October 16, 2001

Galileo Successfully Completes Io Flyby

NASA's Galileo spacecraft successfully completed a close flyby to study Jupiter's moon Io at 0123 Universal Time today (6:23 p.m. Oct. 15, Pacific Daylight Time), during the long-lived spacecraft's 32nd orbit around Jupiter.

Galileo passed closer to Io than ever before, within about 181 kilometers (112 miles) of ground level near Io's south pole.

Engineers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., said that signals confirming the spacecraft's basic health arrived within an hour after the flyby. The signals were received via JPL's Deep Space Network antenna facility near Madrid, Spain.

"Jupiter's radiation belts make flying near Io risky, but Galileo has come through for us again," said JPL's Dr. Eilene Theilig, Galileo project manager.

Galileo has been orbiting Jupiter since 1995. It has already endured more than three times as much radiation as it was designed to tolerate. NASA has extended Galileo's original two- year orbital mission three times to take advantage of the spacecraft's continuing ability to make scientific discoveries.

As of 1500 UT (8 a.m. PDT) today, the spacecraft had recorded about 70 percent of the scientific data that its instruments had been programmed to collect during this swing through the inner portion of the Jovian system. The encounter period that began Oct. 13 includes more-distant observations of Jupiter and the moon Europa, as well as the close-up examination of Io.

The images and other scientific data from the encounter will be transmitted to Deep Space Network antennas in Spain, Australia and California over the next three months.

Engineering data already received, such as voltage readings, suggest that Galileo's solid-state camera functioned properly during the flyby. However, the camera's performance won't be known for sure until transmission of the pictures, which is due to begin in late October. The camera has malfunctioned intermittently in the past year because radiation has degraded its electronics. Galileo engineers sent new software to the camera two weeks ago designed to minimize chances for recurrence of the problem.

Among the high-priority science observations for the flyby are magnetic-field measurements near Io's south pole, useful for understanding the moon's interior and interactive processes within Jupiter's large magnetic environment. Other instruments were scheduled to observe details and changes in several volcanic areas on Io's surface, including a new hot spot and plume eruption discovered on the most recent flyby, in August.

Io is the most volcanically active world known. It orbits closest to Jupiter of the planet's four major moons. Tidal stress from the gravitational pull of Jupiter and the outlying moons heats Io's interior and sustains the volcanism.

Additional information about Galileo and the discoveries it has made since it was launched from NASA's Space Shuttle Atlantis in 1989 is available at JPL, a division of the California Institute of Technology in Pasadena, manages the Galileo mission for NASA's Office of Space Science, Washington, D.C.

Today on Galileo

Tuesday, October 16, 2001

The hectic pace of the Io closest approach and its jam-packed observing sequence is now fully five hours behind us. Galileo has sped on to a distance of 160,000 kilometers (99,440 miles) from Io. But there are still good science observing opportunities to be taken advantage of. And take advantage of them we do!

At 12:10 a.m. PDT [See Note 1] the Photopolarimeter Radiometer instrument (PPR) turns its focus on Jupiter. The observation scans the instrument back and forth across the limb of the giant planet, mapping the temperatures of the different layers of the atmosphere.

At 12:44 a.m. the Near Infrared Mapping Spectrometer (NIMS) collects a thermal map of the entire visible disk of Io, looking for volcanos and other hot spots.

At 1:41 a.m. PPR maps the temperatures on the dark side of Europa. This is the only observation made of this icy moon on this orbit. Europa was the central focus of a series of nine orbits earlier in the Galileo mission (the orbits labelled E11 though E19). At that time, we steered the spacecraft to within 200 kilometers (124 miles) of the surface of that satellite. During this orbit, we only close to within 340,000 kilometers (211,000 miles), at 2:27 a.m., but that is still close enough to detect temperature variations across the surface features.

At this point we are finally able to take a breather, and the next observation doesn't come along until 6:58 a.m., when the Solid State Imaging camera (SSI) snaps a global color image of Io, which now fits within a single SSI frame. This view includes the volcanos Pele, Zamama, and Isum, with the Prometheus volcano appearing near the limb.

At 8:54 a.m. PPR again views Jupiter, scanning again the same patch of real estate that it viewed just after midnight. By now, that portion of the planet has rotated around so that it appears in the center of the visible disk of the planet.

By 12:29 p.m. the spacecraft has receded far enough from Jupiter (out to 15 Jupiter radii, over 1 million kilometers or 666,000 miles) that the radiation environment has cooled down considerably. At this point, the electronic noise induced in the circuitry and sensors of the star scanner detector has faded, and the control software is again instructed to look for three stars to guide it, instead of the single bright star it has relied upon for the past 47 hours.

At 12:58 p.m. SSI again images Io in color, this time with the satellite filling only a quarter of the field of view. This vista will include the volcano Loki near the terminator, or day-night boundary on the dynamic body.

At 2:31 p.m. SSI snaps another picture of the small inner satellite Amalthea. This view will also include several background stars, and will be used by the Navigation Team to help refine our knowledge of the orbit of that body. This will help us fine-tune the trajectory of the spacecraft to achieve the desired fly-by of Amalthea in November 2002.

At 2:53 p.m., and again at 8:27 p.m., NIMS acquires temperature maps of the entire visible hemisphere of Jupiter. With these two views, the scientists will be able to study the dynamics of the turbulent region in the wake of the Great Red Spot, and will have a view of both the north and south auroral regions of the planet.
Finally, at 10:21 p.m., a performance test of the spacecraft gyroscopes is executed. The electronics in the gyroscope system have proven to be very sensitive to the radiation environment sensed by the spacecraft as it flies close to Jupiter. This test will determine the extent of the degradation in the circuitry due to this most recent pass, in preparation for the planning of a spacecraft maneuver coming up on Friday.

