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== Physical characteristics ==
== 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.
[[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.
Io is much less heavily cratered than are the other three Galilean satellites. In [[Uniformitarianism|uniformitarian]] terms, therefore, Io is considered "young," in contrast to the much "older" [[Ganymede]].<ref name=Arnett>Arnett, Bill. "[http://www.nineplanets.org/io.html Entry for Io]." ''The <s>Nine</s> 8 Planets'', January 10, 2001. Accessed February 19, 2008</ref>
=== Interior ===
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>
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 ===
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/>
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.
[[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/>
Yet the tremendous heat of Io's volcanoes is not fully understood or explained. If uniformitarian theories are correct and Io is millions of years old, the innumerable tidal-flexing cycles should have lowered Io's melting point so that it would not produce such hot lava flows. The high temperatures observed are in fact consistent with basaltic lava flows that most uniformitarians think occurred billions of years ago on [[Earth]]. No volcano on Earth today is as hot as are some of the volcanoes now on Io.<ref name=Phillips>Phillips, Tony. "[http://science.nasa.gov/newhome/headlines/ast04oct99_1.htm Io's Alien Volcanoes: Galileo Heads for a Sizzling Encounter]." ''Space Science News'', October 4, 1999. Accessed February 18, 2008.</ref>
=== 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.
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/>
=== Atmosphere ===
[[Image:Gal io1 47935.jpg|thumb|200px|left|False-color grid and grayscale image of Io showing volcanic hot spots and auroras]]Io has no atmosphere in the traditional sense, despite the tremendous amounts of [[sulfur dioxide]] and other gases that Io's many volcanoes release. Io does retain trace amounts of sulfur dioxide at a pressure of one micro-atmosphere or less. This is thick enough to generate [[aurora]]s as the radiation from the Jovian radiation belt strikes the gas, but is not thick enough to have any refractive properties that would be detectable with stellar occultation.
The most likely explanations are Io's deep cold and Jupiter's magnetic field. Most of the released sulfur dioxide falls back to the surface as frost. The rest is stripped away as Io moves through Jupiter's magnetic field and radiation belt.<ref name=solarviews3/>
== Problems for Uniformitarianism Posed by Io ==
* Io is radiating more heat than [[tidal heating]] could generate. Therefore Io must have another source of heat, or else the observed heat flow is unusually high and not something that Io could maintain for long.<ref name=Arnett/> And even in the latter case, stating that the current heat flow is unusual begs the question of what drove the heat flow to present levels.
* As mentioned before, the volcanic eruptions on Io have one-third the absolute temperature at the surface of the [[Sun]]. This defies explanation, because according to all current uniformitarian theories, Io's mantle ought to melt at far lower temperatures after millions of years of constant reworking by tidal flexes.<ref name=Phillips/>
== Observation and Exploration ==
== Observation and Exploration ==