Volcanism on Io
Encyclopedia
Io (moon)
Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....
, a moon of Jupiter, produces lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
flows, volcanic
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
pits, and plumes of sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
and sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
hundreds of kilometres high. This volcanic activity was discovered in 1979 by Voyager 1
Voyager 1
The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of...
imaging scientists
Imaging science
Imaging science is a multidisciplinary field concerned with the generation, collection, duplication, analysis, modification, and visualization of images . As an evolving field it includes research and researchers from physics, mathematics, electrical engineering, computer vision, computer science,...
. Observations of Io
Io (moon)
Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....
by passing spacecraft (the Voyagers
Voyager program
The Voyager program is a U.S program that launched two unmanned space missions, scientific probes Voyager 1 and Voyager 2. They were launched in 1977 to take advantage of a favorable planetary alignment of the late 1970s...
, Galileo, Cassini, and New Horizons
New Horizons
New Horizons is a NASA robotic spacecraft mission currently en route to the dwarf planet Pluto. It is expected to be the first spacecraft to fly by and study Pluto and its moons, Charon, Nix, Hydra and S/2011 P 1. Its estimated arrival date at the Pluto-Charon system is July 14th, 2015...
) and Earth-based astronomers have revealed more than 150 active volcanoes. Up to 400 such volcanoes are predicted to exist based on these observations. Io's volcanism makes the satellite one of only five known currently volcanically active worlds in the solar system (the other four being Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
, Saturn's
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
moon Enceladus
Enceladus (moon)
Enceladus is the sixth-largest of the moons of Saturn. It was discovered in 1789 by William Herschel. Until the two Voyager spacecraft passed near it in the early 1980s very little was known about this small moon besides the identification of water ice on its surface...
, and Neptune's
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
moon Triton
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...
).
First predicted shortly before the Voyager 1 flyby, the heat source for Io's volcanism
Volcanism
Volcanism is the phenomenon connected with volcanoes and volcanic activity. It includes all phenomena resulting from and causing magma within the crust or mantle of a planet to rise through the crust and form volcanic rocks on the surface....
comes from tidal heating produced by its forced orbital eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...
. This differs from Earth's internal heating, which is derived primarily from radioactive isotope decay
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
. Io's eccentric orbit leads to a slight difference in Jupiter's gravitational pull on the satellite between its closest and farthest points on its orbit, causing a varying tidal bulge. This variation in the shape of Io causes frictional heating in its interior. Without this tidal heating, Io might 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 body in the Solar System. Three different types of volcanic eruptions
Types of volcanic eruptions
During a volcanic eruption, lava, tephra , and various gases are expelled from a volcanic vent or fissure. Several types of volcanic eruptions have been distinguished by volcanologists. These are often named after famous volcanoes where that type of behavior has been observed...
have been identified, differing in duration, intensity, lava effusion
Effusive eruption
An effusive eruption is a volcanic eruption characterized by the outpouring of lava onto the ground...
rate, and whether the eruption occurs within a volcanic pit (known as a patera). Lava flows on Io, tens or hundreds of kilometres long, have primarily basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
ic composition, similar to lavas seen on Earth at shield volcano
Shield volcano
A shield volcano is a type of volcano usually built almost entirely of fluid lava flows. They are named for their large size and low profile, resembling a warrior's shield. This is caused by the highly fluid lava they erupt, which travels farther than lava erupted from more explosive volcanoes...
es such as Kīlauea
Kilauea
Kīlauea is a volcano in the Hawaiian Islands, and one of five shield volcanoes that together form the island of Hawaii. Kīlauea means "spewing" or "much spreading" in the Hawaiian language, referring to its frequent outpouring of lava. The Puu Ōō cone has been continuously erupting in the eastern...
in Hawaii
Hawaii
Hawaii is the newest of the 50 U.S. states , and is the only U.S. state made up entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of...
