Exploration of Io
{{Short description|Overview of the exploration of Io, Jupiter's innermost Galilean and third-largest moon}}
File:Artwork Galileo-Io-Jupiter.JPG
The exploration of Io, Jupiter's innermost Galilean and third-largest moon, began with its discovery in 1610 and continues today with Earth-based observations and visits by spacecraft to the Jupiter system. Italian astronomer Galileo Galilei was the first to record an observation of Io on January 8, 1610, though Simon Marius may have also observed Io at around the same time. During the 17th century, observations of Io and the other Galilean satellites helped with the measurement of longitude by map makers and surveyors, with validation of Kepler's Third Law of planetary motion, and with measurement of the speed of light. Based on ephemerides produced by astronomer Giovanni Cassini and others, Pierre-Simon Laplace created a mathematical theory to explain the resonant orbits of three of Jupiter's moons, Io, Europa, and Ganymede. This resonance was later found to have a profound effect on the geologies of these moons. Improved telescope technology in the late 19th and 20th centuries allowed astronomers to resolve large-scale surface features on Io as well as to estimate its diameter and mass.
The advent of uncrewed spaceflight in the 1950s and 1960s provided an opportunity to observe Io up-close. In the 1960s the moon's effect on Jupiter's magnetic field was discovered. 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 atmosphere. In 1979, the two Voyager spacecraft flew through the Jupiter system. Voyager 1, during its encounter in March 1979, observed active volcanism on Io for the first time and mapped its surface in great detail, particularly the side that faces Jupiter. The Voyagers observed the Io plasma torus and Io's sulfur dioxide ({{chem|SO|2}}) atmosphere for the first time. NASA launched the Galileo spacecraft in 1989, which entered Jupiter's orbit in December 1995. Galileo allowed detailed study of both the planet and its satellites, including six flybys of Io between late 1999 and early 2002 that provided high-resolution images and spectra of Io's surface, confirming the presence of high-temperature silicate volcanism on Io. Distant observations by Galileo allowed planetary scientists to study changes on the surface that resulted from the moon's active volcanism.
In 2016, Juno arrived at Jupiter, and while the mission was designed to study Jupiter’s atmosphere and interior, it has performed several distant observations of Io using its visible-light telescope, JunoCAM, and its near-infrared spectrometer and imager, JIRAM.{{cite web |last=Anderson |first=Paul Scott |title=New Juno images of Io's fiery volcanoes |url=https://earthsky.org/space/juno-spacecraft-io-volcanoes-from-a-distance |date=January 6, 2019 |work=EarthSky |access-date=February 14, 2020}}
NASA and the European Space Agency (ESA) have made plans to return to the Jupiter system in the 2020s. ESA launched the Jupiter Icy Moon Explorer (Juice) to explore Ganymede, Europa, and Callisto in 2023, while NASA launched the Europa Clipper in 2024. Both will arrive in the Jupiter system in the late 2020s and early 2030s and should be able to acquire distant observations of Io. The proposed NASA Discovery mission Io Volcano Observer, currently going through a competitive process to be selected, would explore Io as its primary mission.{{cite news |title=NASA Selects Four Possible Missions to Study the Secrets of the Solar System |url=https://www.nasa.gov/press-release/nasa-selects-four-possible-missions-to-study-the-secrets-of-the-solar-system |newspaper=NASA |date=13 Feb 2020}}{{cite conference |title=Io Volcano Observer (IVO) |conference=Satellites panel of 2009 Decadal Survey |last=McEwen |first=A. S. |date=24 August 2009 |url=http://www.spacepolicyonline.com/pages/images/stories/PSDS%20Sat1%20McEwen-IVO.pdf |access-date=2010-02-20 |conference-url=http://sites.nationalacademies.org/SSB/CurrentProjects/ssb_052412 |archive-date=2012-02-29 |archive-url=https://web.archive.org/web/20120229054950/http://www.spacepolicyonline.com/pages/images/stories/PSDS%20Sat1%20McEwen-IVO.pdf |url-status=dead }} In the meantime, Io continues to be observed by the Hubble Space Telescope as well as by Earth-based astronomers using improved telescopes such as Keck and the European Southern Observatory.
Discovery: 1610
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. 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.{{cite book |last=Galilei |first=Galileo |author-link1=Galileo Galilei |title=Sidereus Nuncius |trans-title=The Starry Messenger |orig-year=First published 1610 |date=2004 |pages=17–28 |url=http://hsci.cas.ou.edu/images/barker/5990/Sidereus-Nuncius-whole.pdf |access-date=2010-01-07 |publisher=University of Padua |location=Venice |editor1-last=Carlos |editor1-first=E. S. |editor2-last=Barker |editor2-first=P. |url-status=dead |archive-url=https://web.archive.org/web/20051220010647/http://hsci.cas.ou.edu/images/barker/5990/Sidereus-Nuncius-whole.pdf |archive-date=2005-12-20 }} 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.{{cite web |url=http://home.comcast.net/~erniew/astro/sidnunj.html |title=Galileo's First Jupiter Observations |work=Astronomy Stuff: Observation and Simulation |last=Wright |first=E. |access-date=2010-02-17 |date=2004 |url-status=dead |archive-url=https://web.archive.org/web/20090208103112/http://home.comcast.net/~erniew/astro/sidnunj.html |archive-date=2009-02-08 }} 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 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. During this observation, the three stars in a line to the west of Jupiter were (from east to west): Io, Europa, and Ganymede. 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.{{cite web |last=Blue |first=J. |date=November 9, 2009 |url=http://planetarynames.wr.usgs.gov/append7.html |title=Planet and Satellite Names and Discoverers |publisher=USGS | access-date= 2010-01-13}} Galileo continued to observe the Jupiter system for the next month and a half.{{cite book |last=Drake |first=S. |author-link1=Stillman Drake |title=Galileo at Work: His Scientific Biography |date=1978 |isbn=978-0-226-16226-3 |pages=[https://archive.org/details/galileoatwork00stil/page/134 134–156] |chapter-url=https://books.google.com/books?id=OwOlRPbrZeQC |access-date=2010-02-17 |publisher=University of Chicago Press |location=Chicago |chapter=Eight: 1609–10 |url=https://archive.org/details/galileoatwork00stil/page/134 }} 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. 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." These were the first moons of a planet other than the Earth to be discovered.
