Exploration of Io
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.[1] 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.[1] 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
2) atmosphere for the first time.[1] 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.[2]
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.[3]
NASA and the
Discovery: 1610
![A portrait of the head and upper body of a middle-aged man with a receding hairline and brown beard. He is wearing a black, Italian Renaissance outfit. The text "GAILILEVS GAILILEVS – MATHVS:" is painted to the left of the man's head.](http://upload.wikimedia.org/wikipedia/commons/thumb/9/94/Galileo_Galilei_2.jpg/220px-Galileo_Galilei_2.jpg)
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.[7] During his observation of Jupiter on the evening of January 7, Galileo spotted two stars to the east of Jupiter and another one to the west.[8] Jupiter and these three stars appeared to be in a line parallel to the ecliptic. The star furthest to the east from Jupiter turned out to be Callisto while the star to the west of Jupiter was Ganymede.[9] The third star, the closest one to the east of Jupiter, was a combination of the light from Io and Europa as Galileo's telescope, while having a high magnification for a telescope from his time, was too low-powered to separate the two moons into distinct points of light.[7][9] Galileo observed Jupiter the next evening, January 8, 1610, this time seeing three stars to the west of Jupiter, suggesting that Jupiter had moved to the west of the three stars.[8] During this observation, the three stars in a line to the west of Jupiter were (from east to west): Io, Europa, and Ganymede.[9] This was the first time that Io and Europa were observed and recorded as distinct points of light so this date, January 8, 1610 is used as the discovery date for the two moons by the International Astronomical Union.[10] Galileo continued to observe the Jupiter system for the next month and a half.[7] On January 13, Galileo observed all four of what would later be known as the Galilean moons of Jupiter for the first time in a single observation, though he had observed all four at various times in the preceding days.[9] On January 15, he observed the motions of three of these satellites, including Io, and came to the conclusions that these objects were not background stars, but were in fact, "three stars in the heavens moving about Jupiter, as Venus and Mercury round the Sun."[8] These were the first moons of a planet other than the Earth to be discovered.
The discoveries of Io and the other Galilean satellites of Jupiter were published in Galileo's
In his book Mundus Iovialis ("The World of Jupiter"), published in 1614,
Io as a tool: 1610–1809
![A brass, clock-like mechanical device in a museum display case, with a small card with the number 8 printed on it. The face of the device is split into several rings, with the Roman numerals I through XI (and 0) on one of these rings.](http://upload.wikimedia.org/wikipedia/commons/thumb/9/96/Apparatus_to_demonstrate_the_motion_of_Jupiter%27s_satellites_in_Putnam_Gallery%2C_2009-11-24.jpg/220px-Apparatus_to_demonstrate_the_motion_of_Jupiter%27s_satellites_in_Putnam_Gallery%2C_2009-11-24.jpg)
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.[12] Simon Marius' estimate was only one minute longer in the data published in Mundus Iovalis.[13] The orbital periods generated for Io and the other Jovian satellites provided an additional validation for Kepler's Third Law of planetary motion.[1]
From these estimates of the orbital periods of Io and the other Galilean moons, astronomers hoped to generate ephemeris tables predicting the positions of each moon with respect to Jupiter, as well as when each moon would transit the face of Jupiter or be eclipsed by it. One benefit of such predictions, particularly those of satellite eclipses by Jupiter since they were subject to less observer error, would be determining an observer's longitude on Earth with respect to the prime meridian.[16] By observing an eclipse of a Jovian satellite, an observer could determine the current time at the prime meridian by looking up the eclipse in an ephemeris table. Io was particularly useful for this purpose since its shorter orbital period and closer distance to Jupiter made eclipses more 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.[16] Galileo attempted to produce a table predicting the positions of the Jovian satellites and eclipse timings after he negotiated first with Spain and then with The Netherlands to create a system for measuring longitude at sea using eclipse timings. However, he was never able to generate accurate predictions far enough ahead in time to be useful so he never published his tables.[16] This left the tables published by Simon Marius in Mundus Iovialis and Giovanni Battista Hodierna in 1654 as the most accurate ephemeris tables available, even though they too were unable to predict the moons' positions with sufficient accuracy.[16]
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
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.[1] Laplace also found that the slight difference between these exact ratios and reality was due to their mean motions accounting for the precession of the periapse for Io and Europa. This resonance was later found to have a profound effect on the geologies of the three moons.
