Volcanism on Io

:"This is a sub-article to Io (moon)Volcanism on Io, a moon of Jupiter, produces lava flows, volcanic pits, and plumes of sulfur and sulfur dioxide hundreds of kilometres high. This volcanic activity was discovered in 1979 by "Voyager 1" imaging scientists.cite journal |title=Discovery of currently active extraterrestrial volcanism |journal=Science |last=Morabito |first=L. A. |coauthors="et al." |pages=972 |volume=204 |issue= |year=1979 |doi=10.1126/science.204.4396.972] Observations of Io by passing spacecraft, such as the Voyagers, "Galileo", "Cassini", and "New Horizons", and Earth-based astronomers have revealed more than 150 active volcanoes, with as many as 400 expected based on these observations.cite journal |title=Lava lakes on Io: Observations of Io’s volcanic activity from Galileo NIMS during the 2001 fly-bys |journal=Icarus |last=Lopes |first=R. M. C. |coauthors="et al." |pages=140–174 |volume=169 |issue= |year=2004 |doi=10.1016/j.icarus.2003.11.013] Io's volcanism makes the satellite one of only four known volcanically active worlds in the Solar System (the other three being Earth, Saturn's moon Enceladus, and Neptune's moon Triton).

First predicted shortly before the "Voyager 1" flyby, the heat source for Io's volcanism comes from tidal heating produced by Io's forced orbital eccentricity.cite journal |title=Melting of Io by Tidal Dissipation |journal=Science |last=Peale |first=S. J. |coauthors="et al." |pages=892–894 |volume=203 |issue= |year=1979 |doi=10.1126/science.203.4383.892] This differs from the heat source for Earth's volcanism, which comes primarily from radioactive isotope decay.cite web |last=Watson |first=J. M. |date=May 5, 1999 |title=Some Unanswered Questions |publisher=United States Geological Survey |url=http://pubs.usgs.gov/gip/dynamic/unanswered.html|accessdate=2008-10-11] Io's eccentric orbit leads to a slight difference in Jupiter's gravitational pull on Io between its closest and furthest points on its orbit, causing a varying tidal bulge. This variation in the shape of Io causes frictional heating in Io's interior. Without this tidal heating, Io may have been similar to the Earth's moon, a world of similar size and mass, geologically dead and covered with numerous impact craters.

Io's volcanism has led to the formation of hundreds of volcanic centres and extensive lava formations, making the moon the most volcanically active world in the solar system. Three different types of volcanic eruptions have been identified, differing in duration, intensity, lava effusion rate, and whether the eruption occurs within a volcanic pit (known as a "patera"). Lava flows on Io, some tens or hundreds of kilometres long, are composed primarily of basaltic silicate lavas, similar to lavas seen on Earth at shield volcanoes such as Kilauea in Hawaii.cite journal |title=New estimates for Io eruption temperatures: Implications for the interior |journal=Icarus |last=Keszthelyi |first=L. |coauthors="et al." |pages=491–502 |volume=192 |issue= |year=2007 |doi=10.1016/j.icarus.2007.07.008] While most lava flows on Io are consistent with basalt, several flows consisting of sulfur and sulfur dioxide are also seen, as well as eruption temperatures that can be explained by high-temperature ultramafic silicate lavas.cite book |title=Io after Galileo |publisher=Springer-Praxis |chapter=Active volcanism: Effusive eruptions |last=Williams |first=D. A. |coauthors=Howell, R. R. |editor=Lopes, R. M. C. and Spencer, J. R. (eds.) |pages=pp. 133–161 |year=2007 |isbn=3-540-34681-3]

As a result of the presence of significant quantities of sulfurous materials in Io's crust and on its surface, during some eruptions, sulfur, sulfur dioxide gas, and pyroclastic material are blown up to convert|500|km into space, producing large, umbrella-shaped volcanic plumes.cite journal |title=Galileo observations of volcanic plumes on Io |journal=Icarus |last=Geissler |first=P. E. |coauthors=M. T. McMillan |pages=505–518 |volume=197 |issue= |year=2008 |doi=10.1016/j.icarus.2008.05.005] This material paints the surrounding terrain in red, black, and white, and provides for Io's patchy atmosphere and Jupiter's extensive magnetosphere. Spacecraft that have flown by Io since 1979 have observed numerous surface changes as a result of Io's volcanic activity.cite journal |title=Surface changes on Io during the Galileo mission |journal=Icarus |last=Geissler |first=P. |coauthors="et al." |pages=29–64 |volume=169 |issue= |year=2004 |doi=10.1016/j.icarus.2003.09.024]

