6 kilometers per second (35.1 miles per second), and when it is farthest away it travels 38.7 kilometers per second (24 miles per second).
Mercury's rotational period is 58.6 Earth days and its orbital period is 87.9 Earth days. It has a unique relationship between its rotational and orbital periods: It rotates exactly three times on its axis for every two orbits around the Sun. Because of this relationship, a solar day (sunrise to sunrise) lasts two Mercurian years, or 176 Earth days.
Because Mercury is so close to the Sun, has no insulating atmosphere, and has such a long solar day, it experiences the greatest daily range in surface temperatures (633°C [1,171°F]) of any planet or moon in the solar system. Mercury's maximum surface temperature is about 450°C (842°F) at the equator when it is closest to the Sun, but drops to about -183°C (-297°F) at night.
Interior and Magnetic Field
Mercury's internal structure is unique in the solar system. Mercury's small size and relatively large mass (3.3 × 1023 kilograms [7.3 × 1023 pounds]) means that it has a very large density of 5.44 grams per cubic centimeters (340 pounds per cubic foot), which is only slightly less than Earth's (5.52 grams per cubic centimeter [345 pounds per cubic foot]) and larger than Venus's (5.25 grams per cubic centimeter [328 pounds per cubic foot]). Because of Earth's large internal pressures, however, its uncompressed density is only 4.4 grams per cubic centimeter (275 pounds per cubic foot), compared to Mercury's uncompressed density of 5.3 grams per cubic centimeter (331 pounds per cubic foot). This means that Mercury contains a much larger fraction of iron than any other planet or moon in the solar system. The iron core must be about 75 percent of the planet diameter, or some 42 percent of its volume. Thus, its rocky outer region is only about 600 kilometers (370 miles) thick.
Mercury is the only terrestrial planet , aside from Earth, with a significant magnetic field. The maintenance of terrestrial planet magnetic fields is thought to require an electrically conducting fluid outer core surrounding a solid inner core. Therefore, Mercury's magnetic field suggests that Mercury currently has a fluid outer core of unknown thickness.
Exosphere
Mercury has an extremely tenuous atmosphere with a surface pressure a trillion times less than Earth's. This type of tenuous atmosphere is called an exosphere because atoms in it rarely collide. Mariner 10 identified the presence of hydrogen, helium, and oxygen in the atmosphere and set upper limits on the abundance of argon. These elements are probably derived from the solar wind . Later Earth-based telescopic observations detected sodium and potassium in quantities greater than the elements previously known. Sodium and potassium could be released from surface rocks by their interaction with solar radiation or by impact vaporization of micrometeoroid material. Both sodium and potassium show day-to-day changes in their global distribution.
Polar Deposits
High-resolution radar observations show highly reflective material concentrated in permanently shadowed portions of craters at the polar regions. These deposits have the same radar characteristics as water ice. Mercury's rotation axis is almost perpendicular to its orbit, and therefore Mercury does not experience seasons. Thus, temperatures in permanently shaded polar areas should be less than -161°C (-258°F). At this temperature, water ice is stable, that is, it is not subject to evaporation for billions of years. If the deposits are water ice, they could originate from comet or water-rich asteroid impacts that released the water, which was then cold-trapped in the permanently shadowed craters. Sulfur has also been suggested as a possible material for these deposits.
Geology and Composition
In general, the surface of Mercury can be divided into four major terrains: heavily cratered regions, intercrater plains , smooth plains , and hilly and lineated terrain. The heavily cratered uplands record the period of heavy meteoroid bombardment that ended about 3.8 billion years ago.
The largest relatively fresh impact feature seen by Mariner 10 is the Caloris basin, which has a diameter of 1,300 kilometers (806 miles). The floor structure consists of closely spaced ridges and troughs.
Directly opposite the Caloris basin (the antipodal point) is the unusual hilly and lineated terrain that disrupts preexisting landforms, particularly crater rims (see top image on following page). The hilly and lineated terrain is thought to be the result of seismic waves generated by the Caloris impact and focused at the antipodal region.
