Overview
With over 400 active volcanoes, Io is the most geologically active object in the Solar System. This extreme geologic activity is the result of tidal heating from friction generated within Io's interior as it is pulled between Jupiter and the other Galilean satellites—Europa, Ganymede and Callisto. Several volcanoes produce plumes of sulfur and sulfur dioxide that climb as high as 500 km (310 mi) above the surface. Io's surface is also dotted with more than 100 mountains that have been uplifted by extensive compression at the base of the moon's silicate crust. Some of these peaks are taller than Earth's Mount Everest. Unlike most satellites in the outer Solar System, which are mostly composed of water ice, Io is primarily composed of silicate rock surrounding a molten iron or iron sulfide core. Most of Io's surface is characterized by extensive plains coated with sulfur and sulfur dioxide frost.
Io's volcanism is responsible for many of that satellite's unique features. Its volcanic plumes and lava flows produce large surface changes and paint the surface in various shades of yellow, red, white, black, and green, largely due to allotropes and compounds of sulfur. Numerous extensive lava flows, several longer than 500 kilometres (311 mi) in length, also mark the surface. These volcanic processes have given rise to a comparison of the visual appearance of Io's surface to a pizza. The materials produced by this volcanism provide material for Io's thin, patchy atmosphere and Jupiter's extensive magnetosphere.
Io played a significant role in the development of astronomy in the 17th and 18th centuries. It was discovered in 1610 by Galileo Galilei, along with the other Galilean satellites. This discovery furthered the adoption of the Copernican model of the Solar System, the development of Kepler's laws of motion, and the first measurement of the speed of light. From Earth, Io remained nothing more than a point of light until the late 19th and early 20th centuries, when it became possible to resolve its large-scale surface features, such as the dark red polar and bright equatorial regions. In 1979, the two Voyager spacecraft revealed Io to be a geologically active world, with numerous volcanic features, large mountains, and a young surface with no obvious impact craters. The Galileo spacecraft performed several close flybys in the 1990s and early 2000s, obtaining data about Io's interior structure and surface composition. These spacecraft also revealed the relationship between the satellite and Jupiter's magnetosphere and the existence of a belt of radiation centered on Io's orbit. Io receives about 3,600 rem of radiation per day.
Further observations have been made by Cassini–Huygens in 2000 and New Horizons in 2007, as well as from Earth-based telescopes and the Hubble Space Telescope as their technology has advanced.
Nomenclature
While Simon Marius is not credited with the sole discovery of the Galilean satellites, his names for the moons have stuck. In his 1614 publication Mundus Iovialis anno M.DC.IX Detectus Ope Perspicilli Belgici, he proposed several possible names for the innermost of the large moons of Jupiter, including The Mercury of Jupiter or The First of the "Jovian Planets". Based on a suggestion from Johannes Kepler in October 1613, he also generated a naming scheme so that each moon was given its own name based on the lovers of the Greek mythological Zeus or his Roman equivalent, Jupiter. In this case, he named the innermost large moon of Jupiter after the Greek mythological figure Io. The most common adjectival form of the name is Ionian. Marius' names fell out of favor, and were not revived in common use until the mid-20th century. In much of the earlier astronomical literature, Io is simply referred to by its Roman numeral designation (a system introduced by Galileo) as "Jupiter I", or simply as "the first satellite of Jupiter".
Features on Io are named after characters and places from the Io myth, as well as deities of fire, volcanoes, the Sun, and thunder from various myths, and characters and places from Dante's Inferno, names appropriate to the volcanic nature of the surface. Since the surface was first seen up close by Voyager 1 the International Astronomical Union has approved 225 names for Io's volcanoes, mountains, plateaus, and large albedo features. The approved feature categories used for Io for different types of volcanic features include patera (volcanic depression), fluctus (lava flow), vallis (lava channel), and active eruptive center (location where plume activity was the first sign of volcanic activity at a particular volcano). Named mountains, plateaus, layered terrain, and shield volcanoes use the terms mons, mensa, planum, tholus, respectively. Named, bright albedo regions use the term regio. Examples of named features include Prometheus, Pan Mensa, Tvashtar Paterae, and Tsũi Goab Fluctus.
The Exploration of Io
The advent of unmanned spaceflight in the 1950s and 1960s provided an opportunity to observe Io up-close. The flybys of the two Pioneer probes, Pioneer 10 and 11 in 1973 and 1974, provided the first accurate measurement of Io's mass and size. Data from the Pioneers also revealed an intense belt of radiation near Io and suggested the presence of an Ionian atmosphere. In 1979, the two Voyager spacecraft flew through the Jupiter system. Voyager 1, during its encounter in March 1979, observed active volcanism on Io for the first time and mapped its surface in great detail, particularly the side that faces Jupiter. The Voyagers observed the Io plasma torus and Io's sulfur dioxide (SO2) atmosphere for the first time. NASA launched the Galileo spacecraft in 1989, which entered Jupiter's orbit in December 1995. Galileo allowed detailed study of both the planet and its satellites, including six flybys of Io between late 1999 and early 2002 that provided high-resolution images and spectra of Io's surface, confirming the presence of high-temperature silicate volcanism on Io. Distant observations by Galileo allowed planetary scientists to study changes on the surface that resulted from the moon's active volcanism.