. While most lavas on Io are made of basalt, a few lava flows consisting of sulfur and sulfur dioxide have been seen. In addition, eruption temperatures as high as 1600 kelvin were detected, which can be explained by the eruption of high-temperature ultramafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
silicate lavas.
As a result of the presence of significant quantities of sulfurous materials in Io's crust and on its surface, some eruptions propel sulfur, sulfur dioxide gas, and pyroclastic material
Tephra
200px|thumb|right|Tephra horizons in south-central [[Iceland]]. The thick and light coloured layer at center of the photo is [[rhyolitic]] tephra from [[Hekla]]....
up to 500 kilometres (310.7 mi) into space, producing large, umbrella-shaped volcanic plumes. This material paints the surrounding terrain in red, black, and/or white, and provides material 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.
Discovery
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
. The discovery of a cloud of sodium surrounding Io led to theories that the satellite would be covered in evaporite
Evaporite
Evaporite is a name for a water-soluble mineral sediment that result from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporate deposits, marine which can also be described as ocean deposits, and non-marine which are found in standing bodies of...
s.
Hints of discoveries to come arose from Earth-based infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
observations taken in the 1970s. An anomalously high thermal flux
Heat flux
Heat flux or thermal flux is the rate of heat energy transfer through a given surface. The SI derived unit of heat rate is joule per second, or watt. Heat flux is the heat rate per unit area. In SI units, heat flux is measured in W/m2]. Heat rate is a scalar quantity, while heat flux is a vectorial...
, compared to the other Galilean satellites
Galilean moons
The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei in January 1610. They are the largest of the many moons of Jupiter and derive their names from the lovers of Zeus: Io, Europa, Ganymede and Callisto. Ganymede, Europa and Io participate in a 1:2:4 orbital resonance...
, was discovered during measurements taken at an infrared wavelength of 10 μm
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
while Io was in Jupiter's shadow. At the time, this heat flux was attributed to the surface having a much higher thermal inertia than Europa
Europa (moon)
Europa Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and...
and Ganymede
Ganymede (moon)
Ganymede is a satellite of Jupiter and the largest moon in the Solar System. It is the seventh moon and third Galilean satellite outward from Jupiter. Completing an orbit in roughly seven days, Ganymede participates in a 1:2:4 orbital resonance with the moons Europa and Io, respectively...
. 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. 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 8000 square kilometres (3,088.8 sq mi) in size at 600 kelvin. 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
Science (journal)
Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....
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
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
alone. This effect would be even greater with a differentiated Io.
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
s, suggesting a very young surface. Craters are used by geologists to estimate the age of a planetary surface
Geology of solar terrestrial planets
The geology of solar terrestrial planets mainly deals with the geological aspects of four planets of the Solar system namely, Mercury, Venus, Earth, and Mars and one terrestrial dwarf planet, Ceres...
; the number of impact structures increase with the age of the planetary surface. Instead, Voyager 1 observed a multi-coloured surface, pockmarked with irregular-shaped depressions, which lacked the raised rims characteristic of impact craters. Voyager 1 also observed flow features formed by low-viscosity fluid and tall, isolated mountains that did not resemble terrestrial volcanoes. The surface observed suggested that, just as Peale and colleagues had theorized, Io was heavily modified by volcanism.
On March 8, 1979, three days after passing Jupiter, Voyager 1 took 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
Linda A. Morabito
Linda A. Morabito , also known as Linda Kelly, Linda Hyder, and Linda Morabito-Meyer, is the astronomer who made the discovery of volcanic activity on Io, a moon of Jupiter, on March 9, 1979, at NASA's Jet Propulsion Laboratory...
found a 300 kilometres (186.4 mi) tall cloud along the moon's limb. 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
Pele (volcano)
Pele is an active volcano on the surface of Jupiter's moon Io. It is located on Io's trailing hemisphere at A large, tall volcanic plume has been observed at Pele by various spacecraft starting with Voyager 1 in 1979, though it has not been persistent. The discovery of the Pele plume on March...
