Office of News and Public Affairs
University of Wisconsin-Madison

January 9, 1998

Hubble Finds One More Oddity on an Already Strange Moon

Jupiter's moon Io, whose strange surface is defined by active volcanoes, lakes of molten sulfur and vast fields of sulfur dioxide snow, has revealed another oddity to scientists: caps of glowing hydrogen gas at the moon's poles.

Using a new instrument aboard the Hubble Space Telescope, the Space Telescope Imaging Spectrograph, UW-Madison scientist Frederick Roesler and his colleagues have found evidence of glowing hydrogen gas, something never before seen on the moon first discovered in 1610 by Galileo Galilei.

The discovery was a surprise, said Roesler, a UW-Madison professor of physics who reported his findings at a meeting of the American Astronomical Society.

"It adds to the mystery. What is the hydrogen's origin? Why is it glowing? We're not sure," said Roesler who's studied Io, Jupiter's third largest and closest Galilean moon, for many years.

Roesler said the new observations may indicate that Io's poles are swathed in a frost of molecular hydrogen sulfide, a toxic gas that requires temperatures on the order of minus 130 F. to freeze. Or, said Roesler, there might be other hydrogen-bearing frosts concentrated at Io's poles.

Alternatively, the glowing hydrogen gas seen by Roesler in the Jovian moon's atmosphere may be the result of a large electrical current flowing between Jupiter and Io where hydrogen atoms are propelled to the moon from the hydrogen-rich Jovian atmosphere.

In any case, the discovery of hydrogen in Io's atmosphere adds another unusual feature to a moon chock-full of strange phenomena, including lakes of liquid sulfur, active volcanoes with plumes 200 miles high, and fantastic extremes of surface temperature.

The observations were made last October using one of Hubble's new instruments, the Space Telescope Imaging Spectrograph.

Although resolution of Io's latest mystery will most likely require further observations, scientists' best guess at the moment is that hydrogen is somehow concentrated at the poles in the form of hydrogen sulfide frost which evaporates off the surface. The hydrogen sulfide molecules, Roesler explained, may then be broken up into their constituent hydrogen and sulfur atoms with the hydrogen atoms made to glow by ultraviolet light from the sun.

The discovery, according to Roesler, is of further interest to scientists because the hydrogen glow seen by Hubble seems to have no relationship to glowing belts of oxygen and sulfur atoms observed in the equatorial regions of the Jovian moon.

"It behaves completely differently from the oxygen and sulfur glows," said Roesler, explaining that finding new features of Io, while initially complicating scientists' picture of the moon, may ultimately help them untangle its mysteries.

To make the new discovery Roesler and his colleagues, including UW- Madison physics Professor Frank Scherb, Ron Oliversen of NASA's Goddard Space Flight Center in Greenbelt, Md., and Warren Moos of Johns Hopkins University needed to cut through an obscuring corona of glowing hydrogen that surrounds the Earth.

The new Space Telescope Imaging Spectrograph, installed last February during NASA's last servicing mission to the orbiting telescope, is able to sort through the soup of chemical compounds in space and in planetary atmospheres by its ability to localize and identify different atomic species through the characteristic signatures they emit in different ultraviolet wavelengths.

Astronomy Picture of the Day: Dark Volcano Active on Io

NASA Headquarters, Washington, DC
Jet Propulsion Laboratory, Pasadena, CA

November 5, 1997


Observations taken by NASA's Galileo spacecraft five months apart reveal a new dark spot the size of Arizona on Jupiter's moon Io, indicating that dramatic volcanic activity occurred during that time.

"This is the largest surface change on Io observed by Galileo during its entire two-year tour of the Jovian system," said Galileo imaging team member Dr. Alfred McEwen, a research scientist at the University of Arizona in Tucson.

The visible change took place during the five months between Galileo's seventh and tenth orbits of Jupiter. The change is manifested as a dark spot about 249 miles in diameter, surrounding a volcanic center named Pillan Patera, which is named after the South American god of thunder, fire and volcanoes. Dark features at the center of the deposits may be new lava flows.