Following Galileo and a distant encounter by the Pluto-bound New Horizons spacecraft in 2007, NASA and the European Space Agency (ESA) made plans to return to the Jupiter system and Io. In 2009, NASA approved a plan to send an orbiter to Europa called the Jupiter Europa Orbiter as part of a joint program with ESA called the Europa/Jupiter System Mission. The ESA component of the project, the Jupiter Ganymede Orbiter, is on their shortlist of large-scale missions to be launched in the next decade, with final approval expected in 2011. While these missions will perform Io science as ancillary to their primary missions, the proposed NASA Discovery mission Io Volcano Observer, which still needs to go through a competitive process to be approved, would explore Io as its primary mission. In the meantime, Io continues to be observed by the Hubble Space Telescope as well as by Earth-based astronomers using improved telescopes such as Keck and the European Southern Observatory, which use new technologies such as adaptive optics.
Discovery: 1610
In his book Mundus Iovialis ("The World of Jupiter"), published in 1614, Simon Marius, the court astronomer to the Margraves of Brandenburg-Ansbach, claimed to have discovered Io and the other moons of Jupiter in 1609, one week before Galileo's discovery. According to Marius, he began observing the Jupiter system in late November 1609. He continued to observe the moons of Jupiter through December 1609, but did not record his observations until December 29, 1609 when he came to the conclusion "that these stars moved round Jupiter, just as the five solar planets, Mercury, Venus, Mars, Jupiter, and Saturn revolve round the sun." However, Marius' observations were dated based on the Julian calendar, which was 10 days behind the Gregorian calendar used by Galileo. So Marius' first recorded observation from December 29, 1609 equates to Galileo's second observation of the Jupiter system on January 8, 1610. Galileo doubted this claim and dismissed the work of Marius as plagiarism. Given that Galileo published his work before Marius and that his first recorded observation came one day before Marius', Galileo is credited with the discovery. Despite this, it is one of Marius' naming schemes for the moons of Jupiter that is regularly used today. Based on a suggestion from Johannes Kepler in October 1613, he proposed that each moon was given its own name based on the lovers of the Greek mythological Zeus or his Roman equivalent, Jupiter. He named the innermost large moon of Jupiter after the Greek mythological figure Io.
Io as a tool: 1610–1809
From these estimates of the orbital periods of Io and the other Galilean moons, astronomers hoped to generate ephemeris tables predicting the positions of each moon with respect to Jupiter, as well as when each moon would transit the face of Jupiter or be eclipsed by it. One benefit of such predictions, particularly those of satellite eclipses by Jupiter since they were subject to less observer error, would be determining an observer's longitude on Earth with respect to the prime meridian. By observing an eclipse of a Jovian satellite, an observer could determine the current time at the prime meridian by looking up the eclipse in an ephemeris table. Io was particularly useful for this purpose since its shorter orbital period and closer distance to Jupiter made eclipses more frequent and less affected by Jupiter's axial tilt. Knowing the time at the prime meridian and the local time, the observer's longitude could then be calculated. Galileo attempted to produce a table predicting the positions of the Jovian satellites and eclipse timings after he negotiated first with Spain and then with The Netherlands to create a system for measuring longitude at sea using eclipse timings. However, he was never able to generate accurate predictions far enough ahead in time to be useful so he never published his tables. This left the tables published by Simon Marius in Mundus Iovialis and Giovanni Battista Hodierna in 1654 as the most accurate ephemeris tables available, even though they too were unable to predict the moons' positions with sufficient accuracy.
Giovanni Cassini published a much more accurate ephemeris table in 1668 using his observations from the previous 16 years. Using this table, Cassini generated a more accurate map of France by observing eclipses of the Jovian satellites at various locations across the country. This showed that previous maps had depicted some shorelines as extending farther than they really did, which caused the apparent area of France to shrink, and led King Louis XIV to comment that "he was losing more territory to his astronomers than to his enemies." Eclipse timings of the Jovian moons would continue to be used to determine longitude for another hundred years for tasks such as surveying the Mason-Dixon Line and geodesy measurements. Efforts were made to utilize this method for marine navigation, but it proved to be impossible to make the necessary observations with sufficient accuracy from the moving deck of a ship; it would not be until the invention of the marine chronometer in the mid-18th century that determining longitude at sea became practical.
In 1788, Pierre-Simon Laplace used Cassini's ephemerides and those produced by other astronomers in the preceding century to create a mathematical theory explaining the resonant orbits of Io, Europa, and Ganymede. The ratios of the orbital periods of the inner three Galilean moons are simple integers: Io orbits Jupiter twice every time Europa orbits once, and four times for each revolution by Ganymede; this is sometimes referred to as the Laplace resonance. Laplace also found that the slight difference between these exact ratios and reality was due to their mean motions accounting for the precession of the periapse for Io and Europa. This resonance was later found to have a profound effect on the geologies of the three moons.
Io as a world: 1805–1973
Beginning in the 1890s, larger telescopes allowed astronomers to directly observe large scale features on the surfaces of the Galilean satellites including Io. In 1892, William Pickering measured Io's shape using a micrometer, and similar to his measurement of Ganymede, found it to have an elliptical outline aligned with the direction of its orbital motion. Other astronomers between 1850 and 1895 noted Io's elliptical shape. Edward Barnard observed Io while it transited across the face of Jupiter, finding the poles of Io to be dark compared to a brighter equatorial band. Initially, Barnard concluded that Io was in fact a binary of two dark bodies, but observations of additional transits against Jovian cloud bands of different brightness and the round shape of Io's shadow on the Jovian cloud tops caused him to change his interpretation. The egg-shape of Io reported by Pickering was the result of measuring only the bright equatorial band of Io, and mistaking the dark poles for background space. Later telescopic observations confirmed Io's distinct reddish-brown polar regions and yellow-white equatorial band. Observations of variations in the brightness of Io as it rotated, made by Joel Stebbins in the 1920s, showed that Io's day was the same length as its orbital period around Jupiter, thus proving that one side always faced Jupiter just as the Moon's near-side always faces the Earth. Stebbins also noted Io's dramatic orange coloration, which was unique among the Galilean satellites. Audouin Dollfus used observations of Io in the early 1960s at the Pic du Midi Observatory to create crude maps of the satellite that showed a patchwork of bright and dark spots across the Ionian surface, as well as a bright equatorial belt and dark polar regions.
