Io: Facts about Jupiter’s Volcanic Moon Io: Facts about Jupiter’s Volcanic Moon By Kim Ann Zimmermann | June 16, 2016 03:23pm ET MORE This is a montage of New Horizons images of Jupiter and its volcanic moon Io, taken during the spacecraft’s Jupiter flyby . The image was released in Oct. 2007. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute Jupiter’s fifth moon, Io, is the most volcanically active body in the solar system. Plumes of sulfur spew upward as high as 190 miles (300 kilometers). The surface of Io is splotched with lava lakes and floodplains of liquid rock. Io by the numbers Age: Io is about 4.5 billion years old, about the same age as Jupiter . Distance from Jupiter: Io is the fifth moon from Jupiter. Its orbital distance is about 262,000 miles (422,000 km). Io takes 1.77 Earth-days to orbit Jupiter. Io is tidally locked, so the same side always faces Jupiter.

Size: Io has a mean radius of 1,131.7 miles, making it slightly larger than Earth’s moon. It has a slight elliptical shape, with its longest axis directed toward Jupiter. Among the Galilean satellites Io ranks third, behind Ganymede and Callisto but ahead of Europa , in both mass and volume. Temperature: Io's surface temperature averages about minus 202 degrees Fahrenheit (minus 130 Celsius), resulting in the formation of sulfur dioxide snowfields. But Io’s volcanoes can reach 3,000 F (1,649 C). Io is often referred to as a celestial body of fire and ice. The Discovery of Io Artist's concept of the internal structure of Jupiter's moon Io. A global magma "ocean" (shown in orange) lies beneath a crust 30 to 50 kilometers thick. The rest of Io's mantle is shown in gold, while the moon's core is rendered in silver. Credit: Xianzhe Jia (University of Michigan) and Krishan Khurana (UCLA) Io was the first of Jupiter’s moons discovered by Galileo Galilei on Jan. 8, 1610.

He actually discovered the moon the day prior, but could not differentiate between Io and Europa, another Jupiter moon, until the next night. The discovery, along with three other Jovian moons, was the first time a moon was discovered orbiting a planet other than Earth. Galileo’s discovery eventually led to the understanding that planets orbit the sun, instead of our solar system revolving around Earth. Galileo first referred to this moon as Jupiter I. In the mid-1800s, the moon was renamed Io. In Greek mythology, Io was a priestess of Hera (Zeus' wife) and the daughter of Inachus, the king of Argos. Zeus (the counterpart for the Roman god Jupiter) fell in love with her, but turned her into a cow to avoid being caught with her by his wife, Hera (or Juno). Several spacecraft have flown by Jupiter and its moons. Pioneer 10 arrived first, in 1973, followed by Pioneer 11 in 1974. Voyager 1 and Voyager 2 returned striking photos during their flybys.

The Galileo spacecraft passed as low as 162 miles (261 km) over the surfaces of the Galilean moons and produced detailed images. Characteristics of Io Hubble Space Telescope ultraviolet image of the northern pole of Jupiter. Among many other auroral structures, the Io footprint is the most equator-ward feature close to the centre of the image. This spot is always located close to the feet of the magnetic field lines connected to the satellite Io. Credit: LPAP/Université de Liège The interior of Io is composed of an iron or iron sulfide core and a brown silicate outer layer, giving the planet a splotchy orange, yellow, black, red, and white appearance. Based on data from scientific computer models, Io formed in a region around Jupiter where water ice was plentiful. Io’s heat, combined with the possibility that there was water on Io shortly after it was formed, could have made life possible, although Jupiter’s radiation would have removed the water from the surface. The moon's most distinctive features are its volcanoes. Aside from Earth, Io is the only known body in the solar system to have observed active volcanoes.

While Galileo had made some cryptic notes inferring possible volcanic activity, NASA's Voyager spacecraft discovered Io's volcanoes in 1979. The volcanic activity is a result of Io being stretched and squeezed as it orbits Jupiter. Io's rock surface bulges up and down by as much as 100 meters (328 feet) during the process. This affects Io’s volcanic activity in a similar way to which Earth’s oceans react to the moon. Io’s irregularly elliptical orbit also heightens the tidal activity. Because of the volcanic activity, Io’s atmosphere contains mostly sulphur dioxide. Io’s orbit cuts across Jupiter’s powerful magnetic lines of force, turning Io into an electric generator. As Jupiter rotates, the magnetic forces strip away about a ton (1,000 kg) of Io’s material every second. The material becomes ionized and forms a doughnut-shaped cloud of radiation called a plasma torus. Some of the ions are pulled into Jupiter’s upper atmosphere and create auroras . Explosive mysteries Io's environment has come under intense scrutiny in recent years as scientists try to explain why the moon has the most active volcanoes in the solar system.

The recent findings involve both observations of the moon and modeling of the interior, which can only be inferred. A 2015 study explained why Io's most prolific volcanoes appear to be offset from where scientists expected they would be. While previous models suggested that the zones with the most intense heating would have more active volcanoes, the actual Io had its most active ones farther east. The study suggests that if there was an ocean mixed with molten and solid rock underneath the moon, the rock portions would rub against each other and create the offset zones scientists observed. Insights into Io's surface may also come from a 2012 map published by the U.S. Geological Survey – the first to include the entire surface. The researchers said that volcanic activity would be among the things that could be studied with this map. The imagery includes observations from the Voyager 1 and Voyager 2 spacecraft, as well as Galileo.

While there is no mission specifically planned to look at Io, there will be other missions in the vicinity of the moon in future years, including Juno (2016), a planned NASA Europa mission (perhaps in the 2030s), and the European Space Agency's JUICE mission (JUpiter ICy Moons Explorer, which will focus on Europa, Ganymede and Callisto). Meanwhile, observations continue from the ground; sometimes Io's volcanoes are violent enough to produce spectacular pictures even from so far away . Additional reporting by contributor Elizabeth Howell. This map, released in March 2012 by the U.S. Geological Survey, is the first ever to chart the Jupiter moon Io's geology on a global scale. Io is the most volcanically active object in the solar system. Credit: USGS Io, Jupiter's Moon Io Discoveries On January 7, 1610 Galileo Galilei observed three pinpoints of light strung out in a line next to Jupiter. The next evening, these stars seemed to have moved the wrong way, which caught his attention. Galileo continued to observe the stars and Jupiter for the next week.

