Taking the Scenic Route to Io

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Galileo zoomed by Callisto this morning in the second of four encounters designed to bring the spacecraft closer to Jupiter's volcanic moon Io

June 30, 1999: This morning at 07:47 UT, NASA's Galileo spacecraft zoomed past Jupiter's moon Callisto at a distance of only 1,047 km. JPL reports that the encounter was a success and that the spacecraft is operating normally.

The main purpose of today's flyby was to modify Galileo's orbit and bring the craft closer to Io, the innermost of Jupiter's large satellites. Io is one of the most exotic places in the solar system. It is literally bursting with volcanoes that spew sulfurous plumes over 300 km high. One called Prometheus may have been active for at least 18 years! In October or November, after a series of four orbit-changing encounters with Callisto (today's was the second), Galileo is scheduled to make two daring close approaches to Io, possibly flying through a volcanic plume.

Although the most dramatic flybys of Io won't take place until later in 1999, scientists will get a closer look at the volcanic moon later this week when the spacecraft passes it at a distance of just 127,000 km. Galileo was within just 900 km of Io in December 1995 but the spacecraft wasn't taking pictures at that time, so this week's encounter may provide some of the best ever pictures of Io's volcanoes.

Left: Ron Baalke and David Seal of NASA's Jet Propulsion Laboratory have prepared several computer-generated animations of today's Callisto flyby. Click here for viewing options.

"We'll also have the first real passage since 1995 through the outer edge of the Io torus during this orbit," says JPL's Dr. Torrence Johnson, the chief scientist for the Galileo mission.

The Io torus is a gigantic ring of ionized gas circling Jupiter formed by sulfurous material ejected from Io's volcanoes. With a diameter the size of Io's orbit it spans 844 thousand km and has an important impact on Jupiter's magnetic environment. As Io moves along its orbit and through this magnetized plasma torus, a huge electrical current flows between Io and Jupiter. Carrying about 2 trillion watts of power, it's the biggest DC electrical circuit in the solar system.

"Galileo will spend nearly a day inside the edge of the torus in an overall region of space between Io and Europa," noted Johnson. "We've been there once before in 1995. We know generally what the Io torus is and what it's made of. Now we're going to look at its detailed structure, to investigate what sort of physics is going on."

Although at this point in the Galileo mission Callisto is primarily a turning point on the road to Io, it is also of considerable interest to scientists.

With a diameter of 4,800 km, Callisto is nearly the same size as the planet Mercury. Its icy surface is the most heavily cratered place in the Solar System, but there are no volcanoes or even any large mountains. It is thought that little has happened to alter the surface for billions of years, other than occasional impacts with asteroids, comets, and other interplanetary debris.

Right: Callisto's many craters are apparent in this contrast-enhanced image taken by the Galileo spacecraft in 1997. Visible near the image center is Valhalla, one of the largest impact craters in the Solar System, measuring about 4,000 kilometers across.

"One of the biggest mysteries about Callisto involves its craters," says Torrence Johnson. "As on other cratered bodies, the craters on Callisto come in all sizes. There are a few really large craters, like Valhalla, and then as you look at smaller and smaller impact features there are more and more of them. On Callisto, craters smaller than about 1 km seem to have been partially obliterated, or 'disaggregated' by some unknown process. On planets like Mars or Earth where there's been weathering or erosion, the obliteration of small craters is expected. But Callisto doesn't have a substantial atmosphere or any obvious sources of erosion, so what's happening to these craters? It's a real puzzle.

"The disaggregation in small craters on Callisto is a theme for all four flybys leading up to the Io encounters," he continued. "All of the science instruments will be online during the flyby. The Solid State Imaging Camera will take high resolution pictures that we can use to study cratering statistics. We'll also be using the Near-infrared Mapping Spectrometer (NIMS) and the UV spectrometer to get a handle on the chemical composition of the surface. We really want to examine places on Callisto that we haven't looked at before to see if the process that wipes out the small craters might be related to big impact features or to variations in the composition of the terrain."

The UV spectrometer will also peer at Callisto's limb during the flyby in hopes of gleaning more information about the satellite's tenuous carbon dioxide atmosphere. Galileo's Dust Detector will be active, too, making measurements of dust particles around Callisto that might be ejecta from ongoing impact events.