These changes appear in images taken by the Solid State Imaging system aboard Galileo, with marked differences between the pictures taken on April 4, 1997 and September 19, 1997. In June of 1997 an active plume was observed over Pillan by Galileo and the Hubble Space Telescope with a height of 75 miles, and both Galileo and ground-based astronomers observed an intense hot spot.

"Most of the volcanic plume deposits on Io show up as white, yellow or red due to sulfur compounds. However, this new deposit is gray, which tells us it has a different composition, possibly richer in silicates than the other regions," McEwen explained. "While scientists knew that silicate volcanism existed on Io from high temperatures, this may provide clues as to the composition of the silicates, which in turn tells us about Io's evolution."

"Io is probably primarily composed of silicates, which is the type of volcanic rock found on Earth, " McEwen added, "but the extreme volcanism of Io may have led to the creation of silicate compositions that are unusual on Earth."

The Io images showing the changes in Pillan Patera also reveal alterations in the plume deposit of Pele, the large red oval southwest of Pillan, which may indicate that both plumes were active at the same time and interacted with one another. A dark region southwest of Pele, which appears similar to the Pillan deposits, has been present since the Voyager flybys in 1979.

Io is the most volcanically active body in the Solar System. Scientists hope to learn more about the fiery satellite when Galileo continues its studies over the next two years, during a mission extension known as the Galileo Europa Mission. The extended mission will include eight additional encounters of Europa, four of Callisto, and two close Io flybys in late 1999, depending on spacecraft health. Galileo will pass very close to Pillan Patera in the first of the two Io flybys, so high- resolution images can be acquired over a small portion of this area.

Galileo was launched in 1989 and entered orbit around Jupiter on Dec. 7, 1995. The final satellite encounter of its two-year primary mission will occur on Thursday, Nov. 6, 1997 at 3:32 p.m. EST, when the spacecraft swoops over Europa at an altitude of 1,269 miles.

"The Galileo Orbiter is performing flawlessly and all 11 of its sophisticated science instruments and the radio science investigations are still providing excellent data," said Galileo Project Manager Bill O'Neil of NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA. "A great bounty of Jupiter system science has been obtained and the continuing study of these data will surely add many important discoveries. While not all of the original objectives could be met due to the antenna failure, I believe that the overall science return from Galileo will easily exceed what was envisioned at project inception 20 years ago, because our team of scientists and engineers has done such a superb job of capturing the most important observations."

The Galileo mission is managed by JPL for NASA's Office of Space Science, Washington, DC. JPL is an operating division of California Institute of Technology, Pasadena, CA.

Images of Io and other data received from Galileo are posted on the Galileo home page on the World Wide Web.

NASA Headquarters, Washington, DC
Jet Propulsion Laboratory, Pasadena, CA

October 23, 1996


Scientists participating in NASA's Galileo mission have discovered that the Galileo spacecraft may have flown though a dense, high-altitude ionosphere during its encounter with Jupiter's volcanic moon Io last December. This discovery suggests that Io's atmosphere is time variable and is made of volcanic gas lofted to very high altitudes.

An ionosphere is a region of electrically charged gas that exists at the top of some planetary atmospheres. The surprising discovery is being reported by Galileo scientists this week at a meeting of the American Astronomical Society's Division of Planetary Sciences held in Tucson, AZ, along with other Galileo results, including remarkable new images of the planet and its moons.

"Sensors on the spacecraft found a very dense region of ionized oxygen, sulfur and sulfur dioxide at 555 miles on Io that must be pumped into that region by Io's relentless volcanic activity," said Dr. Louis A. Frank of the University of Iowa, principal investigator on Galileo's plasma science experiment. "Instead of being swept away by Jupiter's rotating magnetosphere as anticipated, the ionized gases surprisingly remain with Io," he said.

