Difference between revisions of "Io"
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| − | [[ | + | {{Planet |
| + | | image=Io galileo 19951217 sc 0-002-005.jpg | ||
| + | | caption=True-color image of Io, from [[Galileo Project|Galileo]] spacecraft | ||
| + | | date=January 7, 1610<ref name=disc>"[http://www.solarviews.com/eng/galdisc.htm The Discovery of the Galilean Satellites]," [[JPL]], [[NASA]], n.d. Quoted by Hamilton, Calvin J., ''[http://www.solarviews.com/ SolarViews].'' Accessed February 18, 2008.</ref> | ||
| + | | discname=[[Galileo Galilei]], [[Simon Marius]]<ref name=disc/> | ||
| + | | origname=Argive princess and mistress of [[Zeus]] | ||
| + | | primary=Jupiter | ||
| + | | order=5 | ||
| + | | periapsis=420,071 km<ref name=calc>Calculated</ref> | ||
| + | | apoapsis=423,529 km<ref name=calc/> | ||
| + | | semimajor=421,800 km<ref name=ssdelem>"[http://ssd.jpl.nasa.gov/?sat_elem Planetary Satellite Mean Orbital Parameters]," Solar System Dynamics, [[JPL]], [[NASA]]. Accessed February 18, 2008.</ref> | ||
| + | | eccentricity=0.0041<ref name=ssdelem/> | ||
| + | | sidereal=1.769 da<ref name=ssdelem/> | ||
| + | | orbitspeed=17.34 km/s<ref name=solarviews>Hamilton, Calvin J. "[http://www.solarviews.com/eng/ganymede.htm Entry for Io]." ''SolarViews'', 2001. Accessed February 18, 2008.</ref> | ||
| + | | inclination=0.040°<ref name=solarviews/> | ||
| + | | siderealday=1.769 da<ref name=ssdelem/> | ||
| + | | rotatespeed=74.88 m/s<ref name=calc/> | ||
| + | | mass=8.93194 * 10<sup>22</sup> kg (1.49464% earth)<ref name=calc/> | ||
| + | | density=3,528 kg/m³<ref name=ssdphys>"[http://ssd.jpl.nasa.gov/?sat_phys_par Planetary Satellite Physical Parameters]." Solar System Dynamics, [[JPL]], [[NASA]]. Accessed February 16, 2008.</ref> | ||
| + | | meanradius=1,821.6 km<ref name=ssdphys/> | ||
| + | | surfacegrav=1.79611 m/s² (0.183152 ''g'')<ref name=calc/> | ||
| + | | escapespeed=2.55805 km/s<ref name=calc/> | ||
| + | | surfacearea=41,698,065 km² (8.17504% earth)<ref name=calc/> | ||
| + | | meantemp=130 K<ref name=solarviews/> | ||
| + | | composition=silicate rock and iron | ||
| + | | color=Yellow | ||
| + | | albedo=0.63<ref name=ssdphys/> | ||
| + | }}'''Io''' is the innermost of the Galilean moons of [[Jupiter]]. It is the most [[volcano| volcanically]] active object in the [[solar system]] and the most dense of all [[planet]]ary satellites. Io was named for an Argive princess and priestess of [[Hera]] who became a fairly prominent mistress of [[Zeus]], after much suffering. That name, assigned quite by accident in the seventeenth century, might be quite apt today in view of Io's volcanic activity and the likely reason for it. | ||
| − | '''Io''', | + | == Discovery and naming == |
| + | [[Galileo Galilei]] observed Jupiter beginning on January 7, 1610, with his famous telescope. He at first thought he had discovered three stars near Jupiter, but on the next night those "stars" seemed to have moved. (Those "stars" were actually Io, [[Europa]], and [[Callisto]].) He continued to observe Jupiter and its companion "stars" for seven days, during which time a fourth "star" ([[Ganymede]]) appeared and all four of these objects appeared to move with Jupiter. Finally he concluded that these objects were not stars at all, but satellites of Jupiter. This was the first direct observation that provided evidence for [[Nicolaus Copernicus]]'s heliocentric model of the solar system.<ref name=disc/> | ||
| + | |||
| + | [[Simon Marius]] claimed to observe Jupiter and these satellites independently of Galileo and beginning five weeks earlier. However, he did not ''publish'' his findings, while Galileo published his. Furthermore, Galileo's notes were more reliable and extensive than those of Marius, which is why Ganymede and the three other satellites he observed ([[Io]], [[Europa]], and [[Callisto]]) are called the Galilean satellites and not the Galilean-Marian satellites.<ref name=disc/> | ||
| + | |||
| + | Marius does, however, receive credit for providing the names that the satellites have today. He named Ganymede, the largest, for the mythical son of King [[Tros]] of [[Troy]], carried aloft to [[Mount Olympus]] by [[Zeus]] to be the cupbearer to the gods. (The names ''Zeus'' and ''Jupiter'' refer to the same classical deity from whom Jupiter gets its name.) The other three satellites are named for three of Zeus' most famous mistresses.<ref name=disc/> Marius propounded these names after [[Johannes Kepler]] suggested them to him. | ||