On January 11, a fourth star ( Ganymede ) appeared. After a week, Galileo observed that the four stars never left the vicinity of Jupiter, appeared to be carried along with the planet, and changed their position with respect to each other and Jupiter. Finally, Galileo determined that what he was observing were not stars, but planetary bodies that were in orbit around Jupiter. This discovery provided evidence in support of the still heretical Copernican solar system and showed that everything did not revolve around the Earth. In 1676 a Danish astronomer Ole Romer was able to make the first accurate measurement of the speed of light by using eclipse timings of the Galilean satellites with Jupiter's shadow. Another discovery was made by Pierre-Simon de Laplace during the late 1700s when he deduced that the orbital periods of Io, Europa, and Ganymede are nearly in a perfect 1:2:4 ratio. In 1920 this knowledge paved the way for the first estimate of the satellites' masses within an accuracy of 20%.

Finally in 1979, the Voyager spacecraft flew past the Jovian system, took high-resolution pictures of the moons, and conducted experiments that provided the first accurate measurements of the moon's dimensions and mass. These in turn were used to calculate the mean density of Io (3.5 g/cm3), Europa (3.0 g/cm3), Ganymede (1.9 g/cm3), and Callisto (1.8 g/cm3). Left: This cutaway illustration shows the possible internal structure of Io. The interior characteristics of the moon are inferred from gravity field and magnetic field measurements by the Galileo spacecraft. Io has a metallic (iron, nickel) core (shown in gray) drawn to the correct relative size. The core is surrounded by a rock shell (shown in brown). Io's rock or silicate shell extends to the surface. [ more ] Based on density, surface composition analysis, and gravity data, Io appears to be a rocky silicate rich body that has a dense iron, iron sulfide core that extends halfway to the surface with a partially melted silicate rich mantle, and a thin rocky crust.

The lower density of Ganymede and Callisto suggest that they are composed of lighter elements, most likely water in some form. Why are there such differences among the four Galilean satellites? During the early formation of the solar system , Jupiter would have been very hot. This may have prevented lighter elements from condensing at the inner orbits. The mini system of Galilean satellites orbiting Jupiter resembles that of the solar system with the rocky, dense planets at the innermost orbits and the light, least dense planets at the outer orbits. Io Statistics Io Io ) is the fifth of Jupiter 's known satellites and the third largest; it is the innermost of the Galilean moons. Io is slightly larger than Earth's Moon . orbit : 422,000 km from Jupiter diameter : 3630 km mass : 8.93e22 kg The pronunciation "EE oh" is also acceptable. Io was a maiden who was loved by Zeus (Jupiter) and transformed into a heifer in a vain attempt to hide her from the jealous Hera . Discovered by Galileo and Marius in 1610.

In contrast to most of the moons in the outer solar system, Io and Europa may be somewhat similar in bulk composition to the terrestrial planets, primarily composed of molten silicate rock. Recent data from Galileo indicates that Io has a core of iron (perhaps mixed with iron sulfide) with a radius of at least 900 km. Io's surface is radically different from any other body in the solar system. It came as a very big surprise to the Voyager scientists on the first encounter. They had expected to see impact craters like those on the other terrestrial bodies and from their number per unit area to estimate the age of Io's surface. But there are very few, if any, impact craters on Io (left). Therefore, the surface is very young . Instead of craters, Voyager 1 found hundreds of volcanic calderas . Some of the volcanoes are active! Striking photos of actual eruptions with plumes 300 km high were sent back by both Voyagers (right) and by Galileo (bottom left image on this page) This may have been the most important single discovery of the Voyager missions; it was the first real proof that the interiors of other "terrestrial" bodies are actually hot and active.

The material erupting from Io's vents appears to be some form of sulfur or sulfur dioxide. The volcanic eruptions change rapidly. In just four months between the arrivals of Voyager 1 and Voyager 2 some of them stopped and others started up. The deposits surrounding the vents also changed visibly. Recent images taken with NASA's Infrared Telescope Facility on Mauna Kea, Hawaii show a new and very large eruption (right). A large new feature near Ra Patera has also been seen by HST . Images from Galileo also show many changes from the time of Voyager's encounter. These observations confirm that Io's surface is very active indeed. Io has an amazing variety of terrains: calderas up to several kilometers deep, lakes of molten sulfur (below right), mountains which are apparently NOT volcanoes (left), extensive flows hundreds of kilometers long of some low viscosity fluid (some form of sulfur?), and volcanic vents. Sulfur and its compounds take on a wide range of colors which are responsible for Io's variegated appearance. Analysis of the Voyager images led scientists to believe that the lava flows on Io's surface were composed mostly of various compounds of molten sulfur.

However, subsequent ground-based infra-red studies indicate that they are too hot for liquid sulfur. One current idea is that Io's lavas are molten silicate rock. Recent HST observations indicate that the material may be rich in sodium. Or there may be a variety of different materials in different locations. Some of the hottest spots on Io may reach temperatures as high as 2000 K though the average is much lower, about 130 K. These hot spots are the principal mechanism by which Io loses its heat. The energy for all this activity probably derives from tidal interactions between Io, Europa, Ganymede and Jupiter. These three moons are locked into resonant orbits such that Io orbits twice for each orbit of Europa which in turn orbits twice for each orbit of Ganymede. Though Io, like Earth's Moon always faces the same side toward its planet, the effects of Europa and Ganymede cause it to wobble a bit. This wobbling stretches and bends Io by as much as 100 meters (a 100 meter tide!) and generates heat the same way a coat hanger heats up when bent back and forth.