As you can see, the pace has slacked off quite a bit now, but we're still going strong, and there's more to come!


Note 1. Pacific Daylight Time (PDT) is 7 hours behind Greenwich Mean Time (GMT). The time when an event occurs at the spacecraft is known as Spacecraft Event Time (SCET). The time at which radio signals reach Earth indicating that an event has occurred is known as Earth Received Time (ERT). Currently, it takes Galileo's radio signals 41 minutes to travel between the spacecraft and Earth. All times quoted above are in Earth Received Time.

For more information on the Galileo spacecraft and its mission to Jupiter, please visit the Galileo home page at one of the following URL's:

NASA Science News

October 16, 2001

Dashing through the Snows of Io

Today, NASA's Galileo spacecraft is heading for its closest-ever flyby of Jupiter's moon Io -- an alien world where fiery volcanoes belch sulfurous snow.

Oct. 16, 2001: A little more than two months ago NASA's Galileo spacecraft flew past the north pole of Jupiter's moon Io -- and right through a swarm of sulfurous snowflakes hurled into space by a previously unknown volcano. Scientists were delighted ... and surprised. They hadn't expected the spacecraft to encounter volcanic "ash" so fresh and strange. But on restless Io, the most volcanic world in the solar system, the unexpected is ordinary.

Today Galileo is heading back to Io for another encounter -- its closest ever. At 0123 UT on Oct. 16, the spacecraft will skim just 181 kilometers above Io's surface near the moon's south pole. The close encounter will mark the sixth time Galileo has flown by Io since the spacecraft arrived in Jupiter's neighborhood in 1995.

"Io is always changing, so we're eager to learn what Galileo might show us this time," said Eilene Theilig, project manager for Galileo at NASA's Jet Propulsion Laboratory (JPL). "Maybe it will surprise us as much as it did in the last flyby."

When Galileo sped past Io's north pole on August 6, scientists were watching for activity from a polar volcano named Tvashtar, which had been spewing a plume several hundred km high only seven months earlier. But Tvashtar was quiet. Instead, the spacecraft spotted a new eruption from a previously-unknown volcano 600 km away. The plume, the tallest on record, soared approximately 500 km above Io's surface as Galileo glided through the outskirts of the billowing ejecta.

It proved a fortuitous encounter for scientists who have long sought a fresh sample of Io's volcanic material. Galileo's onboard plasma science instrument detected particles that had rushed out of a vent on the ground no more than a few minutes earlier. "This was totally unexpected," said the leader of that experiment, Louis Frank of the University of Iowa. "We've had wonderful images and other remote sensing of the volcanoes on Io before, but we've never caught the breath from one of them until now."

The particles Galileo caught weren't hot embers, but rather snowflakes. Snow on Io is made of sulfur-dioxide that condenses within high-flying plumes. Although Io's volcanic vents are very hot, much of the moon's surface is frigid (150 degrees or more below 0 C) and Io's thin atmosphere is space-cold. As soon as volcanic gases rise into the air they quickly begin to freeze. Snow forms in the plumes and frosts collect on the surface. Researchers think Galileo detected sulfur-dioxide snowflakes, each consisting of 15 to 20 molecules clumped together.

As Galileo heads for today's flyby, mission planners don't expect to sample another volcanic plume -- although on Io anything is possible. Instead, the primary goal of the encounter, like August's north polar flyby, is to collect magnetic data. Magnetic readings above Io's poles might reveal whether the satellite generates a magnetic field of its own.

Before August, "all of our previous magnetic measurements at Io had been on equatorial passes," noted Galileo project scientist Torrence Johnson in a recent Science@NASA article. "From those measurements we can't tell whether the field at Io is induced by Jupiter's strong magnetic field or produced by Io itself." Polar measurements may give enough additional information to distinguish between those two possibilities. If Io proves to have its own global magnetic field, it could mean that the moon harbors a self-sustaining magnetic dynamo deep within its core -- just as Earth does.

Discovering Io's magnetic field (if it exists) and finding its form could reveal much about the moon's internal structure. Is Io solid throughout? Or mushy and molten? Does Io's crust float above a global ocean of magma? Planetary scientists who wonder why Io's lava is so hot and how Io's non-volcanic mountains rise (and fall) would love to know the answers.

Galileo's southern flight path is not only good for magnetic readings. It will also provide opportunities to study several of Io's interesting volcanic features -- including a recently discovered hot spot in the far south and Loki, the most powerful volcano in the solar system.

The trajectory will also carry Galileo back inside the hazardous environment of Jupiter's intense radiation belts. Electronic components in Galileo's camera and elsewhere in the spacecraft have been degraded by repeated exposure to energetic-particle radiation near the giant planet. Indeed, Galileo has endured more than three times the cumulative dose of radiation it was designed to tolerate. Mission planners marvel at the craft's resiliency.

Galileo's mission was originally scheduled to end in 1997, but has been extended three times to take advantage of the spacecraft's durability. Even Galileo can't last forever, though. The craft is running low on the propellant it uses both for tweaking its trajectory and for adjusting its orientation to point its antenna. And each pass near Jupiter peppers Galileo's electronics with radiation that could eventually disable the spacecraft's sensors.

After today's encounter, say mission planners, Galileo is slated for a final flyby of Io next January -- one last chance to dash through alien snow -- followed by a visit to Jupiter's inner moon Amalthea in November 2002. Galileo's trajectory will carry it past the tiny satellite and away from the giant planet -- but not for long. Galileo's looping orbit will bring it back to Jupiter for a headlong plunge into the crushing pressure of that planet's atmosphere in September 2003. All good things come to an end ... and Galileo is no exception!

