Volcanism on Io

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Volcanism on Io produces extensive lava flows, hundreds of volcanic pits, and plumes of sulfur and sulfur dioxide hundreds of kilometers in height on this satellite of Jupiter. The tidal heating produced by Io's forced orbital eccentricity has led the moon to become one of the most volcanically active worlds in the solar system, with hundreds of volcanic centers and extensive lava flows. During a major eruption, lava flows tens or even hundreds of kilometers long can be produced, consisting mostly of basaltic or ultramafic silicate lavas. As a by-product of this activity, sulfur and sulfur dioxide gas and pyroclastic material are blown up to 500 km (310 mi) into space producing large, umbrella-shaped plumes, painting the surrounding terrain in red, black, and white, and providing material for Io's patchy atmosphere and Jupiter's extensive magnetosphere.

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[edit] Discovery

Discovery image of active volcanism on Io
Discovery image of active volcanism on Io

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 latest theories suggested that Io would be a world covered in evaporites due to the discovery of a cloud of sodium surrounding Io.[1] Hints of the discoveries to come did come from observations in the Infrared in the 1970s, as Io was found to have an anomalously high thermal flux during an eclipse compared to the other Galilean satellites. At the time, this heat flux was attributed to the surface having a much higher thermal inertia than Europa and Ganymede.[2] But these results, taken at a wavelength of 10-μm, were considerably different from similar measurements at longer wavelengths (at 20-μm), which suggested that Io had similar surface properties to the other Galilean satellites.[3] In retrospect, the greater flux at shorter wavelengths was due to the combined flux from Io's volcanoes and solar heating, while solar heating provides a a much greater fraction of the flux at longer wavelengths.[4] A sharp increase in Io's thermal emission at 5-μm was observed on February 20, 1978 by Witteborn et al..[5] The group considered volcanic activity at the time, in which case the data was fit a region on Io 8000 km² in size at 600 K. However, the authors considered that hypothesis unlikely, and instead focused on emission from Io's interaction with Jupiter's magnetosphere.

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.[6] 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 Io'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.

When Voyager 1's first images of Io came back in early March 1979, an obvious lack of impact craters was noted.[7] Impact craters are used by geologists to estimate the age of a planetary surface. While Callisto, another Galilean satellite, was found to be saturated with impact craters and thus has an ancient surface, no obvious impact craters could be found on Io in Voyager's images, suggesting a very young surface. Instead of impact structures, Voyager 1 observed a multi-colored surface, pockmarked with irregularly-shaped depressions, which lacked the raised rims characteristic of impact craters, flow features formed by some low-viscosity fluid, and tall, isolated massifs that did not resemble terrestrial volcanoes. The surface observed suggested that, just as Peale et al. had suggested, the surface was heavily modified by volcanism on the surface.

On March 8, 1979, shortly after the encounter, Voyager 1 took several images of Jupiter's satellites for optical navigation, to determine the position of the spacecraft by comparing the position of the satellites to background stars. Navigation engineer Linda Morabito, while processing Io images to enhance the visibility of the background stars, found a 300-km tall cloud along the limb of Io.[8] Once the possibility of background solid body was ruled out, the feature was determined to be a plume generated by active volcanism at a dark depression later named Pele. Following this discovery, other plumes were discovered in earlier Voyager images of Io, as well as thermal emission, indicative of cooling lava, from several others.[9]

[edit] 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 gravity pull, rather than radioactive isotope decay.[6] Such heating is dependent on Io's distance from Jupiter, its orbital eccentricity, the composition of its interior, and its physical state.[10] 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 100 m (330 ft). The friction produced in Io's interior due to the varying tidal pull from Jupiter between the periapsis and apoapsis points in Io's orbit is enough to cause significant tidal heating within Io's interior and creating a significant amount of melt. This heat is then released from the interior in the form of volcanic activity and generates its high heat flow (global total: 0.6-1.6×1014 W).[10] 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.[10]

[edit] Composition

Analysis of Voyager images led scientists to believe that the lava flows on Io were composed mostly of various compounds of molten sulfur.[11] Their coloration was found to be similar to various allotropes of sulfur, differences in the color and brightness of polyatomic sulfur as a function of temperature and the packing and bonding of the sulfur atoms. An analysis of the lava flows that radiate out from Ra Patera revealed dark albedo material (associated with liquid sulfur at 525 K) close to the vent, red material (associated with liquid sulfur at 450 K) in the central part of each flow, and orange material (associated with liquid sulfur at 425 K) at the distal ends of each flow.[11] This color pattern would match 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 by Voyager 1's IRIS instrument were consistent with sulfur volcanism.[12] However, the wavelengths detected by the IRIS instrument precluded the detection of higher temperature components. While temperatures consistent with silicate volcanism were not detected by Voyager for this reason, Io's high density and the need for silicates to support the steep slopes observed along patera walls by Voyager suggested that silicates did play a role in Io's youthful appearance.[13]