Discovery

Prior to the "Voyager 1" encounter with Io on March 5, 1979, Io was thought to be a dead world much like the Earth's Moon. The discovery of a cloud of sodium surrounding Io led to theories that the satellite would be covered in evaporites.cite journal |title=Io: A Surface Evaporite Deposit? |journal=Science |last=Fanale |first=F. P. |coauthors="et al." |pages=pp. 922–925 |volume=186 |issue=4167 |year=1974 |doi=10.1126/science.186.4167.922 |pmid=17730914]

Hints of discoveries to come arose from Earth-based infrared observations taken in the 1970s. An anomalously high thermal flux, compared to the other Galilean satellites, was discovered during measurements taken at 10 μm while Io was in Jupiter's shadow.cite journal |title=Thermal Properties of the Galilean satellites |journal=Icarus |last=Morrison |first=J |coauthors=Cruikshank, D. P. |pages=223–236 |volume=18 |issue= |year=1973 |doi=10.1016/0019-1035(73)90207-8] At the time, this heat flux was attributed to the surface having a much higher thermal inertia than Europa and Ganymede.cite journal |title=Ten-micron eclipse observations of Io, Europa, and Ganymede |journal=Icarus |last=Hansen |first=O. L. |pages=237–246 |volume=18 |issue= |year=1973 |doi=10.1016/0019-1035(73)90208-X] These results were considerably different from measurements taken at wavelengths of 20 μm which suggested that Io had similar surface properties to the other Galilean satellites. It has since been determined that the greater flux at shorter wavelengths was due to the combined flux from Io's volcanoes and solar heating, while solar heating provides a much greater fraction of the flux at longer wavelengths.cite book |title=Io after Galileo |publisher=Springer-Praxis |chapter=A history of the exploration of Io |last=Cruikshank |first=D. P. |coauthors=Nelson, R. M. |editor=Lopes, R. M. C. and Spencer, J. R. (eds.) |pages=pp. 5–33 |year=2007 |isbn=3-540-34681-3] A sharp increase in Io's thermal emission at 5 μm was observed on February 20, 1978 by Witteborn, "et al". The group considered volcanic activity at the time, in which case the data was fit into a region on Io convert|8000|km2 in size at convert|600|K. However, the authors considered that hypothesis unlikely, and instead focused on emission from Io's interaction with Jupiter's magnetosphere.cite journal |title=Io: An Intense Brightening Near 5 Micrometers |journal=Science |last=Witteborn |first=F. C. |coauthors="et al." |pages=643–646 |volume=203 |issue= |date=1979 |doi=10.1126/science.203.4381.643]

Shortly before the "Voyager 1" encounter, Stan Peale, Patrick Cassen, and R. T. Reynolds published a paper in the journal "Science" predicting a volcanically-modified surface and a differentiated interior, with distinct rock types rather than a homogeneous blend. They based this prediction on models of Io's interior that took into account the massive amount of heat produced by the varying tidal pull of Jupiter on Io caused by the moon's slightly eccentric orbit. Their calculations suggested that the amount of heat generated for an Io with a homogeneous interior would be three times greater than the amount of heat generated by radioactive isotope decay alone. This effect would be even greater with a differentiated Io.

"Voyager 1"'s first images of Io revealed a lack of impact craters, suggesting a very young surface. Craters are used by geologists to estimate the age of a planetary surface; the more impact structures a planetary surface has, the older it is. Instead of impact structures, "Voyager 1" observed a multi-coloured surface, pockmarked with irregular-shaped depressions, which lacked the raised rims characteristic of impact craters. Also seen were flow features formed by some low-viscosity fluid and tall, isolated mountains that did not resemble terrestrial volcanoes. The surface observed suggested that, just as Peale, "et al.", had theorized, Io was heavily modified by volcanism.cite journal |title=The Jupiter system through the eyes of Voyager 1 |journal=Science |last=Smith |first=B. A. |coauthors="et al." |pages=951–972 |volume=204 |issue= |year=1979 |doi=10.1126/science.204.4396.951]