The discoveries of Io and the other Galilean satellites of Jupiter were published in Galileo's Sidereus Nuncius in March 1610.{{cite book |last=Cruikshank |first=D. P. |author2=Nelson, R. M. |editor=Lopes, R. M. C. |editor1-link=Rosaly Lopes |editor2=Spencer, J. R. |title=Io after Galileo |date=2007 |publisher=Springer-Praxis |isbn=978-3-540-34681-4 |pages=5–33 |chapter=A history of the exploration of Io }} 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.{{cite web |url=http://galileo.rice.edu/sci/observations/jupiter_satellites.html |title=Satellites of Jupiter |work=The Galileo Project |last=Van Helden |first=A. |access-date=2010-02-17 |date=2003 |publisher=Rice University}} However, Galileo did not name each of the four moons individually beyond a numerical system in which Io was referred to as Jupiter I.{{cite book |last=Drake |first=S. |author-link1=Stillman Drake |title=Galileo at Work: His Scientific Biography |date=1978 |isbn=978-0-226-16226-3 |pages=[https://archive.org/details/galileoatwork00stil/page/157 157–176] |chapter-url=https://books.google.com/books?id=OwOlRPbrZeQC |access-date=2010-02-17 |publisher=University of Chicago Press |location=Chicago |chapter=Nine: 1610–11 |url=https://archive.org/details/galileoatwork00stil/page/157 }} 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.
In his book Mundus Iovialis ("The World of Jupiter"), published in 1614, Simon Marius, the court astronomer to the Margraves of Brandenburg-Ansbach, claimed to have discovered Io and the other moons of Jupiter in 1609, one week before Galileo's discovery. According to Marius, he began observing the Jupiter system in late November 1609.{{cite journal |last=Marius |first=S. |author-link1=Simon Marius |title=Mundus Iovialis anno M.DC.IX Detectus Ope Perspicilli Belgici |journal=The Observatory |volume=39 |trans-title=The World of Jupiter discovered in the year 1609 by means of a Dutch spy-glass |orig-year=First published 1614 |date=1916 |publisher=Johann Laur |pages=367–381 |location=Nuremberg |editor1-last=Prickard |editor1-first=A. O. |bibcode=1916Obs....39..367. }} He continued to observe the moons of Jupiter through December 1609, but did not record his observations until December 29, 1609 when he came to the conclusion "that these stars moved round Jupiter, just as the five solar planets, Mercury, Venus, Mars, Jupiter, and Saturn revolve round the Sun." However, Marius' observations were dated based on the Julian calendar, which was 10 days behind the Gregorian calendar used by Galileo. So Marius' first recorded observation from December 29, 1609 equates to Galileo's second observation of the Jupiter system on January 8, 1610.{{cite web |last=Van Helden |first=Albert |url=http://galileo.rice.edu/sci/marius.html |date=14 January 2004 |title=Simon Marius |work=The Galileo Project|publisher=Rice University |access-date=2010-01-07 }} Galileo doubted this claim and dismissed the work of Marius as plagiarism. Given that Galileo published his work before Marius and that his first recorded observation came one day before Marius', Galileo is credited with the discovery.{{cite web |last=Baalke |first=Ron |url=http://www2.jpl.nasa.gov/galileo/ganymede/discovery.html |archive-url=https://web.archive.org/web/19970106010432/http://www.jpl.nasa.gov/galileo/ganymede/discovery.html |url-status=dead |archive-date=1997-01-06 |title=Discovery of the Galilean Satellites |publisher=Jet Propulsion Laboratory |access-date=2010-01-07 }} 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
File:Apparatus to demonstrate the motion of Jupiter's satellites in Putnam Gallery, 2009-11-24.jpg of the Jovian system, built {{c.|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. 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. Simon Marius' estimate was only one minute longer in the data published in Mundus Iovalis. The orbital periods generated for Io and the other Jovian satellites provided an additional validation for Kepler's Third Law of planetary motion.
From these estimates of the orbital periods of Io and the other Galilean moons, astronomers hoped to generate ephemeris tables predicting the positions of each moon with respect to Jupiter, as well as when each moon would transit the face of Jupiter or be eclipsed by it. One benefit of such predictions, particularly those of satellite eclipses by Jupiter since they were subject to less observer error, would be determining an observer's longitude on Earth with respect to the prime meridian.{{cite web |url=http://galileo.rice.edu/sci/observations/longitude.html |title=Longitude at Sea |work=The Galileo Project |last=Van Helden |first=Albert |access-date=2010-02-17 |date=2004 |publisher=Rice University}} By observing an eclipse of a Jovian satellite, an observer could determine the current time at the prime meridian by looking up the eclipse in an ephemeris table. Io was particularly useful for this purpose since its shorter orbital period and closer distance to Jupiter made eclipses more frequent and less affected by Jupiter's axial tilt. Knowing the time at the prime meridian and the local time, the observer's longitude could then be calculated. Galileo attempted to produce a table predicting the positions of the Jovian satellites and eclipse timings after he negotiated first with Spain and then with The Netherlands to create a system for measuring longitude at sea using eclipse timings. However, he was never able to generate accurate predictions far enough ahead in time to be useful so he never published his tables. This left the tables published by Simon Marius in Mundus Iovialis and Giovanni Battista Hodierna in 1654 as the most accurate ephemeris tables available, even though they too were unable to predict the moons' positions with sufficient accuracy.
Giovanni Cassini published a much more accurate ephemeris table in 1668 using his observations from the previous 16 years.{{cite web |author1=O'Connor, J. J. |author2=Robertson, E. F. |date=February 1997 |url=http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/Longitude1.html | title=Longitude and the Académie Royale |publisher=University of St. Andrews |access-date= 2007-06-14 }} Using this table, Cassini generated a more accurate map of France by observing eclipses of the Jovian satellites at various locations across the country. This showed that previous maps had depicted some shorelines as extending farther than they really did, which caused the apparent area of France to shrink, and led King Louis XIV to comment that "he was losing more territory to his astronomers than to his enemies." Eclipse timings of the Jovian moons would continue to be used to determine longitude for another hundred years for tasks such as surveying the Mason–Dixon line and geodesy measurements. Efforts were made to utilize this method for marine navigation, but it proved to be impossible to make the necessary observations with sufficient accuracy from the moving deck of a ship; it would not be until the invention of the marine chronometer in the mid-18th century that determining longitude at sea became practical.
File:Galilean moon Laplace resonance animation 2.gif between Io, Europa and Ganymede (conjunctions are highlighted by color changes)|alt=Io, Europa, and Ganymede move counter-clockwise along three concentric circles around Jupiter. Every time Europa reaches the top of its orbit, Io goes around twice in its orbit. Every time Ganymede reaches the top of its orbit, Io goes around four times in its orbit.]]
