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Exploration of Io

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Painting of the Galileo spacecraft (with fully extended, umbrella-like high-gain antenna dish) in front of Io (with several active volcanic plumes visible) and Jupiter and its Great Red Spot storm
Painting illustrating a flyby of Io by the Galileo spacecraft

The exploration of Io, one of Jupiter's four largest moons, began with its discovery in 1610 and continues today with Earth-based observations and visits by spacecraft to the Jupiter system. The 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 served a variety of purposes, such as helping mariners determine their longitude, validating Kepler's Third Law of planetary motion, and measuring the speed of light.[1] Based on ephemerides produced by astronomer Giovanni Cassini and others, Pierre-Simon Laplace created a mathematical theory to explain the resonant orbits of Io, Europa, and Ganymede.[1] This resonance was later found to have a profound effect on the geologies of the three moons. Improved telescope technology in the late 19th and 20th centuries allowed astronomers to resolve (that is, see) large-scale surface features on Io. New technologies also allowed astronomers to estimate Io's diameter, mass, and surface composition, as well as discover the moon's effect on Jupiter's magnetic field.[1]

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.[1] 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, particularly the side that faces Jupiter, in great detail. The Voyagers also observed the Io plasma torus and Io's sulfur dioxide (SO
2
) atmosphere for the first time.[1] In order to study the Jovian study in better detail and over a longer period of time, NASA launched the Galileo spacecraft in 1989, which entered orbit in December 1995 following the first close flyby of Io by an unmanned spacecraft. Galileo orbited Jupiter until crashing into the giant planet in September 2003. In between, Galileo flew by Io six more times between late 1999 and early 2002, providing high-resolution images and spectra of Io surface, confirming the presence of high-temperature silicate volcanism on Io. Distant observations by Galileo during parts of the mission allowed planetary scientists to study surface changes on Io's surface as a result of the moon's active volcanism.[2]

Following Galileo and a distant encounter by the Pluto-bound New Horizons spacecraft in 2007, NASA and the European Space Agency (ESA) generated 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.[3] The ESA component of the project, the Jupiter Ganymede Orbiter, is on their short list of large-scale missions to be launched in the next decade with final approval coming in 2011.[4] While these missions will perform Io science as ancillary to their primary mission targets, the proposed NASA Discovery mission, the Io Volcano Observer, would explore Io as part of its primary mission, though this project still needs to go through a competition process to be approved.[5] In the meantime, Io continues to be observed by Earth-based astronomers, utilizing new technologies such as adaptive optics and improved telescopes such as Keck, the European Southern Observatory, and the Hubble Space Telescope.[6]

Discovery: 1610

Portrait painting of a middle-aged Galileo Galilei. The text "GAILILEVS GAILILEVS - MATHVS:" painted to the left of Galileo's head.
Galileo Galilei, the discoverer of Io

The first recorded observation of Io was made by Tuscan astronomer Galileo Galilei on January 7, 1610 using a 20x-power, refracting telescope at the University of Padua in the Republic of Venice. The discovery was made possible by the invention of the telescope in The Netherlands a little more than a year earlier and Galileo's innovations to improve the magnification of the new instrument.[7] During his observation of Jupiter on the evening of January 7, Galileo spotted two stars to the east of Jupiter and another one to the west.[8] Jupiter and these three stars appeared to be in a line parallel to the ecliptic. The star furthest to the east from Jupiter turned out to be Callisto while the start to the west of Jupiter was Ganymede.[9] The third star, the closest one to the east of Jupiter, was a combination of the light from Io and Europa as Galileo's telescope, while having a high-magnification for a telescope from his time, was too low-powered to separate the two moons into distinct points of light.[9][7] Galileo observed Jupiter the next evening, January 8, 1610, this time seeing three stars to the west of Jupiter, suggesting that Jupiter had moved to the west of the three stars.[8] During this observation, the three stars in a line to the west of Jupiter were (from east to west): Io, Europa, and Ganymede.[9] This was the first time that Io and Europa were observed and recorded as distinct points of light so this date, January 8, 1610 is used as the discovery date for the two moons by the International Astronomical Union.[10] Galileo continued to observe the Jupiter system for the next month and a half.[7] On January 13, Galileo observed all four of the what would later be known as the Galilean moons of Jupiter for the first time in a single observation, though he had observed all four at various times in the preceding days.[9] On January 15, he observed the motions of three of these satellites, including Io, and came to the conclusions that these objects were not background stars, but were in fact, "three stars in the heavens moving about Jupiter, as Venus and Mercury round the Sun."[8] These were the first moons of a planet other than the Earth to be discovered.

Page of handwritten notes with several drawings of Galileo's observations of the Jovian satellites
Galileo's notes of his discoveries at Jupiter

The discoveries of Io and the other Galilean satellites of Jupiter were published in Galileo's Sidereus Nuncius in March 1610.[1] While the Jovian moons he discovered would later be known as the Galilean satellites, after himself, he proposed the name Medicea Sidera (Medicean Stars) after his new patrons, the de'Medici family of his native Florence. Initially, he proposed the name Cosmica Sidera (Cosmic Stars), after the head of the family, Cosimo II de'Medici, however both Cosimo and Galileo decided on the change to honor the family as a whole.[11] However, Galileo did not name each of the four moons individually beyond a numerical system in which Io was referred to as Jupiter I.[12] By December 1610, thanks to the publication of Sidereus Nuncius the news of Galileo's discovery had spread throughout Europe. With high-powered telescopes like Galileo's becoming more available, other astronomers, such as Thomas Harriot in England, Nicolas-Claude Fabri de Peiresc and Joseph Gaultier de la Vallette in France, Johannes Kepler in Bavaria, and Christopher Clavius in Rome, were able to observe Io and the other Medicean Stars during fall and winter of 1610-1611.[12]

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.[7] According to Marius, he began observing the Jupiter system in late November 1609.[13] 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."[13] 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.[14] Galileo doubted this claim and dismissed the work of Marius as plagiarism.[7] 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.[15] Despite this, it is one of Marius' naming scheme for the moons of Jupiter that is regularly used today. Based on a suggestion from Johannes Kepler in October 1613, he 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. He named the innermost large moon of Jupiter after the Greek mythological figure Io.[14]

Io as a tool: 1610–1809

Photo of a clock-like mechanical device in a museum display case, with a small card with the number 8 printed on it
Dutch Orrery of the Jovian system, built circa 1750, used by Harvard professor John Winthrop

For the next two and a half centuries, because of the satellite's small size and distance, Io remained a featureless, 5th-magnitude point of light in astronomers' telescopes. However, the determination of its orbital period, along with those of the other Galilean satellites, was an early focus for astronomers as Io was too small and distant to determine much about its physical properties. By June 1611, Galileo himself had determined that Io's orbital period was 42.5 hours long, only 2.5 minutes longer than the modern estimate.[12] Simon Marius published an even closer result in Mundus Iovalis, only one minute longer than modern estimates.[13] The orbital periods generated for Io and the other Jovian satellites provided an additional validation for Kepler's Third Law of planetary motion.[1]