"Passage of the Galileo spacecraft through an ionosphere was not expected because images of the volcanic plumes previously taken with the Voyager spacecraft indicated that the plume heights extended only to a few hundred kilometers or less," Frank said. A radio occultation by the Pioneer 10 spacecraft in 1973 indicated ionospheric heights only about 30 to 60 miles above the surface. "No one expected to see this at 900 kilometers' altitude," he added. The difference between what Pioneer saw and what Galileo has observed indicates that Io's atmosphere and ionosphere are variable and may grow and shrink with more or less volcanic activity.

The results may lend credence to previous theories proposed by Galileo project scientist Dr. Torrence Johnson of NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA, that invisible "stealth plumes" deliver volcanic gases to great heights above Io. Io's weak gravity field apparently permits the invisible gases emanating from the volcanoes to reach extraordinary heights far beyond the lower altitudes achieved by the dust and other volcanic ejecta that reflect sunlight and can be seen in images, Frank said.


In a related finding, the energetic particle detector on the spacecraft measured intense, bi-directional electron beams that are aligned with Galileo's magnetic field in Io's vicinity. The beams are similar to those that impinge on Earth's atmosphere to produce aurora and also positive ions and electrons in Earth's atmosphere.

Dr. Donald J. Williams, principal investigator on the energetic particle experiment from the Johns Hopkins Applied Physics Laboratory, said the electron beams span the energy range of 15 kiloelectron volts to 190 kiloelectron volts and represent an energy deposition into Jupiter's atmosphere of up to one billion watts.

"This is sufficient energy input into the Jovian atmosphere to produce visible auroral emissions," Williams said. "These beams are a signature of remarkable particle acceleration processes that occur in the vicinity of Io - processes that are thought to be linked to Io's motion through Jupiter's plasma and magnetic field environment." Additional work is required to determine whether the beams play a role in producing some of the auroral emissions observed in Jupiter or if they are related to radio emissions that have been correlated with Io's orbital motion.

The electron beams also must have a role in maintaining the Io torus, the doughnut-shaped cloud of ionized gases that flows between Jupiter and Io, Galileo scientists said. Auroras in Io's atmosphere is one likely result of the electron beams, they reported, and the two-way electron highway that the beams produce between Jupiter and Io must contribute to some of the auroras observed in Jupiter's atmosphere as well.


Several images recently returned by Galileo show new details of surface features on the moons Ganymede and Io. One new image of the active volcano Prometheus on Io has been compared to one of the same feature taken by NASA's Voyager spacecraft 17 years ago, and shows that the plume is now erupting from a position about 46.5 miles west of where the hot spot resided in 1979. It is not known if the plume source is the same or if the plume is now emanating from a new source. Overall, scientists studying Galileo images of Io are observing that a wide variety of surface changes have occurred in the nearly two decades since a spacecraft last visited Jupiter's system.

NASA Headquarters, Washington, DC

May 3, 1996


Jupiter's volcano-pocked moon Io has been found by NASA's Galileo spacecraft to have a giant iron core that takes up half its diameter, scientists report in today's issue of Science magazine.

The spacecraft also has detected a large "hole" in Jupiter's magnetic field near Io, leading to speculation about whether Io possesses its own magnetic field. If so, it would be the first planetary moon known to have one.

These newly identified characteristics of Io may be related to the intense heating of the moon caused by the constant squeezing and distortion of Io in Jupiter's powerful gravitational grip, according to Galileo Project Scientist Dr. Torrence Johnson of NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA. Io is the most geologically active body in the Solar System, and though it is less than a third of Earth's size, it generates twice as much heat as the Earth.

"Jupiter's massive gravity field distorts the shape of Io in the same way that tides are raised in Earth's oceans by the gravitational tugs of the Sun and Moon," Johnson said. As Io orbits Jupiter, these so-called "body tides" rise and fall due to subtle changes in Io's orbit which in turn are caused by the gravitational nudges from Europa and Ganymede, other moons of Jupiter.