During the 17th and 18th centuries astronomers 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 observed eclipse times 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. Ole Rømer never published his findings, but he sent along his measurements to Dutch mathematician Christiaan Huygens. Huygens used Rømer's estimate of 22 minutes for light to traverse the diameter of the Earth's orbit to calculate that light traveled 220,000 km/s, 26% less than the modern value.{{cite web |last=Huygens |first=C. |author-link1=Christiaan Huygens |date=8 January 1690 |url=http://www.gutenberg.org/catalog/world/readfile?fk_files=164378 | title=Treatise on Light |publisher=Project Gutenberg etext |access-date=2007-04-29 |editor1-last=Thompson |editor1-first=S. P.}} 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 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 {{convert|300,000|km|mi|sigfig=3|sp=us|lk=on}} per second.{{cite web |url=http://www.stats.uwaterloo.ca/~rwoldfor/papers/sci-method/paperrev/node4.html |title=The first evidence |work=Scientific Method, Statistical Method, and the Speed of Light |last=Oldford |first=R.W |access-date=2010-02-17 |date=2000 |publisher=University of Waterloo}}
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
File:Jupiter-io-transit feb 10 2009.gif
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{{E-sp|−5}} of the mass of Jupiter, which was one-quarter of the modern value.{{cite journal |last=de Sitter |first=W. |author-link=Willem de Sitter |date=1931 |title=Jupiter's Galilean satellites (George Darwin Lecture) |journal=Monthly Notices of the Royal Astronomical Society |volume=91 |issue= 7|pages=706–738 |bibcode=1931MNRAS..91..706D | doi = 10.1093/mnras/91.7.706 |doi-access=free }}{{cite book |last=Alexander |first=C. |display-authors=etal |title=Europa |date=2009 |publisher=University of Arizona Press |isbn=978-0-8165-2844-8 |pages=3–26 |chapter=The Exploration History of Europa |editor1-last=Pappalardo |editor1-first=R. T. |editor2-last=McKinnon |editor2-first=W. B. |editor3-last=Khurana |editor3-first=K. }} 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{{E-sp|−5}} of the mass of Jupiter, which was 4% less than the currently accepted mass. Io's diameter was estimated using micrometer measurements and occultations of background stars by Io. Edward E. Barnard used a micrometer at the Lick Observatory in 1897 to estimate a diameter of {{convert|3,950|km|mi|sigfig=3|abbr=on}}, 8.5% larger than the accepted modern value, while Albert A. Michelson, also using the Lick telescope, came up with a better estimate of {{convert|3,844|km|mi|sigfig=4|abbr=on}}. The best pre-spacecraft estimate of Io's diameter and shape came from the observations of an occultation of the star Beta Scorpii C on May 14, 1971, where a diameter of {{convert|3,636|km|mi|sigfig=4|abbr=on}} was found, slightly less the accepted modern value.{{cite journal | last=O'Leary |first=B. |author2=T. C. Van Flandern |title=Io's Triaxial Figure |journal=Icarus |volume=17 |issue= 1|pages=209–215 |date=1972 |doi=10.1016/0019-1035(72)90057-7 |bibcode=1972Icar...17..209O}} 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.
Beginning in the 1890s, larger telescopes allowed astronomers to directly observe large scale features on the surfaces of the Galilean satellites including Io. In 1892, William Pickering measured Io's shape using a micrometer, and similar to his measurement of Ganymede, found it to have an elliptical outline aligned with the direction of its orbital motion.{{cite journal |last=Dobbins |first=T. |author2=Sheehan, W. |date=2004 |title=The Story of Jupiter's Egg Moons |journal=Sky & Telescope |volume=107 |issue=1 |pages=114–120 |bibcode=2004S&T...107a.114D }} Other astronomers between 1850 and 1895 noted Io's elliptical shape. Edward Barnard observed Io while it transited across the face of Jupiter, finding the poles of Io to be dark compared to a brighter equatorial band.{{cite journal |last=Barnard |first=E. E. |author-link=Edward Emerson Barnard |date=1891 |title=Observations of the Planet Jupiter and his Satellites during 1890 with the 12-inch Equatorial of the Lick Observatory |journal=Monthly Notices of the Royal Astronomical Society |volume=51 |issue=9 |pages=543–556 |bibcode=1891MNRAS..51..543B | doi = 10.1093/mnras/51.9.543 |doi-access=free }} 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.{{cite journal |last=Barnard |first=E. E. |date=1894 |title=On the Dark Poles and Bright Equatorial Belt of the First Satellite of Jupiter |journal=Monthly Notices of the Royal Astronomical Society |volume=54 |issue=3 |pages=134–136 |bibcode=1894MNRAS..54..134B |doi=10.1093/mnras/54.3.134|doi-access=free }} The egg-shape of Io reported by Pickering was the result of measuring only the bright equatorial band of Io, and mistaking the dark poles for background space. Later telescopic observations confirmed Io's distinct reddish-brown polar regions and yellow-white equatorial band.{{cite journal |last=Minton |first=R. B. |date=1973 |title=The Red Polar Caps of Io |journal=Communications of the Lunar and Planetary Laboratory |volume=10 |pages=35–39 |bibcode=1973CoLPL..10...35M }} 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.{{cite journal |doi=10.1086/123621 |title=The Light Variations of the Satellites of Jupiter and their Application to Measures of the Solar Constant |journal=Publications of the Astronomical Society of the Pacific |last=Stebbins |first=J. P. |pages=321–322 |volume=38 |issue=226 |date=1926 |bibcode=1926PASP...38..321S|doi-access=free }} Stebbins also noted Io's dramatic orange coloration, which was unique among the Galilean satellites. Audouin Dollfus used observations of Io in the early 1960s at the Pic du Midi Observatory to create crude maps of the satellite that showed a patchwork of bright and dark spots across the Ionian surface, as well as a bright equatorial belt and dark polar regions.{{cite journal |title=History of planetary science. The Pic du Midi Planetary Observation Project : 1941–1971 |journal=Planetary and Space Science |last=Dollfus |first=A. |pages=1037–1073 |volume=46 |issue=8 |date=1998 |doi=10.1016/S0032-0633(98)00034-8 |bibcode=1998P&SS...46.1037D}}
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.{{cite journal |last=Lee |first=T. |date=1972 |title=Spectral Albedos of the Galilean Satellites |journal=Communications of the Lunar and Planetary Laboratory |volume=9 |issue=3 |pages=179–180 |bibcode=1972CoLPL...9..179L }} The lack of water on Io was consistent with the moon's estimated density, although, abundant water ice was found on the surface of Europa, a moon thought to have the same density as Io. Lee concluded that the spectrum was consistent with the presence of sulfur compounds. Binder and Cruikshank (1964) reported that Io's surface was brighter coming out of Jupiter's shadow than when it entered it.{{cite journal |last=Binder |first=A. B. |author2=Cruikshank, D. P. |date=1964 |title=Evidence for an atmosphere on Io |journal=Icarus |volume=3 |issue=4 |pages=299–305 |doi=10.1016/0019-1035(64)90038-7 |bibcode=1964Icar....3..299B}} 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 {{chem|SO|2}} atmosphere froze out enough to produce a layer several millimeters thick, which seemed unlikely. Radio telescopic observations revealed Io's influence on the Jovian magnetosphere, as demonstrated by decametric wavelength bursts tied to the orbital period of Io (Io-DAM), suggesting an electrodynamic coupling between the two worlds.{{cite journal |last=Bigg |first=E. K. |date=1964 |title=Influence of the Satellite Io on Jupiter's Decametric Emission |journal=Nature |volume=203 |issue=4949 |pages=1008–1010 |doi=10.1038/2031008a0 |bibcode = 1964Natur.203.1008B |s2cid=12233914 }}
''Pioneer'' era: 1973–1979
File:Pioneer 10 at Jupiter.gif
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. 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 uncrewed mission to the outer Solar System.