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 a prime meridian.[16] 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 common and less affected by Jupiter's axial tilt. Knowing the time at the prime meridian and the local time, the observer could then calculate his longitude.[16] 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.[16] This left the tables published by Simon Marius in Mundus Iovialis and Giovanni Battista Hodierna in 1654 as the most accurate ephemeris tables available, but these were not able to predict the moons' positions to sufficient accuracy.[16]

Giovanni Cassini published a much more accurate ephemeris table in 1668 using his observations from the previous 16 years.[17] Using this table, Cassini was able to generate a more accurate map of France by observing eclipses of the Jovian satellites at various locations across the country. The map produced showed that some shorelines were too extended, shrinking the apparent area of France, to which King Louis XIV said that "he was losing more territory to his astronomers than to his enemies."[16] Eclipse timings of the Jovian moons would continue to be used to determine longitude for another hundred years, including measuring the Mason-Dixon Line and geodesy measurements. However, it was not until the invention of the marine chronometer in the mid-18th century that determining longitude at sea became practical.[16]

Animated diagram of the orbits of three of the moons of Jupiter. The ratio of each of the moons' orbital periods to that of Io's is also shown.
Animation showing Io's Laplace resonance with Europa and Ganymede

Astronomers during the 17th and 18th centuries also used the ephemeris tables generated by Cassini to better understand the nature of the Jovian system and light. In 1675, Danish astronomer Ole Rømer found that eclipse timings for Io were earlier than predicted when Jupiter was closest to Earth at opposition and later than predicted when Jupiter was furthest from Earth at conjunction. He determined that these discrepancies were due to light having a finite speed.[1] Ole Rømer never published his findings, but he sent along his measurements to Dutch mathematician Christiaan Huygens, who calculated that light traveled 16+23 Earth diameters per second, misinterpreting Rømer's value of 22 minutes as the time in which light traverses the diameter of the Earth's orbit.[18] Using Ole Rømer's measurements and a modern value for the astronomical unit, his measurement that light takes 16.44 minutes to travel the distance of the diameter of Earth's orbit was only 2% greater than the modern-day value, though this was not calculated at the time.[1] In 1809, again making use of observations of Io, but this time with the benefit of more than a century of increasingly precise observations, the French astronomer Jean Baptiste Joseph Delambre reported the time for light to travel from the Sun to the Earth as 8 minutes and 12 seconds. Depending on the value assumed for the astronomical unit, this yields the speed of light as just a little more than 300,000 kilometers (186,000 mi) per second.[19]

In 1788, Pierre-Simon Laplace used Cassini's ephemerides and those produced by other astronomers in the preceding century to create a mathematical theory to explain the resonant orbits of Io, Europa, and Ganymede. In the Laplace resonance, 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.[1] 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

Animation simulating the orbital motion of Io as it passes from left to right in front of Jupiter. A time code indicating the date and time of each frame in the animation is also shown.
Simulation of a transit of Jupiter by Io. Io's shadow precedes Io on Jupiter's cloud tops

Improved telescope technology and mathematical techniques allowed astronomers in the 19th and 20th centuries to estimate many of Io's physical properties, such as its mass, diameter, and albedo, as well as resolve large-scale surface features on Io. In his 1805 book Celestial Mechanics, in addition to laying out his mathematical argument for the resonant orbits of Io, Europa, and Ganymede, Laplace was able to use perturbations on the orbit of Io by Europa and Ganymede to provide the first estimate of Io's mass, 1.73 of the mass of Jupiter, which was one-quarter of the modern value.[20][21] Through the mid-20th century, additional mass estimates using this technique would be performed by Marie-Charles Damoiseau, John Couch Adams, Ralph Allen Sampson, and Willem de Sitter, all of which were less than modern estimates with the closest being Sampson's 1921 estimate of 4.5 of the mass of Jupiter, which was 4% less than the modern value.[20] Io's diameter was estimated using micrometer measurements and the occultation of background stars by Io. Edward E. Barnard used a micrometer at the Lick Observatory in 1897 to estimate a diameter of 3,950 kilometers, 8.5% larger than the accepted modern value, while Albert Michelson, also using the Lick telescope, came up with a better estimate of 3,844 km (2,389 mi).[1] The best pre-spacecraft estimate of Io's diameter and shape came from the measurements of an occultation of the star Beta Scorpii C on May 14, 1971, where a diameter of 3,636 km (2,259 mi) was found, slightly less the accepted modern value.[22] These measurements allowed astronomers to estimate Io's density, given as 2.88 g/cm3 following the Beta Scorpii occultation. While this is 20% less than the currently accepted value, it was enough for astronomers to note the differences between the densities of the inner two Galilean satellites (Io and Europa) versus the outer two Galilean satellites (Ganymede and Callisto). The densities of Io and Europa suggested that they were composed primarily of rock while Ganymede and Callisto contained more ices.[21]

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, finding it to have an elliptical outline aligned with its direction of motion in its orbit, similar to his measurement of Ganymede.[23] Other astronomers between 1850 and 1895 noted Io's elliptical shape.[21] 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.[24] 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.[25] 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.[21] Later telescopic observations confirmed Io's distinct reddish-brown polar regions and yellow-white equatorial band.[26] Observations of variations in the brightness of Io as it rotates by Joel Stebbins in the 1920s showed that Io's day was the same length as its orbital period around Jupiter, proving that it kept one side always facing Jupiter just as just as the Moon's near-side always faces the Earth.[27] Stebbins also noted Io's dramatic orange coloration, which was unique among the Galilean satellites.[1] Audouin Dollfus used observations of Io in the early 1960s at the Pic du Midi Observatory to create crude maps of Io that showed a patchwork of bright and dark spots across Io's surface on top of the general bright equatorial belt and dark polar regions pattern.[28]

Telescopic observations in the mid-20th century began to hint at Io's unusual nature. Spectroscopic observations in the near-infrared suggested that Io's surface was devoid of water ice.[29] The lack of water ice on Io was not completely surprising considering its estimated density, however, abundant water ice was measured on the surface of Europa, a moon thought to have the same density as Io.[21] Lee concluded that the spectrum was consistent with the presence of sulfur compounds.[29] Binder and Cruikshank (1964) reported that Io's surface was brighter coming out of Jupiter's shadow than when it entered it.[30] 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, with some researchers reporting 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 SO
2
atmosphere freezes out enough to produce a layer several millimeters thick, which seems unlikely.[1] Radio telescopic observations revealed Io's influence on the Jovian magnetosphere, as demonstrated by decametric wavelength bursts tied to the orbital period of Io, suggesting an electrodynamic coupling between the two worlds.[31]