Mercury's two plains units have been interpreted to be old lava flows. The older intercrater plains are the most extensive terrain on Mercury (see bottom image on this page). The intercrater plains were created during the period of late heavy meteoroid bombardment. They are thought to be volcanic plains erupted through a fractured crust. They are probably about 4 to 4.2 billion years old.
The younger smooth plains are primarily associated with large impact basins. The largest occurrence of smooth plains fill and surround the Caloris basin, and occupy a large circular area in the north polar region that is probably an old impact basin about 1,500 kilometers (930 miles) in diameter. They are similar to the lunar maria and therefore are believed to be lava flows that erupted relatively late in Mercurian history. They may have an average age of about 3.8 billion years. If so, they are, in general, older than the lunar maria.
Three large radar-bright anomalies have been identified on the unimaged side of Mercury. High-resolution radar observations indicate that two of these are similar to the radar signature of a fresh impact crater, and another has a radar signature unlike any other in the solar system. One or both of these craters could account for the polar deposits if they were the result of comets or water-rich asteroid impacts.
Mercury displays a system of compressive faults (or thrust faults ) called lobate scarps . They are more-or-less uniformly distributed over the part of Mercury viewed by Mariner 10. Presumably they occur on a global scale. This suggests that Mercury has shrunk. Stratigraphic evidence indicates that the faults formed after the intercrater plains relatively late in Mercurian history. The faults were probably caused by a decrease in Mercury's size due to cooling of the planet. The amount of radius decrease is estimated to have been about 2 kilometers (1.2 miles).
Very little is known about the surface composition of Mercury. A new color study of Mariner 10 images has been used to derive some compositional information of the surface over some of the regions viewed by Mariner 10. The smooth plains have an iron content of less than 6 percent by weight, which is similar to the rest of the regions imaged by Mariner 10. The surface of Mercury, therefore, may have a more homogeneous distribution of elements that affect color than does the Moon. At the least, the smooth plains may be low-iron basalts . The MESSENGER mission is designed to accurately determine the composition of the surface.
Geologic History
Knowledge about Mercury's earliest history is very uncertain. The earliest known events are the formation of the intercrater plains (more than 4 billion years ago) during the period of heavy meteoroid bombardment. These plains may have been erupted through fractures caused by large impacts in a thin crust. Near the end of heavy bombardment the Caloris basin was formed by a large impact that caused the hilly and lineated terrain from seismic waves focused at the antipodal region. Eruption of lava within and surrounding the large basins formed the smooth plains about 3.8 billion years ago. The system of lobate scarps formed after the intercrater plains, and resulted in a planetary radius decrease of about 2 kilometers (1.2 miles). Scientists will have to await the results of the MESSENGER and Colombo missions to fully evaluate the geologic history of Mercury.
Origins
How Mercury acquired such a large fraction of iron compared to the other terrestrial planets is not well determined. Three hypotheses have been put forward to explain the enormous iron core. One involves an enrichment of iron due to dynamical processes in the innermost part of the solar system. Another proposes that intense bombardment by solar radiation in the earliest phases of the Sun's evolution vaporized and drove off much of the rocky fraction of Mercury, leaving the core intact. A third proposes that a planetsized object impacted Mercury and blasted away much of the planet's rocky mantle, again leaving the iron core largely intact. Discriminating among these hypotheses may be possible from the chemical makeup of the surface because each one predicts a different composition. MESSENGER is designed to measure the composition of Mercury's surface, so it may be possible to answer this vital question in the near future.
see also Exploration Programs (volume 2); Planetary Exploration, Future of (volume 2); Robotic Exploration of Space (volume 2).
Robert G. Strom
Bibliography
"The Planet Mercury: Mariner 10 Mission." (various papers and authors) Journal of Geophysical Research 80, no. 17 (1975): 2342-2514.
Strom, Robert G. Mercury: The Elusive Planet. Washington, DC: Smithsonian Institution Press, 1987.
——. "Mercury: An Overview." Advances in Space Research 19, no. 10 (1997):1,471-1,485.
——. "Mercury." In Encyclopedia of the Solar System, eds. Weissman, P. R., L. Mc-Fadden, and T. V. Johnson. San Diego: Academic Press, 1999.