On March 8, 1979, three days after passing Jupiter, "Voyager 1" took several images of Jupiter's moons to help mission controllers determine the spacecraft's exact location, a process called optical navigation. While processing images of Io to enhance the visibility of background stars, navigation engineer Linda Morabito found a convert|300|km|adj=on tall cloud along the limb of Io. At first, she suspected the cloud to be a moon behind Io, but no suitably sized body would have been in that location. The feature was determined to be a plume generated by active volcanism at a dark depression later named Pele.cite journal |title=Volcanic eruption plumes on Io |journal=Nature |last=Strom |first=R. G. |coauthors="et al." |pages=733–736 |volume=280 |issue= |year=1979 |doi=10.1038/280733a0] Following this discovery, seven other plumes were located in earlier "Voyager" images of Io. Thermal emission from several sources, indicative of cooling lava, were also found.cite journal |title=Infrared Observations of the Jovian System from Voyager 1 |journal=Science |last=Hanel |first=R. |coauthors="et al." |pages=972–976 |volume=204 |issue=4396 |year=1979 |doi=10.1126/science.204.4396.972-a] Later several surface changes were observed in images acquired by "Voyager 2", when they were compared to images acquired 4 months prior by "Voyager 1", including new plume deposits at Aten Patera and Surt.cite journal |title=The Galilean Satellites and Jupiter: Voyager 2 Imaging Science Results |journal=Science |last=Smith |first=B. A. |coauthors="et al." |pages=927–950 |volume=206 |issue=4421 |year=1979 |doi=10.1126/science.206.4421.927]

Heat source

Unlike the Earth and the Moon, Io's main source of internal heat comes from the dissipation of tidal forces generated by Jupiter's gravitational pull This external heating differs from the internal heat source for volcanism on Earth, the result of decompression in upwelling mantle convection currents, radioactive isotope decay, and residual heat from accretion. Such heating is dependent on Io's distance from Jupiter, its orbital eccentricity, the composition of its interior, and its physical state.cite book |title=Io after Galileo |publisher=Springer-Praxis |chapter=The Interior of Io |last=Moore |first=W. B. |coauthors="et al." |editor=Lopes, R. M. C. and Spencer, J. R. (eds.) |pages=89–108 |year=2007 |isbn=3-540-34681-3] Its Laplace-resonant orbit with Europa and Ganymede maintains Io's eccentricity and prevents tidal dissipation within Io from circularizing its orbit. The eccentricity leads to vertical differences in Io's tidal bulge of as much as convert|100|m|ft|-1 as Jupiter's gravitational pull varies between the periapsis and apoapsis points in Io's orbit. This varying tidal pull also produces friction in Io's interior, enough to cause significant tidal heating and melting. Unlike Earth, where most of its internal heat is released by conduction through the crust, on Io internal heat is released in the form of volcanic activity and generates the satellite's high heat flow (global total: 0.6–1.6esp|14 W). Models of its orbit suggest that the amount of tidal heating within Io changes with time, and that the current heat flow is not representative of the long-term average. The amount of heat released from Io's interior is greater than current estimates for the amount presently generated from tidal heating, suggesting that Io is cooling off from a period of greater tidal heating.cite book |last=Davies |first=A. |title=Volcanism on Io: A Comparison with Earth |year=2007 |publisher=Cambridge University Press |isbn=0-521-85003-7 |pages=pp. 53–72 |chapter=Io and Earth: formation, evolution, and interior structure ]

Composition

Analysis of "Voyager" images led scientists to believe that the lava flows on Io were composed mostly of various compounds of molten sulfur.cite journal |title=Sulphur flows on Io |journal=Nature |last=Sagan |first=C. |pages=750–753 |volume=280 |issue= |year=1979 |doi=10.1038/280750a0] The colouration of the flows was found to be similar to various allotropes of sulfur. Differences in the lava colour and brightness are a function of the temperature of polyatomic sulfur and the packing and bonding of sulfur atoms. An analysis of the lava flows that radiate out from Ra Patera revealed dark albedo material (associated with liquid sulfur at convert|525|K) close to the vent, red material (associated with liquid sulfur at convert|450|K) in the central part of each flow, and orange material (associated with liquid sulfur at convert|425|K) at the furthest ends of each flow. This colour pattern corresponds to flows radiating out from a central vent, cooling as the lava travels away from the vent. In addition, temperature measurements of thermal emission at Loki Patera taken by "Voyager 1"'s Infrared Interferometer Spectrometer and Radiometer (IRIS) instrument were consistent with sulfur volcanism. However, the IRIS instrument was not capable of detecting wavelengths that are indicative of higher temperature components. This meant that temperatures consistent with silicate volcanism were not discovered by "Voyager". Despite this, it was determined that silicates must play a role in Io's youthful appearance as suggested by the moon's high density and the need for silicates to support the steep slopes along patera walls.cite journal |title=Stability of sulfur slopes on Io |journal=Icarus |last=Clow |first=G. D. |coauthors=Carr, M. H. |pages=268–279 |volume=44 |issue= |year=1980 |doi=10.1016/0019-1035(80)90022-6 ] The contradiction between the structural evidence and the spectral and temperature data following the Voyager flybys led to a debate in the planetary science community regarding the composition of Io's lava flows, whether they were composed of silicate or sulfurous materials.cite journal |title=Io on the Eve of the Galileo Mission |journal=Annu. Rev. Earth Planet. Sci.|last=Spencer |first=J. R. |coauthors=Schneider, N. M. |pages=125–190 |volume=24 |year=1996 |doi=10.1146/annurev.earth.24.1.125]