Pioneer 10 became the first spacecraft to reach the Jupiter system on December 3, 1973. It passed within {{convert|357,000|km|mi|sigfig=3|abbr=on}} of Io.{{cite web |url=http://www.dmuller.net/spaceflight/mission.php?mission=pioneer10&appear=black&mtype=scet&showimg=yes&dispwide=no |title=Pioneer 10 Full Mission Timeline |access-date=2010-02-18 |last=Muller |first=D. |date=2010 |work=Interplanetary Space Missions: Realtime Simulations, Full Timelines and Maps }} 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{{E-sp|−7}} bar, though the composition was not determined.{{cite journal |last=Kliore |first=A. J. |display-authors=etal |date=1975 |title=Atmosphere of Io from Pioneer 10 radio occultation measurements |journal=Icarus |volume=24 |issue=4 |pages=407–410 |doi=10.1016/0019-1035(75)90057-3 |bibcode=1975Icar...24..407K}} 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.{{cite web |url=https://history.nasa.gov/SP-349/ch5.htm |title=First into the Outer Solar System |access-date=2007-06-05 |last=Fimmel |first=R. O. |display-authors=etal |date=1977 |work=Pioneer Odyssey |publisher=NASA }} Pioneer 10 also discovered a hydrogen ion torus at the orbit of Io.{{cite journal | last=Judge |first=D. L. |author2=R. W. Carlson |title=Pioneer 10 Observations of the Ultraviolet Glow in the Vicinity of Jupiter |journal=Science |volume=183 |issue=4122|pages=317–318 |date=1974 | pmid=17821094|doi=10.1126/science.183.4122.317 |bibcode = 1974Sci...183..317J |s2cid=38074374 }}
Pioneer 11 encountered the Jupiter system nearly one year later on December 2, 1974, approaching to within {{convert|314,000|km|mi|sigfig=3|abbr=on}} of Io.{{cite web |url=http://www.dmuller.net/spaceflight/mission.php?mission=pioneer11&appear=black&mtype=scet&showimg=yes&dispwide=no |title=Pioneer 11 Full Mission Timeline |access-date=2010-02-18 |last=Muller |first=D. |date=2010 |work=Interplanetary Space Missions: Realtime Simulations, Full Timelines and Maps |archive-url=https://web.archive.org/web/20120303151608/http://www.dmuller.net/spaceflight/mission.php?mission=pioneer11&appear=black&mtype=scet&showimg=yes&dispwide=no |archive-date=2012-03-03 |url-status=dead }} Pioneer 11 provided the first spacecraft image of Io, a {{convert|357|km|mi|sigfig=3|abbr=on}} per pixel frame (D7) over Io's north polar region taken from a distance of {{convert|470,000|km|mi|sigfig=2|abbr=on}}.{{cite web |url=http://www2.jpl.nasa.gov/galileo/io/pioio.html |archive-url=https://web.archive.org/web/19970409095549/http://www.jpl.nasa.gov/galileo/io/pioio.html |url-status=dead |archive-date=1997-04-09 |title=Pioneer 11 Images of Io |work=Galileo Home Page |access-date=2007-04-21 }} This low-resolution image revealed dark patches on Io's surface akin to those hinted at in maps by Audouin Dollfus. Observations by both Pioneers revealed that Jupiter and Io were connected by an electrical conduit known as the Io flux tube, which consists of magnetic field lines trending from the Jupiter's poles to the satellite. Pioneer 11's closer encounter with Jupiter allowed the spacecraft to discover Jupiter's intense radiation belts similar to Earth's Van Allen Belts. One of the peaks in charged particle flux was found near the orbit of Io. 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 and calculating the mass necessary to produce the deviations. When this estimate was combined with the best available information on 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. The high density of Io (3.5 g/cm3) indicated that it was composed primarily of silicate rock rather than water ice.{{cite journal | last=Anderson |first=J. D. |display-authors=etal |title=Gravitational parameters of the Jupiter system from the Doppler tracking of Pioneer 10 |journal=Science |volume=183 |issue=4122|pages=322–323 |date=1974 |doi=10.1126/science.183.4122.322 | pmid=17821098 |bibcode = 1974Sci...183..322A |s2cid=36510719 }}
Following the Pioneer encounters and in the lead up to the Voyager fly-bys in 1979, interest in Io and the other Galilean satellites grew, with the planetary science and astronomy communities going so far as to convene a week of dedicated Io observations by radio, visible, and infrared astronomers in November 1974 known as "Io Week." New observations of Io from Earth and by the Pioneers during the mid-1970s caused a paradigm shift in thinking about its surface chemistry and formation. The trend in the densities of the four Galilean satellites found by Pioneer 10 suggested that the satellites formed as part of a collapsing nebula, like a miniature version of what took place in the Solar System as a whole. The initial hot Jupiter prevented the condensation of water at the orbits of Io and Europa, leading those bodies to have higher densities than the outer two moons.{{cite journal | last=Pollack |first=J. B. |author2=R. T. Reynolds |title=Implications of Jupiter's early contraction history for the composition of the Galilean satellites |journal=Icarus |volume=21 |issue= 3|pages=248–253 |date=1974 |doi=10.1016/0019-1035(74)90040-2 |bibcode=1974Icar...21..248P}} 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.{{cite journal |title=Io: A Surface Evaporite Deposit? |journal=Science |last=Fanale |first=F. P. |display-authors=etal |pages=922–925 |volume=186 |issue=4167 |date=1974 |doi=10.1126/science.186.4167.922 |pmid=17730914 |bibcode=1974Sci...186..922F|s2cid=205532 }} This was consistent with Io lacking water ice either on its surface or in its interior, in contrast with the other Galilean satellites. An absorption band near 560 nm was identified with the radiation-damaged form of the mineral halite. It was thought that deposits of the mineral on Io's surface were the origin of a cloud of sodium atoms surrounding Io, created through energetic-particle sputtering.