As a result, Io is squeezed like a rubber ball. Friction created by this action heats and melts rock within Io to produce the volcanoes and lava flows seen all over its surface, and huge geysers that spew sulfur dioxide onto Io's landscape.

The large, dense core Galileo found within Io was deduced from data taken during the spacecraft's flyby within 559 miles of the moon last Dec. 7, as Galileo passed by the moon on its way to enter orbit around Jupiter. Precise measurements of the spacecraft's radio signal revealed small deviations in Galileo's trajectory caused by the effects of Io's own gravity field.

From these data, Galileo scientists have determined that Io has a two-layer structure. At the center is a metallic core, probably made of iron and iron sulfide, about 560 miles in radius, which is overlain by a mantle of partially molten rock and crust, according to JPL's Dr. John Anderson, team leader of Galileo's celestial mechanics experiment and principal author of the paper published in Science today. The core was probably formed from heating in the interior of the moon, either when it originally formed or as a result of the perpetual tidal heating driving its volcanoes.

Galileo scientists also are trying to determine the cause of the hole they found in Jupiter's magnetic field when the spacecraft was closest to Io. "Instead of increasing continuously as the spacecraft neared Jupiter, the magnetic field strength took a sudden drop of about 30 percent," Johnson said.

"It's an astonishing result and completely unexpected," said Dr. Margaret Kivelson of the University of California at Los Angeles, who heads Galileo's magnetic fields investigation team. Preliminary analyses of these data are currently being prepared for formal publication.

"The data suggest that something around Io -- possibly a magnetic field generated by Io itself -- is creating a bubble or hole in Jupiter's own powerful magnetic field," Kivelson said. "But it's not clear to us just how Io can dig such a deep and wide magnetic hole."

Possible explanations for this signature can only be sorted out using data from all the other space physics instruments onboard Galileo, Johnson said. "We're eagerly awaiting the return of data from the magnetospheric measurements taken during the Io flyby to see if we can resolve this mystery," he said. This data, recorded on board the spacecraft, will be transmitted back to Earth in June or July.

If analysis of this data eventually proves that Io indeed has a magnetic field of its own, it would be the first moon shown to have one. Io would join the Earth, planet Mercury and the outer giant planets as bodies in our Solar System that generate their own magnetic fields.

Other studies conducted by Galileo during its December flyby of Io have provided new evidence that Io is most likely the source of high-velocity dust streams littering millions of miles of space around Jupiter.

In July 1994, Galileo's dust detector began sensing dust streams more powerful than those previously discovered by the Ulysses spacecraft. Dust detectors on Galileo sensed more and more particles during its approach to Jupiter, reaching a peak of 20,000 impacts per day during the longest and most intense interplanetary dust storm ever observed.

These fast-moving particles travel at speeds from 30 to 60 miles per second away from Jupiter -- fast enough to escape the Solar System. These dust impacts continued up to the time of Galileo's Io flyby and then ceased, said Dr. Eberhard Grun of Germany's Max Planck Institute in Heidelberg, principle investigator for Galileo's dust detector experiment.

"My preliminary interpretation of these observations is that they support the idea that Io is in some way the source of the Jupiter dust streams," Grun said.

One theory proposed after the NASA Voyager spacecraft flybys in the late 1970s is that dust particles emitted from Io's volcanoes could become electrically charged and then swept away by Jupiter's rotating magnetic field. Recent modifications to this theory suggest that the dust is subsequently accelerated in the magnetosphere and flung outward from Jupiter at high velocity, creating dust streams.

Galileo's next close encounter with a moon of Jupiter will occur June 27, when the spacecraft will pass about 530 miles above the surface of Ganymede. Larger than Mercury, Ganymede is the largest moon in the Solar System. Galileo will make repeated close flybys of Ganymede, Callisto and Europa during its two-year mission in orbit around Jupiter.

Galileo was launched aboard Space Shuttle Atlantis on Oct. 18, 1989. The mission is managed by JPL for the NASA Office of Space Science, Washington, DC.

Additional information on the Galileo mission and its results can be found on the World Wide Web at URL:

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