(Lacking another body to perturb it, the Moon is not heated by Earth in this way.) Io also cuts across Jupiter's magnetic field lines, generating an electric current. Though small compared to the tidal heating, this current may carry more than 1 trillion watts. It also strips some material away from Io which forms a torus of intense radiation around Jupiter. Particles escaping from this torus are partially responsible for Jupiter's unusually large magnetosphere . Recent data from Galileo indicate that Io may have its own magnetic field as does Ganymede. Io has a thin atmosphere composed of sulfur dioxide and perhaps some other gases. Unlike the other Galilean satellites, Io has little or no water. This is probably because Jupiter was hot enough early in the evolution of the solar system to drive off the volatile elements in the vicinity of Io but not so hot to do so farther out. More about Io Io Moon - Io The Moon - Jupiters Moon Io They cannot be seen with the naked eye. Figure 1. Jupiter and the 4 Galilean moons seen through a small telescope Of these four moons Io is the moon which is closest to Jupiter and, like all the other moons, orbits around the planet .

“Moon” is another word for “satellite”, which means a body in outer space which revolves, or turns around another body. Io is a little larger than our own moon , with a diameter of about 2262 miles (3643 kilometres) and is made of rock. Io orbits Jupiter at a distance of about 262,094 miles (421,800 kilometres) from the planet. Io orbits round Jupiter at a speed of 38,764 miles(62,423 kilometres) per hour which is faster than the other moons . The speed of its orbit is due to both Europa and Ganymede exerting a pull of gravity on Io. Io orbits Jupiter four times for every single orbit that Ganymede makes and every two orbits that Europa makes. She travels 1645,796 miles (2650,236 kilometres)in her orbit and takes only 1.7 of our earth days to complete a single orbit. As Io turns round the planet, she keeps the same face turned towards Jupiter , so in the course of one orbit, Io only turns once on her axis.

Several NASA space missions to Jupiter have given close up photographs and information about these “Galilean moons”, such as this photograph taken from Apollo Mission 4 . Figure 2. Images of the four Galilean moons from the Apollo spacecraft. The edge of Jupiter can be seen in the bottom right. Io is the large moon in the bottom left, closest to Jupiter . Io is so close to Jupiter that the pull of Jupiter’s gravity causes continual disturbance to the moon’s surface. Volcanic eruptions constantly cover the surface of Io with new coatings of sulphuric lava. Io is the most volcanically active body in the whole solar system. The volcanoes are driven by silicate magma. With volcanoes constantly erupting and spilling out hot lava, Io’s surface is so hot that any moisture is driven off it. Because of the constant volcanic eruptions, the appearance of Io is constantly changing. NASA describes Io as “Looking like a giant pizza covered with melted cheese and splotches of tomato and ripe olives”. Figure 3. A NASA image of Io's surface Io, in Greek mythology, was a Princess of Argos. She was loved by Zeus, the sky god of the Greeks (known to the Romans as Jupiter).

Zeus’ wife, Hera (known to the Romans as Juno) was jealous of Io, so Zeus turned Io into a young cow to escape from Hera, but Hera set Argus, a herdsman with 100 eyes, to watch Io. Zeus (Jupiter) killed Argus and Hera (Juno) then sent a gadfly, an insect that stings cattle, to drive Io away. In the heavens, however, the moon Io no Io Moon Facts The colorful Jupiter moon Io, the most volcanically active body in the solar system Io Facts Io is the fifth moon in distance from the planet Jupiter. It orbits Jupiter at an average distance of 262,000 miles (422,000 km,) similar to the distance the Moon orbits Earth. Io is the third largest of Jupiter's moons and very similar in size to our own moon. It is the most volcanically active object in the solar system. Volcanic plumes rise almost 190 miles (300 km) above the surface. Io has an iron core just like Earth, meaning it could have its own magnetic field.

The moon's orbit cuts across Jupiter's powerful magnetic field producing 400,000 volts of electricity across its surface. Jupiter's magnetic field strips off 1 ton of material from Io every second! The moon was discovered in January 1610 by the Italian astronomer Galileo Galilei. Io is often compared to a pizza due to its unusual surface which is made up of sulfur dioxide deposits. A giant plume erupting from Io's northern polar Tvashtar volcano Io's Surface Jupiter's huge gravity causes Io's surface to bulge up and down by as much as 100 meters! This also generates tremendous heat inside Io, turning its subsurface into liquid. This molten lava frequently escapes causing its surface to constantly change. Io's Atmosphere Io's atmosphere is roughly 1 millionth as dense as Earth's atmosphere and made primarily of sulfur. Io's Temperature Temperatures on Io vary from extremely hot to extremely cold. Areas which feature volcanic activity can reach temperatures of over 2000C (4000F). Away from these areas however will see temperatures plunge to -143C (-230F).

Origin of Name Io is named from Greek mythology after the priestess of Hera who became a lover of Zeus. Life on Io Io is an extremely hostile environment with little or no chance for the existence of life. Io - The Volcanic Moon The spectacular surface of Io with volcanic plume As Voyager 1 and 2 approached the Jupiter system in the late 1970's NASA scientists fully expected the craft to discover that its moons would be cold, dead and not particularly interesting, instead what they found was quite astonishing. One moon in particular stood out, Io, scientists were amazed to find out that it was in fact geologically active with volcanoes constantly erupting on its surface. The sulfur deposits from the eruptions also made the small moon strikingly colorful. Scientists initially scratched their heads and wondered how a small moon in one of the coldest parts of the solar system could be generating so much heat. The explanation was in fact very simple, Io's orbit around Jupiter is extremely elliptical, meaning that sometimes it's close to the giant planet and other times it's further away. This causes Jupiter’s strong gravity to push and pull the small moon, producing immense heat inside Io and creating molten lava below its surface.