Io as a world: 1805–1973
![An animation simulating the orbital motion of a small, planetary body as it passes from left to right in front of Jupiter. A dark, circular spot is seen on Jupiter, moving left to right with the same speed, and to the right, of the smaller body.](http://upload.wikimedia.org/wikipedia/commons/thumb/f/f8/Jupiter-io-transit_feb_10_2009.gif/220px-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
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.[23] Other astronomers between 1850 and 1895 noted Io's elliptical shape.[21] Edward Barnard observed Io while it transited across the face of Jupiter, finding the poles of Io to be dark compared to a brighter equatorial band.[24] Initially, Barnard concluded that Io was in fact a binary of two dark bodies, but observations of additional transits against Jovian cloud bands of different brightness and the round shape of Io's shadow on the Jovian cloud tops caused him to change his interpretation.[25] The egg-shape of Io reported by Pickering was the result of measuring only the bright equatorial band of Io, and mistaking the dark poles for background space.[21] Later telescopic observations confirmed Io's distinct reddish-brown polar regions and yellow-white equatorial band.[26] Observations of variations in the brightness of Io as it 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.[27] Stebbins also noted Io's dramatic orange coloration, which was unique among the Galilean satellites.[1] Audouin Dollfus used observations of Io in the early 1960s at the Pic du Midi Observatory to create crude maps of 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.[28]
Telescopic observations in the mid-20th century began to hint at Io's unusual nature. The
2 atmosphere froze out enough to produce a layer several millimeters thick, which seemed unlikely.[1] Radio telescopic observations revealed Io's influence on the Jovian magnetosphere, as demonstrated by decametric wavelength bursts tied to the orbital period of Io (Io-DAM), suggesting an electrodynamic coupling between the two worlds.[31]
Pioneer era: 1973–1979
![A painting of a spacecraft in front of a crescent Jupiter, the distant Sun, and the stars of the Milky Way in the background. The night-side of Jupiter is illuminated.](http://upload.wikimedia.org/wikipedia/commons/thumb/9/90/Pioneer_10_at_Jupiter.gif/220px-Pioneer_10_at_Jupiter.gif)
In the late 1960s, a concept known as the
Pioneer 10 became the first spacecraft to reach the Jupiter system on December 3, 1973. It passed within 357,000 km (222,000 mi) of Io.[32] During Pioneer 10's fly-by of Io, the spacecraft performed a radio occultation experiment by transmitting an S-band signal as Io passed between it and Earth. A slight attenuation of the signal before and after the occultation showed that Io had an ionosphere, suggesting the presence of a thin atmosphere with a pressure of 1.0 × 10−7 bar, though the composition was not determined.[33] This was the second atmosphere to be discovered around a moon of an outer planet, after Saturn's moon Titan. Close-up images using Pioneer's Imaging Photopolarimeter were planned as well, but were lost because of the high-radiation environment.[34] Pioneer 10 also discovered a hydrogen ion torus at the orbit of Io.[35]
![Two versions of the same image of an orange planetary body; the bottom left half of both is illuminated. The image on the right is darker, so dark features on the surface of the body are more visible.](http://upload.wikimedia.org/wikipedia/commons/thumb/0/03/Pioneer11_Io.gif/220px-Pioneer11_Io.gif)
Pioneer 11 encountered the Jupiter system nearly one year later on December 2, 1974, approaching to within 314,000 km (195,000 mi) of Io.[36] Pioneer 11 provided the first spacecraft image of Io, a 357 km (222 mi) per pixel frame (D7) over Io's north polar region taken from a distance of 470,000 km (290,000 mi).[37] This low-resolution image revealed dark patches on Io's surface akin to those hinted at in maps by Audouin Dollfus.[1] Observations by both Pioneers revealed that Jupiter and Io were connected by an electrical conduit known as the Io flux tube, 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.[1] Radio tracking during the encounters of both Pioneers with Io provided an improved estimate of the moon's mass. This was accomplished by analyzing slight changes in trajectory of the two probes due to the influence of Io's gravity 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.[38] The high density of Io (3.5 g/cm3) indicated that it was composed primarily of silicate rock rather than water ice.[38]
Following the Pioneer encounters and in the lead up to the Voyager 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."