Earth-based infrared studies and later Galileo spacecraft observations in the 1980s and 1990s shifted the paradigm from one of primarily sulfur volcanism to one where silicate volcanism dominated, with sulfur acting in a secondary role. In 1986, measurements of a bright eruption on Io's leading hemisphere revealed temperatures of at least 900 K, greater than the boiling point of sulfur (at 715 K), indicating a silicate composition for at least some of Io's lava flows.[14] Similar temperatures were also observed at Surt in 1979 between the two Voyager encounters, and at the eruption observed by Witteborn et al. in 1978.[15],[5] In addition, modeling silicate lava flows on Io suggested that they cooled rapidly, causing their thermal emission to be dominated by lower temperature components.[16] 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 hotspots, or locations where thermal emission was detected, as well spectral measurements of Io's dark material. Temperatures measurements from Galileo's Solid-State Imager (SSI) and Near-Infrared Mapping Spectrometer (NIMS) revealed numerous hotspots with high-temperature components of at least 1200 K and some as high as 1600 K, like at the Pillan Patera eruption in 1997.[17] Spectral observations of Io's dark material, often seen in fresh lava flows or in pyroclastic deposits surrounding recent, explosive volcanic eruptions, suggested the presence of orthopyroxenes, such as enstatite, magnesium-rich silicate minerals common in mafic and ultramafic materials. Based on the measured temperature of the lava and the spectral measurements, at least some of the lava on Io maybe analogous to terrestrial komatiites.[18]

While temperature measurements of Io's volcanoes seems to have settled the sulfur vs. 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 the primary constituent of Pele-type plumes. These, the larger of the two plume types, are generated as sulfur and sulfur dioxide is exsolved from erupting magma.[19] Also, several bright flows have been identified on Io, such as Tsũi Goab Fluctus, Emakong Patera, and Balder Patera, that are suggestive of effusive sulfur or sulfur dioxide volcanism.[20] These sites are also notable for the low-temperature, thermal emission.

[edit] Eruptions Styles

[edit] Intra-Patera

Tupan Patera, an example of a volcanic depression on Io
Tupan Patera, an example of a volcanic depression on Io

Intra-patera eruptions occur within volcanic depressions known as paterae.[21] Paterae generally have flat floors bounded by steep walls. These features resemble terrestrial calderas, but it is unknown if they are produced through collapse over an emptied lava chamber as with 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.[22] Unlike similar features on Earth and Mars, these depressions generally do not lie at the peak of shield volcanoes and are normally larger, with an average diameter of 41 km (25½ mi), the largest being Loki Patera at 202 km (125½ mi).[21] 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.[21]

Intra-patera eruptions can take the form of either lava flows spreading across the floor of the paterae or a lava lake.[23],[24] Observations of thermal emission at several of these 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 patera margins. Overtime, because the solidified lava is denser than the still molten magma below, this crust can flounder, triggering an increase in thermal emission detected at the volcano. 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 on Io.[25] At other sites, such as at Loki Patera, this can occur episodically.[26] During an overturning episode, Loki can emit as much 10 times as much heat than when it is quieter.

[edit] Flow-dominated

Culann Patera, an example of a flow-dominated eruption on Io
Culann Patera, an example of a flow-dominated eruption on Io

Lava flows represent another major volcanic terrain on Io. Magma erupts onto the surface from vents on the floor of paterae or on the plains from fissures, producing inflated, compound lava flows similar to those seen at Kilauea in Hawaii. Images from the Galileo spacecraft revealed that many of Io's major lava flows, like those at at Prometheus and at Amirani are produced by the build-up of small breakouts of lava flows on top of older flows.[27] Larger outbreaks of lava have also been observed on Io. For example, the leading edge of the Prometheus flow moved 75 to 95 km (46½ to 59 mi) between Voyager in 1979 and the first Galileo observations in 1996. A major eruption in 1997 produced more than 3,500 km² (1,350 sq mi) of fresh lava as well as flooding the floor of the adjacent Pillan Patera.[17]

[edit] Explosion-dominated

Two Galileo images showing the effects of an explosion-dominated eruption at Pillan Patera in 1997. Pele is the small volcano to the southwest of Pillan surrounded by a large, red ring
Two Galileo images showing the effects of an explosion-dominated eruption at Pillan Patera in 1997. Pele is the small volcano to the southwest of Pillan surrounded by a large, red ring

Explosion-dominated eruptions are the most dramatic of Io's eruption styles. These eruptions are often short-lived, last only few weeks or months, and are characterized by large volumetric flow rates, producing large lava flows over a short period of time, and high thermal emission.[28] These eruptions often lead to a significant increase in Io's overall brightness in the near-infrared. Earth-based detections of these eruptions led to these being called "outburst" eruptions.