| + | |||
| + | Galileo, for his part, called them the "Medicean planets" after the [[Medici]] family and simply numbered the moons I, II, III, and IV. The satellites carried these names for two centuries until the discovery of other moons of Jupiter made that naming system untenably confusing. | ||
| + | |||
| + | == Orbital and rotational characteristics == | ||
| + | [[Image:Galilean moon Laplace resonance animation.gif|thumb|150 px|left|Orbital resonance of Io, Europa, and Ganymede]]Io is fifth in order of all of Jupiter's moons, and orbits Jupiter at a semi-major axis of 421,800 km. Its orbit is very slightly eccentric and slightly inclined to Jupiter's equator. Io is in [[tidal lock]] (thus keeping the same face toward Jupiter as it revolves). | ||
| + | |||
| + | Io maintains a three-part [[orbital resonance]] or Laplace resonance with Europa and Ganymede. These three satellites complete their orbits around Jupiter in the ratio 1:2:4. They also conjoin with one another at their apsides, in a mutually self-correcting manner that allows the resonance to persist. (Most such resonances are unstable and end with one or all bodies changing their orbits.) | ||
| + | |||
| + | The origin of the resonance itself is controversial. Peale and Lee, in 2002, presented a model suggesting that this resonance is primordial and thus was part of the Jovian system since its formation.<ref name=Peale>Peale, S. J., and Lee, Man Hoi. "[http://adsabs.harvard.edu/abs/2002Sci...298..593P A Primordial Origin of the Laplace Relation Among the Galilean Satellites]." ''Science'', 298(5593):593-597, October 2002. Accessed February 18, 2008.</ref> Their model, however, derives from the [[nebular hypothesis]] of the formation of the solar system and thus depends on [[uniformitarianism|uniformitarian]] assumptions. Showman and Malhotra suggest a model by which the resonance developed after the solar system had formed.<ref name=Showman/> | ||
| + | |||
| + | This orbital resonance, together with tidal stresses imposed by Jupiter itself, is widely held to be the most likely cause of Io's volcanism (see below). | ||
| + | |||
| + | == Physical characteristics == | ||
| + | [[Image:Io interior.jpg|thumb|150px|left|Io cutaway]]Io is slightly more massive than is [[Earth]]'s [[Moon]], though smaller in radius. These two factors, a consequence of Io's greater density, give Io a slightly higher surface gravity than the Moon has. | ||
| + | |||
| + | === Volcanism === | ||
| + | Io's most striking physical characteristic is its tremendous volcanic activity. The volcanoes of Io, which [[Voyager 1]] helped discover in 1979, were the first extraterrestrial volcanoes ever discovered.<ref name=Lee>Lee, Gentry. "[http://www.space.com/opinionscolumns/gentrylee/gentrylee_column_000705.html Spectacular Io]." ''[http://www.space.com Space.com]]'', July 7, 2000. Accessed February 18, 2008.</ref> | ||
| + | |||
| + | == Observation and Exploration == | ||
| + | Until the advent of space flight, observations of Io were perforce limited to those by Earth-based telescopes. Galileo Galilei had shown that Io was present, of course, but only in the late 19th and 20th centuries did telescope technology allow for any resolution of surface features on Io. Astronomers like [[Edward E. Bernard]] noted the differing brightness between Io's equatorial and polar regions and finally determined that they were strictly due to differences in color and albedo (a measure of surface reflectivity), and not because Io had an odd shape and not because Io was in fact two companion objects.<ref name=Dobbins>Dobbins, T., and Sheehan, W. "The Story of Jupiter's Egg Moons". ''Sky & Telescope'' 107 (1):114–120, 2004.</ref><ref name=Barnard1>Barnard, Edward E. "[http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1894MNRAS..54..134B&db_key=AST&data_type=HTML&format=&high=45f9f8ee3007558 On the dark poles and bright equatorial belt of the first satellite of Jupiter]." ''Monthly Notices of the Royal Astronomical Society'', 54:134, January 1894. Accessed February 18, 2008.