Scientists had expected to find something similar to our moon but what they had in fact discovered was the most geologically active body in the entire solar system. Missions to Io Launch: 2020 Agency: NASA/ESA Summary: A joint venture by NASA and the ESA (Europe) will study Io using two separate spacecraft. Even though they will be focusing mainly on another of Jupiter's moons Europa, they will make four close flybys of Io. Juno Summary: Will provide monitoring of Io's volcanic activity using its near-infrared spectrometer. New Horizons Arrival: January 2007 Agency: NASA Summary: During its flyby of the Jovain system New Horizons captured spectacular images of Io including the eruption of its north polar volcano. Galileo Arrival: December 1995 Agency: NASA Summary: During its study of the Jovian system Galileo sent back a lot of new data on Io. Including the discovery of several active volcanoes on the moon, the existence of an iron core, and significant changes to the surface since the earlier Voyager missions.

Voyager 1 and 2 Arrival: April-August 1979 Agency: NASA Summary: As part of its "Grand Tour" of the solar system Voyager 1 and 2 took high resolution images of Io revealing an oddly multi colored moon with a curiously craterless surface. After the images were sent back to Earth a young female engineer noticed a plume emanating from the surface on one of the pictures proving that Io was volcanically active. Pioneer 10 and 11 Launch: March 1972 - April 1973 Arrival: December 1973 - December 1974 Agency: NASA Summary: The first spacecrafts to explore the outer solar system. They provided little data on Io apart from a better estimate of its mass. Pioneer 11 managed to take only one image of any quality pictured left. Share this page The Galilean Moons of Jupiter The Galilean Moons of Jupiter Summary: Jupiter has more than 60 known moons, but understanding the geology of its four largest will hopefully lead to some groundbreaking discoveries.

Sections: The Galilean Moons Each of the Jovian planets has a number of moons, although Jupiter has the most with more than 60 catalogued to date. Jupiter's 4 largest moons exhibit some of the most interesting geology in the solar system. They were discovered by Galileo Galilei and are known as the Galilean moons. Jupiter's largest moon, Ganymede, is larger than Mercury while the other three are larger than Pluto. Outward from Jupiter: Io: Volcanoes and All Jupiter's Moon Io The Voyager spacecraft took the first close-up images of Io more than 300 years after the moon's discovery. The images showed a surface with no signs of craters from past impacts. What we saw instead was a surface almost entirely covered with large volcanoes. Cameras on Voyager actually captured volcanic eruptions in progress. The frequency of these sulfuric eruptions has filled in almost all of the impact craters and left Io with one of the youngest looking surfaces in the solar system. Io Close-Up (click to enlarge) Close-up photos of eruptions in progress show powerfully hot lava glowing orange and red.

Photos taken on the night side of Io show not only the hot volcanic vents, but also a thin sulfur dioxide atmosphere produced by constant outgassing. Io's unusual red and orange colors come primarily from sulfur, which condenses on the surface after being outgassed by the volcanoes. Although there is no direct evidence of tectonic activity on Io, scientists feel confident it exists since the processes that fuel volcanism also fuel tectonics. The volcanic eruptions are so frequent and cover the surface so thoroughly that any clear evidence of tectonic activity is likely to be buried. Europa: What Lies Beneath? Europa Europa's surface and crust are made almost entirely of water ice, and its bizarre, fractured appearance is proof enough that tidal heating has acted there. The icy surface is nearly devoid of impact craters and may be only a few million years old. Europa's Interior Observations made by the Galileo spacecraft show that Europa has a metallic core and a rocky mantle. Surrounding the rocky interior appears to be an icy layer 100 kilometers thick, the top few kilometers of which seem to be frozen solid.

The stretching and squeezing of tidal friction should provide enough heat to melt some of this into liquid water beneath a thin ice shell. If it does, then Europa may have an ocean with more than twice as much liquid water as all of Earth's oceans combined. Analyzing Europa's Cracked Surface Close-up photos of the surface of Europa support the idea of a liquid ocean beneath the surface. These photos, taken by the Galileo spacecraft, show what appears to be icebergs stuck in a layer of ice. Other evidence comes from double-ridged cracks on the surface. Tidal flexing that allows water to well up and build ridges may create these cracks. Ganymede: Largest Moon in the Solar System Two Regions of Ganymede's Surface (click to enlarge) The surface of Ganymede shares many similarities with Europa. Ganymede's surface is also made of water ice, but unlike Europa's surface, it shows signs of varying age. The darker regions are heavily cratered, suggesting they are billions of years old. The lighter regions show no signs of craters and it is thought that eruptions of water covered the surface before freezing over. These areas are geologically younger than the darker regions.

If liquid water occasionally makes its way to the surface to fill in craters, could that suggest a liquid ocean similar to the one that might exist on Europa? Callisto: The Outermost Galilean Moon Callisto Callisto is the stereotypical outer solar system satellite. It is one of the largest and most heavily cratered satellites in the solar system. The surface is very icy and dates back four billion years. Beneath the icy crust is possibly a salty ocean supported by a deeper rocky interior. Callisto's Surface Callisto doesn't have any large mountains, show evidence of volcanic or tectonic activity or have any appreciable level of internal heat. Nonetheless, observations of Callisto's magnetic field may cause scientists to add the large moon to the list of possible worlds with subsurface salty oceans. Io Crescent Io seen in approximately "natural" color by Voyager 2 in July 1979 from a distance of 1.1 million kilometers. IO The innermost of Jupiter's four planet-sized moons, Io is also one of the most volcanically active bodies in the solar system.

Voyager observed eight active volcanic plumes in March and July 1979 and some of these have been monitored since using infrared telescopes on Earth. Lava flows and irregularly shaped mountains are also observed, but no craters (consistent with ongoing volcanic activity, which would erase impact craters not long after they form). The composition of these lavas and plumes remains uncertain. Are they silicate lavas similar to those seen on Earth (for example Hawai'i and Mt. Etna)? Or are they composed of liquid sulfur (which might account for Io's yellowish color) or a combination? Io is the same size and density as Earth's Moon. Why, then, is Io so volcanically active? The answer is that tides on Io raised by both Europa and massive Jupiter are so great that they continually heat and partially melt the interior of Io. [ Return to Satellites Page ] Io & Jupiter Stereo image of Io floating above the cloud decks of Jupiter, showing the true difference in size between the planet and its moon. These images were taken three minutes apart, and the stereo effect results from the orbital motion of Io about Jupiter.

