Thor (volcano)
Encyclopedia
Thor is an active volcano
on Jupiter's
moon
Io
. It is located on Io's anti-Jupiter hemisphere at 39.15°N 133.14°W. 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. 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. 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. 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.
Thor was named
in 2006 by the International Astronomical Union
(IAU) after the Norse
god of thunder, Thor
.
observations of the region in 1979 through the Galileo mission until December 2000 at the latest. 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. 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. 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.
plume at high-resolution and sample the material in the plume directly. 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. The plume at Thor had two components: an inner dust plume 100–125 km (62.1–77.7 mi) tall and a larger, fainter halo 440 km (273.4 mi) tall. This outer plume is one of the largest observed on Io (only the Grian Patera plume seen in July 1999 was larger). The outer halo was composed of sulfur dioxide
gas and fine, dust grains 0.5-10 nanometers in size. 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).
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
. 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. 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.
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. In some areas of the white plume deposit, the areal coverage of frost had increased from 60-70% to 100% as a result of this eruption. The size of the plume deposit is consistent with being formed by Thor's inner dust plume. NIMS data suggests that the outer plume may form a deposit of very fine-grained that is transparent at visible wavelength
s, while the inner plume deposit is thicker and contains larger frost grains, which would appear bright at visible wavelengths. 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.
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 metres (1,095.8 ft) per pixel
. The image revealed several new dark, silicate lava flows, many surrounded by dark, pyroclastic flow
deposits. 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 in size. This fissure maybe a patera, or volcanic depression, in the process of forming. Distant color imaging taken a few hours after the flyby showed that the volcanic plume at Thor was still visible.
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. 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.
on December 22, 2001. 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.
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...
on Jupiter's
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
moon
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....
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....
. It is located on Io's anti-Jupiter hemisphere at 39.15°N 133.14°W. 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
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...
encounter in February 2007, Thor was still active, with the spacecraft observing thermal emission in the near-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...
and a volcanic plume at the volcano.
Thor was named
Planetary nomenclature
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. The task of assigning official names to features is taken up by the International...
in 2006 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...
(IAU) after the Norse
Norse mythology
Norse mythology, a subset of Germanic mythology, is the overall term for the myths, legends and beliefs about supernatural beings of Norse pagans. It flourished prior to the Christianization of Scandinavia, during the Early Middle Ages, and passed into Nordic folklore, with some aspects surviving...
god of thunder, Thor
Thor
In Norse mythology, Thor is a hammer-wielding god associated with thunder, lightning, storms, oak trees, strength, the protection of mankind, and also hallowing, healing, and fertility...
.
2001 eruption
Prior to 2001, no active volcanic activity had been observed at Thor. The appearance of the region had remained stable from VoyagerVoyager 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...
observations of the region in 1979 through the Galileo mission until December 2000 at the latest. In the first detailed observation of Thor, taken in July 1999, several bright, yellow flows were mapped. These flows either consist primarily 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...
, or are cooled silicate
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...
flows coated in sulfur that has condensed on them. 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. 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.
August 2001
On August 6, 2001, the Galileo spacecraft flew over Io's north polar region at an altitude of 194 kilometres (120.5 mi). The goal of the flyby was to image the source of the TvashtarTvashtar Paterae
Tvashtar Paterae compose an active volcanic region of Jupiter's moon Io located near the moon's north pole. It is a series of paterae, or volcanic craters. It is named after Tvashtar, the Hindu god of blacksmiths....
plume at high-resolution and sample the material in the plume directly. 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
Lunar phase
A lunar phase or phase of the moon is the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. The lunar phases change cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon, and Sun...
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. The plume at Thor had two components: an inner dust plume 100–125 km (62.1–77.7 mi) tall and a larger, fainter halo 440 km (273.4 mi) tall. This outer plume is one of the largest observed on Io (only the Grian Patera plume seen in July 1999 was larger). The outer halo was composed of 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...
gas and fine, dust grains 0.5-10 nanometers in size. 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).
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
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
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
Atomic mass unit
The unified atomic mass unit or dalton is a unit that is used for indicating mass on an atomic or molecular scale. It is defined as one twelfth of the rest mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state, and has a value of...
. 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. 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.
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. In some areas of the white plume deposit, the areal coverage of frost had increased from 60-70% to 100% as a result of this eruption. The size of the plume deposit is consistent with being formed by Thor's inner dust plume. NIMS data suggests that the outer plume may form a deposit of very fine-grained that is transparent at visible wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It 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...
s, while the inner plume deposit is thicker and contains larger frost grains, which would appear bright at visible wavelengths. 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.
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 spectrometerSpectrometer
A spectrometer is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the light's intensity but could also, for instance, be the polarization...
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 metres (1,095.8 ft) per pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
. The image revealed several new dark, silicate lava flows, many surrounded by dark, pyroclastic flow
Pyroclastic flow
A pyroclastic flow is a fast-moving current of superheated gas and rock , which reaches speeds moving away from a volcano of up to 700 km/h . The flows normally hug the ground and travel downhill, or spread laterally under gravity...
deposits. 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
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...
50 by in size. This fissure maybe a patera, or volcanic depression, in the process of forming. Distant color imaging taken a few hours after the flyby showed that the volcanic plume at Thor was still visible.
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. 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
Sublimation
Sublimation may refer to:* Sublimation , the change from solid to gas without entering liquid phase* Sublimation , the transformation of emotions* Sublimation , a music album by Canvas Solaris-See also:...
of sulfur deposits seen by the camera on Galileo. Activity at nearby volcanoes suggested that the magma plumbing system
Magma chamber
A magma chamber is a large underground pool of molten rock found beneath the surface of the Earth. The molten rock in such a chamber is under great pressure, and given enough time, that pressure can gradually fracture the rock around it creating outlets for the magma...
below Thor extended to these features as well, producing renewed volcanic activity on a regional scale.
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 HawaiiHawaii
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...
on December 22, 2001. Volcanic activity continued even into the 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...
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.