Telescopic observations in the mid-20th century began to hint at Io's unusual nature. The near-infrared spectroscopy suggested that Io's surface was devoid of water ice. The lack of water on Io was consistent with the moon's estimated density, although, abundant water ice was found on the surface of Europa, a moon thought to have the same density as Io. Lee concluded that the spectrum was consistent with the presence of sulfur compounds. Binder and Cruikshank (1964) reported that Io's surface was brighter coming out of Jupiter's shadow than when it entered it. The authors suggested that this anomalous brightening after an eclipse was the result of an atmosphere partially freezing out onto the surface during the eclipse darkness with the frost slowly sublimating away after the eclipse. Attempts to confirm this result met with mixed results: some researchers reported a post-eclipse brightening, while others did not. Later modeling of Io's atmosphere would show that such brightening would only be possible if Io's SO2 atmosphere froze out enough to produce a layer several millimeters thick, which seemed unlikely. Radio telescopic observations revealed Io's influence on the Jovian magnetosphere, as demonstrated by decametric wavelength bursts tied to the orbital period of Io (Io-DAM), suggesting an electrodynamic coupling between the two worlds.
Pioneer era: 1973–1979
Pioneer 10 became the first spacecraft to reach the Jupiter system on December 3, 1973. It passed within 357,000 km (222,000 mi) of Io. During Pioneer 10's fly-by of Io, the spacecraft performed a radio occultation experiment by transmitting an S-band signal as Io passed between it and Earth. A slight attenuation of the signal before and after the occultation showed that Io had an ionosphere, suggesting the presence of a thin atmosphere with a pressure of 1.0 × 10−7 bar, though the composition was not determined. This was the second atmosphere to be discovered around a moon of an outer planet, after Saturn's moon Titan. Close-up images using Pioneer's Imaging Photopolarimeter were planned as well, but were lost because of the high-radiation environment. Pioneer 10 also discovered a hydrogen ion torus at the orbit of Io.
Following the Pioneer encounters and in the lead up to the Voyager fly-bys in 1979, interest in Io and the other Galilean satellites grew, with the planetary science and astronomy communities going so far as to convene a week of dedicated Io observations by radio, visible, and infrared astronomers in November 1974 known as "Io Week." New observations of Io from Earth and by the Pioneers during the mid-1970s caused a paradigm shift in thinking about its surface chemistry and formation. The trend in the densities of the four Galilean satellites found by Pioneer 10 suggested that the satellites formed as part of a collapsing nebula, like a miniature version of what took place in the solar system as a whole. The initial hot Jupiter prevented the condensation of water at the orbits of Io and Europa, leading those bodies to have higher densities than the outer two moons. Spectroscopic measurements of the light reflected from Io and its surrounding space were made with increasing spectral resolution during the 1970s, providing new insights into its surface composition. Other observations suggested that Io had a surface dominated by evaporites composed of sodium salts and sulfur. This was consistent with Io lacking water ice either on its surface or in its interior, in contrast with the other Galilean satellites. An absorption band near 560 nm was identified with the radiation-damaged form of the mineral halite. It was thought that deposits of the mineral on Io's surface were the origin of a cloud of sodium atoms surrounding Io, created through energetic-particle sputtering.
Measurements of Io's thermal radiation in the mid-infrared spectrum in the 1970s led to conflicting results that were not explained accurately until after the discovery of the active volcanism by Voyager 1 in 1979. An anomalously high thermal flux, compared to the other Galilean satellites, was observed at an infrared wavelength of 10 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 and Ganymede. 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. NASA researchers observed a sharp increase in Io's thermal emission at 5 μm on February 20, 1978, possibly due to an interaction between the satellite and Jupiter's magnetosphere, though volcanism was not ruled out.
A few days before the Voyager 1 encounter, Stan Peale, Patrick Cassen, and R. T. Reynolds published a paper in the journal Science predicting a volcanically modified surface and a differentiated interior, with distinct rock types rather than a homogeneous blend. They based this prediction on models of Io's interior that took into account the massive amount of heat produced by the varying tidal pull of Jupiter on Io resulting from Io's Laplace resonance with Europa and Ganymede not allowing its orbit to circularize. Their calculations suggested that the amount of heat generated for an Io with a homogeneous interior would be three times greater than the amount of heat generated by radioactive isotope decay alone. This effect would be even greater with a differentiated Io.
Voyager era: 1979–1995
Voyager 1 was first of the two probes to encounter the Jupiter system in March 1979. On approach to Jupiter in late February and early March 1979, Voyager imaging scientists noticed that Io appeared distinct from the other Galilean satellites. Its surface was orange in color and marked by dark spots, which were initially interpreted as the sites of impact craters. Among the most intriguing features was a heart-shaped, dark ring 1,000 km (600 mi) across that would later turn out to be the plume deposit of the volcano Pele. The data from the Ultraviolet Spectrometer (UVS) revealed a torus of plasma composed of sulfur ions at the orbit of Io, but tilted to match the equator of Jupiter's magnetic field. The Low-Energy Charged Particle (LECP) detector encountered streams of sodium, sulfur, and oxygen ions prior to entering Jupiter's magnetosphere, material that the LECP science team suspected originated from Io. In the hours prior to Voyager 1's encounter with Io, the spacecraft acquired images for a global map with a resolution of at least 20 km (12 mi) per pixel over the satellite's leading hemisphere (the side that faces the moon's direction of motion around Jupiter) down to less than 1 km (0.6 mi) per pixel over portions of the sub-Jovian hemisphere (the "near" side of Io). The images returned during the approach revealed a strange, multi-colored landscape devoid of impact craters, unlike the other planetary surfaces imaged to that point such as the Moon, Mars, and Mercury. The dark spots in earlier images resembled volcanic calderas more than they did the impact craters seen on those other worlds. Stunned by the oddity of Io's surface, Voyager imaging scientist Laurence Soderblom at a pre-encounter press conference joked, "this one we got all figured out...[Io] is covered with thin candy shells of anything from sulfates and sulfur and salts to all kinds of strange things."
