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}}'''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''' 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.

== Discovery and naming ==

== 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 (moon)|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/>

[[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 (moon)|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/>

[[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.

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.

== Orbital and rotational characteristics ==

== 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).

[[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&nbsp;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.)

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.)

Io is more dense even than Earth's Moon and in fact is more dense than any other satellite of Jupiter (or any other planet).<ref name=ssdphys/> For that reason, and taking into account mass-concentration and other findings from flybys of all the Galilean satellites, Io is ''not'' suspected of having an underground saltwater ocean. Instead, it likely has a metal core of [[iron]] and/or [[iron sulfide]]. According to the best model available so far, the radius of this core likely extends to half the total radius of Io itself. Surrounding this would be a partially molten silicate-rich mantle with a thin rocky surface crust.<ref name=solarviews/><ref name=Anderson>Anderson, J. D.; ''et al''. "Galileo Gravity Results and the Internal Structure of Io". ''Science'' 272:709–712, 1996.</ref>

Io is more dense even than Earth's Moon and in fact is more dense than any other satellite of Jupiter (or any other planet).<ref name=ssdphys/> For that reason, and taking into account mass-concentration and other findings from flybys of all the Galilean satellites, Io is ''not'' suspected of having an underground saltwater ocean. Instead, it likely has a metal core of [[iron]] and/or [[iron sulfide]]. According to the best model available so far, the radius of this core likely extends to half the total radius of Io itself. Surrounding this would be a partially molten silicate-rich mantle with a thin rocky surface crust.<ref name=solarviews/><ref name=Anderson>Anderson, J. D.; ''et al''. "Galileo Gravity Results and the Internal Structure of Io". ''Science'' 272:709–712, 1996.</ref>

Further evidence that Io has an iron core comes from magnetometric measurements taken by Galileo. The magnetic flux density at Io's equator is about 13 milligauss<ref name=Kevelson/>. This corresponds to a magnetic dipole moment of 7.86 * 10¹⁹ N-m/T. Considering Io's mass, Io likely had a magnetic dipole moment ''at creation'' of 2.1 * 10²² N-m/T, according to [[Russell Humphreys]]' model for the creation of celestial magnetic fields.<ref name=Humphreys>Humphreys, D. R. "[http://www.creationresearch.org/crsq/articles/21/21_3/21_3.html The Creation of Planetary Magnetic Fields]." ''[[Creation Research Society Quarterly]]'' 21(3), December 1984. Accessed April 29, 2008.</ref> This in turn suggests a decay time of 1098 Julian years and a half-life of 761 Julian years. The corresponding decay time and half-life of the magnetic field of [[Ganymede]] agrees with this to within four significant digits, and suggests that both satellites have comparable core radii and conductivities.

Further evidence that Io has an iron core comes from magnetometric measurements taken by Galileo. The magnetic flux density at Io's equator is about 13 milligauss.<ref name=Kevelson/> This corresponds to a magnetic dipole moment of 7.86 * 10¹⁹ N-m/T. Considering Io's mass, Io likely had a magnetic dipole moment ''at creation'' of 2.1 * 10²² N-m/T, according to [[Russell Humphreys]]' model for the creation of celestial magnetic fields.<ref name=Humphreys>Humphreys, D. R. "[http://www.creationresearch.org/crsq/articles/21/21_3/21_3.html The Creation of Planetary Magnetic Fields]." ''[[Creation Research Society Quarterly]]'' 21(3), December 1984. Accessed April 29, 2008.</ref> This in turn suggests a decay time of 1098 Julian years and a half-life of 761 Julian years. The corresponding decay time and half-life of the magnetic field of [[Ganymede]] agrees with this to within four significant digits, and suggests that both satellites have comparable core radii and conductivities.

The marked difference between Io's interior and those of its three outer counterparts has not gone unremarked. [[Uniformitarianism|Uniformitarian]] thinkers suggest that Jupiter was at its hottest early in its initial accretion, and that lighter [[element]]s and [[compound]]s would not have been able to accrete as close to Jupiter as Io's orbit was. In this scenario, Jupiter is held to be the center of a "mini-solar system" with heavier elements and compounds accreting inward and lighter substances accreting outward.<ref name=solarviews/><ref name=Harmsworth>The concept of Jupiter as a proto-[[star]] that somehow failed to ignite is fairly widely held. This model states that Jupiter, composed as it is of [[hydrogen]] and [[helium]], would have ignited had it been sufficiently massive. But in that event, life on [[Earth]] could never have developed. See Harmsworth, Andrew, "[http://www.harmsy.freeuk.com/jupiter.html Entry for Jupiter]," ''Spacetech's Orrery: the Solar Sytem in Action'', last update July 2, 2007. Accessed February 18, 2008.</ref>

The marked difference between Io's interior and those of its three outer counterparts has not gone unremarked. [[Uniformitarianism|Uniformitarian]] thinkers suggest that Jupiter was at its hottest early in its initial accretion, and that lighter [[element]]s and [[compound]]s would not have been able to accrete as close to Jupiter as Io's orbit was. In this scenario, Jupiter is held to be the center of a "mini-solar system" with heavier elements and compounds accreting inward and lighter substances accreting outward.<ref name=solarviews/><ref name=Harmsworth>The concept of Jupiter as a proto-[[star]] that somehow failed to ignite is fairly widely held. This model states that Jupiter, composed as it is of [[hydrogen]] and [[helium]], would have ignited had it been sufficiently massive. But in that event, life on [[Earth]] could never have developed. See Harmsworth, Andrew, "[http://www.harmsy.freeuk.com/jupiter.html Entry for Jupiter]," ''Spacetech's Orrery: the Solar Sytem in Action'', last update July 2, 2007. Accessed February 18, 2008.</ref>