Pioneer era: 1973–1979

Painting of the Pioneer spacecraft with a crescent Jupiter, the Sun, and the stars of the Milky Way in the background. The nightside of Jupiter in the painting appears brighter than it would in real life.
Artist's rendition of the Pioneer 10 encounter with Jupiter

Starting in the early 1960s, the United States and Soviet Union launched a number of unmanned, robotic spacecraft to Earth's nearest celestial neighbors: the Moon, Venus, and Mars. In the late 1960s, an opportunity known as the Planetary Grand Tour was identified by the United States' NASA and Jet Propulsion Laboratory (JPL) that would allow a single spacecraft to travel past the asteroid belt and onto each of the outer planets, including Jupiter, if such a mission were launched in 1976 or 1977. However, there was of uncertainty over whether a spacecraft could survive a passage through the asteroid belt, where micrometeoroids could cause physical damage to a spacecraft, or the intense Jovian magnetosphere, where charged particles could cause damage to sensitive electronics.[21] To resolve these questions before sending the more ambitious Voyager missions, NASA and the Ames Research Center launched a pair of twin probes, Pioneer 10 and Pioneer 11 on March 3, 1972 and April 6, 1973, respectively, on the first unmanned mission to the outer solar system.

Pioneer 10 became the first spacecraft to encounter the Jupiter system on December 3, 1973, encountering Io at a distance of 357,000 km (222,000 mi).[32] During Pioneer 10's encounter with Io, the spacecraft performed a radio occultation of Io, when Pioneer transmitted an S-band signal to antennas on Earth as Io passed between the spacecraft 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 bar, though the composition was not constrained.[33] 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 during its encounter with Io, but those observations were lost because of the high-radiation environment.[34] Pioneer 10 also discovered a hydrogen ion torus at the orbit of Io.[35]

Two versions of the same image of an orange planetary body, half of which is illuminated. The image on the right has been darkened so that dark features on Io's surface are more visible
Only image of Io returned from Pioneer 11

Pioneer 11 encountered the Jupiter system nearly one year later on December 2, 1974, encountering Io at a distance of 314,000 km (195,000 mi).[36] Pioneer 11 provided the first spacecraft image of Io, a 357 km (222 mi) per pixel frame (D7) over Io's north polar region taken from a distance of 470,000 km (290,000 mi).[37] The low-resolution image provided confirmation for dark patches on Io's surface akin to those hinted at in maps by Audouin Dollfus.[1] Observations by both Pioneers revealed that Jupiter and Io were connected by an electrical conduit known as the Io flux tube that followed along magnetic field lines in Jupiter' magnetosphere. Pioneer 11's closer encounter with Jupiter allowed the spacecraft to find evidence that Jupiter had intense radiation belts similar to Earth's Van Allen Belts. One of the peaks in charged particle radiation intensity was found near the orbit of Io.[1] Radio tracking during the encounters of both Pioneers with Io provided an improved estimate of the moon's mass. This was accomplished by analyzing slight changes in trajectory of the two probes due to the influence of Io's gravity. The magnitude of these deviations is proportional to the mass of Io. Combined with the best available information of Io's size, Io was found to have the highest density of the four Galilean satellites and that the densities of the four Galilean satellites trended downward with increasing distance from Jupiter.[38] The high density of Io (3.5 g/cm3) indicated that it was composed primarily of silicate rock rather than water ice.[38]

Following the Pioneer encounters and in the lead up to the Voyager encounters in 1979, interest in Io and the other Galilean satellites grew within the planetary science and astronomy communities, even 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."[1] New observations of Io from Earth and by the Pioneers during the mid-1970s provided a paradigm shift in the thinking of Io's 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.[39] 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 Io's surface composition. Observations by Fanale et al. suggested Io had a surface dominated by evaporites composed of sodium salts and sulfur.[40] This fit with the observation that Io had no water ice on its surface nor 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.[40]

Measurements of Io's thermal emission in the mid-infrared in the 1970s led to rather conflicting results that were not explained accurately until after the discovery of active volcanism by Voyager 1 in 1979. 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.[41] At the time, this heat flux was attributed to the surface having a much higher thermal inertia than Europa and Ganymede.[42] 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.[41] A sharp increase in Io's thermal emission at 5 μm was observed on February 20, 1978 by Witteborn, et al., possibly due to an interaction between Io and Jupiter's magnetosphere, though volcanism was not ruled out.[43]

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 Io's 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.[44]

Voyager era: 1979–1995

Spacecraft gray-scale photo of a planetary body (Io) covered in numerous dark spots in front of the bright and dark clouds of Jupiter.
Voyager 1 approach image of Io, with Jupiter's clouds in the background

The first close-up investigation by an unmanned space probe, which included high-resolution imaging, was performed by the twin probes, Voyager 1 and Voyager 2, launched on September 5 and August 20, 1977, respectively. Both spacecraft were part of NASA and JPL's Voyager program to explore the giant outer planets through a series of encounters in the late 1970s and 1980s, a scaled-down version of the earlier Planetary Grand Tour concept. Both probes contained more sophisticated instrumentation than the precursor Pioneer missions, including a camera capable of taking much higher resolution images, important for accessing the geologies of Jupiter's Galilean moons and the cloud features of Jupiter itself. They also had spectrometers with a combined spectral range from the far-ultraviolet to the mid-infrared, useful for examining Io's surface and atmospheric composition and to search for thermal emission sources on Io's surface.

Voyager 1 was first of the two probes to encounter the Jupiter system in March 1979.[45] On approach to Jupiter in late February and early March 1979, Voyager imaging scientists noticed that Io appeared distinct from the other Galilean satellites, being orange in color and marked by dark spots across its surface. The dark spots were initially interpreted as the sites of impact craters.[46] 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.[47] Also on approach, 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.[47][48] The Low-Energy Charged Particle (LECP) detector encountered streams of sodium, sulfur, and oxygen ions prior to entering Jupiter's magnetosphere, material the LECP science team suspected originated from Io.[49] In the hours prior to Voyager 1's encounter with Io, the spacecraft acquired images for a global map of Io with a resolution of at least 20 km (12 mi) per pixel over Io'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).[46] 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.[1] The dark spots in earlier images resembled volcanic calderas more than they did the impact craters seen on those other worlds.[46] 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."[47]

Aerial color image of a volcanic landscape with numerous flow-like features, irregular shaped, flat-floored pits, tall mountains, and shorter mesas. These features are surrounded by greyish orange-colored plains, with several areas of bright and orange terrain surrounding some mountains and pits. The boundary between the dayside and nightside cuts across the image from upper right to bottom center. The upper left and lower left corner are black, outside the area of the mosaic.
Mosaic of Voyager 1 images covering Io's south polar region