Io in "Natural" Color Io appears in shades of pastel yellow and white to the naked eye. This view, using orange, green, and blue filter images, shows approximately how Io would appear to passing humans. Features as small as 7 kilometers are visible. (Gaps are due to missing data.) Io in "Enhanced" Color This view (of the same area as the image above) takes advantage of Voyager's greater sensitivity in the blue and violet wavelengths to produce an "enhanced-color" view. Color differences on Io are more pronounced at these wavelengths, and using them allows geologists to better discriminate individual features. Volcanos and Mountains This color mosaic extends from near the equator (top of image) to the pole (bottom). A variety of lava flows and mountains can be seen. Haemus Montes is the rectangular-shaped mountain at lower left and Euboea Montes is the elliptical-shaped mountain at center. Creidne Patera, a large dark volcanic complex at image center, is a site of effusive volcanism similar to volcanic calderas on Earth (such as Hawai'i).

Just above Creidne is Maasaw Patera, a series of radiating brownish lava flows emanating from a volcanic caldera (or collapse pit). For scale, this image is approximately 1900 kilometers from top to bottom, or slightly larger than the state of Texas. The Io Movie This first-cut movie is based on stereo-derived topography of Io (generated at LPI) combined with the above color mosaic to provide this 3-D rendering of the surface. An improved version is in production. Haemus Montes Haemus Montes, located near Io's south pole, rise approximately 10 kilometers above the surrounding plains (Mount Everest rises almost 9 kilometers above sea level). The origin of these and other similar mountains, which cover about 2% of Io's surface, is uncertain. They are possible evidence of tectonic deformation predating most of the volcanic material that surrounds them. Haemus Montes in 3-D Red-green stereo glasses can be used with this image to see Haemus Montes in 3-D.

More Volcanos and Mountains Enhanced color mosaic of a portion of Io from approximately 0° to 60° latitude and from -60° to 10° longitude. A persistent thermal hotspot named "Kanehekili" is located just north of the rectangular bright orange patch in the upper left quadrant of the scene. Voyager also imaged an active volcanic plume in the vicinity of the large Y-shaped black patch (a lava flow?) at bottom left. ACTIVE VOLCANOS ON IO Loki & Loki Patera Loki is one of the most persistent volcanic areas on Io and has been monitored from Earth almost continuously since it was discovered by Voyager 1 in March 1979. The dark, horseshoe-shaped feature is thought to be a lava lake, partially filled with dark, possibly basaltic, lava. An active volcanic plume, arcing over 150 kilometers into space, can be seen near image center (the blue-gray wispy features extending from the left end of the long, dark, gash-like feature). This is one of eight active plumes observed by Voyager in 1979.

Prometheus Prometheus, another plume observed by Voyager in 1979, is seen in this triptych of three views taken over a 6-hour span. The Promethus plume extends upward 70 kilometers into space before falling on the surface 125 kilometers from the central vent. The plume somewhat resembles an umbrella, with several prominent dark radial spokes comprised of optically thick concentrations of plume material. The plume may be composed of sulfur dioxide explosively heated within the crust of Io. The first image is a black-and-white image, the second shows the area in approximately natural color, the third view shows the area in enhanced color (emphasizing blues and violets where color difference between geologic units on Io is more pronounced). Prometheus can also be viewed in 3-D. Focus on two adjacent images, and allow your eyes to cross until the two images merge. (Red-green stereo glasses do not work on this image.) Prometheus in 3-D Red-green stereo glasses can be used on this image to see the Prometheus plume in 3-D. Pele Vent source of the Pele plume, observed by Voyager in March 1979.

This was the largest plume observed by Voyager, extending ~500 kilometers in all directions from the source (the plume itself extends beyond the limits of this image). The eruption source is the small dark spot at the eastern (right) end of the linear fissure near the center of the frame. Ra Patera Ra Patera is the largest shield volcano on Io. Shield volcanos, like Hawai'i, are large volcanic edifices composed of numerous overlapping lava flows. They are called shields because they form broad convex domes with low slopes. Dark linear flows radiate from the center of Ra. Some have speculated that these flows are composed of sulfur, although many do not agree with this interpretation. Ra Patera was not active when Voyager passed in 1979, but Hubble Space Telescope observations in 1995 and 1996 indicate that a major change has taken place at this location, quite likely a major volcanic eruption. Galileo should obtain new images of this area that will hopefully reveal the origin of these changes. All images by Paul M. Schenk, Lunar and Planetary Institute, Houston, TX.

©Lunar and Planetary Institute, 1997 Supplemental Reading Materials Beatty J. K., O'Leary B., and Chaikin A., eds. (1990) The New Solar System. Sky Publishing Corporation, Cambridge, Massachusetts; Cambridge University Press, New York. 326 pp. Moons and Rings (1991) Voyage Through the Universe series. Time-Life Books, Alexandria, Virginia. 144 pp. Rothery D. (1992) Satellites of the Outer Planets. Clarendon Press, Oxford. 208 pp. McEwen A. et al. (1989) Dynamic geophysics of Io. In Time-Variable Phenomenon in the Jovian System, pp. 11-46, NASA Special Publication 494. Morrison D., ed. (1982) Satellites of Jupiter. University of Arizona Press, Tucson. 972 pp. Nash D. et al. (1986) Io. In Satellites (J. Burns and M. Matthews, eds.), p. 629, University of Arizona Press, Tucson.