Measurements of Io's thermal radiation in the mid-infrared spectrum in the 1970s led to conflicting results that were not explained accurately until after the discovery of the active volcanism by Voyager 1 in 1979. An anomalously high thermal flux, compared to the other Galilean satellites, was observed at an infrared wavelength of 10 μm while Io was in Jupiter's shadow.{{cite journal |title=Thermal Properties of the Galilean satellites |journal=Icarus |last=Morrison |first=J |author2=Cruikshank, D. P. |pages=223–236 |volume=18 |issue= 2|date=1973 |doi=10.1016/0019-1035(73)90207-8 |bibcode=1973Icar...18..224M}} At the time, this heat flux was attributed to the surface having a much higher thermal inertia than Europa and Ganymede.{{cite journal |title=Ten-micron eclipse observations of Io, Europa, and Ganymede |journal=Icarus |last=Hansen |first=O. L. |pages=237–246 |volume=18 |issue= 2|date=1973 |doi=10.1016/0019-1035(73)90208-X |bibcode=1973Icar...18..237H}} These results were considerably different from measurements taken at wavelengths of 20 μm which suggested that Io had similar surface properties to the other Galilean satellites. NASA researchers observed a sharp increase in Io's thermal emission at 5 μm on February 20, 1978, possibly due to an interaction between the satellite and Jupiter's magnetosphere, though volcanism was not ruled out.{{cite journal |title=Io: An Intense Brightening Near 5 Micrometers |journal=Science |last=Witteborn |first=F. C. |display-authors=etal |pages=643–646 |volume=203 |issue= 4381|date=1979 |pmid=17813373 |doi=10.1126/science.203.4381.643|bibcode = 1979Sci...203..643W |s2cid=43128508 }}
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.{{cite journal |title=Melting of Io by Tidal Dissipation |journal=Science |last=Peale |first=S. J. |display-authors=etal |pages=892–894 |volume=203 |issue= 4383|date=1979 |pmid=17771724 |doi=10.1126/science.203.4383.892|bibcode = 1979Sci...203..892P |s2cid=21271617 }}
''Voyager'' era: 1979–1995
File:Io At 5 Million Miles.jpg
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.{{cite web|title=First Pictures: Voyager 1 Images Io's Volcanic Plumes – March 8, 1979|date=March 8, 2021|url=https://www.drewexmachina.com/2021/03/08/first-pictures-voyager-1-images-ios-volcanic-plumes-march-8-1979/|publisher=DREWExMachina|access-date=May 14, 2023}}
Voyager 1 was first of the two probes to encounter the Jupiter system in March 1979.{{cite web |url=http://pds-rings.seti.org/voyager/mission/ |title=Voyager Mission Description |access-date=2007-04-21 |date= 1997-02-19 |work=PDS Rings Node |publisher=NASA }} On approach to Jupiter in late February and early March 1979, Voyager imaging scientists noticed that Io appeared distinct from the other Galilean satellites. Its surface was orange in color and marked by dark spots, which were initially interpreted as the sites of impact craters. Among the most intriguing features was a heart-shaped, dark ring {{convert|1,000|km|mi|sigfig=1|abbr=on}} across that would later turn out to be the plume deposit of the volcano Pele.{{cite book |last=Morrison |first=David.; Samz, Jane |title=Voyager to Jupiter |date=1980 |publisher=National Aeronautics and Space Administration |pages=74–102 |chapter=The First Encounter |chapter-url=http://www.eric.ed.gov/ERICWebPortal/custom/portlets/recordDetails/detailmini.jsp?_nfpb=true&_&ERICExtSearch_SearchValue_0=ED312131&ERICExtSearch_SearchType_0=no&accno=ED312131 }} 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.{{cite journal | last=Broadfoot |first=A. L. |display-authors=etal |title=Extreme ultraviolet observations from Voyager 1 encounter with Jupiter |journal=Science |volume=204 |issue= 4396|pages=979–982 |date=1979 |doi=10.1126/science.204.4396.979 | pmid=17800434 |bibcode = 1979Sci...204..979B |s2cid=1442415 }} 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.{{cite journal | last=Krimigis |first=S. A. |display-authors=etal |title=Low-energy charged particle environment at Jupiter: A first look |journal=Science |volume=204 |issue=4396 |pages=998–1003 |date=1979 | pmid=17800439|doi=10.1126/science.204.4396.998 |bibcode = 1979Sci...204..998K |s2cid=32838223 }} 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 {{convert|20|km|mi|sigfig=2|abbr=on}} per pixel over the satellite's leading hemisphere (the side that faces the moon's direction of motion around Jupiter) down to less than {{convert|1|km|mi|sigfig=1|abbr=on}} per pixel over portions of the sub-Jovian hemisphere (the "near" side of Io).{{cite journal |title=The Jupiter system through the eyes of Voyager 1 |journal=Science |last=Smith |first=B. A. |display-authors=etal |pages=951–972 |volume=204 |issue= 4396|date=1979 |doi=10.1126/science.204.4396.951 |pmid=17800430 |bibcode=1979Sci...204..951S|s2cid=33147728 }} The images returned during the approach revealed a strange, multi-colored landscape devoid of impact craters, unlike the other planetary surfaces imaged to that point such as the Moon, Mars, and Mercury. The dark spots in earlier images resembled volcanic calderas more than they did the impact craters seen on those other worlds. Stunned by the oddity of Io's surface, Voyager imaging scientist Laurence Soderblom at a pre-encounter press conference joked, "this one we got all figured out...
File:Io VGR South polar color mosaic.jpg images covering Io's south polar region|alt=An aerial image of a landscape with numerous flow-like features, irregular shaped, flat-floored pits, tall mountains, and shorter mesas. These features are surrounded by smooth plains, with several areas of bright terrain surrounding some mountains and pits. The boundary between the day-side and night-side 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.]]