. Following this discovery, seven other plumes were located in earlier Voyager images of Io. Thermal emission from multiple sources, indicative of cooling lava, were also found. Surface changes were observed when images acquired by Voyager 2
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...
were compared to those taken four months previously by Voyager 1, including new plume deposits at Aten Patera and Surt
Surt (volcano)
Surt is an active volcano on Jupiter's moon Io. It is located on Io's Jupiter-facing hemisphere at . Surt consists of an oblong volcanic pit , in diameter, surrounded by reddish sulfur and bright sulfur dioxide deposits to its south and east. The volcano was first observed in images acquired by...
.
Heat source
Io's main source of internal heat comes from the dissipationTidal acceleration
Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite , and the primary planet that it orbits . The "acceleration" is usually negative, as it causes a gradual slowing and recession of a satellite in a prograde orbit away from the primary, and a corresponding...
of tidal force
Tidal force
The tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force per unit mass exerted on one body by a second body is not constant across its diameter, the side nearest to the second being more attracted by it than the side...
s generated by Jupiter's gravitational pull. This external heating differs from the internal heat source for volcanism on Earth, which is a result of radioactive isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
decay and residual heat from accretion
Accretion (astrophysics)
In astrophysics, the term accretion is used for at least two distinct processes.The first and most common is the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disc. Accretion discs are common around smaller stars or stellar remnants...
. In the Earth, these internal heat sources drive mantle convection
Mantle convection
Mantle convection is the slow creeping motion of Earth's rocky mantle caused by convection currents carrying heat from the interior of the Earth to the surface. The Earth's surface lithosphere, which rides atop the asthenosphere , is divided into a number of plates that are continuously being...
, which in turn causes volcanism through plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
.
The tidal heating of Io is dependent on the moon's distance from Jupiter, its orbital eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...
, the composition of its interior, and its physical state. Its Laplace orbital resonance 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 metres (328.1 ft) as Jupiter's gravitational pull varies between the periapsis and apoapsis
Apsis
An apsis , plural apsides , is the point of greatest or least distance of a body from one of the foci of its elliptical orbit. In modern celestial mechanics this focus is also the center of attraction, which is usually the center of mass of the system...
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 via volcanic activity and generates the satellite's high heat flow
Heat transfer
Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...
(global total: 0.6–1.6 W
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
). 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 observed release of heat from Io's interior is greater than estimates for the amount presently generated from tidal heating, suggesting that Io is cooling after a period of greater flexing.
Composition
Allotropes of sulfur
There are a large number of allotropes of sulfur. In this respect, sulfur is second only to carbon.The most common form found in nature is yellow orthorhombic α-sulfur, which contains puckered rings of . Chemistry students may have seen "plastic sulfur"; this is not an allotrope but a mixture of...
. The colouration of the flows was found to be similar to its various allotropes
Allotropy
Allotropy or allotropism is the property of some chemical elements to exist in two or more different forms, known as allotropes of these elements...
. Differences in the lava colour and brightness are a function of the temperature of polyatomic sulfur and the packing and bonding of its atoms. An analysis of the flows that radiate out from Ra Patera revealed differently colored materials, all associated with liquid sulfur, at different distances from the vent: dark albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
material close to the vent at 525 K, red material in the central part of each flow at 450 K, and orange material at the farthest ends of each flow at 425 K. This colour pattern corresponds to flows radiating out from a central vent, cooling as the lava travels away from it. In addition, temperature measurements of thermal emission at Loki Patera
Loki Patera
Loki Patera is the largest volcanic depression on Jupiter's moon Io, in diameter. It contains an active lava lake, with an episodically overturning crust. The level of activity seen is similar to a superfast spreading mid-ocean ridge on Earth...
taken by Voyager 1
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
s must play a role in Io's youthful appearance, from the moon's high density and the need for silicates to support the steep slopes along patera walls. 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.
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 900 kelvin. This is higher than the boiling point of sulfur (715 K), indicating a silicate composition for at least some of Io's lava flows. Similar temperatures were also observed at the Surt eruption in 1979 between the two Voyager encounters, and at the eruption observed by Witteborn and colleagues in 1978. 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.