Spencer J. and Schneider N. (1996) Io on the eve of the Galileo mission. Annual Reviews of Earth and Planetary Science, 24, 125. Schenk P. et al. (1997) Geology and topography of Ra Patera, Io, in the Voyager era: Prelude to eruption. Geophys. Res. Lett., 24, 2467-2470. Moons of Jupiter: Io Io Moons of Jupiter: Io Io is one of the 4 large Galilean moons of the planet Jupiter . For a listing of other moons, see Moons of the Planets . Io Stats Jupiter The "Jovian" Planet: Jupiter During the last few weeks we discussed the terrestrial planets--that is those planets that are most like the Earth in size, composition, and structure. While the geologies of the inner four planets have some things in common, we have found that each one is quite different from the others.

The next four planets we will talk about--Jupiter, Saturn, Uranus, and Neptune--are very different to the terrestrial planets in that they have no real solid surfaces, being dominated by thick, deep atmospheres. Thus, there is no "geology" to speak of, and we talk more about the meteorology (weather) of these gas giants. Note their low densities: Jupiter Jupiter is the fifth planet from the Sun, and is the largest planet in the solar system. Its average distance from the Sun is 5.2 AU (778,330,000 km), and its equatorial radius is 68,700 km (almost 11 times that of Earth): Though Jupiter is very massive (having 318 times the mass of the Earth), it is not very dense: it has a mean density of 1.4 gm/cm3. Just slightly more dense than water! Jupiter is 75% hydrogen, 24% helium, almost identical to the Sun. Suprisingly, even though Jupiter is enormous, it spins very rapidly, completing one rotation in slightly under 10 hours. It takes 12 years for Jupiter to circle the Sun.

Jupiter is a swirling ball of clouds arranged in rotating storms and in complex bands: These bands are caused by rising and descending air, as shown in the textbook: Atmospheric temperature profile: Jupiter is mostly a large ball of hydrogen and helium gas with a few trace elements: This picture illustrates the internal structure of Jupiter. The outer layer is primarily composed of molecular hydrogen. At greater depths the hydrogen starts resembling a liquid. At 10,000 kilometers below Jupiter's cloud top liquid hydrogen reaches a pressure of 1,000,000 bar with a temperature of 6,000 K. At this state hydrogen changes into a phase of liquid metallic hydrogen. In this state, the hydrogen atoms break down yielding ionized protons and electrons similar to the Sun's interior. Below this is a layer dominated by ice where "ice" denotes a soupy liquid mixture of water, methane, and ammonia under high temperatures and pressures. Finally at the center is a rocky or rocky-ice core of up to 10 Earth masses.1 The central pressure of Jupiter is 80 Million Bars, and the temperature is 25,000 K.

In fact, Jupiter radiates about twice as much energy as it receives from the Sun. This is mostly believed to be heat left over from formation, but "shrinking" can also supply some of this heat. The rapid rotation of Jupiter creates an incredible amount of shear in the atmosphere, and all kinds of rotating structures form. The biggest of these, the "great red spot" was discovered by Cassini in the 17th century: The red spot is simply a high pressure area like those seen on the Earth: While it was first observed about 300 years ago, it has begun to shrink and may disappear in the next decade or two. Jupiter has no real solid surface, with the clouds gradually compressing to a liquid state, and this further compresses into a "metallic" state that surrounds the rocky core. The temperature of Jupiter at the cloud tops is very cold, 152 K, but increases dramatically as you go inwards, reaching to 25,000 K in the core.

Because Jupiter spins so rapidly, and it is a big ball of gas, it is actually a lot larger in diameter at the equator than at the poles--this is called "oblateness", and all of the gas giants are oblate. Here's the physics: Jupiter has an equatorial radius of 71,492 km, but a polar radius of 66,854 km! The polar radius is only 94% of the equatorial radius (this can easily be noticed in a small telescope). The Earth is also oblate, but not as dramatically: REq = 6378 km; RPol = 6357 km (oblateness = 6357/6378 = 0.997). Magnetic Field Jupiter has an enormously strong magnetic field--it is about 14 times the strength of the Earth's field ("14 Gauss"). It is believed the rapid rotation leads to the generation of this field, enabling electric currents in its central regions (liquid/metallic hydrogen conducts electricity like a normal metal). The magnetosphere of Jupiter is one of the largest structures in the solar system, extending all of the way to Saturn!

Where this magnetic field enters the atmosphere we get aurorae (like those on Earth): The Solar System's Vacuum Cleaner Because Jupiter is so massive, its gravity influences many of the smaller bodies in the solar system---it also eats the ones that get too close. In 1994 a comet, Shoemaker-Levy, got too close, first being "captured" by Jupiter, and then breaking-up into a bunch of pieces: And then these pieces crashed into Jupiter one at a time: Eventually they all did, leaving dark plumes of material in Jupiter's atmosphere: This has certainly happened numerous times in Jupiter's past, as this "crater chain" on one of Jupiter's moons (Ganymede) reveals: The Moons of Jupiter Jupiter has a large number of known moons (about 67 of them), but most of these are tiny little rocks (< 5 km in radii) and most of the outer ones are probably captured asteroids.

There are four major moons of Jupiter: the "Galilean satellites" Io , Europa , Ganymede , and Callisto (shown in order, below--that tiny oblong rock to the left of Io is one of the smaller moons Almathea, whose orbit is inside that of Io). The densities of these four moons are Io: 3.5 g/cm3, Europa: 3.0 g/cm3, Ganymede: 1.9 g/cm3, and Callisto: 1.8 g/cm3. Just like the terrestrial planets, the densities decrease with distance. This is surely due to the fact that when Jupiter formed it was much hotter, thus driving away the water and other low density atoms/molecules to more distant reaches. The innermost of these four moons is Io, and its surface is covered with lava flows from the numerous volcanoes on its surface (go to pages 323 to 325 of the textbook for more images). In fact, Io has the youngest surface in the solar system, being constantly reshaped by volcanic eruptions. The yellowish-orange color of Io is due to the large amount of sulfur that is discharged in these eruptions.