On March 5, 1979, Voyager 1 performed the closest encounter with Io of the Voyager mission from a distance of {{convert|20,600|km|mi|sigfig=3|abbr=on}} over its south pole. 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 {{convert|0.5|km|mi|sigfig=1|abbr=on}} per pixel. Unfortunately, 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 the Voyager{{'}}s scan platform was moving between targets. The highest-resolution images showed a relatively young surface punctuated by oddly shaped pits that appeared 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. Even in the highest resolution images, no impact craters were observed, suggesting that Io's surface was being regularly renewed by the present-day volcanic activity. The encounter over one of Io's poles allowed Voyager 1 to directly sample the edge of the Io flux tube, finding an intense electric current of 5{{E-sp|6}} amperes.{{cite journal | last=Hess |first=N. F. |display-authors=etal |title=Magnetic Field Studies at Jupiter by Voyager 1: Preliminary Results |journal=Science |volume=204 |issue=4396 |pages=982–987 |date=1979 | pmid=17800435|doi=10.1126/science.204.4396.982 |bibcode = 1979Sci...204..982N |hdl=2060/19790019933 |s2cid=38847163 |hdl-access=free }} The color data from Voyager's cameras showed that Ionian surface was dominated by sulfur and sulfur dioxide ({{chem|SO|2}}) frosts.{{cite journal | last=Soderblom |first=L. A. |display-authors=etal |title=Spectrophotometry of Io: Preliminary Voyager 1 results |journal=Geophys. Res. Lett. |volume=7 |issue= 11|pages=963–966 |date=1980 |doi=10.1029/GL007i011p00963 |bibcode=1980GeoRL...7..963S}} 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.{{cite journal | last=Sagan |first=C. |author-link1=Carl Sagan |title=Sulphur flows on Io |journal=Nature |volume=280 |issue= 5725|pages=750–753 |date=1979 |doi=10.1038/280750a0 |bibcode = 1979Natur.280..750S |s2cid=32086788 }}
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 {{convert|300|km|adj=on|sigfig=2|sp=us}} tall cloud along the moon's limb.{{cite journal | last=Morabito |first=L. A. |display-authors=etal |title=Discovery of currently active extraterrestrial volcanism |journal=Science |volume=204 |issue=4396 |pages=972 |date=1979 |doi=10.1126/science.204.4396.972 |pmid=17800432|bibcode = 1979Sci...204..972M |s2cid=45693338 }} 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.{{cite journal |title=Volcanic eruption plumes on Io |journal=Nature |last=Strom |first=R. G. |display-authors=etal |pages=733–736 |volume=280 |issue= 5725|date=1979 |doi=10.1038/280733a0|bibcode = 1979Natur.280..733S |s2cid=8798702 |doi-access=free }} Analysis of other Voyager 1 images showed nine such plumes scattered across the surface, proving that Io was volcanically active. The Infrared Interferometer Spectrometer (IRIS) on Voyager 1 discovered thermal emission from multiple sources, indicative of cooling lava. This showed that some of the lava flows visible on Io's surface were active.{{cite journal |title=Infrared Observations of the Jovian System from Voyager 1 |journal=Science |last=Hanel |first=R. |display-authors=etal |pages=972–976 |volume=204 |issue=4396 |date=1979 |pmid=17800431 |doi=10.1126/science.204.4396.972-a|s2cid=43050333 }} IRIS also measured gaseous {{chem|SO|2}} within the Loki plume, providing additional evidence for an atmosphere on Io.{{cite journal | last=Pearl |first=J. C. |display-authors=etal |title=Identification of gaseous {{chem|SO|2}} and new upper limits for other gases on Io |journal=Nature |volume=288 |issue= 5725|pages=757–758 |date=1979 |doi=10.1038/280755a0 |bibcode = 1979Natur.280..755P |s2cid=4338190 |doi-access=free }} These results confirmed the prediction made by Peale et al. shortly before the encounter.
Voyager 2 passed Io on July 9, 1979 at a distance of {{convert|1,130,000|km|mi|sigfig=3|abbr=on}}, approaching Jupiter between the orbits of Europa and Ganymede.{{cite book |last=Morrison |first=David.; Samz, Jane |title=Voyager to Jupiter |date=1980 |publisher=National Aeronautics and Space Administration |pages=104–126 |chapter=The Second Encounter: More Surprises from the "Land" of the Giant |chapter-url=http://www.eric.ed.gov/ERICWebPortal/custom/portlets/recordDetails/detailmini.jsp?_nfpb=true&_&ERICExtSearch_SearchValue_0=ED312131&ERICExtSearch_SearchType_0=no&accno=ED312131 }} 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.{{cite journal |title=The Galilean Satellites and Jupiter: Voyager 2 Imaging Science Results |journal=Science |last=Smith |first=B. A. |display-authors=etal |pages=927–950 |volume=206 |issue=4421 |date=1979 |pmid=17733910 |doi=10.1126/science.206.4421.927|bibcode = 1979Sci...206..927S |s2cid=22465607 }} 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. As a result of the discovery of active volcanic plumes 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. 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.{{cite book |last=Strom |first=R. G. |author2=Schneider, N. M. |editor=Morrison, D. |title=Satellites of Jupiter |date=1982 |publisher=University of Arizona Press |isbn=978-0-8165-0762-7 |pages=[https://archive.org/details/satellitesofjupi0000unse/page/598 598–633] |chapter=Volcanic eruptions on Io |chapter-url=https://archive.org/details/satellitesofjupi0000unse/page/598 }} 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.