On March 5, 1979, Voyager 1 performed the closest encounter with Io of the Voyager mission from a distance of 20,600 km (12,800 mi) over Io's south pole.[45][47] The close distance of the encounter allowed Voyager to acquire images of the sub-Jovian and south polar regions of Io with a best resolution of less than 0.5 km (0.3 mi) per pixel.[46] However, many of the close-up images were limited by smear as the result of problems with Voyager's internal clock due to the high radiation environment, causing some narrow-angle-camera exposures of Io to be acquired while Voyager's scan platform was moving between targets.[47] The highest-resolution images showed a relatively young surface punctuated by oddly shaped pits that appears more akin to volcanic calderas than to impact craters, mountains taller than Mount Everest, and features resembling volcanic lava flows. The majority of the surface was covered in smooth, layered plains, with scarps marking the boundary between different layers.[46] Even in the highest resolution images, no impact craters were observed, suggesting that Io's surface was being regularly renewed by present-day volcanic activity.[46] The encounter over one of Io's poles also allowed Voyager 1 to directly sample the edge of the Io flux tube, finding an intense electrical current of 5 amperes.[50] The color data from Voyager's cameras showed that its surface was dominated by sulfur and sulfur dioxide (SO
2
) frosts.[51] Io's multi-colored surface was explained as being the result of various sulfur allotropes, caused by liquid sulfur being heated to different temperatures, changing its color and viscosity.[52]

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.[53] 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.[54] Analysis of other Voyager 1 images showed nine such plumes scattered across the surface, proving that Io was volcanically active.[54] The Infrared Interferometer Spectrometer (IRIS) on Voyager 1 discovered thermal emission from multiple sources, indicative of cooling lava. This data showed that in addition to the gaseous volcanic plumes, some of the lava flows visible on Io's surface were also active.[55] IRIS also measured gaseous SO
2
within the Loki plume, providing additional evidence for an atmosphere on Io.[56] These results confirmed the prediction made by Peale et al. shortly before the encounter.[44]

Thin crescent (open to the right) of the full disk of a planetary body (Io) with two bright clouds along the upper left edge of Io and another along the right edge.
Three volcanic plumes seen by Voyager 2 along the limb of Io

Voyager 2 passed Io on July 9, 1979 at a distance of 1,130,000 km (702,000 mi), approaching Jupiter between the orbits of Europa and Ganymede.[57] Though it did not approach nearly as close to Io as Voyager 1, comparisons between images taken by the two spacecraft showed several surface changes that had occurred in the four months between the encounters, including new plume deposits at Aten Patera and Surt.[58] The Pele plume deposit had changed shape, from a heart-shape during the Voyager 1 encounter to an oval during the Voyager 2 flyby. Additional changes were observed at Loki, with changes in the distribution of diffuse plume deposits and additional dark material in the southern portion of Loki Patera, the result of a volcanic eruption there.[58] As a result of the discovery of active volcanic plumes on Io by Voyager 1, a ten-hour "Io Volcano Watch" was added to the departure leg of the Voyager 2 encounter to monitor Io's plumes.[57] Observations of Io's crescent during this monitoring campaign revealed that seven of the nine plumes observed in March were still active in July 1979, with only the volcano Pele shutting down between flybys (no images were available to confirm continued activity at Volund), and no new plumes were observed.[59] The blue color of the plumes observed during this monitoring period (Amirani, Maui, Masubi, and Loki) suggested that the reflected light from the plumes came from fine grained particles approximately 1 μm in diameter.[58]

Following the Voyager encounters and leading up to the Galileo mission, planetary scientists developed new theories to explain the data and images returned from Voyager and continued to observe Io with Earth-based and new space-based telescopes to follow up on Voyager's discoveries and to monitor's Io volcanic activity. Initially, the accepted theory was that Io's lava flows were composed of sulfurous compounds based on 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) and the color of volcanic terrains.[60] 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.[60] 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.[61] 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.[43][62] 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.[63] 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, from the eruption of gases at volcanic vents, or both.[60]

Galileo era: 1995–2003

A multi-colored image of the full-disk of a planetary body (Io), dotted with numerous dark spots. Much of the middle portion of the planetary body is yellow to white/gray, while the polar regions at the top and bottom are generally reddish in color.
Mosaic of images from Galileo acquired in November 1996

Just as the two Voyager probes launched in 1977, planning began for the next NASA mission to Jupiter. Rather than performing a flyby of the Jupiter system like all the missions preceding it, the Galileo spacecraft would orbit Jupiter to perform close-up observations of Jupiter and its many moons, including Io, as well as deliver an Jovian atmospheric probe. Originally scheduled to launched via the Space Shuttle in 1982, delays resulting from developement issues with the shuttle and upper stage motor pushed the launch to 1986, when the Challenger disaster delayed Galileo's launch even further. Finally, on October 18, 1989, Galileo began its journey aboard the shuttle Atlantis.[64] En route to Jupiter, the high-gain antenna, folded up like an umbrella to allow the spacecraft to fit in the shuttle cargo bay, failed to open completely. For the rest of the mission, data from the spacecraft would have to be transmitted back to Earth at a much lower data rate using the low-gain antenna. Despite this setback, data compression algorithms uploaded to Galileo allowed it to complete most of its science goals at Jupiter.[2]

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.[2] The encounter did yield significant results from lower data rate experiments. Analysis of the Doppler shift of Galileo's radio signal showed that Io was differentiated with a large iron core, similar to that found in the rocky planets of the inner solar system.[65] Magnetometer data from the encounter, combined with the discovery of an iron core, suggested that Io might have a magnetic field, though this result was quite tenuous at the time.[66]

Two spacecraft images, displayed side-by-side, showing a red, diffuse ring with a darker, gray region in the middle. In the image on the left, this red ring is interrupted on its upper right side by a hexagonal dark gray region.
Two Galileo images showing the effects of a major eruption at Pillan Patera in 1997

Jupiter's intense radiation belts near the orbit of Io forced Galileo to come no closer than the orbit of Europa until the end of the first extended mission in 1999. Despite the lack of close-up imaging and mechanical problems that greatly restricted the amount of data returned, several significant discoveries at Io were made during Galileo's two-year, primary mission. During the first several orbits, Galileo mapped Io's surface in search of surface changes that occurred since the Voyager encounters 17 years earlier. This included the appearance of a new lava flow, Zamama, and the shifting of the Prometheus plume by 75 km (47 mi) to the west, tracking the end of a new lava flow at Prometheus.[67] Starting with Galileo's first orbit, the spacecraft's camera, the Solid-State Imager (SSI), began taking one or two images per orbit of Io while the moon was in Jupiter's shadow. This allowed Galileo to monitor high-temperature volcanic activity on Io by observing thermal emission sources across Io's surface.[67] These same eclipse images also allowed Galileo scientists to observe aurorae created by the interaction between Io's atmosphere and volcanic plumes with the Io flux tube and the plasma torus.[68] During Galileo's ninth orbit, the spacecraft observed a major eruption at Pillan Patera, observing high-temperature thermal emission and a new volcanic plume. The temperatures observed at Pillan and other eruptions confirmed that volcanic eruptions on Io consist of silicate lavas with magnesium-rich mafic and ultramafic compositions, with volatiles like sulfur and sulfur dioxide serving a similar role to water and carbon dioxide on Earth.[69] During the following orbit, Galileo found that Pillan was surrounded by a new, dark pyroclastic deposit composed of silicate minerals such as orthopyroxene.[69] The Near-Infrared Mapping Spectrometer (NIMS) also observed Io on a number occasions during the primary mission, mapping Io's volcanic thermal emission and the distribution of sulfur dioxide frost, whose absorption bands dominate Io's near-infrared spectrum.[70][71]