If you would like to learn more about Jupiter, Io, and the ongoing Galileo mission, please visit the Jet propulsion Laboratory's Galileo home page: JPL, a division of the California Institute of Technology in Pasadena, manages Galileo for NASA's Office of Space Science in Washington, D.C.

NASA Science News for May 19, 2000

The latest images of Io from NASA's Galileo spacecraft reveal a bizarre world of hot volcanoes, sulfurous snowfields, and slip-sliding mountains.

FULL STORY at The Secret Lives of Alien Volcanoes


May 18, 2000


Detailed analysis of Jupiter's moon Io reveals a colorful, active world full of surprises, according to five reports published in the May 19 issue of Science, and based on new results from NASA's Galileo spacecraft and Hubble Space Telescope.

The reports describe giant, erupting plumes migrating with lava flows, red and green deposits that change as unstable sulfur compounds condense from huge plumes, and mountains that may split and slide sideways for hundreds of kilometers, or miles.

Galileo observations of Prometheus reveal a volcanic field similar to Hawaii's volcanoes, but more active and much larger. Prometheus features an 80-kilometer (50-mile) tall plume of gas and particles erupting from near the end of the lava flows, like where Hawaiian flows enter the ocean. This is Io's most consistently active plume. Its size and shape have remained constant since at least 1979, but the plume location wandered about 85 kilometers (53 miles) to the west between 1979 and 1996.

"The main vent of the volcano didn't move, but the plume did," said Dr. Rosaly Lopes-Gautier of NASA's Jet Propulsion Laboratory, Pasadena, Calif., lead author of one of the reports.

"This type of behavior has never been seen on Earth," said Dr. Susan Kieffer of Kieffer Science Consulting, Inc., Ontario, Canada, lead author of a Science report. Kieffer and her colleagues suggest that the Prometheus plume is fed when a "snowfield" of sulfur dioxide and/or sulfur vaporizes under the lava flow and material erupts through a rootless conduit in the flow.

Scientists had speculated that bright red material on Io came from unstable forms of sulfur condensing from sulfur gas. By combining Galileo and Hubble Space Telescope results, scientists have learned more about the role of sulfur in Io's volcanoes. While Galileo carried out the first of three recent Io flybys in October 1999, Hubble scanned Io with its ultraviolet spectrograph to measure the composition of gases escaping from volcanoes. Hubble detected a surprise -- a 350 kilometer (220 mile) high cloud of gaseous sulfur in the plume ejected by the volcano Pele. The sulfur gas is a specific type, with sulfur atoms joined in pairs, that had never before been seen on Io; it is stable only at the very high temperatures found in the throats of Io's volcanoes. When these molecules fall onto Io's frigid surface (about -160 Celsius or -250 Fahrenheit) away from the volcanoes, they probably recombine into larger molecules with three or four sulfur atoms. The latter types of sulfur are red, so the Hubble results explain the 1,200-kilometer (750-mile) wide, red debris ring around Pele.

"These Hubble findings should help scientists understand the chemistry of Io's interior," said Dr. John Spencer of Lowell Observatory, Flagstaff, Ariz., lead author of two of the Science papers.

Galileo has found many other smaller, red patches near Io's active volcanoes, where this sulfur conversion process probably also occurs. The red deposits are found near calderas or shield volcanoes where lava first reaches the surface, often distant from plumes like Prometheus where lava flows apparently vaporize surface materials.

The composition of bright green materials on Io has been puzzling. In some places, it appears that when red material is deposited onto fresh lava flows, especially on caldera floors, it transforms into green material. It is possible that the surfaces are still warm, which accelerates the transformation of the red types of sulfur and the sublimation of sulfur dioxide. Eventually both red and green materials acquire the pale yellow color that is characteristic of ordinary yellow sulfur, made of rings of eight sulfur atoms.

Although Io is the most volcanically active body in the solar system, the mountains (up to 16 kilometers or 10 miles high) are not volcanoes. They have no volcanic vents or flows; instead, they appear to be giant tilted blocks of crust. Giant depressions on Io are thought to be calderas formed by collapse over empty magma chambers. Unlike Earth's calderas, many Io depressions have very straight margins, sharp corners, and are located next to mountains. In new images of the Hi'iaka Patera depression and adjacent mountains, it looks as though two mountain blocks have split and slid apart by 145 kilometers (90 miles), forming a pull-apart basin like California's Death Valley or Salton Sea. This is surprising because such large-scale lateral movements on Earth are caused by plate tectonics, but there are no indications of a similar process on Io.

"We consider it more likely that lateral movements may be driven by deep 'mantle plumes' of rising hot rock masses within Io," said Dr. Alfred McEwen of the University of Arizona, Tucson, lead author of one of the papers.

New images are available at

Galileo has been studying Jupiter and its moons for 4-1/2 years. It completed a two-year primary mission in December 1997 and a two-year extended mission in December 1999. Galileo is continuing its studies under yet another extension, the Galileo Millennium Mission. On Sat., May 20, the spacecraft will fly by Jupiter's moon Ganymede, the largest moon in the solar system, for the first time since May 7, 1997. JPL, a division of the California Institute of Technology in Pasadena, manages the Galileo mission for NASA's Office of Space Science, Washington, D.C.

University of Arizona

18 May 2000

Galileo's sharp new pictures of Io detail amazing volcanism

By Lori Stiles Images: Planetary Imaging Research Laboratory

When the Galileo spacecraft flew close by Jupiter's moon, Io, late last year and early this year, it took sharpest-ever pictures of the best volcanic show in the solar system.