Just after the Voyager encounters, the accepted theory was that Io's lava flows were composed of sulfurous compounds. This was based on the color of volcanic terrains, and the low temperatures measured by the IRIS instrument (though IRIS was not sensitive to the high-temperatures associated with active silicate volcanism, where thermal emission peaks in the near-infrared).{{cite journal |title=Io on the Eve of the Galileo Mission |journal=Annual Review of Earth and Planetary Sciences |last=Spencer |first=J. R. |author2=Schneider, N. M. |pages=125–190 |issue=1 |volume=24 |date=1996 |doi=10.1146/annurev.earth.24.1.125|bibcode = 1996AREPS..24..125S }} However, Earth-based infrared studies in the 1980s and 1990s shifted the paradigm from one of primarily sulfur volcanism to one where silicate volcanism dominates, and sulfur acts in a secondary role. In 1986, measurements of a bright eruption on Io's leading hemisphere revealed temperatures higher than the boiling point of sulfur, indicating a silicate composition for at least some of Io's lava flows.{{cite journal |title=Io: Evidence for Silicate Volcanism in 1986 |journal=Science |last=Johnson |first=T. V. |display-authors=etal |pages=1280–1283 |volume=242 |issue= 4883|date=1988 |pmid=17817074 |doi=10.1126/science.242.4883.1280|bibcode = 1988Sci...242.1280J |s2cid=23811832 }} 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.{{cite journal |title=Io: Ground-Based Observations of Hot Spots |journal=Science |last=Sinton |first=W. M. |display-authors=etal |pages=1015–1017 |volume=210 |issue= 4473|date=1980 |pmid=17797493 |doi=10.1126/science.210.4473.1015|bibcode = 1980Sci...210.1015S }} 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.{{cite journal |title=Silicate volcanism on Io |journal=Journal of Geophysical Research |last=Carr |first=M. H. |pages=3521–3532 |volume=91 |issue= B3|date=1986 |doi=10.1029/JB091iB03p03521 |bibcode=1986JGR....91.3521C|url=https://zenodo.org/record/1231402 |doi-access=free }} 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.{{cite book |last=Harland |first=D. M |title=Jupiter Odyssey: The Story of NASA's Galileo Mission |date=2000 |publisher=Springer-Praxis |isbn=978-1-85233-301-0 |pages=1–25 |chapter=Early days }} 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.{{cite book |last=Perry |first=J. |display-authors=etal |editor=Lopes, R. M. C. |editor2=Spencer, J. R. |title=Io after Galileo |date=2007 |publisher=Springer-Praxis |isbn=978-3-540-34681-4 |pages=35–59 |chapter=A Summary of the Galileo mission and its observations of Io }}
Galileo arrived at Jupiter on December 7, 1995, after a six-year journey from Earth during which it used gravity assists with Venus and Earth to boost its orbit out to Jupiter. Shortly before Galileo{{'}}s Jupiter Orbit Insertion maneuver, the spacecraft performed the only targeted flyby of Io of its nominal mission. High-resolution images were originally planned during the encounter, but problems with the spacecraft's tape recorder, used to save data taken during encounters for later playback to Earth, required the elimination of high-data-rate observations from the flyby schedule to ensure the safe recording of Galileo atmospheric probe data. The encounter did yield significant results from lower data rate experiments. Analysis of the Doppler shift of Galileo{{'}}s radio signal showed that Io is differentiated with a large iron core, similar to that found in the rocky planets of the inner Solar System.{{cite journal | last=Anderson |first=J. D. |display-authors=etal |title=Galileo Gravity Results and the Internal Structure of Io |journal=Science |volume=272 |issue=5262 |pages=709–712 |date=1996 |doi=10.1126/science.272.5262.709 | pmid=8662566 |bibcode = 1996Sci...272..709A |s2cid=24373080 }} Magnetometer data from the encounter, combined with the discovery of an iron core, suggested that Io might have a magnetic field.{{cite journal | last=Kivelson |first=M. G. |display-authors=etal |title=A Magnetic Signature at Io: Initial Report from the Galileo Magnetometer |journal=Science |volume=273 |issue=5273 |pages=337–340 |date=1996 | pmid=8662516|doi=10.1126/science.273.5273.337 |bibcode = 1996Sci...273..337K |s2cid=33017180 }}
File:Io Pillan Patera comparison.jpg
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
| class="wikitable" style="float:center; font-size:75%; margin:0 0 1em 1em; " cellpadding="0" cellspacing="2" |
| bgcolor="#a0ffa0" colspan="5" | Galileo encounters with Io with altitudes less than {{convert|300,000|km|mi|sigfig=3|abbr=on}} |
|---|
| Orbit
!Date !colspan="2" | Altitude !Notes |
| J0
|December 7, 1995 |{{convert|897|km|mi|sigfig=3|abbr=on|disp=table}} |No remote sensing; Gravity measurements reveal differentiated interior, large iron core; magnetic field? |
| C3
|November 4, 1996 |{{convert|244,000|km|mi|sigfig=3|abbr=on|disp=table}} |Clear-filter imaging of anti-Jovian hemisphere; near-IR spectra of {{chem|SO|2}} frost |
| E14
|March 29, 1998 |{{convert|252,000|km|mi|sigfig=3|abbr=on|disp=table}} |Multi-spectral imaging of anti-Jovian hemisphere |
| C21
|July 2, 1999 |{{convert|127,000|km|mi|sigfig=3|abbr=on|disp=table}} |Global color mosaic of anti-Jovian hemisphere |
| I24
|October 11, 1999 |{{convert|611|km|mi|sigfig=3|abbr=on|disp=table}} |High-resolution imaging of Pillan, Zamama, and Prometheus flows; Camera and Near-IR spectrometer suffer radiation damage |
| I25
|November 26, 1999 |{{convert|301|km|mi|sigfig=3|abbr=on|disp=table}} |Spacecraft safing event precludes high-resolution observations; images of Tvashtar outburst eruption |
| I27
|February 22, 2000 |{{convert|198|km|mi|sigfig=3|abbr=on|disp=table}} |Change detection at Amirani, Tvashtar, and Prometheus; Stereo imaging over Tohil Mons |
| I31
|August 6, 2001 |{{convert|194|km|mi|sigfig=3|abbr=on|disp=table}} |Camera problems preclude high-resolution imaging; Near-IR spectrometer observes eruption at Thor |
| I32
|October 16, 2001 |{{convert|184|km|mi|sigfig=3|abbr=on|disp=table}} |High-resolution observations of Thor, Tohil Mons, Gish Bar |
| I33
|January 17, 2002 |{{convert|102|km|mi|0|abbr=on|disp=table}} |Spacecraft safing event precludes observations; almost all remote sensing lost |
| A34
|November 7, 2002 |{{convert|45,800|km|mi|sigfig=3|abbr=on|disp=table}} |No remote sensing due to budget constraints |
File:021206 Galileo Io at sunset.jpg
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. Starting in May 1999, Galileo used four flybys (20 to 23) with Callisto to lower its periapse, setting up a chance for it to fly by Io twice in late 1999. During Galileo
Following the February 2000 encounter, Galileo
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.{{cite journal | last=Marchis |first=F. |display-authors=etal |title=High-Resolution Keck Adaptive Optics Imaging of Violent Volcanic Activity on Io |journal=Icarus |volume=160 |issue= 1|pages=124–131 |date=2002 |doi=10.1006/icar.2002.6955 |bibcode=2002Icar..160..124M}}{{cite web |url=http://planetary.org/blog/article/00000874/ |title=Here We Go! |access-date=2007-06-03 |last=Spencer |first=John |date=2007-02-23 |archive-url=https://web.archive.org/web/20070227125731/http://www.planetary.org/blog/article/00000874/|archive-date=2007-02-27|url-status=dead }} 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. 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
=''New Horizons'' (2007)=
File:Tvashtarvideo.gif 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 {{convert|2,239,000|km|mi|sigfig=4|abbr=on}}.{{cite web |url=http://www.dmuller.net/spaceflight/mission.php?mission=newhorizons&appear=black&mtype=scet&showimg=yes&dispwide=no |title=New Horizons Full Mission Timeline |access-date=2010-02-20 |last=Muller |first=D. |date=2010 |work=Interplanetary Space Missions: Realtime Simulations, Full Timelines and Maps }} During the encounter, numerous remote observations of Io were obtained, including visible imaging with a peak resolution of {{convert|11.2|km|mi|sigfig=3|abbr=on}} per pixel.{{cite web |url=http://pirlwww.lpl.arizona.edu/~perry/New_Horizons/ |title=New Horizons Io Observations |access-date=2010-02-20 |last=Perry |first=J. |date=2008 |publisher=Planetary Image Research Laboratory }} 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.{{cite journal |title=Io Volcanism Seen by New Horizons: A Major Eruption of the Tvashtar Volcano |journal=Science |last=Spencer |first=J. R. |display-authors=etal |pages=240–243 |volume=318 |issue= 5848|date=2007 |doi=10.1126/science.1147621 |pmid=17932290|bibcode = 2007Sci...318..240S |s2cid=36446567 }} 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.