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 found a 300-kilometer (190 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, the feature surrounded by a dark, footprint-shaped ring seen in approach images. Analysis of other Voyager 1 images showed nine such plumes scattered across the surface, proving that Io was volcanically active. The Infrared Interferometer Spectrometer (IRIS) on Voyager 1 discovered thermal emission from multiple sources, indicative of cooling lava. This showed that some of the lava flows visible on the Io's surface were active. IRIS also measured gaseous SO2 within the Loki plume, providing additional evidence for an atmosphere on Io. These results confirmed the prediction made by Peale et al. shortly before the encounter.
Just after the Voyager encounters, the accepted theory was that Io's lava flows were composed of sulfurous compounds. This was based on the color of volcanic terrains, and the low temperatures measured by the IRIS instrument (though IRIS was not sensitive to the high-temperatures associated with active silicate volcanism, where thermal emission peaks in the near-infrared). However, 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 higher than the boiling point of sulfur, indicating a silicate composition for at least some of Io's lava flows. Similar temperatures were observed at the Surt eruption in 1979 between the two Voyager encounters, and at the eruption observed by NASA researchers 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. Spectra from Earth-based observations confirmed the presence of an atmosphere at Io, with significant density variations across Io's surface. These measurements suggested that Io's atmosphere was produced by either the sublimation of sulfur dioxide frost, or from the eruption of gases at volcanic vents, or both.
Galileo era: 1995–2003
Galileo arrived at Jupiter on December 7, 1995, after a six-year journey from Earth during which it used gravity assists with Venus and Earth to boost its orbit out to Jupiter. Shortly before Galileo's Jupiter Orbit Insertion maneuver, the spacecraft performed the only targeted flyby of Io of its nominal mission. High-resolution images were originally planned during the encounter, but problems with the spacecraft's tape recorder, used to save data taken during encounters for later playback to Earth, required the elimination of high-data-rate observations from the flyby schedule to ensure the safe recording of Galileo atmospheric probe data. The encounter did yield significant results from lower data rate experiments. Analysis of the Doppler shift of Galileo's radio signal showed that Io is differentiated with a large iron core, similar to that found in the rocky planets of the inner solar system. Magnetometer data from the encounter, combined with the discovery of an iron core, suggested that Io might have a magnetic field.
| Galileo encounters with Io with altitudes less than 300,000 km (186,000 mi) | ||||
|---|---|---|---|---|
| Orbit | Date | Altitude | Notes | |
| J0 | December 7, 1995 | 897 km | 557 mi | No remote sensing; Gravity measurements reveal differentiated interior, large iron core; magnetic field? |
| C3 | November 4, 1996 | 244,000 km | 152,000 mi | Clear-filter imaging of anti-Jovian hemisphere; near-IR spectra of SO2 frost |
| E14 | March 29, 1998 | 252,000 km | 157,000 mi | Multi-spectral imaging of anti-Jovian hemisphere |
| C21 | July 2, 1999 | 127,000 km | 78,900 mi | Global color mosaic of anti-Jovian hemisphere |
| I24 | October 11, 1999 | 611 km | 380 mi | High-resolution imaging of Pillan, Zamama, and Prometheus flows; Camera and Near-IR spectrometer suffer radiation damage |
| I25 | November 26, 1999 | 301 km | 187 mi | Spacecraft safing event precludes high-resolution observations; images of Tvashtar outburst eruption |
| I27 | February 22, 2000 | 198 km | 123 mi | Change detection at Amirani, Tvashtar, and Prometheus; Stereo imaging over Tohil Mons |
| I31 | August 6, 2001 | 194 km | 121 mi | Camera problems preclude high-resolution imaging; Near-IR spectrometer observes eruption at Thor |
| I32 | October 16, 2001 | 184 km | 114 mi | High-resolution observations of Thor, Tohil Mons, Gish Bar |
| I33 | January 17, 2002 | 102 km | 63 mi | Spacecraft safing event precludes observations; almost all remote sensing lost |
| A34 | November 7, 2002 | 45,800 km | 28,500 mi | No remote sensing due to budget constraints |
In order to prevent potential biological contamination of the possible Europan biosphere, the Galileo mission ended on September 23, 2003 when the spacecraft was intentionally crashed into Jupiter.
Post-Galileo and New Horizons: 2003–present
The New Horizons spacecraft, en route to Pluto and the Kuiper belt, flew by the Jupiter system on February 28, 2007, approaching Io to a distance of 2,239,000 km (1,391,000 mi). During the encounter, numerous remote observations of Io were obtained, including visible imaging with a peak resolution of 11.2 km (6.96 mi) per pixel. Like Galileo during its November 1999 flyby of Io and Cassini during encounter in December 2000, New Horizons caught Tvashtar during a major eruption at the same site as the 1999 lava curtain. Owing to Tvashtar's proximity to Io's north pole and its large size, most images of Io from New Horizons showed a large plume over Tvashtar, providing the first detailed observations of the largest class of Ionian volcanic plumes since observations of Pele's plume in 1979. New Horizons also captured images of a volcano near Girru Patera in the early stages of an eruption, and surface changes from several volcanic eruptions that have occurred since Galileo, such as at Shango Patera, Kurdalagon Patera, and Lerna Regio.