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.[41] At the time, this heat flux was attributed to the surface having a much higher thermal inertia than Europa and Ganymede.[42] These results were considerably different from measurements taken at wavelengths of 20 μm which suggested that Io had similar surface properties to the other Galilean satellites.[41] 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.[43]
A few days before the Voyager 1 encounter,
Voyager era: 1979–1995
![Photo a planetary body covered in numerous dark spots in front of the bright and dark clouds of Jupiter.](http://upload.wikimedia.org/wikipedia/commons/thumb/1/1f/Io_At_5_Million_Miles.jpg/220px-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.[45]
Voyager 1 was first of the two probes to encounter the Jupiter system in March 1979.[46] 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.[47] Among the most intriguing features was a heart-shaped, dark ring 1,000 km (600 mi) across that would later turn out to be the plume deposit of the volcano Pele.[48] 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.[48][49] 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.[50] In the hours prior to Voyager 1's encounter with Io, the spacecraft acquired images for a global map with a resolution of at least 20 km (12 mi) per pixel over the satellite's leading hemisphere (the side that faces the moon's direction of motion around Jupiter) down to less than 1 km (0.6 mi) per pixel over portions of the sub-Jovian hemisphere (the "near" side of Io).[47] The images returned during the approach revealed a strange, multi-colored landscape devoid of impact craters, unlike the other planetary surfaces imaged to that point such as the Moon, Mars, and Mercury.[1] The dark spots in earlier images resembled volcanic calderas more than they did the impact craters seen on those other worlds.[47] Stunned by the oddity of Io's surface, Voyager imaging scientist Laurence Soderblom at a pre-encounter press conference joked, "this one we got all figured out...[Io] is covered with thin candy shells of anything from sulfates and sulfur and salts to all kinds of strange things."[48]
![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.](http://upload.wikimedia.org/wikipedia/commons/thumb/2/22/Io_VGR_South_polar_color_mosaic.jpg/220px-Io_VGR_South_polar_color_mosaic.jpg)
On March 5, 1979, Voyager 1 performed the closest encounter with Io of the Voyager mission from a distance of 20,600 km (12,800 mi) over its south pole.[46][48] The close distance of the encounter allowed Voyager to acquire images of the sub-Jovian and south polar regions of Io with a best resolution of less than 0.5 km (0.3 mi) per pixel.[47] 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.[48] 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.[47] 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.[47] 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 × 106 amperes.[51] The color data from Voyager's cameras showed that Ionian surface was dominated by sulfur and sulfur dioxide (SO
2) frosts.[52] 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.[53]
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
![The thin crescent (open to the right) of the full disk of a planetary body with two bright clouds along the upper left edge of the object and another along the right edge.](http://upload.wikimedia.org/wikipedia/commons/thumb/d/d0/VGR2_Io_plumes2.jpg/220px-VGR2_Io_plumes2.jpg)
Voyager 2 passed Io on July 9, 1979 at a distance of 1,130,000 km (702,000 mi), approaching Jupiter between the orbits of Europa and Ganymede.[58] 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.[59] 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.[59] 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.[58] 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.[60] 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.[59]
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).[61] 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.[61] 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.[62] 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.[43][63] 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.[64] 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.[61]
Galileo era: 1995–2003