Earth-based infrared studies in the 1980s and 1990s shifted the paradigm from one of primarily sulfur volcanism to one where silicate volcanism dominates, and sulfur acts in a secondary role. In 1986, measurements of a bright eruption on Io's leading hemisphere revealed temperatures of at least convert|900|K. This is higher than the boiling point of sulfur (convert|715|K|disp=s), indicating a silicate composition for at least some of Io's lava flows.cite journal |title=Io: Evidence for Silicate Volcanism in 1986 |journal=Science |last=Johnson |first=T. V. |coauthors="et al." |pages=1280–1283 |volume=242 |issue= |year=1988 |doi=10.1126/science.242.4883.1280] Similar temperatures were also observed at the Surt eruption in 1979 between the two "Voyager" encounters, and at the eruption observed by Witteborn, "et al.", in 1978.cite journal |title=Io: Ground-Based Observations of Hot Spots |journal=Science |last=Sinton |first=W. M. |coauthors="et al." |pages=1015–1017 |volume=210 |issue= |year=1980 |doi=10.1126/science.210.4473.1015] In addition, modeling of silicate lava flows on Io suggested that they cooled rapidly, causing their thermal emission to be dominated by lower temperature components, such as solidified flows, as opposed to the small areas covered by still molten lava near the actual eruption temperature.cite journal |title=Silicate volcanism on Io |journal=J. Geophys. Res. |last=Carr |first=M. H. |pages=3521–3532 |volume=91 |issue= |year=1986 |doi=10.1029/JB091iB03p03521]

Silicate volcanism, involving basaltic lava with mafic to ultramafic (magnesium-rich) compositions, was confirmed by the "Galileo" spacecraft in the 1990s and 2000s from temperature measurements of Io's numerous hot spots, locations where thermal emission is detected, and from spectral measurements of Io's dark material. Temperature measurements from "Galileo"'s Solid-State Imager (SSI) and Near-Infrared Mapping Spectrometer (NIMS) revealed numerous hot spots with high-temperature components of at least convert|1200|K and some as high as convert|1600|K, like at the Pillan Patera eruption in 1997. Spectral observations of Io's dark material suggested the presence of orthopyroxenes, such as enstatite, magnesium-rich silicate minerals common in mafic and ultramafic basalt. This dark material is seen in volcanic pits, fresh lava flows, and pyroclastic deposits surrounding recent, explosive volcanic eruptions.cite journal |title=Global Color Variations on Io |journal=Icarus |last=Geissler |first=P. E. |coauthors="et al." |pages=265–282 |volume=140 |year=1999 |doi=10.1006/icar.1999.6128] Based on the measured temperature of the lava and the spectral measurements, at least some of the lava may be analogous to terrestrial komatiites.cite journal |title=A komatiite analog to potential ultramafic materials on Io |journal=J. Geophys. Res. |last=Williams |first=D. A. |coauthors="et al." |pages=1671–1684 |volume=105 |issue=E1 |year=2000 |doi=10.1029/1999JE001157] Compressional superheating, which could increase the temperature of magma during ascent to the surface during an eruption, may also be a factor in some of the higher temperature eruptions.

While temperature measurements of Io's volcanoes settled the sulfur-versus-silicates debate that persisted between the "Voyager" and "Galileo" missions at Jupiter, sulfur and sulfur dioxide still play a significant role in the phenomena observed on Io. Both sulfur and sulfur dioxide have been detected in the plumes generated at Io's volcanoes, with sulfur being a primary constituent of Pele-type plumes.cite journal |title=Discovery of Gaseous S₂ in Io's Pele Plume |journal=Science |last=Spencer |first=J. |coauthors="et al." |pages=1208–1210 |volume=288 |issue= |year=2000 |doi=10.1126/science.288.5469.1208] Several bright flows have been identified on Io, at Tsũi Goab Fluctus, Emakong Patera, and Balder Patera for example, that are suggestive of effusive sulfur or sulfur dioxide volcanism.cite journal |title=Mapping of the Culann–Tohil region of Io from Galileo imaging data |journal=Icarus |last=Williams |first=D. A. |coauthors="et al." |pages=80–97 |volume=169 |issue= |year=2004 |doi=10.1016/j.icarus.2003.08.024]

Eruptions Styles

Observations of volcanic eruptions on Io by spacecraft and Earth-based astronomers has led to the identification of differences in the types of eruptions seen on the satellite. The three main types identified include "intra-patera", "flow-dominated", and "explosion-dominated" eruptions. These eruptions differ in terms of duration, energy released, brightness temperature (determined from infrared imaging), type of lava flow, and whether it is confined within volcanic pits.