Future exploration
Additional dedicated Io missions have been proposed. One, called the Io Volcano Observer (IVO), would launch in 2015 as a Discovery-class mission and would arrive at Jupiter and Io in July 2021. This mission remains in the concept study phase, but may be submitted as a proposal for the next Discovery Announcement of Opportunity. If selected, IVO would use high-inclination orbits of Jupiter to flyby Io at least six times, perhaps more if an extended mission were approved and the health of the spacecraft holds. The main goals of this proposed mission include measuring Io's volcanic eruption temperatures, determining the composition of Io's lavas, sampling its volcanic plumes through mass spectroscopy, and mapping Io's internal structure using electromagnetic induction sounding.
Orbit and rotation
Io orbits Jupiter at a distance of 421,700 km (262,000 mi) from the planet's center and 350,000 km (217,000 mi) from its cloudtops. It is the innermost of the Galilean satellites of Jupiter, its orbit lying between those of Thebe and Europa. Including Jupiter's inner satellites, Io is the fifth moon out from Jupiter. It takes 42.5 hours to revolve once (fast enough for its motion to be observed over a single night of observation). Io is in a 2:1 mean-motion orbital resonance with Europa and a 4:1 mean-motion orbital resonance with Ganymede, completing two orbits of Jupiter for every one orbit completed by Europa, and four orbits for every one completed by Ganymede. This resonance helps maintain Io's orbital eccentricity (0.0041), which in turn provides the primary heating source for its geologic activity (see the "Tidal heating" section for a more detailed explanation of the process). Without this forced eccentricity, Io's orbit would circularize through tidal dissipation, leading to a geologically less active world.
Like the other Galilean satellites of Jupiter and the Earth's Moon, Io rotates synchronously with its orbital period, keeping one face nearly pointed toward Jupiter. This synchronicity provides the definition for Io's longitude system. Io's prime meridian intersects the north and south poles, and the equator at the sub-Jovian point. The side of Io that always faces Jupiter is known as the subjovian hemisphere, while the side that always faces away is known as the antijovian hemisphere. The side of Io that always faces in the direction that the moon travels in its orbit is known as the leading hemisphere, while the side that always faces in the opposite direction is known as the trailing hemisphere.
Interaction with Jupiter's magnetosphere
Surrounding Io (up to a distance of 6 Io radii from the moon's surface) is a cloud of neutral sulfur, oxygen, sodium, and potassium atoms. These particles originate in Io's upper atmosphere but are excited from collisions with ions in the plasma torus (discussed below) and other processes into filling Io's Hill sphere, which is the region where the moon's gravity is predominant over Jupiter. Some of this material escapes Io's gravitational pull and goes into orbit around Jupiter. Over a 20-hour period, these particles spread out from Io to form a banana-shaped, neutral cloud that can reach as far as 6 Jovian radii from Io, either inside Io's orbit and ahead of the satellite or outside Io's orbit and behind the satellite. The collisional process that excites these particles also occasionally provides sodium ions in the plasma torus with an electron, removing those new "fast" neutrals from the torus. However, these particles still retain their velocity (70 km/s, compared to the 17 km/s orbital velocity at Io), leading these particles to be ejected in jets leading away from Io.
Io orbits within a belt of intense radiation known as the Io plasma torus. The plasma in this doughnut-shaped ring of ionized sulfur, oxygen, sodium, and chlorine originates when neutral atoms in the "cloud" surrounding Io are ionized and carried along by the Jovian magnetosphere. Unlike the particles in the neutral cloud, these particles co-rotate with Jupiter's magnetosphere, revolving around Jupiter at 74 km/s. Like the rest of Jupiter's magnetic field, the plasma torus is tilted with respect to Jupiter's equator (and Io's orbital plane), meaning Io is at times below and at other times above the core of the plasma torus. As noted above, these ions' higher velocity and energy levels are partly responsible for the removal of neutral atoms and molecules from Io's atmosphere and more extended neutral cloud. The torus is composed of three sections: an outer, "warm" torus that resides just outside Io's orbit; a vertically extended region known as the "ribbon", composed of the neutral source region and cooling plasma, located at around Io's distance from Jupiter; and an inner, "cold" torus, composed of particles that are slowly spiraling in toward Jupiter. After residing an average of 40 days in the torus, particles in the "warm" torus escape and are partially responsible for Jupiter's unusually large magnetosphere, their outward pressure inflating it from within. Particles from Io, detected as variations in magnetospheric plasma, have been detected far into the long magnetotail by New Horizons. To study similar variations within the plasma torus, researchers measure the ultraviolet-wavelength light it emits. While such variations have not been definitively linked to variations in Io's volcanic activity (the ultimate source for material in the plasma torus), this link has been established in the neutral sodium cloud.
During an encounter with Jupiter in 1992, the Ulysses spacecraft detected a stream of dust-sized particles being ejected from the Jupiter system. The dust in these discrete streams travel away from Jupiter at speeds upwards of several hundred kilometres per second, have an average size of 10 μm, and consist primarily of sodium chloride. Dust measurements by Galileo showed that these dust streams originate from Io, but the exact mechanism for how these form, whether from Io's volcanic activity or material removed from the surface, is unknown.
