Thor (volcano)

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Galileo image of Thor taken in October 2001 Galileo I32 Thor SSI.png
Galileo image of Thor taken in October 2001

Thor is an active volcano on Jupiter's moon Io. It is located on Io's anti-Jupiter hemisphere at 39°09′N133°08′W / 39.15°N 133.14°W / 39.15; -133.14 [2] Coordinates: 39°09′N133°08′W / 39.15°N 133.14°W / 39.15; -133.14 [2] . A major eruption with high thermal emission and a large, volcanic plume was observed during a Galileo flyby on August 6, 2001, when the spacecraft flew through the outer portions of the plume allowing for direct sampling. The eruption continued into Galileo's next flyby in October 2001. [1] [3] As seen during high-resolution images taken during the eruption, Thor consists of a series of dark lava flows emanating from a set of nearby volcanic depressions. [1] Before the eruption, the area consisted of red-brown plains, composed of irradiated sulfur, typical of Io's mid- to high-northern latitudes and a set of yellow flows, possibly consisting of sulfur or silicate flows covered by diffuse sulfur deposits. [4] During the New Horizons encounter in February 2007, Thor was still active, with the spacecraft observing thermal emission in the near-infrared and a volcanic plume at the volcano. [5]

Volcano A rupture in the crust of a planetary-mass object that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface

A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.

Jupiter Fifth planet from the Sun in the Solar System

Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a giant planet with a mass one-thousandth that of the Sun, but two-and-a-half times that of all the other planets in the Solar System combined. Jupiter and Saturn are gas giants; the other two giant planets, Uranus and Neptune, are ice giants. Jupiter has been known to astronomers since antiquity. It is named after the Roman god Jupiter. When viewed from Earth, Jupiter can reach an apparent magnitude of −2.94, bright enough for its reflected light to cast shadows, and making it on average the third-brightest natural object in the night sky after the Moon and Venus.

Natural satellite astronomical body that orbits a planet

A natural satellite or moon is, in the most common usage, an astronomical body that orbits a planet or minor planet.


Thor was named in 2006 by the International Astronomical Union (IAU) after the Norse god of thunder, Thor. [2]

Planetary nomenclature system of uniquely identifying features on the surface of a planet or natural satellite

Planetary nomenclature, like terrestrial nomenclature, is a system of uniquely identifying features on the surface of a planet or natural satellite so that the features can be easily located, described, and discussed. Since the invention of the telescope, astronomers have given names to the surface features they have discerned, especially on the Moon and Mars. To standardize planetary nomenclature, the International Astronomical Union (IAU) was assigned in 1919 the task of selecting official names for features on Solar System bodies.

International Astronomical Union Association of professional astronomers

The International Astronomical Union is an international association of professional astronomers, at the PhD level and beyond, active in professional research and education in astronomy. Among other activities, it acts as the internationally recognized authority for assigning designations and names to celestial bodies and any surface features on them.

Norse mythology body of mythology of the North Germanic people stemming from Norse paganism and continuing after the Christianization of Scandinavia and into the Scandinavian folklore of the modern period

Norse mythology is the body of myths of the North Germanic peoples, stemming from Norse paganism and continuing after the Christianization of Scandinavia, and into the Scandinavian folklore of the modern period. The northernmost extension of Germanic mythology, Norse mythology consists of tales of various deities, beings, and heroes derived from numerous sources from both before and after the pagan period, including medieval manuscripts, archaeological representations, and folk tradition.

2001 eruption

Prior to 2001, no active volcanic activity had been observed at Thor. [3] The appearance of the region had remained stable from Voyager observations of the region in 1979 through the Galileo mission until December 2000 at the latest. [6] In the first detailed observation of Thor, taken in July 1999, several bright, yellow flows were mapped. These flows either consist primarily of sulfur, or are cooled silicate flows coated in sulfur that has condensed on them. [4] Either way, no changes at these flows were observed in their size, color, or distribution through the end of 2000, suggesting that these flows were emplaced before the Voyager encounters. [4] [6] No thermal emission had ever been observed at Thor as late as May 2001, so the eruption observed later that year must have started after those observations. [3]