These eruptions can produce extensive volcanic lava flows over a short-period of time. An eruption from a fissure northwest of Pillan Patera produced a 3,500 km² lava flow, including the flooding of the Pillan Patera floor, over a 2.5-5.5 month period in mid-1997.[17] Similar, rapidly emplaced lava flows was also observed by Galileo at Thor in 2001.[24] Such flow rates are similar to those seen at Iceland's Laki eruption in 1783 and in terrestrial flood basalt eruptions.[28] Explosion-dominated eruptions can also produce large pyroclastic and plume deposits surrounding the eruption site. The Pillan eruption in 1997 produced a 400-km wide deposit of dark, silicate material and plume-deposit, bright sulfur dioxide. The Tvashtar eruptions of 2000 and 2007 generated a 330-km tall plume, which produced a 1,200-km wide ring of red sulfur and sulfur dioxide. Finally, these eruptions can produce dramatic displays such as lava fountain. Outburst eruptions at Tvashtar in November 1999 and February 2007 centered around a 25-km long, 750-meter tall lava "curtain" produced at a small patera nested within the larger Tvashtar Paterae complex.[29]

The highest temperatures measured on Io have been at explosion-dominated eruptions. These temperatures (~1500 K) suggest an ultramafic lava composition, similar to Pre-Cambrian komatiites, is dominant at these eruptions.[18]

[edit] Plumes

Sequence of New Horizons images showing Io's volcano Tvashtar spewing material 330 km above its surface.
Sequence of New Horizons images showing Io's volcano Tvashtar spewing material 330 km above its surface.

The discovery of plumes at the volcanoes Pele and Loki were the first sign that Io is geologically active.[8] Generally, these plumes are formed when volatiles like sulfur and sulfur dioxide are ejected skyward from Io's volcanoes at speeds reaching 1 km/s (0.62 mps). Additional material that might be found in these volcanic plumes include sodium, potassium, and chlorine.[30],[31] These plumes appear to be formed in one of two ways.[32] Io's largest plumes are created when sulfur and sulfur dioxide gas evaporate from erupting magma at volcanic vents or lava lakes, often dragging silicate pyroclastic material with them. These plumes form red (from the short-chain sulfur) and black (from the silicate pyroclastics) deposits on the surface. Plumes formed in this manner are among the largest observed at Io, forming red rings more than 1000 km (620 mi) in diameter. Examples of this plume type include Pele, Tvashtar, and Dazhbog. Another type of plume is produced when encroaching lava flows vaporize underlying sulfur dioxide, sending the sulfur dioxide skyward. This type of plume often forms bright circular deposits consisting of sulfur dioxide. These plumes are often less than 100 km (62 mi) tall, and are among the most long-lived plumes on Io. Examples include Prometheus, Amirani, and Masubi.