</ref><ref name=Barnard2>Barnard, Edwin E. "[http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1891MNRAS..51..543B&db_key=AST&data_type=HTML&format=&high=45f9f8ee3010624 Observations of the Planet Jupiter and his Satellites during 1890 with the 12-inch Equatoreal of the Lick Observatory]." ''Monthly Notices of the Royal Astronomical Society'', 51:543, June 1890. Accessed February 18, 2008.</ref> More detailed observations followed, as recently (to the Voyager missions) as 1973.<ref name=Minton>Minton, R. B. "[http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1973CoLPL..10...35M&db_key=AST&data_type=HTML&format=&high=45f9f8ee3013123 The Polar Caps of Io]." ''Communications of the Lunar and Planetary Laboratory'', 10:35-39, 1973. Accessed February 18, 2008.</ref> | ||
| + | |||
| + | The Pioneer missions 10 and 11 flew by Io in 1973 and 1974. They revealed little beyond a better estimate of Io's mass, and one good image from Pioneer 11. The heavy radiation in the torus of radioactive particles that surrounds Io compromised Pioneer's instruments, and all its observations were lost. | ||
| + | |||
| + | [[Voyager 1]] established Io's volcanism. Moreover, [[Voyager 2]], which passed Io five months later, demonstrated pronounced surface changes that showed that the volcanoes were highly active and constantly reforming Io's surface. | ||
| + | |||
| + | The [[Galileo Project]] spacecraft did not make any close passes at Io for years, primarily because mission controllers were concerned with the safety of the spacecraft and desired to examine several far less dangerous targets. The mission was extended twice, however, and in the last years of the mission, ''Galileo'' made several flybys of Io and took surface temperature measurements. These measurements clearly showed that Io's volcanoes tended to be hotter even than many volcanoes on Earth. | ||
| + | |||
| + | In September of 2003, the Galileo controllers deliberately crashed the spacecraft on Jupiter, in order to avoid crashing it on [[Europa]] and possibly doing irreparable damage to Europa's subsurface ocean. Since that time, observations have been limited largely to Earth-based telescopes (like the Keck Observatory in [[Hawaii]] and the [[Hubble Space Telescope]]--except that the [[New Horizons mission]] made a flyby of Io on February 28, 2007, and captured many remarkable images. No other missions to Io are planned for at least ten more years. | ||
==References== | ==References== | ||
| + | {{reflist|2}} | ||
| + | ==Related reference== | ||
John Gribbin, ''Companion to the Cosmos'' (Little, Brown & Company, 1996) | John Gribbin, ''Companion to the Cosmos'' (Little, Brown & Company, 1996) | ||
{{Solarsystem}} | {{Solarsystem}} | ||
| − | |||
Revision as of 03:51, February 19, 2008
| Io | |
|---|---|
 True-color image of Io, from Galileo spacecraft | |
| Date of discovery | January 7, 1610[1] |
| Name of discoverer | Galileo Galilei, Simon Marius[1] |
| Name origin | Argive princess and mistress of Zeus |
| Orbital characteristics | |
| Primary | Jupiter |
| Order from primary | 5 |
| Perizene | 420,071 km[2] |
| Apozene | 423,529 km[2] |
| Semi-major axis | 421,800 km[3] |
| Orbital eccentricity | 0.0041[3] |
| Sidereal month | 1.769 da[3] |
| Avg. orbital speed | 17.34 km/s[4] |
| Inclination | 0.040°[4] to Jupiter's equator |
| Rotational characteristics | |
| Sidereal day | 1.769 da[3] |
| Rotational speed | 74.88 m/s[2] |
| Physical characteristics | |
| Mass | 8.93194 * 1022 kg (1.49464% earth)[2] |
| Density | 3,528 kg/m³[5] |
| Mean radius | 1,821.6 km[5] |
| Surface gravity | 1.79611 m/s² (0.183152 g)[2] |
| Escape speed | 2.55805 km/s[2] |
| Surface area | 41,698,065 km² (8.17504% earth)[2] |
| Mean temperature | 130 K[4] |
| Composition | silicate rock and iron |
| Color | Yellow |
| Albedo | 0.63[5] |
Io is the innermost of the Galilean moons of Jupiter. It is the most volcanically active object in the solar system and the most dense of all planetary satellites. Io was named for an Argive princess and priestess of Hera who became a fairly prominent mistress of Zeus, after much suffering. That name, assigned quite by accident in the seventeenth century, might be quite apt today in view of Io's volcanic activity and the likely reason for it.