Galileo took more than 100 high-resolution images during Io flybys on Oct. 11, Nov. 26 and Feb. 22. Results include pictures of an active lava flow as long as the Grand Canyon, a mile-high curtain of burning lava, a unique long-lived "wandering" volcanic plume, and a 6-mile-diameter lava lake that shines steady as a beacon. The Galileo imaging team reports the results in the May 19 issue of Science.

"Io is absolutely fascinating because every single thing we see is completely new and completely unexpected," said Jani Radebaugh, a graduate student at the University of Arizona who helps analyze the photographs. "You can take what you know about volcanology and about planetary processes, but in every picture we see, there's something new going on that we don't understand. There's something brand new, every time."

"Io is like a laboratory for large-scale volcanic experiments," said Alfred S. McEwen of the Galileo imaging team. McEwen directs the Planetary Imaging Research Laboratory (PIRL) at the UA Lunar and Planetary Lab (LPL). "We don't have controlled laboratories big enough to study these processes. And large-scale volcanic fields on Earth are very modified. So here's a place we can watch the changes and really understand how this activity might happen at this scale. With that insight, we can go back and reconsider what happened in Earth's past."

"I like Io as a model of what might be going on under the crust of Europa," said Paul Geissler, senior research associate at LPL and member of the Galileo imaging team. "The whole (jovian moon) system is a great exercise in comparative planetology," Geissler said. "You have four separate moons, separated at birth, and they've grown up completely differently. So it's very important to study them from that point of view."

The Galileo imaging scientists are trying to understand how Io's extraordinary volcanism fits within the big geologic picture of the moon and the jovian system. They'd like to discover the composition of Io's crust. They'd like to know if Io's interior holds a magma ocean -- a question images alone won't answer. They use Galileo to explore how volcanoes erupt and modify the landscape, what the lavas are made of, and how Io's strange terrain forms and evolves.

Results discussed in the Science article include --

The sharp new Galileo photos show that several mountains have calderas cut into their sides. The photos also suggest that mountains and calderas are geologically related in other ways. For example, Turtle said, rising plumes of new material in Io's mantle may concentrate stresses acting in Io's crust, causing it to fail, forming mountains by thrust faulting. As the mountains are tilted up, layers of weak, sulfur-rich material deposited by Io's plumes may fail, resulting in tremendous landslides.

From a mosaic of new pictures of the lava-filled depression called Hi'iaka Patera and its two bordering mountains, UA graduate student Windy Jaeger speculates that Io's crust there might have been laterally pulled apart. It may be only coincidence that the north and south parts, if pushed together, would fit together like pieces of a puzzle, Jaeger said. At this point, evidence is only circumstantial.

But if Hi'iaka Patera was once pulled apart, McEwen said, "that would be very surprising, because on Earth a movement on that scale is associated with plate tectonics. But we see no evidence for plate tectonics on Io. There again might be some unique Ionian process involved."

Caught in the act

W.M. Keck Observatory

12 January 2000


MAUNA KEA, Hawaii -- The successful installation and operation of a 21-st century adaptive-optics system at the W.M. Keck Observatory has ushered in a new era of ultra high-resolution astronomical imaging capability.

Recent observations with the Keck adaptive-optics (AO) system have produced the highest spatial-resolution images ever obtained from ground or space by an optical-infrared telescope. Funded by the W.M. Keck Foundation, the $7.4 million AO system was installed on the 10-meter Keck II telescope in February 1999. A clone of the instrument will be installed on the twin Keck I telescope this year.

While adaptive optics previously have been demonstrated on 4-meter class telescopes, the Keck AO system is the first to be installed on the new generation of 8-10 meter telescopes -- thus taking advantage of the larger apertures' diffraction limit.

The Keck AO system senses and adjusts for distortions in starlight introduced by the Earth's atmosphere. The system splits off the optical portion of the light to sense the distortion while transmitting the corrected infrared light to the science instrument. Corrections are sent to a flexible mirror at up to 670 times per second.

The resultant images have an improved resolution more than tenfold -- from about half an arc-second to tens of milli-arc seconds.

"The Keck AO system has opened up new astronomical frontiers by realizing the full angular resolution capability of a 10-meter telescope combined with its light-gathering power," said Keck optics manager Dr. Peter Wizinowich. "We can now observe objects at a level of detail 10 to 20 times higher than without an AO system."

Since AO first light, astronomers at Keck have used the system to image Neptune, Saturn's moon Titan, the asteroid Vesta, Jupiter's volcanic moon Io; explore binary-star systems previously beyond resolution, the heart of our Milky Way galaxy, and distant galaxies. The recent images of Io caught an apparent limb eruption and the surface albedo variations are clearly visible. A series of images of Vesta have been made into a movie that clearly shows the asteroid's rotation. (See images.)

The first phase of the Keck AO system currently uses natural guide stars to sense atmospheric distortion. An artificial guide-star system using a sodium-wavelength laser designed by a team from Lawrence Livermore National Lab will open the full sky to AO observations when it comes online in the next year. Currently less than 10 percent of the sky has a guide-star bright enough for AO correction.

The Keck AO system now uses an engineering-grade camera (K-Cam) with a 256x256 indium-antimonide detector for science imaging. The addition of the Near Infrared Spectrometer (NIRSPEC) and the next-generation Near Infrared Camera (NIRC 2) this year with their 1024 x 1024 InSb arrays and improved sensitivities will add powerful new tools to the Keck II repertoire.

The installation of the Keck I AO system later this year will be an important step toward combining the light of the two Keck telescopes to achieve even higher resolution.

"We are currently working with JPL (Jet Propulsion Laboratory) to build an interferometer that will have 10 times the angular resolution of an individual Keck telescope," said Wizinowich.