![A multi-colored image of the full disk of a planetary body, dotted with numerous dark spots. Much of the middle portion of the planetary body is yellow to white/gray, while the polar regions at the top and bottom are generally reddish in color.](http://upload.wikimedia.org/wikipedia/commons/thumb/8/8c/Iosurface_gal.jpg/220px-Iosurface_gal.jpg)
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 arrived at Jupiter on December 7, 1995, after a six-year journey from Earth during which it used
![Two images, displayed side-by-side, showing a red, diffuse ring with a darker, gray region in the middle. In the image on the right, this red ring is interrupted on its upper right side by a hexagonal dark gray region.](http://upload.wikimedia.org/wikipedia/commons/thumb/5/5e/Io_Pillan_Patera_comparison.jpg/220px-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'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,
Galileo encounters with Io with altitudes less than 300,000 km (186,000 mi)[2] | ||||
---|---|---|---|---|
Orbit | Date | Altitude | Notes | |
J0 | December 7, 1995 | 897 km | 557 mi | No remote sensing; Gravity measurements reveal differentiated interior, large iron core; magnetic field? |
C3 | November 4, 1996 | 244,000 km | 152,000 mi | Clear-filter imaging of anti-Jovian hemisphere; near-IR spectra of SO 2 frost |
E14 | March 29, 1998 | 252,000 km | 157,000 mi | Multi-spectral imaging of anti-Jovian hemisphere |
C21 | July 2, 1999 | 127,000 km | 78,900 mi | Global color mosaic of anti-Jovian hemisphere |
I24 | October 11, 1999 | 611 km | 380 mi | High-resolution imaging of Pillan, Zamama, and Prometheus flows; Camera and Near-IR spectrometer suffer radiation damage |
I25 | November 26, 1999 | 301 km | 187 mi | Spacecraft safing event precludes high-resolution observations; images of Tvashtar outburst eruption |
I27 | February 22, 2000 | 198 km | 123 mi | Change detection at Amirani, Tvashtar, and Prometheus; Stereo imaging over Tohil Mons |
I31 | August 6, 2001 | 194 km | 121 mi | Camera problems preclude high-resolution imaging; Near-IR spectrometer observes eruption at Thor |
I32 | October 16, 2001 | 184 km | 114 mi | High-resolution observations of Thor, Tohil Mons, Gish Bar |
I33 | January 17, 2002 | 102 km | 63 mi | Spacecraft safing event precludes observations; almost all remote sensing lost |
A34 | November 7, 2002 | 45,800 km | 28,500 mi | No remote sensing due to budget constraints |
![A portion of a planetary body with a pair of large, mountainous ridges on the left side of the image, a shorter, rugged domical mountain at top center, an elliptical pit near bottom center, and the boundary between the dayside (to the left) and the nightside (to the right) running down the right side of the image. Two small mountain peaks are seen near this boundary at lower right.](http://upload.wikimedia.org/wikipedia/commons/thumb/2/2e/021206_Galileo_Io_at_sunset.jpg/220px-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.
![A colorized image, with a multi-colored region in the middle, elongated left-to-right. The text "I32 Pele" is displayed at top left, and at bottom center, and a color chart of the gradient used. A scale bar shows that the image covers an area 60 kilometers across.](http://upload.wikimedia.org/wikipedia/commons/thumb/d/d9/Pele_Io_I32.jpg/220px-Pele_Io_I32.jpg)
Following the February 2000 encounter, Galileo's mission at Jupiter was extended for a second and final time with the Galileo Millennium Mission. The focus of this extended mission was joint observation of the Jovian system by both Galileo and Cassini, which performed a distant flyby of Jupiter en route to Saturn on December 30, 2000.[75] Discoveries during the joint observations of Io revealed a new plume at Tvashtar and provided insights into Io's aurorae.[76] Distant imaging by Galileo during the Cassini flyby revealed a new red ring plume deposit, similar to the one surrounding Pele, around Tvashtar, one of the first of this type seen in Io's polar regions, though Galileo would later observe a similar deposit around Dazhbog Patera in August 2001.[2] Galileo performed three additional flybys of Io, on August 6 and October 16, 2001 and January 17, 2002, during the Galileo Millennium Mission. Both encounters in 2001 allowed Galileo to observe Io's polar regions up-close, though imaging from the August 2001 flyby was lost due to a camera malfunction.