Intra-Patera eruptions

"Intra-patera" eruptions occur within volcanic depressions known as "paterae".cite journal |title=Paterae on Io: A new type of volcanic caldera? |journal=J. Geophys. Res. |last=Radebaugh |first=D. |coauthors="et al." |pages=33005–33020 |volume=106 |issue= |year=2001 |doi=10.1029/2000JE001406] These volcanic centers generally have flat floors bounded by steep walls. They resemble terrestrial calderas, but it is unknown whether they form when an empty lava chamber collapses, like their terrestrial cousins. One hypothesis suggests that these features are produced through the exhumation of volcanic sills, with the overlying material either being blasted out or integrated into the sill.cite journal |title=A Post-Galileo view of Io's Interior |journal=Icarus |last=Keszthelyi |first=L. |coauthors="et al." |pages=271–286 |volume=169 |issue= |year=2004 |doi=10.1016/j.icarus.2004.01.005] Some paterae display evidence for multiple collapses, similar to the calderas atop Olympus Mons on Mars or Kilauea on Earth, suggesting that at least some paterae form like volcanic calderas. Because the formation mechanism is still uncertain, the general term for these features uses the Latin descriptor term used by the International Astronomical Union in naming them, "paterae". Unlike similar features on Earth and Mars, these depressions generally do not lie at the peak of shield volcanoes and are larger, with an average diameter of convert|41|km and depth of convert|1.5|km|1. The largest volcanic depression on Io is Loki Patera at convert|202|km. Whatever the formation mechanism, the morphology and distribution of many paterae suggest that these features are structurally controlled, with at least half bounded by faults or mountains.

This eruption style can take the form of either lava flows, spreading across the floor of the paterae, or lava lakes.cite conference |title=Gish Bar Patera, Io: Geology and Volcanic Activity, 1997–2001 |booktitle=LPSC XXXIV |location=Clear Lake, Texas |last=Perry |first=J. E. |coauthors="et al." |date=2003 |id=Abstract #1720 |url=http://www.lpi.usra.edu/meetings/lpsc2003/pdf/1720.pdf] Except for observations by Galileo during its seven close flybys, it can be difficult to tell the difference between a lava lake and a lava flow eruption on a patera floor, due to the lower resolution and similar thermal emission characteristics. Lava flow eruptions on patera floors can be just as voluminous as those seen spreading out across the Ionian plains, such as the Gish Bar Patera eruption in 2001. Flow-like features have also been observed at a number of paterae, like Camaxtli Patera, suggesting that lava flows are an important resurfacing mechanism on the floors of these depressions.cite journal |title=Imaging of volcanic activity on Jupiter's moon Io by Galileo during the Galileo Europa Mission and the Galileo Millennium Mission |journal=J. Geophys. Res. |last=Keszthelyi |first=L. |coauthors="et al." |pages=33025–33052 |volume=106 |issue= |year=2001 |doi=10.1029/2000JE001383]

Ionian lava lakes are depressions partially filled with molten lava covered by a thin crust of cooled, solidified lava. These lava lakes are directly connected to a magma reservoir lying below the lava lake.cite book |last=Davies |first=A. |title=Volcanism on Io: A Comparison with Earth |year=2007 |publisher=Cambridge University Press |isbn=0-521-85003-7 |pages=pp. 142–152 |chapter=Effusive activity: landforms and thermal emission evolution ] Observations of thermal emission at several Ionian lava lakes reveal glowing lava along the patera margin. These lava lakes are often crusted over by cooled lava, with that crust breaking up along the edge of the patera. Over time, because the solidified lava is denser than the still-molten magma below, this crust can flounder, triggering an increase in thermal emission at the volcano.cite journal |title=Io: Loki Patera as a magma sea |journal=J. Geophys. Res. |last=Matson |first=D. L. |coauthors="et al." |pages=E09002 |volume=111 |issue= |date=2006 |doi=10.1029/2006JE002703] For some lava lakes, like the one at Pele, this occurs continuously, making Pele one of the brightest emitters of heat in the near-infrared spectrum on Io.cite journal |title=Observations and temperatures of Io's Pele Patera from Cassini and Galileo spacecraft images |journal=Icarus |last=Radebaugh |first=J. |coauthors="et al." |pages=65–79 |volume=169 |issue= |year=2004 |doi=10.1016/j.icarus.2003.10.019] At other sites, such as at Loki Patera, this can occur episodically. During an overturning episode, Loki can emit up to ten times as much heat than when it is calmer.cite journal |title=The nature of the volcanic activity at Loki: Insights from Galileo NIMS and PPR data |journal=Icarus |last=Howell |first=R. R. |coauthors=Lopes, R. M. C. |pages=448–461 |volume=186 |issue= |year=2007 |doi=10.1016/j.icarus.2006.09.022] During an eruption at these more quiescent lava lakes, a wave of floundering lava crust spreads out across the patera at the rate of convert|1|km|1 per day, until the entire lava lake crust has been resurfaced. Another eruption would begin once the new crust has cooled and thickened enough for it to no longer be buoyant over the molten lava.cite journal |title=Loki, Io: New ground-based observations and a model describing the change from periodic overturn |journal=Geophys. Res. Lett. |last=Rathbun |first=J. A. |coauthors=J. R. Spencer |pages=L17201 |volume=33 |issue= |year=2006 |doi=10.1029/2006GL026844]