[edit] References

  1. ^ Fanale, F. P.; et al. (1974). "Io: A Surface Evaporite Deposit?". Science 186 (4167): pp. 922–925. doi:10.1126/science.186.4167.922. PMID 17730914. 
  2. ^ Hansen, O. L. (1973). "Ten-micron eclipse observations of Io, Europa, and Ganymede". Icarus 18: 237–246. 
  3. ^ Morrison, J.; Cruikshank, D. P. (1973). "Thermal Properties of the Galilean satellites". Icarus 18: 223–236. 
  4. ^ Cruikshank, D. P.; and Nelson, R. M. (2007). "A history of the exploration of Io", in Lopes, R. M. C.; and Spencer, J. R.: Io after Galileo. Springer-Praxis, pp. 5-33. ISBN 3-540-34681-3. 
  5. ^ a b Witteborn, F. C.; et al. (1979). "Io: An Intense Brightening Near 5 Micrometers". Science 203: 643–646. 
  6. ^ a b Peale, S. J.; et al. (1979). "Melting of Io by Tidal Dissipation". Science 203: 892–894. 
  7. ^ Smith, B. A.; et al. (1979). "The Jupiter system through the eyes of Voyager 1". Science 204: 951–972. 
  8. ^ a b Morabito, L. A.; et al. (1979). "Discovery of currently active extraterrestrial volcanism". Science 204: 972. 
  9. ^ Strom, R. G.; et al. (1979). "Volcanic eruption plumes on Io". Nature 280: 733–736. 
  10. ^ a b c Moore, W. B. et al. (2007). "The Interior of Io.", in R. M. C. Lopes and J. R. Spencer: Io after Galileo. Springer-Praxis, 89-108. 
  11. ^ a b Sagan, C. (1979). "Sulphur flows on Io". Nature 280: 750–753. 
  12. ^ Hanel, R. (1979). "Infrared Observations of the Jovian System from Voyager 1". Science 204 (4396): 972–976. 
  13. ^ Clow, G. D.; and M. H. Carr (1980). "Stability of sulfur slopes on Io". Icarus 44: 268–279. 
  14. ^ Johnson, T. V.; et al. (1988). "Io: Evidence for Silicate Volcanism in 1986". Science 242: 1280–1283. 
  15. ^ Sinton, W. M.; et al. (1980). "Io: Ground-Based Observations of Hot Spots". Science 210: 1015–1017. 
  16. ^ Carr, M. H. (1986). "Silicate volcanism on Io". J. Geophys. Res. 91: 3521–3532. 
  17. ^ a b c McEwen, A. S.; et al. (1998). "High-temperature silicate volcanism on Jupiter's moon Io". Science 281: 87–90. 
  18. ^ a b Williams, D. A.; et al. (2000). "A komatiite analog to potential ultramafic materials on Io". J. Geophys. Res. 105 (E1): 1671–1684. 
  19. ^ Spencer, J.; et al. (2000). "Discovery of Gaseous S2 in Io's Pele Plume". Science 288: 1208–1210. 
  20. ^ Williams, D. A.; et al. (2004). "Mapping of the Culann–Tohil region of Io from Galileo imaging data". Icarus 169: 80–97. 
  21. ^ a b c Radebaugh, D.; et al. (2001). "Paterae on Io: A new type of volcanic caldera?". J. Geophys. Res. 106: 33005–33020. 
  22. ^ Keszthelyi, L.; et al. (2004). "A Post-Galileo view of Io's Interior". Icarus 169: 271–286. 
  23. ^ Perry, J. E.; et al. (2003). "Gish Bar Patera, Io: Geology and Volcanic Activity, 1997-2001". LPSC XXXIV. Abstract#1720. 
  24. ^ a b Lopes, R.; et al. (2004). "Lava lakes on Io: Observations of Io’s volcanic activity from Galileo NIMS during the 2001 fly-bys". Icarus 169: 140–174. 
  25. ^ Radebaugh, J.; et al. (2004). "Observations and temperatures of Io’s Pele Patera from Cassini and Galileo spacecraft images". Icarus 169: 65–79. 
  26. ^ Howell, R. R.; Lopes, R. M. C. (2007). "The nature of the volcanic activity at Loki: Insights from Galileo NIMS and PPR data". Icarus 186: 448–461. 
  27. ^ Keszthelyi, L.; et al. (2001). "Imaging of volcanic activity on Jupiter's moon Io by Galileo during the Galileo Europa Mission and the Galileo Millennium Mission". J. Geophys. Res. 106: 33025–33052. 
  28. ^ a b Williams, D. A.; Howell, R. R. (2007). "Active volcanism: Effusive eruptions", in Lopes, R. M. C.; and Spencer, J. R.: Io after Galileo. Springer-Praxis, pp. 133-161. ISBN 3-540-34681-3. 
  29. ^ Wilson, L.; Head, J. W. (2001). "Lava Fountains from the 1999 Tvashtar Catena fissure eruption on Io: Implications for dike emplacement mechanisms, eruptions rates, and crustal structure". J. Geophys. Res. 106: 32,997–33,004. 
  30. ^ Roesler, F. L.; et al. (1999). "Far-Ultraviolet Imaging Spectroscopy of Io's Atmosphere with HST/STIS". Science 283 (5400): 353–357. 
  31. ^ Geissler, P. E.; et al. (1999). "Galileo Imaging of Atmospheric Emissions from Io". Science 285 (5429): 448–461. 
  32. ^ McEwen, A. S.; Soderblom, L. A. (1983). "Two classes of volcanic plume on Io". Icarus 58: 197–226. 

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