[2] The data from the magnetometer confirmed that Io lacked an intrinsic magnetic field, though later analysis of this data in 2009 did reveal evidence for an induced magnetic field generated by the interaction between Jupiter's magnetosphere and a silicate magma ocean in Io's asthenosphere.[2][77] During the August 2001 flyby, Galileo flew through the outer portions of the newly formed Thor volcanic plume, allowing for the first direct measurement of composition of Io's volcanic material.[2] During the October 2001 encounter, Galileo imaged the new Thor eruption site, a major new lava flow at Gish Bar Patera,[78] and the lava lake at Pele.[2] Due to a safing event prior to the encounter, nearly all of the observations planned for the January 2002 flyby were lost.[2]
In order to prevent potential biological contamination of the possible Europan biosphere, the Galileo mission ended on September 23, 2003 when the spacecraft was intentionally crashed into Jupiter.[21]
Post-Galileo Era: 2003–2016
![In the New Horizons image (from 2007), a small area of dark material is present in a bright region near the bottom; this area was not present in the Galileo image (from 1999).](http://upload.wikimedia.org/wikipedia/commons/thumb/0/08/Iosurface.jpg/220px-Iosurface.jpg)
Following the end of the Galileo mission, astronomers have continued monitoring Io's active volcanoes with
New Horizons (2007)
![](http://upload.wikimedia.org/wikipedia/commons/thumb/1/12/Tvashtarvideo.gif/220px-Tvashtarvideo.gif)
The New Horizons spacecraft, en route to Pluto and the Kuiper belt, flew by the Jupiter system on February 28, 2007, approaching Io to a distance of 2,239,000 km (1,391,000 mi).[82] During the encounter, numerous remote observations of Io were obtained, including visible imaging with a peak resolution of 11.2 km (6.96 mi) per pixel.[83] 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.[84] 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.[84]
A study with the
Juno Era: 2016–2025
![](http://upload.wikimedia.org/wikipedia/commons/thumb/d/db/Juno_JunoCam_image_of_Io_from_October_15%2C_2023.png/220px-Juno_JunoCam_image_of_Io_from_October_15%2C_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.[88] 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.[89] 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 are planned for Juno's extended mission on December 30, 2023 and February 3, 2024, both with altitudes of 1,500 kilometers.[90] 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.[91]
Juno encounters with Io with altitudes less than 100,000 km (62,100 mi) | ||||
---|---|---|---|---|
Orbit | Date | Altitude | Notes | |
PJ25 | February 17, 2020 | 195,104 km | 121,000 mi | Closest encounter to Io during primary mission |
PJ43 | July 5, 2022 | 86,141 km | 53,500 mi | |
PJ47 | December 14, 2022 | 63,771 km | 39,600 mi | Safe mode after encounter[92] |
PJ49 | March 1, 2023 | 51,547 km | 32,000 mi | |
PJ51 | May 16, 2023 | 35,555 km | 22,100 mi | |
PJ53 | July 31, 2023 | 22,202 km | 13,800 mi | |
PJ55 | October 15, 2023 | 11,640 km | 7,230 mi | |
PJ57 | December 30, 2023 | 1,500 km | 932 mi | |
PJ58 | February 3, 2024 | 1,500 km | 932 mi | |
PJ60 | April 9, 2024 | 17,347 km | 10,779 mi | |
PJ67 | November 25, 2024 | 85,736 km | 53,300 mi | |
PJ72 | May 8, 2025 | 92,957 km | 57,761 mi |
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.[3][89] 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.[93]
Future missions
There are two forthcoming missions planned for the Jovian system. The
A dedicated mission to Io, called the Io Volcano Observer (IVO), has been proposed for the Discovery Program as a Jupiter orbiter that would perform at least ten flybys of Io over 3.5 years.[96] In 2020, as part of the 2019 Discovery mission call, IVO was selected as one of four missions to continue to a Phase A study.[4] If selected to fly, it would explore Io's active volcanism and impact on the Jupiter system as a whole by measuring its global heat flow, its induced magnetic field, the temperature of its lava, and the composition of its atmosphere, volcanic plumes, and lavas.[97] With its primary launch window, it would launch in January 2029 and arrive at Jupiter on August 2, 2033.[98]
See also
- Exploration of Jupiter
- Volcanology of Io
References
- ^ ISBN 978-3-540-34681-4.