The Keck Adaptive Optics Team members included Scott Acton, John Gathright, Olivier Lai, William Lupton, Chris Shelton, Kevin Tsubota and Peter Wizinowich at Keck Observatory; and Jong An, Ken Avicola, Herb Friedman, Don Gavel, Erik Johansson, Bruce Macintosh, Scot Olivier and Claire Max at Lawrence Livermore National Laboratory.


At top, the Galileo satellite image of Io from the proximity of a Jovian orbit shows clearly the dark spots on the moon, which are clearly identifiable in the Keck II telescope/Adaptive Optics image of Io at top right. Note the bright flare on the upper left limb of the moon, an apparent eruption from the volcanically active moon. The three images at the bottom are the various wavelengths in the infrared that were combined and computer processed to make the final image. Wavelengths: J is 1.25 microns, K' is 2.12 microns, H is 1.6 microns. (Visible wavelengths of light are around a half micron). The resolution of Io is on the order of 0.040 arc-seconds, the most detailed image ever made of Io from the Earth. Credit: W.M. Keck Observatory/Adaptive Optics team

The images were captured in late November, 1999.

Lowell Observatory
1400 Mars Hill Road
Flagstaff, Arizona 86001

January 2000

Lowell Observatory Astronomer Spots Eruption on Io while Galileo Captures it in Space

On November 25, 1999, Jupiter expert Dr. John Spencer of Lowell Observatory, working with astronomer Glenn Orton of the Jet Propulsion Laboratory at NASA's Infrared Telescope Facility at Mauna Kea, Hawaii, discovered a large volcanic eruption on Io, one of Jupiter's satellites. Their infrared photo of the eruption was taken just hours after a close Io flyby by NASA's Galileo spacecraft. Spencer's Earth-based observations over many years show that this type of eruption occurs only about 20% of the time anywhere on Io, but by amazing good luck, Galileo caught a close-up view of the eruption in its narrow field of view as it flew past.

The Galileo spacecraft observed the fiery lava fountain shooting more than a mile above the moon's surface. The images show a curtain of lava erupting within a giant volcanic crater. "Catching these fountains was a one-in-500-chance observation," said Galileo scientist Dr. Alfred McEwen from the University of Arizona in Tucson.

Spencer was delighted but not entirely surprised by what Galileo saw. He and former Lowell Observatory astronomer John Stansberry had inferred the existence of lava fountains on Io from their earlier Earth-based observations, but, he said, "it's incredible to see that idea confirmed so spectacularly by Galileo." The fact that the same eruption was recorded by spacecraft as well as by earth-based observing is a boon to the research being done on Io. By combining the data, scientists have their best chance ever to pin down temperatures of the extremely hot lava on Io.

The attached is a false-color infrared image of the sunlit disk of Jupiter's moon Io. The bright spot at the 1 o'clock position is the same lava fountain seen close-up by Galileo's camera, but in this case it is seen from Earth at a distance of 630 million kilometers (390 million miles).

The Galileo images are available at

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

December 17, 1999


During a recent close flyby of Jupiter's moon Io, NASA's Galileo spacecraft observed a fiery lava fountain shooting more than a mile above the moon's surface.

The images, showing a curtain of lava erupting within a giant volcanic crater, will be unveiled today during the American Geophysical Union's fall meeting in San Francisco. Galileo took the pictures on Thanksgiving night, November 25.

"We've finally caught a close-up of a massive volcanic eruption in action on Io," said Galileo project scientist Dr. Torrence Johnson of NASA's Jet Propulsion Laboratory, Pasadena, CA. "The erupting lava was so hot and bright, it over-exposed part of the camera picture and left a bright blur in the middle."

These lava fountains were hot enough and tall enough to be observed by the NASA Infrared Telescope atop Mauna Kea, HI. By combining data from this telescope and Galileo observations, scientists have their best chance ever to pin down temperatures of the extremely hot lava on Io.

The images show a region of giant calderas, or crater depressions, in Io's northern latitudes. They came from two of Galileo's onboard instruments -- the camera and near-infrared mapping spectrometer, which observes wavelengths invisible to the unaided eye.

Lava fountains provide the most spectacular volcanic show on Earth, although the fountains found in Hawaii and elsewhere on Earth rarely exceed a few hundred yards in height. Because their appearances are infrequent and brief, it is very difficult to target these events. "Catching these fountains was a one-in-500-chance observation," said Galileo scientist Dr. Alfred McEwen from the University of Arizona in Tucson.

New results from the most powerful volcano in the solar system, Loki, will also be discussed at the press conference. These include recent observations of Io by infrared telescopes in Hawaii and Wyoming, and two other Galileo instruments, the photopolarimeter radiometer and near-infrared mapping spectrometer. These data show large changes in the output of heat at Loki over time, with huge portions of the lava surface appearing to be of a uniform temperature.

The telescope observations show that Loki began a period of major eruption in early September, and Galileo caught the eruption in full force during its October flyby of Io. While observing Loki's 120-mile (193-kilometer) wide caldera, one Galileo instrument found a sharply defined region that was much hotter than the rest.

"We think the hot region is the site of the eruption that began in September," said Dr. John Spencer of Lowell Observatory, Flagstaff, AZ, a co-investigator for the photopolarimeter radiometer, which maps surface temperatures by measuring heat radiation. "Eventually the new lava may spill out to cover the rest of the caldera."

The Io flybys were challenging and risky, because Io lies in an area of intense radiation from Jupiter's radiation belts, and radiation can harm spacecraft components. In fact, radiation- related problems garbled some of the pictures taken by Galileo during its October 10 Io flyby. Galileo team members thought the images were a lost cause, but engineers at JPL's Measurement Technology Center were able to fix them with the help of LabVIEW software from National Instruments in Austin, TX.