Voyager program American scientific program about the probes Voyager 1 and Voyager 2

The Voyager program is an American scientific program that employs two robotic probes, Voyager 1 and Voyager 2, to study the outer Solar System. The probes were launched in 1977 to take advantage of a favorable alignment of Jupiter, Saturn, Uranus and Neptune. Although their original mission was to study only the planetary systems of Jupiter and Saturn, Voyager 2 continued on to Uranus and Neptune. The Voyagers now explore the outer boundary of the heliosphere in interstellar space; their mission has been extended three times and they continue to transmit useful scientific data. Neither Uranus nor Neptune has been visited by a probe other than Voyager 2.

Sulfur Chemical element with atomic number 16

Sulfur (also spelled sulphur) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent, and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula S8. Elemental sulfur is a bright yellow, crystalline solid at room temperature.

Silicate class of chemical compounds, salts and esters of silicic acids

In chemistry, a silicate is any member of a family of anions consisting of silicon and oxygen, usually with the general formula [SiO(4−2x)−
, where 0 ≤ x < 2. The family includes orthosilicate SiO4−
, metasilicate SiO2−
, and pyrosilicate Si
. The name is also used for any salt of such anions, such as sodium metasilicate; or any ester containing the corresponding chemical group, such as tetramethyl orthosilicate.

August 2001

Colorized Galileo image from August 2001 showing the Thor plume Galileo I31 Thor plume colorized.png
Colorized Galileo image from August 2001 showing the Thor plume

On August 6, 2001, the Galileo spacecraft flew over Io's north polar region at an altitude of 194 kilometers (121  mi ). [7] The goal of the flyby was to image the source of the Tvashtar plume at high-resolution and sample the material in the plume directly. [1] Imaging during the encounter was prevented by a camera anomaly. Distant imaging acquired a few days before and after the encounter were successful. Images of a crescent Io were taken on August 4, 2001 to image the Tvashtar plume as context for closer and in situ observations during the encounter. Instead of a plume at Tvashtar, the images revealed a volcanic plume over Thor, suggesting that a major eruption was ongoing. [1] The plume at Thor had two components: an inner dust plume 100–125 km (62–78 mi) tall and a larger, fainter halo 440 km (270 mi) tall. This outer plume is one of the largest observed on Io (only the Grian Patera plume seen in July 1999 was larger). [8] The outer halo was composed of sulfur dioxide gas and fine, SO
dust grains 0.5-10 nanometers in size. [8] While the outer halo was fainter than the inner, optically-thick dust plume, the mass of outer halo was actually greater (at least 108 kg compared to 106-107 kg for the typical dust plume).

The kilometre, or kilometer is a unit of length in the metric system, equal to one thousand metres. It is now the measurement unit used officially for expressing distances between geographical places on land in most of the world; notable exceptions are the United States and the road network of the United Kingdom where the statute mile is the official unit used.

Mile Unit of length

The mile is an English unit of length of linear measure equal to 5,280 feet, or 1,760 yards, and standardised as exactly 1,609.344 metres by international agreement in 1959.

Tvashtar Paterae paterae on Io

Tvashtar Paterae compose an active volcanic region of Jupiter's moon Io located near its north pole. It is a series of paterae, or volcanic craters. It is named after Tvashtar, the Hindu god of blacksmiths. Tvashtar was studied by the Galileo spacecraft over several years. During this time, a 25-kilometre (16 mi) long, 1-to-2-kilometre high curtain of lava was seen to erupt from one patera, a lake of superheated silicate lava erupted in the largest patera, and finally a plume of gas burst out, rising 385 kilometres (239 mi) above Io and blanketing areas as far away as 700 kilometres (430 mi).