Flow-dominated eruptions

"Flow-dominated" eruptions are long-lived eruption events that build-up extensive, compound lava flows. The extent of many of these lava flows makes them a major terrain type on Io. In this style of eruption, magma erupts onto the surface from vents on the floor of paterae or from fissures on the plains, producing inflated, compound lava flows similar to those seen at Kīlauea in Hawaii. Images from the "Galileo" spacecraft revealed that many of Io's major lava flows, like those at Prometheus and Amirani are produced by the build-up of small breakouts of lava on top of older flows. Flow-dominated eruptions differ from explosion-dominated eruptions by their longevity and their lower energy output per unit of time. Lava erupts in this style at a generally steady rate, and flow-dominated eruptions can last for years to decades.

Active flow fields more than convert|300|km long have been observed on Io at Amirani and Masubi. A relatively inactive flow field named Lei-Kung Fluctus covers more than convert|125000|km2, an area slightly larger than Nicaragua.cite book |last=Davies |first=A. |title=Volcanism on Io: A Comparison with Earth |year=2007 |publisher=Cambridge University Press |isbn=0-521-85003-7 |pages=pp. 155–177 |chapter=The view from "Galileo" ] The thickness of these flow fields was not determined by Galileo, but the individual breakouts on the surface of these fields are likely convert|1|m thick. In many cases, active lava breakouts flow out onto the surface tens to hundreds of kilometres from the lava source vent, with low amounts of thermal emission observed between the breakout and the vent. This suggests that lava flows through lava tubes between the source vent and the breakout.cite journal |title=Galileo at Io: Results from High-Resolution Imaging |journal=Science |last=McEwen |first=A. S. |coauthors="et al." |pages=1193–1198 |volume=288 |issue=5469 |year=2000 |doi=10.1126/science.288.5469.1193]

While these eruptions generally have a steady eruption rate, larger outbreaks of lava have also been observed at many flow-dominated eruption sites. For example, the leading edge of the Prometheus flow field moved convert|75|to|95|km between "Voyager" in 1979 and the first "Galileo" observations in 1996.cite journal |title=Active Volcanism on Io as Seen by Galileo SSI |journal=Iacrus |last=McEwen |first=A. S. |coauthors="et al." |pages=181–219 |volume=135 |issue= |year=1998 |doi=10.1006/icar.1998.5972] While dwarfed by many explosion-dominated eruptions on Io, average flow rate at these compound lava flow fields is much greater than what is observed at similar, contemporary lava flows on Earth. Average surface coverage rates of convert|35|-|60|m2 per second were observed at Prometheus and Amirani during the "Galileo" mission, compared to convert|0.6|m2 per second at Kilauea.cite book |last=Davies |first=A. |title=Volcanism on Io: A Comparison with Earth |year=2007 |publisher=Cambridge University Press |isbn=0-521-85003-7 |pages=pp. 208–216 |chapter=Prometheus and Amirani: effusive activity and insulated flows ]

Explosion-dominated eruptions

"Explosion-dominated" eruptions are the most pronounced of Io's eruption styles. These eruptions, sometimes called "outburst" eruptions from their Earth-based detections, are characterized by their short duration (lasting only a few weeks or months), rapid onset, large volumetric flow rates, and high thermal emission.cite book |last=Davies |first=A. |title=Volcanism on Io: A Comparison with Earth |year=2007 |publisher=Cambridge University Press |isbn=0-521-85003-7 |pages=pp. 27–38 |chapter=Between "Voyager" and "Galileo": 1979-1995 ] These eruptions lead to a short-lived, significant increase in Io's overall brightness in the near-infrared. The most powerful volcanic eruption observed in historical times was an "outburst" eruption at Surt, observed by Earth-based astronomers on February 22, 2001.cite journal |title=High-Resolution Keck Adaptive Optics Imaging of Violet Volcanic Activity on Io |journal=Icarus |last=Marchis |first=F. |coauthors="et al." |pages=124–131 |volume=160 |issue= |year=2002 |doi=10.1006/icar.2002.6955 |laysummary=http://www.keckobservatory.org/article.php?id=54 |laysource=Keck Observatory Press Release |laydate=November 13, 2002]

