Exploration 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 by Galileo's innovations to improve the magnification of the new instrument. 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 star to the west of Jupiter was Ganymede. 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. Galileo continued to observe the Jupiter system for the next month and a half. On January 13, Galileo observed all four of 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. 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. However, Galileo did not name each of the four moons individually beyond a numerical system in which Io was referred to as Jupiter I. 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. 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." Galileo doubted this claim and dismissed the work of Marius as plagiarism. 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==

of the Jovian system, built , 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. So, the determination of its orbital period, along with those of the other Galilean satellites, was an early focus for astronomers. 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. 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. 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." Using Ole Rømer's data 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. 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==

Improved telescopes 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 to resolve large-scale surface features on it. 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. 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 the modern value with the closest being Sampson's 1921 estimate of 4.5 of the mass of Jupiter, which was 4% less than the currently accepted mass. 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. Other astronomers between 1850 and 1895 noted Io's elliptical shape. 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. 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. 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. 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 atmosphere froze out enough to produce a layer several millimeters thick, which seemed unlikely. ==Pioneer era: 1973–1979==

In the late 1960s, a concept known as the Planetary Grand Tour was developed in the United States by NASA and the Jet Propulsion Laboratory (JPL). It would allow a single spacecraft to travel past the asteroid belt and onto each of the outer planets, including Jupiter, if the mission was launched in 1976 or 1977. However, there was uncertainty over whether a spacecraft could survive passage through the asteroid belt, where micrometeoroids could cause it physical damage, or the intense Jovian magnetosphere, where charged particles could harm sensitive electronics. 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 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. Pioneer 11 encountered the Jupiter system nearly one year later on December 2, 1974, approaching to within of Io. Pioneer 11 provided the first spacecraft image of Io, a per pixel frame (D7) over Io's north polar region taken from a distance of . This low-resolution image revealed dark patches on Io's surface akin to those hinted at in maps by Audouin Dollfus. 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." 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. 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. 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==

The first close-up investigation of Io using high-resolution imaging was performed by the twin probes, Voyager 1 and Voyager 2, launched on September 5 and August 20, 1977, respectively. These two spacecraft were part of NASA and JPL's Voyager program to explore the giant outer planets through a series of missions in the late 1970s and 1980s. This was a scaled-down version of the earlier Planetary Grand Tour concept. Both probes contained more sophisticated instrumentation than the previous Pioneer missions, including a camera capable of taking much higher resolution images. This was important for viewing the geologic features of Jupiter's Galilean moons as well as 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 its surface. 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. 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 per pixel over the satellite's leading hemisphere (the side that faces the moon's direction of motion around Jupiter) down to less than 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 color data from Voyager's cameras showed that Ionian surface was dominated by sulfur and sulfur dioxide () frosts. Different surface colors were thought to correspond to distinct sulfur allotropes, caused by liquid sulfur being heated to different temperatures, changing its color and viscosity. 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 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. IRIS also measured gaseous 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. 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. The Pele plume deposit had changed shape, from a heart-shape during the Voyager 1 encounter to an oval during the Voyager 2 flyby. Changes in the distribution of diffuse plume deposits and additional dark material were observed in the southern portion of Loki Patera, the consequence of a volcanic eruption there. The blue color of the plumes observed (Amirani, Maui, Masubi, and Loki) suggested that the reflected light from them came from fine grained particles approximately 1 μm in diameter. 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. 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==

Planning for the next NASA mission to Jupiter began in 1977, just as the two Voyager probes were launched. 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 the planet and its many moons, including Io, as well as deliver a Jovian atmospheric probe. Originally scheduled to be launched via the Space Shuttle in 1982, delays resulting from development issues with the shuttle and upper-stage motor pushed the launch back, and in 1986 the Challenger disaster delayed ''Galileo's launch even further. Finally, on October 18, 1989, Galileo began its journey aboard the shuttle Atlantis. 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. 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 Galileos 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. Magnetometer data from the encounter, combined with the discovery of an iron core, suggested that Io might have a magnetic field. 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 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 to the west, tracking the end of a new lava flow at Prometheus. 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 its surface. During Galileo's ninth orbit, the spacecraft observed a major eruption at Pillan Patera, detecting high-temperature thermal emission and a new volcanic plume. The temperatures observed at Pillan and other volcanoes 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. During the following orbit, Galileo found that Pillan was surrounded by a new, dark pyroclastic deposit composed of silicate minerals such as orthopyroxene. 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 new evidence of a possible sub-surface water ocean. Finally, the imaging coverage was limited by 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 by an incident when radiation forced a reset of the spacecraft's computer putting it into safe mode during the November 1999 encounter. Even so, Galileo fortuitously imaged an outburst eruption at Tvashtar Paterae during the November flyby, observing a curtain of lava fountains long and high. An additional encounter was performed on February 22, 2000. 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. 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. Discoveries during the joint observations of Io revealed a new plume at Tvashtar and provided insights into Io's aurorae. 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. 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. and the lava lake at Pele. Due to a safing event prior to the encounter, nearly all of the observations planned for the January 2002 flyby were lost. 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 Era: 2003–2016==