During the encounter, while the camera was not functioning properly, the other scientific instruments on Galileo were able to obtain observations of the Thor eruption. During closest approach, the Plasma Subsystem, an instrument designed to detect plasma in the vicinity of the spacecraft, sampled some of the material in the outer halo of the Thor plume, finding "snowflakes" weighing 500-1000 amu. [9] Assuming a pure sulfur dioxide composition, this suggested that the dust particles inferred by the camera's distant observation were made up of 15 to 20 molecules of sulfur dioxide. [8] [10] The Near-infrared Mapping Spectrometer (NIMS) mapped thermal emission and infrared spectra across Io's anti-Jupiter hemisphere shortly after the encounter, and found an intense thermal hotspot at Thor with a near-infrared spectrum consistent with an explosion-dominated eruption. NIMS found high eruption temperatures at Thor suggesting exposed, silicate lava and a high power output indicating a high flow rate for the lava at Thor. Prior to its official naming by the IAU, NIMS scientists designated the eruption I31A, as the first new eruption detected during Galileo orbit I31. [3]

Plasma (physics) State of matter

Plasma is one of the four fundamental states of matter, and was first described by chemist Irving Langmuir in the 1920s. Plasma can be artificially generated by heating or subjecting a neutral gas to a strong electromagnetic field to the point where an ionized gaseous substance becomes increasingly electrically conductive, and long-range electromagnetic fields dominate the behaviour of the matter.

The unified atomic mass unit or dalton is a standard unit of mass that quantifies mass on an atomic or molecular scale. One unified atomic mass unit is approximately the mass of one nucleon and is effectively numerically equivalent to 1 g/mol. It is defined as one twelfth of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest, and has a value approaching 1.66053906660(50)×10−27 kg, or approximately 1.66 yoctograms. The CIPM has categorised it as a non-SI unit accepted for use with the SI, and whose value in SI units must be obtained experimentally.

Another imaging observation taken on August 8 showed the effects of this eruption on the surface of Io, as a new dark spot was observed surrounding the Thor volcano and a bright ring composed of fresh, fine-grained sulfur dioxide frost deposited by the plume. [1] [11] In some areas of the white plume deposit, the areal coverage of SO
frost had increased from 60-70% to 100% as a result of this eruption. [11] The size of the plume deposit is consistent with being formed by Thor's inner dust plume. [8] NIMS data suggests that the outer plume may form a deposit of very fine-grained SO
that is transparent at visible wavelengths, while the inner plume deposit is thicker and contains larger frost grains, which would appear bright at visible wavelengths. [11] Unlike many large, "outburst" eruptions, no red deposits were observed at Thor, suggesting that the upper lithosphere of Io contains heterogeneities in the distribution of sub-surface sulfur. [4]

Wavelength spatial period of the wave—the distance over which the waves shape repeats, and thus the inverse of the spatial frequency

In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is thus the inverse of the spatial frequency. Wavelength is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings and is a characteristic of both traveling waves and standing waves, as well as other spatial wave patterns. Wavelength is commonly designated by the Greek letter lambda (λ). The term wavelength is also sometimes applied to modulated waves, and to the sinusoidal envelopes of modulated waves or waves formed by interference of several sinusoids.

October 2001

Surface changes at Thor between July 1999 and October 2001 Thor Comparison C21 I32.png
Surface changes at Thor between July 1999 and October 2001

Galileo flew by Io again on October 16, 2001, this time passing over the satellite's south polar region at an altitude of 184 km (114 mi). As a result of the discovery of the Thor eruption during the previous flyby, the observation plan was adjusted so that the camera and near-infrared spectrometer could take high-resolution images and spectra of the new eruption site. The camera acquired a single, clear-filter frame over the volcano with a spatial resolution of 334 meters (1,100 ft) per pixel. [7] The image revealed several new dark, silicate lava flows, many surrounded by dark, pyroclastic flow deposits. [1] The dark flows generally covered over the previously observed yellow flows, though by October 2001, some of those older flows remained visible. The source of a large dark flow on the eastern side of the volcano appears to be a fissure 50 by 17 km (31 by 11 mi) in size. This fissure maybe a patera, or volcanic depression, in the process of forming. [4] Distant color imaging taken a few hours after the flyby showed that the volcanic plume at Thor was still visible. [1] [12]