Explosion-dominated eruptions occur when a body of magma (called a "dike"), from deep within Io's partially molten mantle, reaches the surface at a fissure. This results in a spectacular display of lava fountains.cite journal |title=Lava Fountains from the 1999 Tvashtar Catena fissure eruption on Io: Implications for dike emplacement mechanisms, eruptions rates, and crustal structure |journal=J. Geophys. Res. |last=Wilson |first=L. |coauthors=Head, J. W. |pages=32,997–33,004 |volume=106 |issue= |year=2001 |doi=10.1029/2000JE001323] During the beginning of the eruption, the thermal emission from an outburst eruption is dominated by high, 1-3 μm infrared radiation. This thermal emission is produced by a large amount of exposed, fresh lava within the fountains at the eruption source vent.cite book |last=Davies |first=A. |title=Volcanism on Io: A Comparison with Earth |year=2007 |publisher=Cambridge University Press |isbn=0-521-85003-7 |pages=pp. 192 207 |chapter=Pillan and Tvashtar Paterae: lava fountains and flows ] Outburst eruptions at Tvashtar in November 1999 and February 2007 centered around a convert|25|km|adj=on long, convert|1|km|adj=on tall lava "curtain" produced at a small patera nested within the larger Tvashtar Paterae complex.cite journal |title=Io Volcanism Seen by New Horizons: A Major Eruption of the Tvashtar Volcano |journal=Science |last=Spencer |first=J. R. |coauthors="et al." |pages=240–243 |volume=318 |issue= |year=2007 |doi=10.1126/science.1147621]

The large amount of exposed molten lava at these lava fountains have provided researchers their best opportunity to measure the actual eruption temperatures of Ionian lavas. These temperatures (about convert|1600|K|disp=/), suggestive of an ultramafic lava composition and similar to Pre-Cambrian komatiites, are dominant at such eruptions, though superheating of the magma during ascent to the surface cannot be ruled as a factor in the high eruption temperatures.

While the more explosive, lava fountaining stage may last only a few days to a week, explosion-dominated eruptions can continue for several more weeks to months, producing large, voluminous silicate lava flows. A major eruption in 1997 from a fissure north-west of Pillan Patera produced more than convert|31|km3 of fresh lava over a two and a half to five and a half month period, and later flooded the floor of Pillan Patera.cite journal |title=High-temperature silicate volcanism on Jupiter's moon Io |journal=Science |last=McEwen |first=A. S. |coauthors="et al." |pages=87–90 |volume=281 |issue= |year=1998 |doi=10.1126/science.281.5373.87] Observations by "Galileo" suggest lava coverage rates at Pillan between convert|1000|and|3000|m2 per second during the 1997 eruption. The Pillan lava flow was found to be convert|10|m thick, compared to the 1–metre-thick flows observed at the inflated flow fields at Prometheus and Amirani. Similar, rapidly emplaced lava flows were also observed by "Galileo" at Thor in 2001. Such flow rates are similar to those seen at Iceland's Laki eruption in 1783 and in terrestrial flood basalt eruptions.

Explosion-dominated eruptions can produce dramatic (but often short-lived) surface changes around the eruption site, such as large pyroclastic and plume deposits produced as gas exsolves from lava fountains at these eruptions. The 1997 Pillan eruption produced a convert|400|km|adj=on wide deposit of dark, silicate material and bright sulfur dioxide. The Tvashtar eruptions of 2000 and 2007 generated a convert|330|km|adj=on tall plume, which deposited a ring of red sulfur and sulfur dioxide convert|1200|km wide.cite journal |title=The final Galileo SSI observations of Io: orbits G28-I33 |journal=Icarus |last=Turtle |first=E. P. |coauthors="et al." |pages=3–28 |volume=169 |issue= |year=2004 |doi=10.1016/j.icarus.2003.10.014] Despite their dramatic appearance, without continuous resupply of material, these changes often revert back to their pre-eruption appearance in only a few months (in the case of Grian Patera) to a few years (such as at Pillan Patera).