Galileo encounters with Io with altitudes less than 300,000 km (186,000 mi)[2]
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 SO
2
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
Gray-scale spacecraft image of a portion of a planetary body with a pair of large, mountainous ridges on the left side of the image, a shorter, rugged domical mountain at top center, an elliptical pit near bottom center, and the boundary between the dayside (to the left) and the nightside (to the right) running down the right side of the image. Two small mountain peaks are seen near this boundary at lower right.
Mongibello Mons, as seen by Galileo in February 2000

In December 1997, NASA approved an extended mission for Galileo known as the Galileo Europa Mission, which ran for two years following the end of the primary mission. The focus of this extended mission was to follow-up on the discoveries made at Europa with seven additional flybys to search for additional information on a possible sub-surface water ocean.[21] Starting in May 1999, Galileo used four flybys with Callisto to lower its periapse, setting up a chance for it to fly by Io twice in late 1999.[2] During Galileo's 21st orbit, it acquired a three-color, global mosaic of the anti-Jovian hemisphere (the "far" side of Io), its highest resolution observations of Io to date. This mosaic complemented the coverage obtained by Voyager 1, whose highest resolution observations covered Io's sub-Jovian hemisphere.[2] Galileo's two flybys in late 1999, on October 11 and November 26, provided high-resolution images and spectra of various volcanoes and mountains on Io's anti-Jovian hemisphere. The camera suffered a problem with an image mode used extensively durign the first encounter, causing the majority of images taken to be highly degraded (though a software algoritm was developed to partially recover some of these images).[2] NIMS also had problems due to the high-radiation environment near Io, suffering a hardware failure that limited the number of near-infrared wavelengths it sampled.[72] Finally, the low-data rate playback forcing Galileo to transmit data from each encounter days to weeks later on the apoapse leg of each orbit and a safing event, when radiation forced a reset of spacecraft's computer and putting it into safe mode, during the November 1999 encounter limited the imaging coverage during each flyby. Even so, Galileo fortuitously imaged an outburst eruption at Tvashtar Paterae during this flyby, observing a curtain of lava fountains 25 km (16 mi) long and 1.5 km (0.93 mi) tall.[73] An additional encounter was performed on February 22, 2000 on provisional funding. With no new errors with Galileo's remote sensing instruments, no safing events, and more time after the flyby before the next satellite encounter, Galileo was able to acquire and send back more data from this encounter. This included information on the lava flow rate at Prometheus, Amirani, and Tvashtar, very high resolution imaging of Chaac Patera and layered terrain in Bulicame Regio, and mapping of the mountains and topography around Camaxtli Patera, Zal Patera, and Shamshu Patera.[2]

Colorized image, with a multi-colored region in the middle, elongated left-to-right. The text "I32 Pele" is displayed at top left, and a bottom center, a 30-kilometer scale bar and a color chart connected to brightness in the original 1-micron image is included.
Infrared image showing night-time thermal emission from the lava lake Pele

Following the February 2000 encounter, Galileo's mission at Jupiter was extended for a second and final time with the Galileo Millennium Mission. The focus of this extended mission was joint observation of the Jovian system by both Galileo and Cassini, which performed a distant flyby of Jupiter en route to Saturn on December 30, 2000.[74] Discoveries during the joint observations of Io revealed a new plume at Tvashtar and provided insights into Io's aurorae.[75] Distant imaging by Galileo during the Cassini flyby revealed a new red ring plume deposit, similar to the one surrounding Pele, around Tvashtar, one of the first of this type seen in Io's polar regions, though Galileo would later observe a similar deposit around Dazhbog Patera in August 2001.[2] Galileo performed three additional flybys of Io, on August 6 and October 16, 2001 and January 17, 2002, during the Galileo Millennium Mission. Both encounters in 2001 allowed Galileo to observe Io's polar regions up-close, though imaging from the August 2001 flyby was lost due to a camera malfunction.[2] The data from the magnetometer confirmed that Io lacked a intrinsic magnetic field, though later analysis of this data in 2009 did reveal evidence for an induced magnetic field generated by the interaction between Jupiter's magnetosphere and a silicate magma ocean in Io's asthenosphere.[2][76] During the August 2001 flyby, Galileo flew through the outer portions of the newly formed Thor volcanic plume, allowing for the first direct measurement of composition of Io's volcanic material.[2] During the October 2001 encounter, Galileo imaged the new Thor eruption site, a major new lava flow at Gish Bar Patera,[77] and the lava lake at Pele.[2] Due to a safing event prior to the encounter, nearly all of the observations planned for the January 2002 flyby were lost.[2]

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.[21]

Post-Galileo and New Horizons: 2003–present

Two gray-scale spacecraft images, displayed side-by-side, of a planetary body (Io). The image on the right was taken by the Galileo spacecraft in 1999. The image on the right was taken by New Horizons' LORRI camera in 2007. Approximately, the same hemisphere of Io is displayed in both images. A bright region near the bottom of both images is circled. In the image on the right, a small area of dark material is seen.
Changes in surface features in the eight years between Galileo and New Horizons observations

Following the end of the Galileo mission, new observations of Io and its volcanic activity came from Earth-based and space-based telescopes. In particular, adaptive optics imaging from the Keck telescope in Hawaii and the European Southern Observatory in Chile as well as imaging from the Hubble telescope have allowed astronomers to monitor Io's active volcanoes by observing their thermal emission and measuring the composition of gases over active volcanoes like Pele and Tvashtar.[78][79] Imaging from the Keck telescope in February 2001 revealed the most powerful volcanic eruption observed in modern times, either on Io or on Earth, at the volcano Surt.[78] Earth-based telescopes coming online over the next decade, such as the Thirty Meter Telescope at the Mauna Kea Observatory will provide more detailed observations of Io's volcanoes, approaching the resolution achieved by Galileo's Near-IR spectrometer.[6] Hubble ultraviolet, millimeter-wave, and ground-based mid-infrared observations of Io's atmosphere have revealed strong density heterogeneities between bright, frost-covered regions along the satellite's equator and its polar regions, providing further evidence that Io's atmosphere is supported by the sublimation of sulfur dioxide frost on Io's surface.[80]

The New Horizons spacecraft, en route to Pluto and the Kuiper belt, flew by the Jupiter system on February 28, 2007, passing by Io at a distance of 2,239,000 km (1,391,000 mi).[81] During the encounter, numerous distant observations of Io were obtained, including visible imaging with a peak resolution of 11.2 km (6.96 mi) per pixel.[82] 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 plume since observations of Pele's plume in 1979.[83] 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.[83]

Future exploration

Frame from a computer animation of the Jupiter Europa spacecraft with a portion of Io shown in the background. Several dark regions are visible on Io. The bright blue cloud of a volcanic plume is visible along a portion of the horizon.
Simulated view of a Jupiter Europa Orbiter flyby of Io in December 2026.