NIMS also observed Thor at high resolution. It found Thor was still vigorously erupting, though the power output was lower than it was in August 2001. [3] The most intense part of the eruption (in terms of total power output) was centered over the large eastern lava flow observed by the camera team. NIMS also found thermal emission from several nearby paterae, where no volcanic activity had previously been observed. This activity coincided with a darkening of the floors of these volcanoes, as a result of fresh lava flows or the sublimation of sulfur deposits seen by the camera on Galileo. Activity at nearby volcanoes suggested that the magma plumbing system below Thor extended to these features as well, producing renewed volcanic activity on a regional scale. [3]

After Galileo

While the Galileo observations of Thor in October 2001 were the last for the spacecraft, the 2001 eruption continued to be observed by Earth-based astronomers. Thermal emission from Thor was seen from the Keck telescope in Hawaii on December 22, 2001. [13] Volcanic activity continued even into the New Horizons encounter in February 2007, when a thermal hotspot and a faint volcanic plume 100 km (62 mi) tall were spotted at Thor. However, the plume and much of the dark pyroclastic flow deposits had faded or were covered by a new plume at Tvashtar by that time. [5]

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Volcanology The study of volcanoes, lava, magma and associated phenomena

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Io (moon) Innermost of the four Galilean moons of Jupiter

Io is the innermost of the four Galilean moons of the planet Jupiter. It is the fourth-largest moon, has the highest density of all the moons, and has the least amount of water molecules of any known astronomical object in the Solar System. It was discovered in 1610 and was named after the mythological character Io, a priestess of Hera who became one of Zeus' lovers.

Loki Patera The largest volcanic depression on Jupiters moon Io

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Pele (volcano) volcano on Io

Pele is a very active volcano on the surface of Jupiter's moon Io. It is located on Io's trailing hemisphere at 18.7°S 255.3°W. A large, 300-kilometer (190 mi) 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 8, 1979 confirmed the existence of active volcanism on Io. The plume is associated with a lava lake at the northern end of the mountain Danube Planum. Pele is also notable for a persistent, large red circle that surrounds the volcano resulting from sulfurous fallout from the volcanic plume.

Volcanology of Io volcanology of Io, a moon of Jupiter

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Masubi (volcano) volcano on Io

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Tupan Patera patera on Io

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Tawhaki Vallis vallis on Io

Tawhaki Vallis is a shallow valley on Jupiter's moon Io. It is located on Io's leading hemisphere in the equatorial plains of western Media Regio at 0.5°N 72.8°W. The valley is 190 kilometers long, 0.5 to 6 km wide, and 40 to 65 meters deep. Due to the shallow depth and lack of brightness or color variations associated with it, Tawhaki Vallis was seen in only a single, high-spatial-resolution observation taken by Galileo during an Io encounter on November 26, 1999. The northern and southern ends of the valley are cut off by the northern margin of the observation and the dusk terminator, so Tawhaki could be longer than the measured length. The valley was formally named Tawhaki Vallis by the International Astronomical Union in 2006 after a nearby volcano, Tawhaki Patera, and the Māori lightning god, Tāwhaki.

Tawhaki Patera patera on Io

Tawhaki Patera is an active volcano on Jupiter's moon Io. It is located on Io's leading hemisphere at 3.32°N 76.18°W within the equatorial plains of western Media Regio. Tawhaki is an Ionian patera, a type of volcanic crater similar to a caldera, 49.8 kilometers (30.9 mi) wide and 550 meters (1,800 ft) deep.

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Danube Planum planum on Io

Danube Planum is a rifted mesa on the surface of Jupiter's moon Io. It is located on Io's trailing hemisphere at 22.73°S 257.44°W. Danube Planum is 244.22 kilometers across and 5.5 km tall. The mountain is bisected by a 15-to-25-kilometer-wide, northeast-southwest-trending canyon, splitting the mountain into two main east and west mountains, with several additional blocks at the southern end of the fracture. The outer margin of the plateau is marked by 2.6-to-3.4-km-tall scarps. Mass wasting in the form of landslide deposits are visible along the base of the western half of Danube Planum. Two volcanic depressions, known as paterae, lie at northern and southern ends of mountain. The volcano at the northern end, Pele, is one of the most active volcanoes on Io. One of the faults that helped form Danube Planum may also act as a conduit for magma to rise to the surface at Pele.