Another interesting story lies beneath Callisto's surface. Galileo magnetometer data released in 1998 indicate that Callisto, like another of Jupiter's moons Europa, may harbor an underground ocean. Callisto has a magnetic field that fluctuates in time with Jupiter's rotation. So far, the best explanation for Callisto's peculiar magnetism is an underground layer of melted ice. If the liquid is salty like Earth's oceans, it can carry sufficient electrical currents (induced by Jupiter's powerful rotating magnetic field) to produce a fluctuating magnetic field around Callisto.

Above: Voyager and Galileo have returned tantalizing evidence for a liquid water ocean beneath the surface of Europa. Now researchers are reporting telltale indications that the battered Jovian moon Callisto may also harbor a subsurface ocean. This cutaway view of Callisto shows a whitish 200 kilometer thick band of ice just beneath the moon's surface. The hypothetical ocean - indicated by the underlying light blue stripe - is potentially a salty layer of liquid water up to 10 kilometers thick, while the rest of the interior is seen as a jumble of rock and ice. More information...

Scientists are intrigued by the prospects of an ocean on Callisto because liquid water is a prerequisite for life as we know it on Earth.

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"The basic ingredients for life -- what we call 'pre-biotic chemistry' -- are abundant in many solar system objects, such as comets, asteroids and icy moons," explained Dr. Torrence Johnson in 1998. "Biologists believe liquid water and energy are needed to actually support life, so it's exciting to find another place where we might have liquid water. However, energy is another matter, and Callisto may not have enough. Callisto's ocean is only being heated by radioactive elements, whereas Europa has tidal energy as well. That makes Europa a better prospect for life."

There are many questions about Callisto, and scientists hope that the latest series of Galileo flybys will provide some answers. The next two flybys of Callisto are scheduled for August 14 and September 16, 1999, when the spacecraft's orbit will be further altered to bring it closer to Jupiter and Io. Science@NASA will continue to cover Galileo's exploits as the spacecraft heads for its dramatic encounters with Io's volcanoes on October 11 and Nov. 26, 1999.

Above: Scientists are curious about the differences between Callisto and another of Jupiter's satellites, Ganymede. The two are similar in size and mass, yet appear to have a very different geology and internal structure. Galileo data indicate that on Ganymede all the rock and heavy metals that make up the satellite have separated from the ice completely, forming a dense inner core. Callisto is different. There is a shell of differentiated material near the surface (extending to a depth of few hundred km), but inside it's mixed rock and ice. Both Callisto and Ganymede are covered by huge tracts of relatively dark, heavily cratered terrains, shown above at similar image resolution and solar illumination. Ancient dark terrain on Ganymede is complex featuring scarps, furrows, and domes. Dark terrain on Callisto rarely shows these features. Callisto appears to have had a much simpler geologic history than its nearby neighbor Ganymede.

Web Links

Ice, Water and Fire the Galileo Europa Mission

Galileo home page at JPL, with the latest on Europa, Callisto and Io

Jet Propulsion Laboratory home page

Io from the SEDS Nine Planets web site

Callisto from the SEDS Nine Planets web site

Jupiter from the SEDS Nine Planets web site

Io: The Prometheus Plume Aug. 18, 1997 Astronomy Picture of the Day

Close-up of an Io volcano Aug. 4, 1995 Astronomy Picture of the Day

Sizzling Io July 6, 1998 Astronomy Picture of the Day

Related Stories:

Galileo buzzes Europa -- Feb. 2, 1999. Galileo executes a close flyby of Europa for the last time during the current mission.

The Frosty Plains of Europa -- Dec. 3, 1998. As Galileo returns new images of Europa, NASA scientists prepare to study samples from a potentially similar environment here on Earth.

Callisto makes a big splash -- Oct. 22, 1998. Scientists may have discovered a salty ocean and a possible ingredient for life on Jupiter's moon.

Galileo takes a close look at icy Europa -- Oct 2, 1998. The spacecraft flew within 2300 miles of the mysterious satellite last weekend.

Clues to possible life on Europa may lie buried in Antarctic ice -- Mar. 5, 1998. Exotic microbial forms turn up in ice above Antarctica's Lake Vostok.

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