Here is a possible lake of sulfur: All of these eruptions have left a trail of sulphur in orbit around Jupiter: The reason Io has so many volcanoes is that the intense gravity of Jupiter, and that of the other three moons, constantly tugs on Io, stretching and compressing it (see page 325 of the textbook). The the three inner moons are also in orbital resonances: For every four orbits of Io, Europa makes two, and Ganymede one. This means the constantly line up in the same configurations every once in a while, enhancing the tidal stresses. That generates an enormous amount of heat that liquifies the insides of Io. This process is also at work on the next moon out from Jupiter, Europa . Europa has one of the smoothest and brightest surfaces in the solar system. It is believed to be frozen water ice. Because ice is plastic (it flows), features with large elevations (mountains, craters, etc.) are not sustainable, and thus the surface is relatively smooth, but is covered with a large number of cracks: Folded "crust" on Europa that looks like fractured ice sheets in which water flowed into the gap for a new, smoother surface.

"Freckles" on Europa, which are believed to be "upwelling" of warmer ice from below. Thera and Thrace, two of the largest features on Europa, and appear to be larger, more complicated versions of the freckles. An impact crater on Europa--barely recognizable as it just bores through the ice. Here is an image of ice on the Earth's surface in the polar regions, it is remarkably similar to Europa in appearance: It is now believed that Europa has a large ocean of water beneath its frozen crust. This is also probably the case for the largest moon of Jupiter, Ganymede. Here are cutaway drawings of the internal structures of Jupiter's four moons (blue signifies water): Here is a close-up of two possible models for Europa: Recently , geysers of water were discovered on Europa, proving that it has liquid water below the ice layer. Plans are being drawn-up to explore Europa and/or Ganymede as they may be ideal sites for life to have evolved and to currently exist. For more on Europa, go here . A recent mission (arrived in orbit July 5th, 2016), Juno, will probe the gravity of Jupiter and its moons.

The largest planetary satellite in the solar system is Ganymede (radius of 2,600 km). It is mostly rock and ice, here are two close-ups: Ganymede is slightly larger than Mercury, and twice as massive as the Earth's moon. Because of its low density, however, Mercury is much more massive than Ganymede. Thus it is both the largest and most massive moon in the solar system. Ganymede is the only satellite known to have its own magnetic field, suggesting a liquid, iron rich core. Presumably the tidal forces that act on Io are at work on Ganymede helping the keep the internal regions hot. The surface seems to be nearly all water ice. Like Europa, there is much evidence for grooved terrain caused by tidal forces on the icy crust (go here for a big map of Ganymede). The final Gallilean satellite is Callisto, with a radius of 2400 km, about the same size as Mercury (but only 1/3 its mass). It has one of the most battered surfaces of any object in the solar system: It shows no evidence of structures that would have resulted from internal heating like the other Gallilean satellites.

Callisto may have a completely unaltered surface from the time it was created--only impacts appear to exist, nothing else. Its low density means mostly ice with a rocky core. All of the Gallean moons have small "exospheres", where molecules of the rocks and ices that make up their surfaces are blown into space. Moons Moons: Moons are `fossils" into a planet's past.

The major, named moon systems are: Earth: Luna (The Moon) Mars: Deimos, Phobos Jupiter: Adrastea, Amalthea, Ananke, Callisto, Carme, Elara, Europa, Ganymede, Himalia, Io, Leda, Lysithea, Metis, Pasiphae, Sinope, Thebe Saturn: Atlas, Calypso, Dione, Enceladus, Epimetheus, Helene, Hyperion, Iapetus, Janus, Mimas, Pan, Pandora, Phoebe, Prometheus, Rhea, Telesto, Tethys, Titan Uranus: Ariel, Belinda, Bianca, Cordelia, Cressida, Desdemona, Juliet, Miranda, Oberon, Ophelia, Portia, Puck, Rosalind, Titania, Umbriel Neptune: Despina, Galatea, Larissa, Naiad, Nereid, Proteus, Thalassa, Triton Pluto: Charon (note: Pluto/Charon form a binary system, but Charon is the smaller so it is classed as the moon of Pluto) New, smaller moons are being discovered all the

 time with recent space missions.

The total count of moons (as of 12/18/2001) are: Mercury - 0 moons Mars - 2 moons Uranus - 20 moons Venus - 0 moons Jupiter - 28 moons Neptune - 8 moons Earth - 1 moon Saturn - 30 moons Pluto - 1 moon Moons range in shape from highly irregular to spheres. Their shape reflects their formation history, irregular objects are ill-formed moons (captured asteroids or comets) or pieces of a larger moon, spherical objects were once molten spheres, probably at the time of their formation. Moons of Mars: Deimos & Phobos We speculate, from their irregular appearances and low mean densities, that Deimos and Phobos, are captured asteroids. Both Deimos and Phobos are saturated with craters. Deimos has a smoother appearance caused by partial filling of some of its craters. Moons of Jupiter: Jupiter has 63 confirmed moons as of Jan 2009, i.e. moons with known orbits around Jupiter. Eight of these moons are regular, meaning they have prograde, nearly circular orbits.

They are composed of the four Galilean satellites, plus the inner or Amalthea group: Adrastea is a typical small moon Metis is the innermost known satellite of Jupiter Amalthea is one of Jupiter's smaller, irregular moons, an example of moon collecting dust from another moon (Io) The remaining 55 moons are irregular, they have a mixture of prograde and retrograde orbits with high inclinations and eccentricities. Jupiter's regular satellites are believed to have formed from a circumplanetary disk, a ring of accreting gas and solid debris analogous to a protoplanetary disk. The irregular satellites are substantially smaller objects with more distant and eccentric orbits. They form families with shared similarities in orbit (semi-major axis, inclination, eccentricity) and composition; it is believed that these are at least partially collisional families that were created when larger (but still small) parent bodies were shattered by impacts from asteroids captured by Jupiter's gravitational field. Moons of Saturn: The moons of Saturn are numerous and diverse, ranging from tiny moonlets less than 1 kilometer across to the enormous Titan, which is larger than the planet Mercury.