Exploration of Io

The exploration of Io, Jupiter's third-largest moon, began with its discovery in 1610 and continues today with Earth-based observations and visits by spacecraft to the Jupiter system. Italian astronomer Galileo Galilei was the first to record an observation of Io on January 8, 1610, though Simon Marius may have also observed Io at around the same time. During the 17th century, observations of Io and the other Galilean satellites helped with the measurement of longitude by map makers and surveyors, with validation of Kepler's Third Law of planetary motion, and with measurement of the speed of light. Based on ephemerides produced by astronomer Giovanni Cassini and others, Pierre-Simon Laplace created a mathematical theory to explain the resonant orbits of three of Jupiter's moons, Io, Europa, and Ganymede. This resonance was later found to have a profound effect on the geologies of these moons. Improved telescope technology in the late 19th and 20th centuries allowed astronomers to resolve large-scale surface features on Io as well as to estimate its diameter and mass.

Thomagata Patera patera on Io

Thomagata Patera is a volcano on Jupiter's moon Io. It is located on Io's anti-Jupiter hemisphere at 25.67°N 165.94°W, to the east of the nearby active volcanoes Volund and Zamama. Thomagata is a kidney-shaped Ionian patera, a type of volcanic crater similar to a caldera, 56 kilometers (35 mi) long, 26 km (16 mi) wide, and 1.2–1.6 km (0.7–1.0 mi) deep. The volcano is currently inactive as a thermal hotspot has never been observed at Thomagata and the bright floor of the patera suggests that it is cold enough for sulfur dioxide and sulfur to condense. Thomagata is located near the center of a low, 100 km (62 mi) wide mesa. The edge of the mesa rises 200 meters (660 ft) above the surrounding plains, however the slope up to the edge of Thomagata Patera is unknown. If the floor of the patera is at the same level as the surrounding plains, the western slope of the mesa would have a grade of 2°. The morphology of this mesa and the pattern of faded lava flows along its slopes radiating away from Thomagata suggest that Thomagata Patera and the mesa that surrounds it may be a shield volcano, also called a tholus on Io. The irregular margin of the mesa and the lack of debris at the base of its basal scarp suggest that it was modified by sulfur dioxide sapping.

Kanehekili Fluctus fluctus on Io

Kanehekili Fluctus is a lava flow field on Jupiter's moon, Io. This fluctus is located in the sub-Jovian hemisphere at 17.68°S 33.56°W as shown in the picture on the right. Also in the picture is the Kanehekili volcanic center located at 18.21°S 33.6°W. This lava field covers roughly 34,500 square kilometres (13,300 sq mi). The hotspot was detected by the Galileo Solid State Imaging experiment (SSI) on orbits by Galileo.

Zamama (volcano) volcano on Io

Zamama is an active volcanic center on Jupiter's moon Io. This volcanic center erupted after the Voyager 1 flyby in 1979, making it one of the few planetary volcanoes known to have activated during this generation's lifetime. Further analysis and study by the Galileo spacecraft helped with the overall study of Io's volcanism. Galileo located it on Io at 21°N173°W. Zamama has a fissure-fed-type flow that is 150 km (93 mi) long with temperatures of 1,100 K, and the volcanic center site has explosive and effusive eruption characteristics. The flow appears to be emanating from the Promethean-type volcano.

Chaac-Camaxtli region

The Chaac-Camaxtli region is a volcanic region on Jupiter's moon Io, located from approximately 5 to 20°N and 130 to 160°W in its anti-Jovian hemisphere. It consists mainly of the hummocky bright plains that occupy the surface. This area is defined on the west by Chaac Patera, and on the east by Camaxtli Patera. At least 10 distinct volcanic centers are located in the region, making it a volcanically active region on Io's surface. Most of the volcanism here is expressed as paterae, which range in size from circular to elliptical. A patera is defined by the International Astronomical Union as "irregular or complex craters with scalloped edges." The largest volcanic structure here is the Chaac Patera. The paterae found in the Chaac-Camaxtli region are Chaac, Balder Patera, Grannos, Ababinili, Ruaumoko, Steropes, Camaxtli, Tien Mu, Utu, and Mentu.


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