=== Volcanism ===

=== Volcanism ===

[[Image:Gal io1-2 48496.jpg|thumb|350 px|right|Mercator-like projection of Io showing active volcanoes]]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>

[[Image:Gal io1-2 48496.jpg|thumb|350 px|right|Mercator-like projection of Io showing active volcanoes]]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>

Io has hundreds of active volcanoes, and their eruptions and lava flows are constantly changing the surface of Io. The active eruption sites achieve temperatures as high as 1800 K, this although the average surface temperature of Io is about 130 K.<ref name=solarviews/>

Io has hundreds of active volcanoes, and their eruptions and lava flows are constantly changing the surface of Io. The active eruption sites achieve temperatures as high as 1800 K, this although the average surface temperature of Io is about 130 K.<ref name=solarviews/>

The favored theory for explaining this volcanism is [[tidal heating]]. The [[orbital resonance]] that Io has with [[Ganymede]] and especially [[Europa]], combined with the eccentricity of Io's own orbit around [[Jupiter]], combine to flex Io's crust by as much as 100 meters between Io's perizene and apozene.<ref name=solarviews2>Hamilton, Calvin J. "[http://www.solarviews.com/eng/iovolcano.htm Io's Volcanic Features]." ''Solarviews''. Accessed February 19, 2008.</ref><ref name=Arnett/> The resonance, furthermore, helps maintain the eccentricity and also prevent Io from receding from Jupiter.

The favored theory for explaining this volcanism is [[tidal heating]]. The [[orbital resonance]] that Io has with [[Ganymede]] and especially [[Europa]], combined with the eccentricity of Io's own orbit around [[Jupiter]], combine to flex Io's crust by as much as 100 meters between Io's perizene and apozene.<ref name=Arnett/><ref name=solarviews2>Hamilton, Calvin J. "[http://www.solarviews.com/eng/iovolcano.htm Io's Volcanic Features]." ''Solarviews''. Accessed February 19, 2008.</ref> The resonance, furthermore, helps maintain the eccentricity and also prevent Io from receding from Jupiter.

[[Image:Gal io3 48584.jpg|thumb|250 px|left|Sodium cloud released by volcano Prometheus on Io]]The tremendous observed flexing during a sidereal month of less than two Julian days would be expected to heat the mantle tremendously. In fact, Io is known to radiate heat at a power level of about 125 TW on average. This yields a heat flow per surface area of 2.5 W/m².<ref name=solarviews2/>

[[Image:Gal io3 48584.jpg|thumb|250 px|left|Sodium cloud released by volcano Prometheus on Io]]The tremendous observed flexing during a sidereal month of less than two Julian days would be expected to heat the mantle tremendously. In fact, Io is known to radiate heat at a power level of about 125 TW on average. This yields a heat flow per surface area of 2.5 W/m².<ref name=solarviews2/>

=== Mountains ===

=== Mountains ===

Io's mountains average 6 km in altitude, and some of Io's mountains are as high as 16 km, higher even than [[Mount Everest]] on Earth.<ref name=solarviews3>Hamilton, Calvin J. "[http://www.solarviews.com/eng/iomountain.htm Io's Atmosphere, Mountains, and Water Cycle]." ''Solarviews'', accessed February 19, 2008.</ref> Most of these are not volcanoes, because they have no associated lava flow. Most scientists believe that these mountains form by faulting.

Io's mountains average 6&nbsp;km in altitude, and some of Io's mountains are as high as 16&nbsp;km, higher even than [[Mount Everest]] on Earth.<ref name=solarviews3>Hamilton, Calvin J. "[http://www.solarviews.com/eng/iomountain.htm Io's Atmosphere, Mountains, and Water Cycle]." ''Solarviews'', accessed February 19, 2008.</ref> Most of these are not volcanoes, because they have no associated lava flow. Most scientists believe that these mountains form by faulting.

Yet volcano-related depressions, or [[caldera]]s, often surround some of Io's peaks.<ref name=solarviews3/> The locations of many of the calderas violates many accepted assumptions concerning volcanism and mountain formation.<ref name=Myers>Myers, Robert. "[http://www.space.com/scienceastronomy/solarsystem/io_sulfur_snow_001027.html Jovian Cauldron: Io's Volcanoes Revealed in Sharp Detail]." ''[http://www.space.com Space.com]'', October 26, 2000. Accessed February 19, 2008.</ref> Debate on the origins of these calderas continues today, with some observers favoring fractures in Io's crust and others favoring the more traditional explanation of the collapse of vacated magma chambers.<ref name=Myers/>

Yet volcano-related depressions, or [[caldera]]s, often surround some of Io's peaks.<ref name=solarviews3/> The locations of many of the calderas violates many accepted assumptions concerning volcanism and mountain formation.<ref name=Myers>Myers, Robert. "[http://www.space.com/scienceastronomy/solarsystem/io_sulfur_snow_001027.html Jovian Cauldron: Io's Volcanoes Revealed in Sharp Detail]." ''[http://www.space.com Space.com]'', October 26, 2000. Accessed February 19, 2008.</ref> Debate on the origins of these calderas continues today, with some observers favoring fractures in Io's crust and others favoring the more traditional explanation of the collapse of vacated magma chambers.<ref name=Myers/>

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.

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.

In September 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==