Following the end of the Galileo mission, astronomers have continued monitoring Io's active volcanoes with adaptive optics imaging from the Keck telescope in Hawaii and the European Southern Observatory in Chile, as well as imaging from the Hubble telescope. These technologies are used to observe the thermal emissions and measure the composition of gases over volcanoes such as Pele and Tvashtar. 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. 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 Ionian atmosphere is supported by the sublimation of sulfur dioxide frost on Io's surface. New Horizons (2007) '' images showing Io's volcano Tvashtar spewing material 330 km above its surface. 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 . During the encounter, numerous remote observations of Io were obtained, including visible imaging with a peak resolution of 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 Xihe. Post-eclipse brightening, which has been seen at times in the past, was detected in near infrared wavelengths using an instrument aboard the Cassini spacecraft. == Juno Era: 2016–2025 ==

The Juno spacecraft was launched in 2011 and entered orbit around Jupiter on July 5, 2016. Junos mission is primarily focused on improving our understanding of Jupiter's interior, magnetic field, aurorae, and polar atmosphere. Junos 54-day orbit is highly inclined and highly eccentric in order to better characterize Jupiter's polar regions and to limit its exposure to the planet's harsh inner radiation belts, limiting close encounters with Jupiter's moons. During its primary mission, which lasts through June 2021, Junos closest approach to Io to date occurred during Perijove 25 on February 17, 2020, at a distance of 195,000 kilometers, acquiring near-infrared spectrometry with JIRAM while Io was in Jupiter's shadow. In January 2021, NASA officially extended the Juno mission through September 2025. While Junos highly inclined orbit keeps the spacecraft out of the orbital planes of Io and the other major moons of Jupiter, its orbit has been precessing so that its close approach point to Jupiter is at increasing latitudes and the ascending node of its orbit is getting closer to Jupiter with each orbit. This orbital evolution will allow Juno to perform a series of close encounters with the Galilean satellites during the extended mission. Two close encounters with Io occurred during Juno extended mission on December 30, 2023, and February 3, 2024, both with altitudes of 1,500 kilometers. Nine additional encounters with altitudes between 11,500 and 90,000 kilometers are also planned between July 2022 and May 2025. The primary goal of these encounters will be to improve our understanding of Io's gravity field using doppler tracking and to image Io's surface to look for surface changes since Io was last seen up-close in 2007. During several orbits, Juno has observed Io from a distance using JunoCAM, a wide-angle, visible-light camera, to look for volcanic plumes and JIRAM, a near-infrared spectrometer and imager, to monitor thermal emission from Io's volcanoes. ==Future missions==

Two spacecraft are en route to the Jovian system. The Jupiter Icy Moon Explorer (JUICE) is a European Space Agency mission to the Jovian system that is intended to end up in orbit around Ganymede. JUICE was launched in 2023, with arrival at Jupiter planned for July 2031. JUICE will not fly by Io, but it will use its instruments, such as a narrow-angle camera, to monitor Io's volcanic activity and measure its surface composition during the two-year Jupiter-tour phase of the mission prior to Ganymede orbit insertion. Europa Clipper is a NASA mission to the Jovian system focused on Jupiter's moon Europa. Like JUICE, Europa Clipper will not perform any flybys of Io, but distant volcano monitoring is likely. Europa Clipper was launched in October 2024 with an expected arrival at Europa in 2030. A dedicated mission to Io, called the Io Volcano Observer (IVO), was proposed for the Discovery Program as a Jupiter orbiter that would perform at least ten flybys of Io over 3.5 years, but in June 2021 it was passed over in favor of two missions to Venus. • Exploration of Jupiter • Volcanology of Io ==References==