"It would be like watching a scrambled cable signal on television, and then using software to unscramble the signal," Johnson said. "JPL engineers had to break the code that was inadvertently introduced by the radiation near Io."

"They only had one-fourth of the data needed to reconstruct the images," said Dr. Laszlo Keszthelyi, a Galileo research associate at the University of Arizona. "These guys found a way to intelligently guess the missing bits. It seemed to be mathematically impossible, but they pulled it off."

The new Io images are available at:

Additional information and pictures taken by Galileo are available at:

NASA Space Science News for November 19, 1999

A Volcanic Flashback on Io -- With another Io flyby less than a week away, JPL releases new data showing towering mountains, sizzling hot spots, and enormous lava lakes on Jupiter's fiery moon.

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

Nov. 19, 1999


Jupiter's fiery moon Io is providing scientists with a window on volcanic activity and colossal lava flows similar to those that raged on Earth eons ago, thanks to new pictures and data gathered by NASA's Galileo spacecraft.

The sharp images of Io were taken on Oct. 11 during the closest-ever spacecraft flyby of the moon, when Galileo dipped to just 380 miles (611 kilometers) above Io's surface. The new data reveal that Io, the most volcanic body in the solar system, is even more active than previously suspected, with more than 100 erupting volcanoes.

"The latest flyby has shown us gigantic lava flows and lava lakes, and towering, collapsing mountains," said Dr. Alfred McEwen of the University of Arizona, Tucson, a member of the Galileo imaging team. "Io makes Dante's Inferno seem like another day in paradise."

Ancient rocks on Earth and other rocky planets show evidence of immense volcanic eruptions. The last comparable lava eruption on Earth occurred 15 million years ago, and itŐs been over 2 billion years since lava as hot as that found on Io (reaching 2,700 degrees Fahrenheit) flowed on Earth.

"No people were around to observe and document these past events," said Dr. Torrence Johnson, Galileo project scientist at NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA. "Io is the next best thing to traveling back in time to Earth's earlier years. It gives us an opportunity to watch, in action, phenomena long dead in the rest of the solar system."

The new data focus on three of Io's most active volcanoes -- Pele, Loki and Prometheus. The vent region of Pele has an intense high-temperature hot spot that is remarkably steady, unlike lava flows that erupt in pulses, spread out over large areas, and then cool over time. This leads scientists to hypothesize that there must be an extremely active lava lake at Pele that constantly exposes fresh lava. Galileo's camera snapped a close-up picture showing part of the volcano glowing in the dark. Hot lava, at most a few minutes old, forms a thin, curving line more than six miles (10 kilometers) long and up to 150 feet (50 meters) wide. Scientists believe this line is glowing liquid lava exposed as the solidifying crust breaks up along the caldera's walls. This is similar to the behavior of active lava lakes in Hawaii, although Pele's lava lake is a hundred times larger.

Loki, the most powerful volcano in the solar system, consistently puts out more heat than all of Earth's active volcanoes combined. Two of Galileo's instruments -- the photopolarimeter radiometer and near-infrared mapping spectrometer -- have provided detailed temperature maps of Loki. "Unlike the active lava lake at Pele, Loki has an enormous caldera that is repeatedly flooded by lava, over an area larger than the state of Maryland," said Dr. Rosaly Lopes-Gautier of JPL, a member of the spectrometer team.

Observations of Prometheus made early in the Galileo mission showed a new lava flow and a plume erupting from a location about 60 miles (100 kilometers) west of the area where the plume was observed in 1979 by NASA's Voyager spacecraft. New Galileo data clarify where lava is erupting, advancing, and producing plumes. The most unexpected result is that the 50-mile (75 kilometer) tall plume erupts from under a lava flow, far from the main volcano. The plume is fed by vaporized sulfur dioxide-rich snow under the lava flow.

Mountains on Io are much taller than Earth's largest mountains, towering up to 52,000 feet (16 kilometers) high. Paradoxically, they do not appear to be volcanoes. Scientists are not sure how the mountains form, but new Galileo images provide a fascinating picture of how they die. Concentric ridges covering the mountains and surrounding plateaus offer evidence that the mountains generate huge landslides as they collapse under the force of gravity. The ridges bear a striking resemblance to the rugged terrain surrounding giant Olympus Mons on Mars.

Scientists hope to learn more about dynamic Io when Galileo swoops down for an even closer look on Nov. 25 from an altitude of only 186 miles (300 kilometers). Because Io's orbit is bathed in intense radiation from Jupiter's radiation belts, there is a risk of radiation damage to spacecraft components. In fact, several spacecraft systems sustained damage during the October flyby. Given these radiation risks, the Io flybys were scheduled near the end of the spacecraft's two-year extended mission.

New Io images taken by the spacecraft are available at:

Galileo entered orbit around Jupiter and its moons on Dec. 7, 1995, for a two-year prime mission. JPL manages the Galileo mission for NASA's Office of Space Science, Washington, DC. JPL is operated for NASA by the California Institute of Technology, Pasadena, CA.


November 4, 1999


A volcanic crater several times larger than one found at Hawaii's Kilauea volcano has been photographed on Jupiter's moon Io during a close flyby performed by NASA's Galileo spacecraft.

"It appears that the Prometheus volcano on Io has characteristics remarkably similar to those of the Kilauea volcano in Hawaii, although Prometheus is much larger," said Dr. Laszlo Keszthelyi (KEST-ay), a Galileo research associate at the University of Arizona, Tucson, AZ. "Both volcanoes are long- lived eruptions, with flows that apparently travel through lava tubes and produce plumes when they interact with cooler materials."

The sharp images of Prometheus released today come from two of Galileo's onboard instruments -- the camera, and the near- infrared mapping spectrometer which observes in wavelengths not visible to the naked eye. The images were taken during the close flyby of Io by Galileo on October 10, 1999, and are part of a large batch of data currently being transmitted to Earth.