There are currently two forthcoming missions planned for the Jupiter system. Juno, scheduled to launch in 2011, has limited imaging capabilities, but it could provide monitoring of Io's volcanic activity using its near-infrared spectrometer, JIRAM. The Europa/Jupiter System Mission (EJSM), a joint NASA/ESA project approved in February 2009 and scheduled to launch in 2020, would study Io using two spacecraft, NASA's Jupiter Europa Orbiter and ESA's Jupiter Ganymede Orbiter, as part of the Jupiter tour segments of both spacecraft before they go into orbit around their respective targets.[3] While most of the observations of Io would be acquired from a distance as both spacecraft focus primarily on the icy Galilean satellites, the Jupiter Europa Orbiter would perform four close flybys of Io in 2025 and 2026 prior to orbiting Europa. These flybys would not only provide the first close up imaging of Io's volcanic features in 24 years, but would provide unique science, including studies of Io's topography and subsurface via laser altimetry and ground-penetrating radar and direct measurements of the composition of Io's atmosphere and volcanic plumes via mass spectrometry. With the Jupiter Europa Orbiter's greater downlink bandwidth, substantial greater imaging coverage should be obtained during each flyby than during Galileo's flybys.[3] ESA's contribution will still face funding competition from other ESA projects with final approval in 2011.[4]

In addition to these missions already approved by NASA, 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; however, this mission remains in the concept study phase but may be submitted as a proposal for the next Discovery Announcement of Opportunity.[5] If selected, IVO would use polar orbits of Jupiter to encounter 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.[5]