Jupiter's magnetic field lines, which Io crosses, couples Io's atmosphere and neutral cloud to Jupiter's polar upper atmosphere through the generation of an electric current known as the Io flux tube. This current produces an auroral glow in Jupiter's polar regions known as the Io footprint, as well as aurorae in Io's atmosphere. Particles from this auroral interaction act to darken the Jovian polar regions at visible wavelengths. The location of Io and its auroral footprint with respect to the Earth and Jupiter has a strong influence on Jovian radio emissions from our vantage point: when Io is visible, radio signals from Jupiter increase considerably. The Juno mission, planned for the next decade, should help to shed light on these processes. The Jovian magnetic field lines that do get past Io's ionosphere also induce an electric current, which in turn creates an induced magnetic field, within Io's interior. Io's induced magnetic field is thought to be generated within a partially molten, silicate magma ocean 50 kilometers beneath the moon's surface. Similar induced fields were found at the other Galilean satellites by Galileo, generated within liquid water oceans in the interiors of those moons.
Interior
Composed primarily of silicate rock and iron, Io is closer in bulk composition to the terrestrial planets than to other satellites in the outer solar system, which are mostly composed of a mix of water ice and silicates. Io has a density of 3.5275 g/cm3, the highest of any moon in the Solar System; significantly higher than the other Galilean satellites and higher than the Earth's moon. Models based on the Voyager and Galileo measurements of the moon's mass, radius and quadrupole gravitational coefficients (numerical values related to how mass is distributed within an object) suggest that its interior is differentiated between an outer, silicate-rich crust and mantle and an inner, iron- or iron sulfide–rich core. The metallic core makes up approximately 20% of Io's mass. Depending on the amount of sulfur in the core, the core has a radius between 350 and 650 km (220 and 400 mi) if it is composed almost entirely of iron, or between 550 and 900 km (340 and 560 mi) for a core consisting of a mix of iron and sulfur. Galileo's magnetometer failed to detect an internal, intrinsic magnetic field at Io, suggesting that the core is not convecting.
Modeling of Io's interior composition suggests that the mantle is composed of at least 75% of the magnesium-rich mineral forsterite, and has a bulk composition similar to that of L-chondrite and LL-chondrite meteorites, with higher iron content (compared to silicon) than the Moon or Earth, but lower than Mars. To support the heat flow observed on Io, 10–20% of Io's mantle may be molten, though regions where high-temperature volcanism has been observed may have higher melt fractions. However, re-analysis of Galileo magnetometer data in 2009 revealed the presence of an induced magnetic field at Io, requiring a magma ocean 50 km (31 mi) below the surface. It is not known if the 10-20% partial melting percentage for Io's mantle is consistent with the requirement for a significant amount of molten silicates in this possible magma ocean. The lithosphere of Io, composed of basalt and sulfur deposited by Io's extensive volcanism, is at least 12 km (7 mi) thick, but is likely to be less than 40 km (25 mi) thick.
Tidal heating
Unlike the Earth and the Moon, Io's main source of internal heat comes from tidal dissipation rather than radioactive isotope decay, the result of Io's orbital resonance with Europa and Ganymede. Such heating is dependent on Io's distance from Jupiter, its orbital eccentricity, the composition of its interior, and its physical state. Its Laplace resonance with Europa and Ganymede maintains Io's eccentricity and prevents tidal dissipation within Io from circularizing its orbit. The resonant orbit also helps to maintain Io's distance from Jupiter; otherwise tides raised on Jupiter would cause Io to slowly spiral outward from its parent planet. The vertical differences in Io's tidal bulge, between the times Io is at periapsis and apoapsis in its orbit, could be as much as 100 m (330 ft). The friction or tidal dissipation produced in Io's interior due to this varying tidal pull, which, without the resonant orbit, would have gone into circularizing Io's orbit instead, creates significant tidal heating within Io's interior, melting a significant amount of the moon's mantle and core. The amount of energy produced is up to 200 times greater than that produced solely from radioactive decay. This heat is released in the form of volcanic activity, generating its observed high heat flow (global total: 0.6 to 1.6×1014 W). Models of its orbit suggest that the amount of tidal heating within Io changes with time, and that the current heat flow is not representative of the long-term average.Surface
Based on their experience with the ancient surfaces of the Moon, Mars, and Mercury, scientists expected to see numerous impact craters in Voyager 1's first images of Io. The density of impact craters across Io's surface would have given clues to the moon's age. However, they were surprised to discover that the surface was almost completely lacking in impact craters, but was instead covered in smooth plains dotted with tall mountains, pits of various shapes and sizes, and volcanic lava flows. Compared to most worlds observed to that point, Io's surface was covered in a variety of colorful materials (leading Io to be compared to a rotten orange or to pizza) from various sulfurous compounds. The lack of impact craters indicated that Io's surface is geologically young, like the terrestrial surface; volcanic materials continuously bury craters as they are produced. This result was spectacularly confirmed as at least nine active volcanoes were observed by Voyager 1.
Surface composition
Explosive volcanism, often taking the form of umbrella-shaped plumes, paints the surface with sulfurous and silicate materials. Plume deposits on Io are often colored red or white depending on the amount of sulfur and sulfur dioxide in the plume. Generally, plumes formed at volcanic vents from degassing lava contain a greater amount of S2, producing a red "fan" deposit, or in extreme cases, large (often reaching beyond 450 km or 280 mi from the central vent) red rings. A prominent example of a red-ring plume deposit is located at Pele. These red deposits consist primarily of sulfur (generally 3- and 4-chain molecular sulfur), sulfur dioxide, and perhaps Cl2SO2. Plumes formed at the margins of silicate lava flows (through the interaction of lava and pre-existing deposits of sulfur and sulfur dioxide) produce white or gray deposits.