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
-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
Pillan Patera
Pillan Patera is a patera, or a complex crater with scalloped edges, on Jupiter's moon Io. It is located at , south of Pillan Mons and west of Reiden Patera. It is named after the Araucanian thunder, fire, and volcano god. Its name was approved by the International Astronomical Union in 1997.Pillan...
eruption in 1997. Initial estimates during the course of the Galileo mission suggesting eruption temperatures approaching 2000 kelvin have since proven to be overestimates since the wrong thermal models were used to calculate the temperatures. Spectral observations of Io's dark material suggested the presence of orthopyroxenes
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
, such as enstatite
Enstatite
Enstatite is the magnesium endmember of the pyroxene silicate mineral series enstatite - ferrosilite . The magnesium rich members of the solid solution series are common rock-forming minerals found in igneous and metamorphic rocks...
, magnesium-rich silicate minerals common in mafic and ultramafic basalt. This dark material is seen in volcanic pits, fresh lava flows, and pyroclastic
Pyroclastic rock
Pyroclastic rocks or pyroclastics are clastic rocks composed solely or primarily of volcanic materials. Where the volcanic material has been transported and reworked through mechanical action, such as by wind or water, these rocks are termed volcaniclastic...
deposits surrounding recent, explosive volcanic eruptions. Based on the measured temperature of the lava and the spectral measurements, some of the lava may be analogous to terrestrial komatiite
Komatiite
Komatiite is a type of ultramafic mantle-derived volcanic rock. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content...
s. 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 materials have been detected in the plumes generated at Io's volcanoes, with sulfur being a primary constituent of Pele-type plumes. 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.
Eruption styles
Observations of Io by spacecraft and Earth-based astronomers have 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. They 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
Depression (geology)
A depression in geology is a landform sunken or depressed below the surrounding area. Depressions may be formed by various mechanisms.Structural or tectonic related:...
known as paterae, which generally have flat floors bounded by steep walls. Paterae resemble terrestrial caldera
Caldera
A caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption, such as the one at Yellowstone National Park in the US. They are sometimes confused with volcanic craters...
s, but it is unknown whether they form when an empty lava chamber collapses, like their terrestrial cousins. One hypothesis suggests that they are produced through the exhumation of volcanic sills
Sill (geology)
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...
, with the overlying material either being blasted out or integrated into the sill. Some paterae display evidence for multiple collapses, similar to the calderas atop Olympus Mons
Olympus Mons
Olympus Mons is a large volcanic mountain on the planet Mars. At a height of almost , it is one of the tallest mountains in the Solar System, three times as tall as Mount Everest and more than twice the height of Mauna Kea the tallest mountain on Earth. Olympus Mons is the youngest of the large...
on Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
or Kīlauea on Earth, suggesting that they may occasionally form like volcanic calderas. Since the formation mechanism is still uncertain, the general term for these features uses the Latin descriptor term employed by the International Astronomical Union
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
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 41 kilometres (25.5 mi). Patera depths have been measured for a only few paterae and typically exceed 1 km. The largest volcanic depression on Io is Loki Patera
Loki Patera
Loki Patera is the largest volcanic depression on Jupiter's moon Io, in diameter. It contains an active lava lake, with an episodically overturning crust. The level of activity seen is similar to a superfast spreading mid-ocean ridge on Earth...
at 202 kilometres (125.5 mi) across. Whatever the formation mechanism, the morphology and distribution of many paterae suggest that they are structurally controlled, with at least half bounded by faults or mountains.