Discovery and naming
Galileo Galilei observed Jupiter beginning on January 7, 1610, with his famous telescope. He at first thought he had discovered three stars near Jupiter, but on the next night those "stars" seemed to have moved. (Those "stars" were actually Io, Europa, and Callisto.) He continued to observe Jupiter and its companion "stars" for seven days, during which time a fourth "star" (Ganymede) appeared and all four of these objects appeared to move with Jupiter. Finally he concluded that these objects were not stars at all, but satellites of Jupiter. This was the first direct observation that provided evidence for Nicolaus Copernicus's heliocentric model of the solar system.[1]
Simon Marius claimed to observe Jupiter and these satellites independently of Galileo and beginning five weeks earlier. However, he did not publish his findings, while Galileo published his. Furthermore, Galileo's notes were more reliable and extensive than those of Marius, which is why Ganymede and the three other satellites he observed (Io, Europa, and Callisto) are called the Galilean satellites and not the Galilean-Marian satellites.[1]
Marius does, however, receive credit for providing the names that the satellites have today. He named Ganymede, the largest, for the mythical son of King Tros of Troy, carried aloft to Mount Olympus by Zeus to be the cupbearer to the gods. (The names Zeus and Jupiter refer to the same classical deity from whom Jupiter gets its name.) The other three satellites are named for three of Zeus' most famous mistresses.[1] Marius propounded these names after Johannes Kepler suggested them to him.
Galileo, for his part, called them the "Medicean planets" after the Medici family and simply numbered the moons I, II, III, and IV. The satellites carried these names for two centuries until the discovery of other moons of Jupiter made that naming system untenably confusing.
Orbital and rotational characteristics
Io is fifth in order of all of Jupiter's moons, and orbits Jupiter at a semi-major axis of 421,800 km. Its orbit is very slightly eccentric and slightly inclined to Jupiter's equator. Io is in tidal lock (thus keeping the same face toward Jupiter as it revolves).
Io maintains a three-part orbital resonance or Laplace resonance with Europa and Ganymede. These three satellites complete their orbits around Jupiter in the ratio 1:2:4. They also conjoin with one another at their apsides, in a mutually self-correcting manner that allows the resonance to persist. (Most such resonances are unstable and end with one or all bodies changing their orbits.)
The origin of the resonance itself is controversial. Peale and Lee, in 2002, presented a model suggesting that this resonance is primordial and thus was part of the Jovian system since its formation.[6] Their model, however, derives from the nebular hypothesis of the formation of the solar system and thus depends on uniformitarian assumptions. Showman and Malhotra suggest a model by which the resonance developed after the solar system had formed.[7]
This orbital resonance, together with tidal stresses imposed by Jupiter itself, is widely held to be the most likely cause of Io's volcanism (see below).
Physical characteristics
Io is slightly more massive than is Earth's Moon, though smaller in radius. These two factors, a consequence of Io's greater density, give Io a slightly higher surface gravity than the Moon has.