Compositional mapping and Io's high density suggest that Io contains little to no water, though small pockets of water ice or hydrated minerals have been tentatively identified, most notably on the northwest flank of the mountain Gish Bar Mons. This lack of water is likely due to Jupiter being hot enough early in the evolution of the solar system to drive off volatile materials like water in the vicinity of Io, but not hot enough to do so farther out.
Mountains
Despite the extensive volcanism that gives Io its distinctive appearance, nearly all its mountains are tectonic structures, and are not produced by volcanoes. Instead, most Ionian mountains form as the result of compressive stresses on the base of the lithosphere, which uplift and often tilt chunks of Io's crust through thrust faulting. The compressive stresses leading to mountain formation are the result of subsidence from the continuous burial of volcanic materials. The global distribution of mountains appears to be opposite that of volcanic structures; mountains dominate areas with fewer volcanoes and vice versa. This suggests large-scale regions in Io's lithosphere where compression (supportive of mountain formation) and extension (supportive of patera formation) dominate. Locally, however, mountains and paterae often abut one another, suggesting that magma often exploits faults formed during mountain formation to reach the surface.
Mountains on Io (generally, structures rising above the surrounding plains) have a variety of morphologies. Plateaus are most common. These structures resemble large, flat-topped mesas with rugged surfaces. Other mountains appear to be tilted crustal blocks, with a shallow slope from the formerly flat surface and a steep slope consisting of formerly sub-surface materials uplifted by compressive stresses. Both types of mountains often have steep scarps along one or more margins. Only a handful of mountains on Io appear to have a volcanic origin. These mountains resemble small shield volcanoes, with steep slopes (6–7°) near a small, central caldera and shallow slopes along their margins. These volcanic mountains are often smaller than the average mountain on Io, averaging only 1 to 2 km (0.6 to 1.2 mi) in height and 40 to 60 km (25 to 37 mi) wide. Other shield volcanoes with much shallower slopes are inferred from the morphology of several of Io's volcanoes, where thin flows radiate out from a central patera, such as at Ra Patera.
Nearly all mountains appear to be in some stage of degradation. Large landslide deposits are common at the base of Ionian mountains, suggesting that mass wasting is the primary form of degradation. Scalloped margins are common among Io's mesas and plateaus, the result of sulfur dioxide sapping from Io's crust, producing zones of weakness along mountain margins.
Volcanism
First predicted shortly before the Voyager 1 flyby, the heat source for Io's volcanism comes from tidal heating produced by its forced orbital eccentricity. This differs from Earth's internal heating, which is derived primarily from radioactive isotope decay. 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 have been identified, differing in duration, intensity, lava effusion rate, and whether the eruption occurs within a volcanic pit (known as a patera). Lava flows on Io, tens or hundreds of kilometres long, have primarily basaltic composition, similar to lavas seen on Earth at shield volcanoes such as Kīlauea in Hawaii. 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 1,600 K (1,300 °C; 2,400 °F) were detected, which can be explained by the eruption of high-temperature ultramafic 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 up to 500 kilometres (310 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
Hints of discoveries to come arose from Earth-based infrared observations taken in the 1970s. An anomalously high thermal flux, compared to the other Galilean satellites, was discovered during measurements taken at an infrared wavelength of 10 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 and Ganymede. 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 8,000 square kilometres (3,100 sq mi) in size at 600 K (300 °C; 600 °F). However, the authors considered that hypothesis unlikely, and instead focused on emission from Io's interaction with Jupiter's magnetosphere.
Shortly before the Voyager 1 encounter, Stan Peale, Patrick Cassen, and R. T. Reynolds published a paper in the journal Science predicting a volcanically modified surface and a differentiated interior, with distinct rock types rather than a homogeneous blend. They based this prediction on models of Io's interior that took into account the massive amount of heat produced by the varying tidal pull of Jupiter on Io caused by the moon's slightly eccentric orbit. Their calculations suggested that the amount of heat generated for an Io with a homogeneous interior would be three times greater than the amount of heat generated by radioactive isotope decay alone. This effect would be even greater with a differentiated Io.
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 found a 300-kilometre (190 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. 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 were compared to those taken four months previously by Voyager 1, including new plume deposits at Aten Patera and Surt.
Heat source
Io's main source of internal heat comes from the dissipation of tidal forces generated by Jupiter's gravitational pull. This external heating differs from the internal heat source for volcanism on Earth, which is a result of radioactive isotope decay and residual heat from accretion. In the Earth, these internal heat sources drive mantle convection, which in turn causes volcanism through plate tectonics.The tidal heating of Io is dependent on the moon's distance from Jupiter, its orbital eccentricity, 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 (330 ft) as Jupiter's gravitational pull varies between the periapsis and apoapsis points in Io's orbit. This varying tidal pull also produces friction in Io's interior, enough to cause significant tidal heating and melting. Unlike Earth, where most of its internal heat is released by conduction through the crust, on Io internal heat is released via volcanic activity and generates the satellite's high heat flow (global total: 0.6–1.6 × 1014 W). Models of its orbit suggest that the amount of tidal heating within Io changes with time, and that the current heat flow is not representative of the long-term average. The 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
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 K (600 °C; 1,200 °F). This is higher than the boiling point of sulfur (715 K/442 °C; 827 °F), 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.
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
This eruption style can take the form of either lava flows, spreading across the floor of the paterae, or lava lakes. 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 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.