- ^ ISBN 978-3-540-34681-4.
- ^ EarthSky. Retrieved February 14, 2020.
- ^ a b "NASA Selects Four Possible Missions to Study the Secrets of the Solar System". NASA. 13 Feb 2020.
- ^ McEwen, A. S. (24 August 2009). Io Volcano Observer (IVO) (PDF). Satellites panel of 2009 Decadal Survey. Archived from the original (PDF) on 2012-02-29. Retrieved 2010-02-20.
- ^ ISBN 978-3-540-34681-4.
- ^ ISBN 978-0-226-16226-3. Retrieved 2010-02-17.
- ^ a b c Galilei, Galileo (2004) [First published 1610]. Carlos, E. S.; Barker, P. (eds.). Sidereus Nuncius [The Starry Messenger] (PDF). Venice: University of Padua. pp. 17–28. Archived from the original (PDF) on 2005-12-20. Retrieved 2010-01-07.
- ^ a b c d Wright, E. (2004). "Galileo's First Jupiter Observations". Astronomy Stuff: Observation and Simulation. Archived from the original on 2009-02-08. Retrieved 2010-02-17.
- ^ Blue, J. (November 9, 2009). "Planet and Satellite Names and Discoverers". USGS. Retrieved 2010-01-13.
- ^ Van Helden, A. (2003). "Satellites of Jupiter". The Galileo Project. Rice University. Retrieved 2010-02-17.
- ^ ISBN 978-0-226-16226-3. Retrieved 2010-02-17.
- ^ Bibcode:1916Obs....39..367.
- ^ Van Helden, Albert (14 January 2004). "Simon Marius". The Galileo Project. Rice University. Retrieved 2010-01-07.
- ^ a b Baalke, Ron. "Discovery of the Galilean Satellites". Jet Propulsion Laboratory. Archived from the original on 1997-01-06. Retrieved 2010-01-07.
- ^ a b c d e f Van Helden, Albert (2004). "Longitude at Sea". The Galileo Project. Rice University. Retrieved 2010-02-17.
- ^ O'Connor, J. J.; Robertson, E. F. (February 1997). "Longitude and the Académie Royale". University of St. Andrews. Retrieved 2007-06-14.
- ^ Huygens, C. (8 January 1690). Thompson, S. P. (ed.). "Treatise on Light". Project Gutenberg etext. Retrieved 2007-04-29.
- ^ Oldford, R.W (2000). "The first evidence". Scientific Method, Statistical Method, and the Speed of Light. University of Waterloo. Retrieved 2010-02-17.
- ^ .
- ^ ISBN 978-0-8165-2844-8.
- .
- Bibcode:2004S&T...107a.114D.
- .
- .
- Bibcode:1973CoLPL..10...35M.
- doi:10.1086/123621.
- .
- ^ Bibcode:1972CoLPL...9..179L.
- .
- S2CID 12233914.
- ^ Muller, D. (2010). "Pioneer 10 Full Mission Timeline". Interplanetary Space Missions: Realtime Simulations, Full Timelines and Maps. Retrieved 2010-02-18.
- .
- ^ Fimmel, R. O.; et al. (1977). "First into the Outer Solar System". Pioneer Odyssey. NASA. Retrieved 2007-06-05.
- S2CID 38074374.
- ^ Muller, D. (2010). "Pioneer 11 Full Mission Timeline". Interplanetary Space Missions: Realtime Simulations, Full Timelines and Maps. Archived from the original on 2012-03-03. Retrieved 2010-02-18.
- ^ "Pioneer 11 Images of Io". Galileo Home Page. Archived from the original on 1997-04-09. Retrieved 2007-04-21.
- ^ S2CID 36510719.
- .
- ^ S2CID 205532.
- ^ .
- .
- ^ S2CID 43128508.
- ^ S2CID 21271617.