"We've been having a feast looking at the material from Io," said Dr. Rosaly Lopes-Gautier NASA's Jet Propulsion Laboratory, Pasadena, CA. "We have been waiting for such high-resolution images of Io for more than 10 years." Scientists will present an assortment of new images and describe their latest discoveries at a press briefing scheduled for November 19 at NASA Headquarters, Washington, DC.

Prometheus is the "Old Faithful" of Io's many volcanoes. It has been active during every observation over the past 20 years by NASA's Voyager and Galileo spacecraft and the Hubble Space Telescope. The new spectrometer images show two distinct hot spots at Prometheus -- a large one to the west and a fainter, cooler one to the east. The images reveal numerous lava flows near the western hot spot and enable scientists to identify a crater, or caldera, 28 kilometers (17 miles) long and 14 kilometers (9 miles) wide near the hot spot to the east.

Previously, it was thought that the 50 to 100 kilometer- (30 to 60 mile-) tall plume observed at Prometheus formed where the lava erupts onto the surface. Now, however, it now appears that the plume forms at the far end of the lava flows. The caldera and eastern hot spot are thought to be associated with the vent where the molten rock rises to the surface. It appears that after the lava reaches the surface, it is transported westward through lava tubes for about 100 kilometers (60 miles) before breaking out onto the surface again. Here, numerous lava flows wander across a plain covered with sulfur dioxide-rich snow. The plume is created by the interaction of the hot lava with the snow.

This plume feature is just one of several similarities between Prometheus and Hawaii's Kilauea. Volcanologists say that Prometheus has been erupting for more than 20 years and Kilauea has been erupting for more than 16 years. The current vent at Kilauea consists of a small lava lake about 100 meters (330 feet) across that produces a relatively small thermal hot spot. From this vent, lava is transported 10 kilometers (6 miles) in lava tubes to the Pacific Ocean where large steam plumes are generated by the interaction between the hot lava and the ocean. Galileo scientists believe the plume seen on the western end of Prometheus is similar to this Hawaiian steam plume, except the Ionian plume is composed largely of sulfur dioxide and rises much higher because of Io's low atmospheric density and gravity.

Another Io flyby, this time at an altitude of 300 kilometers (186 miles), is planned for November 25 at 8:40 p.m. Pacific Time (11:40 p.m. Eastern Time). (Times given are in Earth-received time -- or the time when the signal of the event is received on Earth.) The Io flybys are challenging and risky, because Io lies in an area of intense radiation from Jupiter's radiation belts, and that radiation can harm spacecraft components. Because of the risk, the flybys were scheduled for the final portion of Galileo's extended mission.

The new Io images are available at Additional information and pictures taken by Galileo are available at the mission homepage.

The spacecraft has been orbiting Jupiter and its moons for nearly four years, with its primary mission running from December 1995 until December 1997, followed by its current two-year extended mission. JPL manages the Galileo mission for NASA's Office of Space Science, Washington, D.C. JPL is operated for NASA by the California Institute of Technology, Pasadena, CA.

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.

The Brown University News Bureau

July 2, 1998


PROVIDENCE, R.I. -- Hundreds of millions of miles from the sun, volcanoes on Jupiter's moon Io sizzle at the highest recorded surface temperatures of any planetary body in the solar system. Planetary scientists from University of Arizona, Brown University and five other institutions report this finding in the cover story of the July 3 issue of the weekly journal Science.

The report provides an important clue to understanding geophysical processes within Io, which may be similar to the early stages in the evolution of Earth, Venus and other planetary bodies.

"The very hot lavas erupting on Io are hotter than anything that has erupted on Earth for billions of years," says lead author Alfred McEwen, director of the Planetary Image Research Lab at the University of Arizona. "They are the highest surface temperatures in the solar system other than the sun itself."

At least 12 different vents on Io spew lava at temperatures greater than 2,200 degrees Fahrenheit. One volcanic vent may be as hot as 3,100 degrees Fahrenheit -- about three times hotter than the hottest sunlit surface of Mercury, the closest planet to the sun. The surface temparatures on Io, which is 1,245 million miles from the sun, stay well below freezing (minus 243 degrees Fahrenheit) except for the volcanic hot spots.

The latest temperature measurements are more than double the highest temperatures recorded by the Voyager spacecraft in 1979 and also exceed more recent measurements made by telescopes.

McEwen and his colleagues calculated the temperatures of Io's volcanoes using two instruments on the Galileo spacecraft. The instruments read the infrared "signatures" of the volcanic vents, which emit light beyond the color red, which is the longest wavelength visible to the human eye. Scientists calculated the lava temperatures needed to fit the infrared signatures. These lava temperatures and the visible color properties of the dark flows are consistent with lava compositions rich in heavy elements like magnesium.

Io's neighbors turn up its internal thermostat. Neighboring moons Europa and Ganymede pull Io into an elliptical orbit, so that Io passes close and then swings farther away from Jupiter. During its orbit, Io actually changes shape slightly, molded by the massive gravitational forces of Jupiter at different distances. Just as metal heats up when it's bent back and forth, scientists believe Io heats up when it changes shape.

"It's almost as if Io is being kneaded by the tidal interactions between Jupiter and the other moons," says co-author James Head, professor of geological sciences at Brown University.

In its chilly corner of the universe, Io needs to release its inner heat, just as a cup of hot coffee cools by releasing steam. Scientists have known for a while that Io is the solar system's most volcanically active planetary body. Yet scientists were surprised by the extreme temperatures.