Lava lake
Lava lakes are large volumes of molten lava, usually basaltic, contained in a volcanic vent, crater, or broad depression. The term is used to describe both lava lakes that are wholly or partly molten and those that are solidified...
s. 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 inadequate resolution and similar thermal emission characteristics. Intra-patera lava flow eruptions, such as the Gish Bar Patera
Gish Bar Patera
Gish Bar Patera is a patera, or a complex crater with scalloped edges, on Jupiter's moon Io. It is 106.3 by 115.0 kilometers and 9,600 km2 in area. It is located at . It is named after the Babylonian sun god Gish Bar. Its name was approved by the International Astronomical Union in 1997. It is...
eruption in 2001, can be just as voluminous as those seen spreading out across the Ionian plains. Flow-like features have also been observed within a number of paterae, like Camaxtli Patera, suggesting that lava flows periodically resurface their floors.
Ionian lava lakes are depressions partially filled with molten lava covered by a thin solidified crust. These lava lakes are directly connected to a magma reservoir lying below. Observations of thermal emission at several Ionian lava lakes reveal glowing molten rock along the patera margin, caused by the lake's 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 founder, triggering an increase in thermal emission 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 spectrum on Io. At other sites, such as at Loki Patera, this can occur episodically. During an overturning episode at these more quiescent lava lakes, a wave of foundering crust spreads out across the patera at the rate of about 1 kilometre (0.621372736649807 mi) per day, with new crust forming behind it until the entire lake has been resurfaced. Another eruption would only begin once the new crust has cooled and thickened enough for it to no longer be buoyant over the molten lava. During an overturning episode, Loki can emit up to ten times more heat than when its crust is stable.
Flow-dominated eruptions
Fissure vent
A fissure vent, also known as a volcanic fissure or simply fissure, is a linear volcanic vent through which lava erupts, usually without any explosive activity. The vent is usually a few meters wide and may be many kilometers long. Fissure vents can cause large flood basalts and lava channels...
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 flows, like those at Prometheus
Prometheus (volcano)
Prometheus is an active volcano on Jupiter's moon Io. It is located on Io's hemisphere facing away from Jupiter at . Prometheus consists of a -wide volcanic pit named Prometheus Patera and a -long compound lava flow, all surrounded by reddish sulfur and circular, bright sulfur dioxide volcanic...
and Amirani
Amirani (volcano)
Amirani is an active volcano on Jupiter's moon Io. It is located on Io's leading hemisphere at .The volcano is responsible for the largest active lava flow in the entire Solar System, with recent flows dwarfing those of even other volcanos on Io...
, 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 at a generally steady rate, and flow-dominated eruptions can last for years or decades.
Active flow fields more than 300 kilometres (186.4 mi) long have been observed on Io at Amirani and Masubi. A relatively inactive flow field named Lei-Kung Fluctus covers more than 125000 square kilometres (48,262.8 sq mi), an area slightly larger than Nicaragua
Nicaragua
Nicaragua is the largest country in the Central American American isthmus, bordered by Honduras to the north and Costa Rica to the south. The country is situated between 11 and 14 degrees north of the Equator in the Northern Hemisphere, which places it entirely within the tropics. The Pacific Ocean...
. The thickness of flow fields was not determined by Galileo, but the individual breakouts on their surface are likely to be 1 metres (3.3 ft) thick. In many cases, active lava breakouts flow out onto the surface at locations tens to hundreds of kilometres from the source vent, with low amounts of thermal emission observed between it and the breakout. This suggests that lava flows through lava tube
Lava tube
Lava tubes are natural conduits through which lava travels beneath the surface of a lava flow, expelled by a volcano during an eruption. They can be actively draining lava from a source, or can be extinct, meaning the lava flow has ceased and the rock has cooled and left a long, cave-like...
s from the source vent to the breakout.
While these eruptions generally have a steady eruption rate, larger outbreaks of lava have been observed at many flow-dominated eruption sites. For example, the leading edge of the Prometheus flow field moved 75 to 95 km (46.6 to 59 ) between observations by Voyager in 1979 and Galileo in 1996. While generally dwarfed by explosion-dominated eruptions, average flow rate at these compound flow fields is much greater than what is observed at similar contemporary lava flows on Earth. Average surface coverage rates of 35–60 m² (41.9–71.8 ) per second were observed at Prometheus and Amirani during the Galileo mission, compared to 0.6 square metre (0.717594027780648 sq yd) per second at Kīlauea.