Villas, Faith, Clark R. Chapman, and Mildred S. Matthews, eds. Mercury. Tucson:University of Arizona Press, 1988.
Mercury (Planet)
Mercury (planet)
Mercury, the closest object to the Sun, is a small, bleak planet. Because of the Sun's intense glare, it is difficult to observe Mercury from Earth. Mercury is visible just above the horizon for only about one hour before sunrise and one hour after sunset.
Mercury is named for the Roman messenger god with winged sandals. The planet was so named because it orbits the Sun quickly, in just 88 days. In contrast to its short year, Mercury has an extremely long day. It takes the planet the equivalent of 59 Earth days to complete one rotation.
Mercury is the second smallest planet in the solar system (only Pluto is smaller). Mercury's diameter is about 3,000 miles (4,800 kilometers), yet it has just 5.5 percent of Earth's mass. (Earth's diameter is about 7,900 miles [12,720 kilometers].) On average, Mercury is 36 million miles (58 million kilometers) from the Sun. The Sun's intense gravitational field tilts Mercury's orbit and stretches it into a long ellipse (oval).
The Mariner exploration
Little else was known about Mercury until the U.S. space probe Mariner 10 photographed the planet in 1975. Mariner first approached the planet Venus in February 1974, then used that planet's gravitational field to send it around like a slingshot in the direction of Mercury. The second leg of the journey to Mercury took seven weeks.
On its first flight past Mercury, Mariner 10 came within 470 miles (756 kilometers) of the planet and photographed about 40 percent of its surface. The probe then went into orbit around the Sun and flew past Mercury twice more in the next year before running out of fuel.
Mariner 10 collected much valuable information about Mercury. It found that the planet's surface is covered with deep craters, separated by plains and huge banks of cliffs. Mercury's most notable feature is an ancient crater called the Caloris Basin, about the size of the state of Texas.
Astronomers believe that Mercury, like the Moon, was originally made of liquid rock that solidified as the planet cooled. Some meteorites hit the planet during its cooling stage and formed craters. Other meteorites,
however, broke through the cooling crust, causing lava to flow up to the surface and cover older craters, forming the plains.
Mercury's very thin atmosphere is made of sodium, potassium, helium, and hydrogen. Temperatures on Mercury reach 800°F (427°C) during its long day and −278°F (−173°C) during its long night. This temperature variation, the largest experienced by any planet in the solar system, is due to the fact that Mercury has essentially no insulating atmosphere to transport the Sun's heat from the day side to the night side.
Mariner 10 also gathered information about Mercury's core, which is nearly solid metal and is composed primarily of iron and nickel. This core, the densest of any in the solar system, accounts for about four-fifths of Mercury's diameter. It may also be responsible for creating the magnetic field that protects Mercury from the Sun's harsh particle wind.
Discovery of water on Mercury
Perhaps one of the most surprising discoveries in recent times was that of ice at Mercury's poles. The finding was made in 1991 when scientists bounced powerful radar signals off the planet's surface. Scientists had previously believed that any form of water on Mercury would rapidly evaporate given the planet's high daytime temperatures.
The polar regions of Mercury are never fully illuminated by the Sun, and it appears that ice managed to collect in the permanently shadowed regions of many polar crater rims. It is not clear where the ice came from, but scientists believe comet crashes may be one source.
Future exploration
In 2004, the National Aeronautics and Space Administration (NASA) plans to launch the $286 million MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) spacecraft. It will reach Mercury five years later, enter orbit, then examine the planet's atmosphere and entire surface for one Earth year with a suite of detectors including cameras, spectrometers, and a magnetometer. MESSENGER will also explore Mercury's atmosphere and determine the size of the planet's core and how much of it is solid. Finally, the spacecraft will try to confirm whether water ice exists in polar craters on Mercury.
The European Space Agency also has ambitious plans to explore Mercury. At some future date, it proposes to send a trio of spacecraft called BepiColombo that, like MESSENGER, will study the planet's atmosphere and search for water ice in polar craters. BepiColombo will include two satellites and a vehicle that will land on the surface, deploying a tiny, tethered rover to gather information.