The findings raise new questions about the composition and evolution of Io. For example, the hot temperatures suggest that the lava is composed of dense material that tends to sink, not rise, within a planet. Typically, lighter material in a volcanically active planetary body tends to melt first and rise to the surface where it cools and forms a crust. The process is called differentiation.

"Given Io's intense vulcanism, we expect extreme differentiation," McEwen says. "The evidence suggests we're seeing heavy magma erupt to the surface. How do we explain that? It's harder for dense material to rise through a low-density crust, although this has occurred on Earth's moon. Perhaps some process mixes the crust back into Io's interior, so the crust has a higher density."

On Earth, the tectonic plates move slowly around the surface, forming new crust at mid-ocean ridges, for example, and recycling oceanic crust into the hot mantle where two plates collide, one diving under the other. Scientists don't know yet how to explain what's happening on Io.

"We have a lot of the same questions about early Earth," McEwen says. "Early Earth is hard to understand because the evidence has been so degraded by an active environment and plate tectonics. I like to think of Io as a grand experiment in planetary vulcanism and differentiation. This experiment may ultimately help us to understand the evolution of Earth and other planets, such as Venus and Mars."

More information:

The Io image on the cover of Science shows about 20 dark volcanic vents, some also shaded red by sulfur. It is available at


At 3 km/pixel, this is the highest resolution color picture of Io yet taken by Galileo. The satellite is seen against a backdrop of Jupiter's cloud tops, which appear blue in this false-color composite constructed from 1-micron, green and violet filter images. Among the surprises that can be seen on the surface of Io are several small, distinctly greenish patches, and subtle violet hues at the cores and margins of bright SO2-rich regions like the one in the lower right. Dark spots, many flagged by bright red pyroclastic deposits, mark the sites of current volcanic activity. Most of Io's riotous color can be explained by sulfur compounds, but the dark materials that make up the flows and calderas are probably silicates. (Image processed by Paul Geissler, University of Arizona)

New images of Io taken by the Galileo spacecraft are available on the Galileo home page


July 2, 1998


New observations by NASA's Galileo spacecraft reveal dozens of volcanic vents on Jupiter's fiery moon Io where lava sizzles hotter than any surface temperatures recorded on any planetary body in our solar system. Temperatures this high are not known to have occurred on Earth for billions of years. At one such volcanic vent, known as Pillan Patera, two of Galileo's instruments have indicated the lava temperature may be 2,000 Kelvin (3,140 degrees Fahrenheit). These results are reported in the July 3 issue of the journal Science.

"The most likely explanation for these very high temperatures is that the eruptions contain magnesium-rich silicates," said Dr. Alfred McEwen of the University of Arizona, Tucson, AZ, a member of Galileo's solid state imaging camera team. "We've tentatively identified magnesium-rich orthopyroxene in lava flows around these hot spots. This leads us to conclude that silicate volcanism is taking place with lava compositions expected to melt at a very high temperature. We must now think of Io's volcanoes in terms of the type of very high-temperature silicate volcanism which was found on Earth during its early days, and which we suspect occurred also on Venus and Mars."

The new findings by the Galileo camera and the spacecraft's near infrared mapping spectrometer have updated scientists' information on Io's volcanic processes. Previously, Io observations made by the Voyager spacecraft in 1979 put the highest temperature estimates at about 650 Kelvin (710 degrees Fahrenheit). This led many scientists to believe that Io's volcanic activity was caused by low-temperature sulfur volcanism. In 1986, ground-based telescope observations increased the temperature estimates to above 900 Kelvin (1,160 degrees Fahrenheit), which suggested that silicate volcanism was occurring at least occasionally, just as it does on Earth today. In 1996 and 1997, Galileo identified 30 locations with temperatures higher than 700 Kelvin (800 degrees Fahrenheit).

"This new data indicate that high-temperature eruptions on Io are a basic and common part of its active volcanic processes," said Dr. Torrence Johnson of JPL, Galileo project scientist. Johnson led the group that found the high temperature eruption in 1986. He is also a member of the near infrared mapping spectrometer team. "Io's current volcanic activity may have a lot in common with ancient volcanic processes on Earth and other planets. Since the geologic record from those times is very sparse, it's quite exciting to be able to study this type of volcanism going on today."

"This discovery of high-temperature silicate volcanism provides us with an extremely important clue to understanding the geophysical processes within Io," McEwen explained. Io is heated by periodic tides as it orbits Jupiter, along with the other Galilean satellites (Europa, Ganymede and Callisto).

Armed with this new information, scientists also hope to learn more about the composition of Io's crust. "Io's extreme volcanic activity is expected to result in a low-density crust rich in silica, sodium and potassium," said McEwen. "However, the high-temperature volcanism suggests that the crust may be composed of heavier lavas."

Galileo's solid state imaging camera observed Io during 11 eclipses in 5 orbits, when Io was in Jupiter's shadow, and sunlight was blocked so the camera could better see the glowing volcanic vents. Io's hot spots were also studied by the spacecraft's near infrared mapping spectrometer during 11 orbits, mostly when Io was not in eclipse. The camera provides high spatial resolution to image the hottest features and map color variations, while the spectrometer can observe at many wavelengths and is sensitive to a wider temperature range. Thus, the combination of both instruments provides a powerful means to study Io's volcanism. The camera and spectrometer together have discovered a total of 41 hot spots on Io.

Scientists hope to gather more detailed information about Io with two planned close flybys in late 1999, as long as the Galileo spacecraft remains healthy. Galileo has been orbiting Jupiter and its four largest moons, including Io, for 2-1/2 years. It is currently in the midst of an extended journey, known as the Galileo Europa Mission, with eight flybys of Europa and four of Callisto, in addition to the Io flybys.

Galileo Europa Mission is managed by JPL, a division of California Institute of Technology, Pasadena, CA.

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