Explosion-dominated eruptions
Surt (volcano)
Surt is an active volcano on Jupiter's moon Io. It is located on Io's Jupiter-facing hemisphere at . Surt consists of an oblong volcanic pit , in diameter, surrounded by reddish sulfur and bright sulfur dioxide deposits to its south and east. The volcano was first observed in images acquired by...
, observed by Earth-based astronomers on February 22, 2001.
Explosion-dominated eruptions occur when a body of magma (called a dike
Dike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
) from deep within Io's partially molten mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
reaches the surface at a fissure. This results in a spectacular display of lava fountain
Lava fountain
A lava fountain is a volcanic phenomenon in which lava is forcefully but non-explosively ejected from a crater, vent, or fissure. Lava fountains may reach heights of up to . They may occur as a series of short pulses, or a continuous jet of lava. They are commonly seen in Hawaiian eruptions.-See...
s. During the beginning of the outburst eruption, thermal emission is dominated by strong, 1–3 μm
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
infrared radiation. It is produced by a large amount of exposed, fresh lava within the fountains at the eruption source vent. Outburst eruptions at Tvashtar in November 1999 and February 2007 centred around a 25 kilometres (15.5 mi) long, 1 kilometre (0.621372736649807 mi) tall lava "curtain" produced at a small patera nested within the larger Tvashtar Paterae complex.
The large amount of exposed molten lava at these lava fountains has provided researchers with their best opportunity to measure the actual temperatures of Ionian lavas. Temperatures suggestive of an ultramafic lava composition similar to Pre-Cambrian komatiite
Komatiite
Komatiite is a type of ultramafic mantle-derived volcanic rock. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content...
s (about 1600 kelvin) are dominant at such eruptions, though superheating of the magma during ascent to the surface cannot be ruled out as a factor in the high eruption temperatures.
Fissure
In anatomy, a fissure is a groove, natural division, deep furrow, elongated cleft, or tear in various parts of the body.-Brain:...
north-west of Pillan Patera
Pillan Patera
Pillan Patera is a patera, or a complex crater with scalloped edges, on Jupiter's moon Io. It is located at , south of Pillan Mons and west of Reiden Patera. It is named after the Araucanian thunder, fire, and volcano god. Its name was approved by the International Astronomical Union in 1997.Pillan...
produced more than 31 cubic kilometres (7.4 cu mi) of fresh lava over a two and a half to five and a half month period, and later flooded the floor of Pillan Patera. Observations by Galileo suggest lava coverage rates at Pillan between 1000 and 3000 m² (1,196 and 3,588 ) per second during the 1997 eruption. The Pillan flow was found to be 10 metres (32.8 ft) thick, compared to the 1 metres (3.3 ft) thick flows observed at the inflated fields at Prometheus and Amirani. Similar, rapidly emplaced lava flows were observed by Galileo at Thor in 2001. Such flow rates are similar to those seen at Iceland
Iceland
Iceland , described as the Republic of Iceland, is a Nordic and European island country in the North Atlantic Ocean, on the Mid-Atlantic Ridge. Iceland also refers to the main island of the country, which contains almost all the population and almost all the land area. The country has a population...
'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. The 1997 Pillan eruption produced a 400 kilometres (248.5 mi) wide deposit of dark, silicate material and bright sulfur dioxide. The Tvashtar eruptions of 2000 and 2007 generated a 330 kilometres (205.1 mi) tall plume that deposited a ring of red sulfur and sulfur dioxide 1200 kilometres (745.6 mi) wide. Despite the dramatic appearance of these features, without continuous resupply of material, the vent surroundings often revert back to their pre-eruption appearance over a period of months (in the case of Grian Patera) or years (as at Pillan Patera).