- ^ "First Pictures: Voyager 1 Images Io's Volcanic Plumes – March 8, 1979". DREWExMachina. March 8, 2021. Retrieved May 14, 2023.
- ^ a b "Voyager Mission Description". PDS Rings Node. NASA. 1997-02-19. Retrieved 2007-04-21.
- ^ S2CID 33147728.
- ^ a b c d e Morrison, David.; Samz, Jane (1980). "The First Encounter". Voyager to Jupiter. National Aeronautics and Space Administration. pp. 74–102.
{{cite book}}
: CS1 maint: multiple names: authors list (link) - S2CID 1442415.
- S2CID 32838223.
- S2CID 38847163.
- .
- S2CID 32086788.
- S2CID 45693338.
- ^ S2CID 8798702.
- S2CID 43050333.
- S2CID 4338190.
- ^ a b Morrison, David.; Samz, Jane (1980). "The Second Encounter: More Surprises from the "Land" of the Giant". Voyager to Jupiter. National Aeronautics and Space Administration. pp. 104–126.
{{cite book}}
: CS1 maint: multiple names: authors list (link) - ^ S2CID 22465607.
- ISBN 978-0-8165-0762-7.
- ^ .
- S2CID 23811832.
- PMID 17797493.
- .
- ISBN 978-1-85233-301-0.
- S2CID 24373080.
- S2CID 33017180.
- ^ .
- PMID 10436151.
- ^ PMID 9651251.
- .
- .
- .
- .
- ^ Atkinson, C. (2001). "Jupiter Millennium Flyby". Jet Propulsion Laboratory. Archived from the original on 2009-01-19. Retrieved 2010-02-17.
- S2CID 20150275.
- PMID 20093451.
- ^ Perry, J. E.; et al. (2003). Gish Bar Patera, Io: Geology and Volcanic Activity, 1997–2001 (PDF). Lunar and Planetary Science Conference XXXIV. Abstract #1720.
- ^ .
- ^ Spencer, John (2007-02-23). "Here We Go!". Archived from the original on 2007-02-27. Retrieved 2007-06-03.
- .
- ^ Muller, D. (2010). "New Horizons Full Mission Timeline". Interplanetary Space Missions: Realtime Simulations, Full Timelines and Maps. Retrieved 2010-02-20.
- ^ Perry, J. (2008). "New Horizons Io Observations". Planetary Image Research Laboratory. Retrieved 2010-02-20.
- ^ S2CID 36446567.
- ^ Tsang, C., et al. 2016. The collapse of Io's primary atmosphere in Jupiter eclipse. Journal of Geophysical Research: Planets: 121, 1400–1410.
- ^ "Space scientists observe Io's atmospheric collapse during eclipse".
- ^ Bellucci, G., et al. 2004. Cassini/VIMS observation of an Io post-eclipse brightening event. Icarus: 172, 141–148.
- ^ Greicius, Tony (September 21, 2015). "Juno – Mission Overview". NASA. Archived from the original on December 25, 2018. Retrieved February 14, 2020.
- ^ S2CID 213970081.
- ^ "NASA's Juno Mission Expands into the Future". January 13, 2021. Retrieved February 1, 2021.
- ^ Bolton, Scott (September 2, 2020). "Juno OPAG Report" (PDF). Retrieved August 31, 2020.
- ^ "Juno Spacecraft Recovering Memory After 47th Flyby of Jupiter". December 22, 2022. Retrieved January 20, 2023.
- S2CID 225456943.
- ^ Jonathan Amos (2 May 2012). "Esa selects 1bn-euro Juice probe to Jupiter". BBC News.
- ^ JUICE assessment study report (Yellow Book), ESA, 2012
- ^ McEwen, A.; et al. (2020). Io Volcano Observer (IVO): Does Io have a Magma Ocean? (PDF). LPSC LI. Abstract #1648.
- ^ Meghan Bartels (27 Mar 2019). "These Scientists Want to Send a NASA Probe to Jupiter's Volcanic Moon Io". Space.com.
- ^ McEwen, A.; et al. (2021). The Io Volcano Observer (IVO) (PDF). LPSC LII. Abstract #2548.