See also

References

  1. ^ a b c d e f g h i j k l m n o p q Cruikshank, D. P. (2007). "A history of the exploration of Io". In Lopes, R. M. C.; and Spencer, J. R. (ed.). Io after Galileo. Springer-Praxis. pp. 5–33. ISBN 3-540-34681-3. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: multiple names: editors list (link)
  2. ^ a b c d e f g h i j k l m n Perry, J.; et al. (2007). "A Summary of the Galileo mission and its observations of Io". In Lopes, R. M. C.; and Spencer, J. R. (ed.). Io after Galileo. Springer-Praxis. pp. 35–59. ISBN 3-540-34681-3. {{cite book}}: Explicit use of et al. in: |first= (help)CS1 maint: multiple names: editors list (link)
  3. ^ a b c Joint Jupiter Science Definition Team (January 30, 2009). "Jupiter Europa Orbiter Mission Study 2008: Final Report" (PDF). Outer Planet Flagship Mission. NASA/ESA. Retrieved 2010-02-20. {{cite web}}: Cite has empty unknown parameter: |month= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  4. ^ a b "Cosmic Vision 2015–2025 Proposals". ESA. 2007-07-21. Retrieved 2009-02-20.
  5. ^ a b c McEwen, A. S. (24 August 2009). Io Volcano Observer (IVO) (PDF). Satellites panel of 2009 Decadal Survey. Retrieved 2010-02-20. {{cite conference}}: Cite has empty unknown parameter: |coauthors= (help); External link in |conferenceurl= (help); Unknown parameter |conferenceurl= ignored (|conference-url= suggested) (help)
  6. ^ a b Marchis, F.; et al. (2007). "Outstanding questions and future exploration". In Lopes, R. M. C.; and Spencer, J. R. (ed.). Io after Galileo. Springer-Praxis. pp. 287–303. ISBN 3-540-34681-3. {{cite book}}: Explicit use of et al. in: |first= (help)CS1 maint: multiple names: editors list (link)
  7. ^ a b c d e Drake, S. (1978). "Eight: 1609–10". Galileo at Work: His Scientific Biography. Chicago: University of Chicago Press. pp. 134–156. ISBN 0-226-16226-5. Retrieved 2010-02-17.
  8. ^ a b c Galilei, Galileo (2004) [First published 1610]. Carlos, E. S.; Barker, P. (eds.). Sidereus Nuncius (pdf). Venice: University of Padua. pp. 17–28. Retrieved 2010-01-07. {{cite book}}: Unknown parameter |trans_title= ignored (|trans-title= suggested) (help)
  9. ^ a b c d Wright, E. (2004). "Galileo's First Jupiter Observations". Astronomy Stuff: Observation and Simulation. Retrieved 2010-02-17.
  10. ^ Blue, J. (November 9, 2009). "Planet and Satellite Names and Discoverers". USGS. Retrieved 2010-01-13.
  11. ^ Van Helden, A. (2003). "Satellites of Jupiter". The Galileo Project. Rice University. Retrieved 2010-02-17.
  12. ^ a b c Drake, S. (1978). "Nine: 1610-11". Galileo at Work: His Scientific Biography. Chicago: University of Chicago Press. pp. 157–176. ISBN 0-226-16226-5. Retrieved 2010-02-17.
  13. ^ a b c Marius, S. (1916) [First published 1614]. Prickard, A. O. (ed.). Mundus Iovialis anno M.DC.IX Detectus Ope Perspicilli Belgici. Nuremberg: Johann Laur. pp. 367–381. Retrieved 2010-01-07. {{cite book}}: Unknown parameter |trans_title= ignored (|trans-title= suggested) (help)
  14. ^ a b Van Helden, Albert (14 January 2004). "Simon Marius". The Galileo Project. Rice University. Retrieved 2010-01-07.
  15. ^ Baalke, Ron. "Discovery of the Galilean Satellites". Jet Propulsion Laboratory. Retrieved 2010-01-07.
  16. ^ a b c d e f Van Helden, Albert (2004). "Longitude at Sea". The Galileo Project. Rice University. Retrieved 2010-02-17.
  17. ^ O'Connor, J. J.; Robertson, E. F. (1997). "Longitude and the Académie Royale". University of St. Andrews. Retrieved 2007-06-14. {{cite web}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  18. ^ Huygens, C. (8 January 1690). "Treatise on Light". Project Gutenberg etext. Retrieved 2007-04-29. {{cite web}}: Unknown parameter |editor1_first= ignored (help); Unknown parameter |editor1_last= ignored (help)
  19. ^ Oldford, R.W (2000). "The first evidence". Scientific Method, Statistical Method, and the Speed of Light. University of Waterloo. Retrieved 2010-02-17.
  20. ^ a b de Sitter, W. (1931). "Jupiter's Galilean satellites (George Darwin Lecture)". Monthly Notices of the Royal Astronomical Society. 91: 706–738.
  21. ^ a b c d e f g h Alexander, C.; et al. (2009). "The Exploration History of Europa". Europa. University of Arizona Press. pp. 3–26. ISBN 978-0-8165-2844-8. {{cite book}}: Explicit use of et al. in: |first= (help); Unknown parameter |editor1_first= ignored (help); Unknown parameter |editor1_last= ignored (help); Unknown parameter |editor2_first= ignored (help); Unknown parameter |editor2_last= ignored (help); Unknown parameter |editor3_first= ignored (help); Unknown parameter |editor3_last= ignored (help)
  22. ^ O'Leary, B. (1972). "Io's Triaxial Figure". Icarus. 17: 209–215. doi:10.1016/0019-1035(72)90057-7. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  23. ^ Dobbins, T. (2004). "The Story of Jupiter's Egg Moons". Sky & Telescope. 107 (1): 114–120. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  24. ^ Barnard, E. E. (1891). "Observations of the Planet Jupiter and his Satellites during 1890 with the 12-inch Equatorial of the Lick Observatory". Monthly Notices of the Royal Astronomical Society. 51 (9): 543–556.
  25. ^ Barnard, E. E. (1894). "On the Dark Poles and Bright Equatorial Belt of the First Satellite of Jupiter". Monthly Notices of the Royal Astronomical Society. 54 (3): 134–136.
  26. ^ Minton, R. B. (1973). "The Red Polar Caps of Io". Communications of the Lunar and Planetary Laboratory. 10: 35–39.
  27. ^ Stebbins, J. P. (1926). "The Light Variations of the Satellites of Jupiter and their Application to Measures of the Solar Constant". Publications of the Astronomical Society of the Pacific. 38 (226): 321–322. {{cite journal}}: Cite has empty unknown parameter: |coauthors= (help)
  28. ^ Dollfus, A. (1998). "History of planetary science. The Pic du Midi Planetary Observation Project : 1941–1971". Planetary and Space Science. 46 (8): 1037–1073. doi:10.1016/S0032-0633(98)00034-8. {{cite journal}}: Cite has empty unknown parameter: |coauthors= (help)
  29. ^ a b Lee, T. (1972). "Spectral Albedos of the Galilean Satellites". Communications of the Lunar and Planetary Laboratory. 9 (3): 179–180.
  30. ^ Binder, A. B. (1964). "Evidence for an atmosphere on Io". Icarus. 3: 299–305. doi:10.1016/0019-1035(64)90038-7. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  31. ^ Bigg, E. K. (1964). "Influence of the Satellite Io on Jupiter's Decametric Emission". Nature. 203: 1008–1010. doi:10.1038/2031008a0.
  32. ^ Muller, D. (2010). "Pioneer 10 Full Mission Timeline". Interplanetary Space Missions: Realtime Simulations, Full Timelines and Maps. Retrieved 2010-02-18.
  33. ^ Kliore, A. J. (1975). "Atmosphere of Io from Pioneer 10 radio occultation measurements". Icarus. 24: 407–410. doi:10.1016/0019-1035(75)90057-3. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  34. ^ Fimmel, R. O. (1977). "First into the Outer Solar System". Pioneer Odyssey. NASA. Retrieved 2007-06-05. {{cite web}}: Cite has empty unknown parameter: |month= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  35. ^ Judge, D. L. (1974). "Pioneer 10 Observations of the Ultraviolet Glow in the Vicinity of Jupiter". Science. 183 (4122): 317–318. doi:10.1126/science.183.4122.317. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  36. ^ Muller, D. (2010). "Pioneer 11 Full Mission Timeline". Interplanetary Space Missions: Realtime Simulations, Full Timelines and Maps. Retrieved 2010-02-18.
  37. ^ "Pioneer 11 Images of Io". Galileo Home Page. Retrieved 2007-04-21.
  38. ^ a b Anderson, J. D. (1974). "Gravitational parameters of the Jupiter system from the Doppler tracking of Pioneer 10". Science. 183 (4122): 322–323. doi:10.1126/science.183.4122.322. PMID 17821098. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  39. ^ Pollack, J. B. (1974). "Implications of Jupiter's early contraction history for the composition of the Galilean satellites". Icarus. 21: 248–253. doi:10.1016/0019-1035(74)90040-2. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  40. ^ a b Fanale, F. P. (1974). "Io: A Surface Evaporite Deposit?". Science. 186 (4167): 922–925. doi:10.1126/science.186.4167.922. PMID 17730914. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  41. ^ a b Morrison, J (1973). "Thermal Properties of the Galilean satellites". Icarus. 18: 223–236. doi:10.1016/0019-1035(73)90207-8. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  42. ^ Hansen, O. L. (1973). "Ten-micron eclipse observations of Io, Europa, and Ganymede". Icarus. 18: 237–246. doi:10.1016/0019-1035(73)90208-X.