Plumes
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
and sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
are ejected skyward from Io's volcanoes at speeds reaching 1 km/s, creating umbrella-shaped clouds of gas and dust. Additional materials that might be found in the volcanic plumes include sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
, potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
, and chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
. While striking in appearance, volcanic plumes are relatively uncommon. Of the 150 or so active volcanoes observed on Io, plumes have only been observed at a couple of dozen of them. The limited area of Io's lava flows suggests that much of the resurfacing needed to erase Io's cratering record must come from 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. Examples of Prometheus-type plumes include Prometheus
Prometheus (volcano)
Prometheus is an active volcano on Jupiter's moon Io. It is located on Io's hemisphere facing away from Jupiter at . Prometheus consists of a -wide volcanic pit named Prometheus Patera and a -long compound lava flow, all surrounded by reddish sulfur and circular, bright sulfur dioxide volcanic...
, Amirani
Amirani (volcano)
Amirani is an active volcano on Jupiter's moon Io. It is located on Io's leading hemisphere at .The volcano is responsible for the largest active lava flow in the entire Solar System, with recent flows dwarfing those of even other volcanos on Io...
, Zamama, and Masubi
Masubi (volcano)
Masubi is an active volcano on Jupiter's moon Io. It is located on Io's leading hemisphere at within a bright terrain region named Tarsus Regio. A volcanic plume has been observed at Masubi by various spacecraft starting with Voyager 1 in 1979, though it has not been persistent like similar...
. These plumes are usually less than 100 kilometres (62.1 mi) tall with eruption velocities around 0.5 km/s. Prometheus-type plumes are dust-rich, with a dense inner core and upper canopy shock zone
Gas Dynamics Shocks
In fluid dynamics, shock refers to an abrupt discontinuity in the flow field. It occurs in flows when the local flow speed exceeds the local sound speed. More specifically, it is a flow whose Mach number exceeds 1.-Explanation of phenomena:...
, giving them an umbrella-like appearance. These plumes often form bright circular deposits, with a radius ranging between 100 and 250 km (62.1 and 155.3 ) 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
New Horizons
New Horizons is a NASA robotic spacecraft mission currently en route to the dwarf planet Pluto. It is expected to be the first spacecraft to fly by and study Pluto and its moons, Charon, Nix, Hydra and S/2011 P 1. Its estimated arrival date at the Pluto-Charon system is July 14th, 2015...
in 2007. While the dust plume can be clearly seen in sunlit visible-light
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
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
Tephra
200px|thumb|right|Tephra horizons in south-central [[Iceland]]. The thick and light coloured layer at center of the photo is [[rhyolitic]] tephra from [[Hekla]]....
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
Pele (volcano)
Pele is an active volcano on the surface of Jupiter's moon Io. It is located on Io's trailing hemisphere at A large, tall volcanic plume has been observed at Pele by various spacecraft starting with Voyager 1 in 1979, though it has not been persistent. The discovery of the Pele plume on March...
, which is associated with a long-lived active lava lake eruption, though the plume is thought to be intermittent. The higher vent temperatures and pressures associated with these plumes generate eruption speeds of up to 1 km/s, allowing them to reach heights of between 300 and 500 km (186.4 and 310.7 ). Pele-type plumes form red (from short-chain sulfur) and black (from silicate pyroclastics) surface deposits, including large 1000 kilometres (621.4 mi)-wide red rings, as seen at Pele. They are generally fainter than Prometheus-type plumes as a result of the low dust content, causing some to be called stealth plumes. These plumes are sometimes only seen in images acquired while Io is in the shadow of Jupiter
Eclipse
An eclipse is an astronomical event that occurs when an astronomical object is temporarily obscured, either by passing into the shadow of another body or by having another body pass between it and the viewer...
or those taken in the ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
range. The little dust that is visible in sunlit images is generated when sulfur and sulfur dioxide condense as the gases reach the top of their ballistic trajectories. That is why these plumes lack the dense central column seen in Prometheus-type plumes, in which dust is generated at the plume source. Examples of Pele-type plumes have been observed at Pele, Tvashtar, and Grian.