Volcanism
Io's most striking physical characteristic is its tremendous volcanic activity. The volcanoes of Io, which Voyager 1 helped discover in 1979, were the first extraterrestrial volcanoes ever discovered.[8]
Observation and Exploration
Until the advent of space flight, observations of Io were perforce limited to those by Earth-based telescopes. Galileo Galilei had shown that Io was present, of course, but only in the late 19th and 20th centuries did telescope technology allow for any resolution of surface features on Io. Astronomers like Edward E. Bernard noted the differing brightness between Io's equatorial and polar regions and finally determined that they were strictly due to differences in color and albedo (a measure of surface reflectivity), and not because Io had an odd shape and not because Io was in fact two companion objects.[9][10][11] More detailed observations followed, as recently (to the Voyager missions) as 1973.[12]
The Pioneer missions 10 and 11 flew by Io in 1973 and 1974. They revealed little beyond a better estimate of Io's mass, and one good image from Pioneer 11. The heavy radiation in the torus of radioactive particles that surrounds Io compromised Pioneer's instruments, and all its observations were lost.
Voyager 1 established Io's volcanism. Moreover, Voyager 2, which passed Io five months later, demonstrated pronounced surface changes that showed that the volcanoes were highly active and constantly reforming Io's surface.
The Galileo Project spacecraft did not make any close passes at Io for years, primarily because mission controllers were concerned with the safety of the spacecraft and desired to examine several far less dangerous targets. The mission was extended twice, however, and in the last years of the mission, Galileo made several flybys of Io and took surface temperature measurements. These measurements clearly showed that Io's volcanoes tended to be hotter even than many volcanoes on Earth.
In September of 2003, the Galileo controllers deliberately crashed the spacecraft on Jupiter, in order to avoid crashing it on Europa and possibly doing irreparable damage to Europa's subsurface ocean. Since that time, observations have been limited largely to Earth-based telescopes (like the Keck Observatory in Hawaii and the Hubble Space Telescope--except that the New Horizons mission made a flyby of Io on February 28, 2007, and captured many remarkable images. No other missions to Io are planned for at least ten more years.
References
- ↑ 1.0 1.1 1.2 1.3 1.4 "The Discovery of the Galilean Satellites," JPL, NASA, n.d. Quoted by Hamilton, Calvin J., SolarViews. Accessed February 18, 2008.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Calculated
- ↑ 3.0 3.1 3.2 3.3 "Planetary Satellite Mean Orbital Parameters," Solar System Dynamics, JPL, NASA. Accessed February 18, 2008.
- ↑ 4.0 4.1 4.2 Hamilton, Calvin J. "Entry for Io." SolarViews, 2001. Accessed February 18, 2008.
- ↑ 5.0 5.1 5.2 "Planetary Satellite Physical Parameters." Solar System Dynamics, JPL, NASA. Accessed February 16, 2008.
- ↑ Peale, S. J., and Lee, Man Hoi. "A Primordial Origin of the Laplace Relation Among the Galilean Satellites." Science, 298(5593):593-597, October 2002. Accessed February 18, 2008.
- ↑ Cite error: Invalid
<ref>tag; no text was provided for refs namedShowman - ↑ Lee, Gentry. "Spectacular Io." Space.com], July 7, 2000. Accessed February 18, 2008.
- ↑ Dobbins, T., and Sheehan, W. "The Story of Jupiter's Egg Moons". Sky & Telescope 107 (1):114–120, 2004.
- ↑ Barnard, Edward E. "On the dark poles and bright equatorial belt of the first satellite of Jupiter." Monthly Notices of the Royal Astronomical Society, 54:134, January 1894. Accessed February 18, 2008.
- ↑ Barnard, Edwin E. "Observations of the Planet Jupiter and his Satellites during 1890 with the 12-inch Equatoreal of the Lick Observatory." Monthly Notices of the Royal Astronomical Society, 51:543, June 1890. Accessed February 18, 2008.
- ↑ Minton, R. B. "The Polar Caps of Io." Communications of the Lunar and Planetary Laboratory, 10:35-39, 1973. Accessed February 18, 2008.
Related reference
John Gribbin, Companion to the Cosmos (Little, Brown & Company, 1996)
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