Flow-dominated eruptions
Active flow fields more than 300 kilometres (190 mi) long have been observed on Io at Amirani and Masubi. A relatively inactive flow field named Lei-Kung Fluctus covers more than 125,000 square kilometres (48,000 sq mi), an area slightly larger than Nicaragua. The thickness of flow fields was not determined by Galileo, but the individual breakouts on their surface are likely to be 1 metre (3 ft 3 in) 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 tubes 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 kilometres (47 to 59 mi) 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 square metres (380–650 sq ft) per second were observed at Prometheus and Amirani during the Galileo mission, compared to 0.6 square metres (6.5 sq ft) per second at Kīlauea.
Explosion-dominated eruptions
Explosion-dominated eruptions occur when a body of magma (called a dike) from deep within Io's partially molten mantle reaches the surface at a fissure. This results in a spectacular display of lava fountains. During the beginning of the outburst eruption, thermal emission is dominated by strong, 1–3 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-kilometre (16 mi) long, 1-kilometre (0.62 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 komatiites (about 1,600 K / 1,300 °C; 2,400 °F) 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.
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-kilometre (250 mi) wide deposit of dark, silicate material and bright sulfur dioxide. The Tvashtar eruptions of 2000 and 2007 generated a 330-kilometre (210 mi) tall plume that deposited a ring of red sulfur and sulfur dioxide 1,200 kilometres (750 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
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, Amirani, Zamama, and Masubi. These plumes are usually less than 100 kilometres (62 mi) tall with eruption velocities around 0.5 kilometres per second (0.31 mi/s). Prometheus-type plumes are dust-rich, with a dense inner core and upper canopy shock zone, giving them an umbrella-like appearance. These plumes often form bright circular deposits, with a radius ranging between 100 and 250 kilometres (62 and 160 mi) and consisting primarily of sulfur dioxide frost. Prometheus-type plumes are frequently seen at flow-dominated eruptions, helping make this plume type quite long-lived. Four out of the six Prometheus-type plumes observed by Voyager 1 in 1979 were also observed throughout the Galileo mission and by New Horizons in 2007. While the dust plume can be clearly seen in sunlit visible-light images of Io acquired by passing spacecraft, many Prometheus-type plumes have an outer halo of fainter, more gas-rich material reaching heights approaching that of the larger, Pele-type plumes.
Io's largest plumes, Pele-type plumes, are created when sulfur and sulfur dioxide gas exsolve from erupting magma at volcanic vents or lava lakes, carrying silicate pyroclastic material with them. The few Pele-type plumes that have been observed are usually associated with explosion-dominated eruptions, and are short-lived. The exception to this is Pele, which is associated with a long-lived active lava lake eruption, though the plume is thought to be intermittent. The higher vent temperatures and pressures associated with these plumes generate eruption speeds of up to 1 kilometre per second (0.62 mi/s), allowing them to reach heights of between 300 and 500 kilometres (190 and 310 mi). Pele-type plumes form red (from short-chain sulfur) and black (from silicate pyroclastics) surface deposits, including large 1,000 kilometres (620 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 or those taken in the ultraviolet 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.
Atmosphere
Gas in Io's atmosphere is stripped by Jupiter's magnetosphere, escaping to either the neutral cloud that surrounds Io, or the Io plasma torus, a ring of ionized particles that shares Io's orbit but co-rotates with the magnetosphere of Jupiter. Approximately one ton of material is removed from the atmosphere every second through this process so that it must be constantly replenished. The most dramatic source of SO2 are volcanic plumes, which pump 104 kg of sulfur dioxide per second into Io's atmosphere on average, though most of this condenses back onto the surface. Much of the sulfur dioxide in Io's atmosphere sustained by sunlight-driven sublimation of SO2 frozen on the surface. The day-side atmosphere is largely confined to within 40° of the equator, where the surface is warmest and most active volcanic plumes reside. A sublimation-driven atmosphere is also consistent with observations that Io's atmosphere is densest over the anti-Jupiter hemisphere, where SO2 frost is most abundant, and is densest when Io is closer to the sun. However, some contribution from volcanic plume are required as the highest observed densities have been seen near volcanic vents. Because the density of sulfur dioxide in the atmosphere is tied directly to surface temperature, Io's atmosphere partially collapses at night or when the satellite is in the shadow of Jupiter. The collapse during eclipse is limited somewhat by the formation of a diffusion layer of sulfur monoxide in the lowest portion of the atmosphere, but the atmosphere pressure of Io's nightside atmosphere is two to four orders of magnitude less than at its peak just past noon. The minor constituents of Io's atmosphere, such as NaCl, SO, S, and S derive either from: direct volcanic outgassing; photodissociation, or chemical breakdown caused by solar ultraviolet radiation, from SO2; or the sputtering of surface deposits by charged particles from Jupiter's magnetosphere.
High-resolution images of Io acquired while the satellite is experiencing an eclipse reveal an aurora-like glow. As on Earth, this is due to radiation hitting the atmosphere, though in this case the charged particles come from Jupiter's magnetic field rather than the solar wind. Aurorae usually occur near the magnetic poles of planets, but Io's are brightest near its equator. Io lacks an intrinsic magnetic field of its own; therefore, electrons traveling along Jupiter's magnetic field near Io directly impact the satellite's atmosphere. More electrons collide with the atmosphere, producing the brightest aurora, where the field lines are tangent to the satellite (i.e., near the equator), since the column of gas they pass through is longer there. Aurorae associated with these tangent points on Io are observed to rock with the changing orientation of Jupiter's tilted magnetic dipole. Fainter aurora from oxygen atoms along the limb of Io (the red glows in the image at right), and sodium atoms on Io's night-side (the green glows in the same image) have also been observed.
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