Plumes

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thumb|Sequence_of_convert">330|km above its surfaceThe discovery of volcanic plumes at Pele and Loki in 1979 provided conclusive evidence that Io was geologically active. Generally, these plumes are formed when volatiles like sulfur and sulfur dioxide are ejected skyward from Io's volcanoes at speeds reaching convert|1|km/s|2. Additional material that might be found in these volcanic plumes include sodium, potassium, and chlorine.cite journal |url= |title=Far-Ultraviolet Imaging Spectroscopy of Io's Atmosphere with HST/STIS |journal=Science |last=Roesler |first=F. L. |coauthors="et al." |pages=353–357 |volume=283 |issue=5400 |year=1999 |doi=10.1126/science.283.5400.353 |format=fee required] cite journal |url= |title=Galileo Imaging of Atmospheric Emissions from Io |journal=Science |last=Geissler |first=P. E. |coauthors="et al." |pages=448–461 |volume=285 |issue=5429 |year=1999 |doi=10.1126/science.285.5429.870 |format=fee required] Despite their dramatic appearance, volcanic plumes are relatively uncommon. Of the 150 or so active volcanoes observed on Io, plumes have only been observed at couple of dozen of these hotspots. The relative confinement of Io's lava flows suggest that much of the resurfacing needed to erase Io's cratering record must come from volcanic plume deposits.

The most common type of volcanic plume on Io are dust plumes, or Prometheus-type plumes, produced when encroaching lava flows vaporize underlying sulfur dioxide frost, sending the material skyward.cite journal |title=Observations and initial modeling of lava-SO₂ interactions at Prometheus, Io |journal=J. Geophys. Res. |last=Milazzo |first=M. P. |coauthors="et al." |pages=33121–33128 |volume=106 |issue= |year=2001 |doi=10.1029/2000JE001410] Examples of Prometheus-type plumes include Prometheus, Amirani, Zamama, and Masubi. These plumes are usually less than convert|100|km tall with eruption velocities around convert|0.5|km/s|2.cite journal |title=Two classes of volcanic plume on Io |journal=Icarus |last=McEwen |first=A. S. |coauthors=Soderblom, L. A. |pages=197–226 |volume=58 |issue= |year=1983 |doi=10.1016/0019-1035(83)90075-1] Prometheus-type plumes are dust-rich, with a dense inner core and upper canopy shock zone, giving them an umbrella-like appearance. These plumes often form bright circular deposits, with a radius from the plume source ranging between convert|100|and|250|km and consisting primarily of sulfur dioxide frost. Prometheus-type plumes are frequently seen at flow-dominated eruptions, helping make this plume type quite long lived. Four out of the six Prometheus-type plumes observed by "Voyager 1" in 1979 were also observed throughout the "Galileo" mission and by "New Horizons" in 2007. While the dust plume can be clearly seen in sunlit visible-light images of Io acquired by passing spacecraft, many Prometheus-type plumes have an outer halo of fainter, more gas-rich material reaching heights approaching that of the larger, Pele-type plumes.

Io's largest plumes, Pele-type plumes, are created when sulfur and sulfur dioxide gas exsolve from erupting magma at volcanic vents or lava lakes, carrying silicate pyroclastic material with them. The few Pele-type plumes that have been observed are usually associated with explosion-dominated eruptions, and are short-lived. The exception to this is Pele, which is associated with a long-lived active lava lake eruption, though the plume is thought to be intermittent. The higher vent temperatures and vent pressures associated with these plumes generate eruption speeds of up to convert|1|km/s|2, allowing these plumes to reach heights of between convert|300|and|500|km above Io's surface. These plumes form red (from the short-chain sulfur) and black (from the silicate pyroclastics) deposits on the surface. Most Pele-type plumes form large, convert|1000|km-wide red ring deposits, as seen at Pele. They are generally fainter than Prometheus-type plumes, sometimes seen only in images acquired while Io is in the shadow of Jupiter or in ultraviolet images, as a result of the low dust content, causing some to be called stealth plumes. The little dust that is visible in these plumes in sunlit images is generated as sulfur and sulfur dioxide condense as the gas reach the top of their ballistic trajectories. That is why these plumes lack a dense central column, seen in Prometheus-type plumes whose dust is generated at the plume vent. Examples of Pele-type plumes have been observed at Pele, Tvashtar, and Grian.

References

External links

* [http://jupiter.berkeley.edu/volnews.html Summary of Known Io Volcanic Activity, 1995-2001]
* [http://pirlwww.lpl.arizona.edu/~geissler/Volcanic_centers/gif/ Io Surface Changes Candidates]
* [http://www.solarviews.com/eng/iovolcano.htm Io's Volcanic Features]