A study with the Gemini telescope found that Io's SO2 atmosphere collapses during eclipse with Jupiter.Tsang, C., et al. 2016. The collapse of Io's primary atmosphere in Jupiter eclipse. Journal of Geophysical Research: Planets: 121, 1400–1410.{{Cite web | url=https://phys.org/news/2016-08-space-scientists-io-atmospheric-collapse.html | title=Space scientists observe Io's atmospheric collapse during eclipse}} 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.Bellucci, G., et al. 2004. Cassini/VIMS observation of an Io post-eclipse brightening event. Icarus: 172, 141–148.
''Juno'' Era: 2016–2025
File:Image of Io from Juno JunoCam from December 2023.png
The Juno spacecraft was launched in 2011 and entered orbit around Jupiter on July 5, 2016. Juno{{'}}s mission is primarily focused on improving our understanding of Jupiter's interior, magnetic field, aurorae, and polar atmosphere.{{cite web |last=Greicius |first=Tony |title=Juno – Mission Overview |url=http://www.nasa.gov/mission_pages/juno/overview/index.html |date=September 21, 2015 |work=NASA |access-date=February 14, 2020 |archive-date=December 25, 2018 |archive-url=https://web.archive.org/web/20181225085049/https://www.nasa.gov/mission_pages/juno/overview/index.html%20 |url-status=dead }} Juno{{'}}s 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, Juno{{'}}s 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.{{cite journal |title=Infrared observations of Io from Juno |journal=Icarus |last=Mura |first=A. |display-authors=etal |volume=341 |date=2020 |page=113607 |doi=10.1016/j.icarus.2019.113607 |bibcode=2020Icar..34113607M |s2cid=213970081 }} In January 2021, NASA officially extended the Juno mission through September 2025. While Juno{{'}}s 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{{'s}} extended mission on December 30, 2023, and February 3, 2024, both with altitudes of 1,500 kilometers.{{cite web |url=https://www.missionjuno.swri.edu/news/nasa-s-juno-mission-expands-into-the-future |title=NASA's Juno Mission Expands into the Future |date=January 13, 2021 |access-date=February 1, 2021}} 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.{{cite web |url=https://www.lpi.usra.edu/opag/meetings/opag2020fall/presentations/Bolton_6011.pdf |title=Juno OPAG Report |first=Scott |last=Bolton |date=September 2, 2020 |access-date=August 31, 2020}}
| class="wikitable" style="float:center; font-size:75%; margin:0 0 1em 1em; " cellpadding="0" cellspacing="2" |
| bgcolor="#a0ffa0" colspan="5" | Juno encounters with Io with altitudes less than {{convert|100,000|km|mi|sigfig=3|abbr=on}} |
|---|
| Orbit
!Date !colspan="2" | Altitude !Notes |
| PJ25
|February 17, 2020 |{{convert|195,104|km|mi|sigfig=3|abbr=on|disp=table}} |Closest encounter to Io during primary mission |
| PJ43
|July 5, 2022 |{{convert|86,141|km|mi|sigfig=3|abbr=on|disp=table}} | |
| PJ47
|December 14, 2022 |{{convert|63,771|km|mi|sigfig=3|abbr=on|disp=table}} |
| PJ49
|March 1, 2023 |{{convert|51,547|km|mi|sigfig=3|abbr=on|disp=table}} | |
| PJ51
|May 16, 2023 |{{convert|35,555|km|mi|sigfig=3|abbr=on|disp=table}} | |
| PJ53
|July 31, 2023 |{{convert|22,202|km|mi|sigfig=3|abbr=on|disp=table}} | |
| PJ55
|October 15, 2023 |{{convert|11,640|km|mi|sigfig=3|abbr=on|disp=table}} | |
| PJ57
|December 30, 2023 |{{convert|1500|km|mi|sigfig=3|abbr=on|disp=table}} | |
| PJ58
|February 3, 2024 |{{convert|1500|km|mi|sigfig=3|abbr=on|disp=table}} | |
| PJ60
|April 9, 2024 |{{convert|17,347|km|mi|0|abbr=on|disp=table}} | |
| PJ67
|November 25, 2024 |{{convert|85,736|km|mi|sigfig=3|abbr=on|disp=table}} | |
| PJ72
|May 8, 2025 |{{convert|92,957|km|mi|0|abbr=on|disp=table}} | |
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. JIRAM near-infrared spectroscopy has so far allowed for the coarse mapping of sulfur dioxide frost across Io's surface as well as mapping minor surface components weakly absorbing sunlight at 2.1 and 2.65 μm.{{cite journal |title=Mapping Io's Surface Composition With Juno/JIRAM |journal=Journal of Geophysical Research: Planets |last=Tosi |first=F. |display-authors=et al |date=2020 |volume=125 |issue=11 |doi=10.1029/2020JE006522 |bibcode=2020JGRE..12506522T |s2cid=225456943 }}
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.{{cite news |title=Esa selects 1bn-euro Juice probe to Jupiter |author=Jonathan Amos |url=https://www.bbc.co.uk/news/science-environment-17917102 |newspaper=BBC News |date=2 May 2012}} JUICE was launched in 2023, with arrival at Jupiter planned for July 2031.{{Citation |year=2012 |title=JUICE assessment study report (Yellow Book) |publisher=ESA |url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=49837}} 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.{{cite conference |title=Io Volcano Observer (IVO): Does Io have a Magma Ocean? |conference=LPSC LI |last=McEwen |first=A. |display-authors=etal |date=2020 |id=Abstract #1648 |url=https://www.hou.usra.edu/meetings/lpsc2020/pdf/1648.pdf}}{{Cite web |title=NASA Selects 2 Missions to Study ‘Lost Habitable’ World of Venus - NASA |url=https://www.nasa.gov/news-release/nasa-selects-2-missions-to-study-lost-habitable-world-of-venus/ |access-date=2025-05-16 |language=en-US}}
References
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