Saturn has sixty-two moons with confirmed orbits, fifty-three of which have names, and only thirteen of which have diameters larger than 50 kilometers. Saturn has seven moons that are large enough to become spherical. Twenty-four of Saturn's moons are regular satellites; they have prograde orbits not greatly inclined to the Saturn's equatorial plane. These include the seven major satellites, four small moons which exist in a Trojan orbit with larger moons, two mutually co-orbital moons and two moons which act as shepherds of Saturn's F Ring. Two other known regular satellites orbit within gaps in Saturn's rings. The relatively large Hyperion is locked in a resonance with Titan. The remaining regular moons orbit near the outer edge of the A Ring, within G Ring and between the major moons Mimas and Enceladus. The regular satellites are traditionally named after Titans and Titanesses or other figures associated with the mythological Saturn. The remaining thirty-eight, all small except one, are irregular satellites, whose orbits are much farther from Saturn, have high inclinations, and are mixed between prograde and retrograde. These moons are probably captured minor planets, or debris from the breakup of such bodies after they were captured, creating collisional families.

The irregular satellites have been classified by their orbital characteristics into the Inuit, Norse, and Gallic groups, and their names are chosen from the corresponding mythologies. The largest of the irregular moons is Phoebe, the ninth moon of Saturn, discovered at the end of the 19th century. The rings of Saturn are made up of objects ranging in size from microscopic to hundreds of meters, each of which is on its own orbit about the planet. Thus, a precise number of Saturnian moons cannot be given, as there is no objective boundary between the countless small anonymous objects that form Saturn's ring system and the larger objects that have been named as moons. At least 150 moonlets embedded in the rings have been detected by the disturbance they create in the surrounding ring material, though this is thought to be only a small sample of the total population of such objects. Daphnis drifts through the Keeler gap Atlas the second of Saturn's known satellites, orbits near the outer edge of the A-ring Enceladus is one of the innermost moons of Saturn.

Enceladus reflects almost 100 percent of the sunlight that strikes it and has evidence of internal heating and recent resurfacing effects Hyperion is one of the smaller moons of Saturn. It has a pock-marked body and is the largest irregularly shaped satellite ever observed.

Iapetus is one of the stranger moons of Saturn, its leading side is dark with a slight reddish color while its trailing side is bright Dione is the densest moon of Saturn other than Titan, and has several usual characteristics: 1) has a rocky core and ice crust, 2) is heavy cratering on trailing hemisphere, 3) has bright, wispy features Rhea is the largest airless satellite of Saturn that has different regions with different crater sizes indicating that parts of the moon have undergone resurfacing since formation Mimas is one of the innermost moons of Saturn with a very large impact crater that came close to fracturing the moon Tethys is an icy body similar in nature to Dione and Rhea Phoebe is the last of the known satellites of Saturn and orbits in a retrograde direction (opposite to the direction of the other satellites' orbits) in a plane much closer to the ecliptic than to Saturn's equatorial plane. Thus, Phoebe may be a captured asteroid with a composition unmodified since the time it was formed in the outer Solar System.

Moons of Uranus: Uranus, the seventh planet of the Solar System, has 27 known moons, all of which are named after characters from the works of William Shakespeare and Alexander Pope. William Herschel discovered the first two moons, Titania and Oberon, in 1787, and the other spherical moons were discovered in 1851 by William Lassell (Ariel and Umbriel) and in 1948 by Gerard Kuiper (Miranda). The remaining moons were discovered after 1985, either during the Voyager 2 flyby mission or with the aid of advanced Earth-based telescopes. A montage of Uranus' large moons and one smaller moon: from left to right Puck, Miranda, Ariel, Umbriel, Titania and Oberon. Uranian moons are divided into three groups: thirteen inner moons, five major moons, and nine irregular moons. The inner moons are small dark bodies that share common properties and origins with the planet's rings. The five major moons are massive enough to have achieved hydrostatic equilibrium (they were once molten spheres), and four of them show signs of internally driven processes such as canyon formation and volcanism on their surfaces.

The largest of these five, Titania, is 1,578 km in diameter and the eighth-largest moon in the Solar System, and about 20 times less massive than Earth's Moon. Uranus's irregular moons have elliptical and strongly inclined (mostly retrograde) orbits at great distances from the planet. Ariel is a relatively small satellite and is the brightest moon of Uranus Miranda with a jumbled surface unlike anything in the Solar System, indicates evidence of violent past with possible multiple shattering and reassembly Titania is the largest moon of Uranus and is marked by a few large impact basins Moons of Neptune: Neptune has thirteen known moons, by far the largest of which is Triton, discovered by William Lassell just 17 days after the discovery of Neptune itself. Over a century passed before the discovery of the second natural satellite, called Nereid. Neptune's moons are named for minor water deities in Greek mythology. Unique among all large planetary moons, Triton is an irregular satellite, as its orbit is retrograde to Neptune's rotation and inclined relative to the planet's equator.

The next-largest irregular satellite in the Solar System, Saturn's moon Phoebe, is only 0.03% Triton's mass. Triton is massive enough to have achieved hydrostatic equilibrium and to retain a thin atmosphere capable of forming clouds and hazes. Both its atmosphere and its surface are composed mainly of nitrogen with small amounts of methane and carbon monoxide. Triton's surface appears relatively young, and was probably modified by internally driven processes within the last few million years. The temperature at its surface is about 38K. Inward of Triton are six regular satellites, all of which have prograde orbits in planes that lie close to Neptune's equatorial plane. Some of these orbit among Neptune's rings. The largest of them is Proteus. Neptune also has six outer irregular satellites, including Nereid, whose orbits are much farther from Neptune, have high inclinations, and are mixed between prograde and retrograde. The two outermost ones, Psamathe and Neso, have the largest orbits of any natural satellites discovered in the Solar System to date. Proteus is one of the darkest objects in the Solar System

Question: Io is one of the moons of which planet?
Answer: JUPITER