  43. ^ a b Witteborn, F. C. (1979). "Io: An Intense Brightening Near 5 Micrometers". Science. 203: 643–646. doi:10.1126/science.203.4381.643. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  44. ^ a b Peale, S. J. (1979). "Melting of Io by Tidal Dissipation". Science. 203: 892–894. doi:10.1126/science.203.4383.892. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  45. ^ a b "Voyager Mission Description". PDS Rings Node. NASA. 1997-02-19. Retrieved 2007-04-21. {{cite web}}: Cite has empty unknown parameters: |month= and |coauthors= (help)
  46. ^ a b c d e f Smith, B. A. (1979). "The Jupiter system through the eyes of Voyager 1". Science. 204 (4396): 951–972. doi:10.1126/science.204.4396.951. PMID 17800430. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  47. ^ a b c d e Morrison, David.; Samz, Jane (1980). "The First Encounter". Voyager to Jupiter. National Aeronautics and Space Administration. pp. 74–102.{{cite book}}: CS1 maint: multiple names: authors list (link)
  48. ^ Broadfoot, A. L. (1979). "Extreme ultraviolet observations from Voyager 1 encounter with Jupiter". Science. 204 (4396): 979–982. doi:10.1126/science.204.4396.979. PMID 17800434. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  49. ^ Krimigis, S. A. (1979). "Low-energy charged particle environment at Jupiter: A first look". Science. 204 (4396): 998–1003. doi:10.1126/science.204.4396.998. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  50. ^ Hess, N. F. (1979). "Magnetic Field Studies at Jupiter by Voyager 1: Preliminary Results". Science. 204 (4396): 982–987. doi:10.1126/science.204.4396.982. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  51. ^ Soderblom, L. A. (1980). "Spectrophotometry of Io: Preliminary Voyager 1 results". Geophys. Res. Lett. 7: 963–966. doi:10.1029/GL007i011p00963. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  52. ^ Sagan, C. (1979). "Sulphur flows on Io". Nature. 280: 750–753. doi:10.1038/280750a0. {{cite journal}}: Cite has empty unknown parameter: |coauthors= (help)
  53. ^ Morabito, L. A. (1979). "Discovery of currently active extraterrestrial volcanism". Science. 204 (4396): 972. doi:10.1126/science.204.4396.972. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  54. ^ a b Strom, R. G. (1979). "Volcanic eruption plumes on Io". Nature. 280: 733–736. doi:10.1038/280733a0. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  55. ^ Hanel, R. (1979). "Infrared Observations of the Jovian System from Voyager 1". Science. 204 (4396): 972–976. doi:10.1126/science.204.4396.972-a. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  56. ^ Pearl, J. C. (1979). "Identification of gaseous SO
    2
    and new upper limits for other gases on Io". Nature. 288: 757–758. doi:10.1038/280755a0.
    {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  57. ^ a b Morrison, David.; Samz, Jane (1980). "The Second Encounter: More Surprises from the "Land" of the Giant". Voyager to Jupiter. National Aeronautics and Space Administration. pp. 104–126.{{cite book}}: CS1 maint: multiple names: authors list (link)
  58. ^ a b c Smith, B. A. (1979). "The Galilean Satellites and Jupiter: Voyager 2 Imaging Science Results". Science. 206 (4421): 927–950. doi:10.1126/science.206.4421.927. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  59. ^ Strom, R. G. (1982). "Volcanic eruptions on Io". In Morrison, D. (ed.). Satellites of Jupiter. University of Arizona Press. pp. 598–633. ISBN 0-8165-0762-7. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  60. ^ a b c Spencer, J. R. (1996). "Io on the Eve of the Galileo Mission". Annual Review of Earth and Planetary Sciences. 24: 125–190. doi:10.1146/annurev.earth.24.1.125. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  61. ^ Johnson, T. V. (1988). "Io: Evidence for Silicate Volcanism in 1986". Science. 242: 1280–1283. doi:10.1126/science.242.4883.1280. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  62. ^ Sinton, W. M. (1980). "Io: Ground-Based Observations of Hot Spots". Science. 210: 1015–1017. doi:10.1126/science.210.4473.1015. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  63. ^ Carr, M. H. (1986). "Silicate volcanism on Io". Journal of Geophysical Research. 91: 3521–3532. doi:10.1029/JB091iB03p03521.
  64. ^ Harland, D. M (2000). "Early days". Jupiter Odyssey: The Story of NASA's Galileo Mission. Springer-Praxis. pp. 1–25. ISBN 978-1852333010. {{cite book}}: Cite has empty unknown parameter: |coauthors= (help)
  65. ^ Anderson, J. D. (1996). "Galileo Gravity Results and the Internal Structure of Io". Science. 272 (5262): 709–712. doi:10.1126/science.272.5262.709. PMID 8662566. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  66. ^ Kivelson, M. G. (1996). "A Magnetic Signature at Io: Initial Report from the Galileo Magnetometer". Science. 273 (5273): 337–340. doi:10.1126/science.273.5273.337. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  67. ^ a b McEwen, A. S. (1998). "Active Volcanism on Io as Seen by Galileo SSI". Icarus. 135 (1): 181–219. doi:10.1006/icar.1998.5972. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  68. ^ Geissler, P. E. (1998). "Galileo Imaging of Atmospheric Emissions from Io". Science. 285 (5429): 870–874. doi:10.1126/science.285.5429.870. PMID 9651251. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  69. ^ a b McEwen, A. S. (1998). "High-temperature silicate volcanism on Jupiter's moon Io". Science. 281 (5373): 87–90. doi:10.1126/science.281.5373.87. PMID 9651251. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  70. ^ Lopes-Gautier, R. (1999). "Active Volcanism on Io: Global Distribution and Variations in Activity". Icarus. 140: 243–264. doi:10.1006/icar.1999.6129. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  71. ^ Carlson, R. W. (1997). "The distribution of sulfur dioxide and other infrared absorbers on the surface of Io". Geophysical Research Letters. 24 (20): 2479–2482. doi:10.1029/97GL02609. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  72. ^ Lopes, R. M. C. (2001). "Io in the near infrared: Near-Infrared Mapping Spectrometer (NIMS) results from the Galileo flybys in 1999 and 2000". J. Geophys. Res. 106 (E12): 33053–33078. doi:10.1029/2000JE001463. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  73. ^ Keszthelyi, L. (2001). "Imaging of volcanic activity on Jupiter's moon Io by Galileo during the Galileo Europa Mission and the Galileo Millennium Mission". Journal of Geophysical Research. 106: 33025–33052. doi:10.1029/2000JE001383. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  74. ^ Atkinson, C. (2001). "Jupiter Millennium Flyby". Jet Propulsion Laboratory. Retrieved 2010-02-17.
  75. ^ Porco, C. C. (2003). "Cassini imaging of Jupiter's atmosphere, satellites, and rings". Science. 299 (5612): 1541–1547. doi:10.1126/science.1079462. PMID 12624258. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  76. ^ Kerr, R. A. (2010). "Magnetics Point to Magma 'Ocean' at Io". Science. 327 (5964): 408–409. doi:10.1126/science.327.5964.408-b.
  77. ^ Perry, J. E. (2003). Gish Bar Patera, Io: Geology and Volcanic Activity, 1997–2001 (PDF). Lunar and Planetary Science Conference XXXIV. Abstract #1720. {{cite conference}}: External link in |conferenceurl= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |conferenceurl= ignored (|conference-url= suggested) (help)
  78. ^ a b Marchis, F. (2002). "High-Resolution Keck Adaptive Optics Imaging of Violent Volcanic Activity on Io". Icarus. 160: 124–131. doi:10.1006/icar.2002.6955. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  79. ^ Spencer, John (2007-02-23). "Here We Go!". Retrieved 2007-06-03. {{cite web}}: Cite has empty unknown parameters: |month= and |coauthors= (help)
  80. ^ Gratiy, S. L. (2009). "Multi-wavelength simulations of atmospheric radiation from Io with a 3-D spherical-shell backward Monte Carlo radiative transfer model". Icarus. in press. doi:10.1016/j.icarus.2009.11.004. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  81. ^ Muller, D. (2010). "New Horizons Full Mission Timeline". Interplanetary Space Missions: Realtime Simulations, Full Timelines and Maps. Retrieved 2010-02-20.
  82. ^ Perry, J. (2008). "New Horizons Io Observations". Planetary Image Research Laboratory. Retrieved 2010-02-20.
  83. ^ a b Spencer, J. R. (2007). "Io Volcanism Seen by New Horizons: A Major Eruption of the Tvashtar Volcano". Science. 318 (5848): 240–243. doi:10.1126/science.1147621. PMID 17932290. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)