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Another half-dozen moons were announced to be orbiting Jupiter by a team at the University of Hawaii including Scott Sheppard and David Jewitt and Cambridge's Jan Kleyna. Jupiter has 58 moons as of this writing, but Jewitt estimates that perhaps 100 can be found with the current method of wide-field mosaic CCD's with today's telescopes and cameras. So the number of moons quoted will go up by a few with some regularity. These newly discovered are mainly in retrograde orbits, and form a class of moon different from the Galileo satellite or other, inner moons. They were presumably captured by Jupiter, rather than formed from a disk of gas and dust around Jupiter. These new moons are very small, only a few kilometers across, much smaller than the Galilean satellites.
To my question as to why these newly discovered moons are largely retrograde, Sheppard responded:
"That's a good question. There are a number of possible reasons for this. First, the prograde satellites are not as stable as retrograde satellites at large distances from Jupiter. Thus no progrades are known at the distances of the Retrogrades because of planetary and Solar perturbations. For this reason retrogrades are easier to find since most have very distant orbits and thus spend more of their time out of the glare of Jupiter while the progrades which are closer in have lots of Jupiter scattered light from Jupiter to deal with.
"Also we find 3 families of Retrogrades and only one big family of progrades plus Themisto (which is a small lone member of the Themisto family). So 3 families to 1 means we expect 3 times more retrogade objects. If we add in that retrogrades are easier to find, this may account for what we are seeing. It is still possible that the prograde family doesn't have many small members, That is one of the things we hope to find out in the next year or so. If true, that would be very interesting."
See http://www.ifa.hawaii.edu/~sheppard/satellites/
Problems with a prior launch of the latest version of the Ariane 5 rocket have grounded all Ariane 5's, delaying the launch of Rosetta, perhaps until 2004.
The delay probably requires finding a new target, and Comet Churyumov-Gerasimenko is under detailed consideration. But its original target remains a possibility. Various characteristics of desirability, including how active the comet nucleus is, are being considered.
See http://sci.esa.int/rosetta
There are now at least 48 known moons of Jupiter, and there are also about 50 new candidates that are being followed up and checked.
Pictures of Jupiter, taken by a NASA spacecraft on its way to Saturn, are flipping at least one long-standing notion about Jupiter upside down.
Stripes dominate Jupiter's appearance. Darker "belts" alternate with lighter "zones." Scientists have long considered the zones, with their pale clouds, to be areas of upwelling atmosphere, partly because many clouds on Earth form where air is rising. On the principle of what goes up must come down, the dark belts have been viewed as areas where air generally descends.
However, pictures from the Cassini spacecraft show that individual storm cells of upwelling bright-white clouds, too small to see from Earth, pop up almost without exception in the dark belts. Earlier spacecraft had hinted so, but not with the overwhelming evidence provided by the new images of 43 different storms.
"We have a clear picture emerging that the belts must be the areas of net-rising atmospheric motion on Jupiter, with the implication that the net motion in the zones has to be sinking," said Dr. Tony Del Genio, an atmospheric scientist at NASA's Goddard Institute for Space Studies, New York. "It's the opposite of expectations for the past 50 years," he said.
Del Genio is one of 24 co-authors from America and Europe reporting diverse results from the Cassini imaging of Jupiter in Friday's edition of the journal Science. Cassini's camera took about 26,000 images of Jupiter, its moons and its faint rings over a six-month period as the spacecraft passed nearby two years ago.
"The range of illumination angles at which Cassini viewed Jupiter's main ring gives insight about particles in the ring by the way they scatter sunlight. The particles appear to be irregularly shaped, not spheres," said camera-team leader Dr. Carolyn Porco of Southwest Research Institute, Boulder, Colo. "They likely come from surfaces of one or more moons being eroded by micrometeoroid impacts," she said.
Spherical particles would suggest an origin as melted droplets, not erosion. In addition, Cassini imaging shows the degree to which the orbits of two small moons near the ring, Metis and Adrastea, are inclined matches the vertical thickness of the ring. That points to those moons as sources of the ring particles said Porco.
One surprise in ultraviolet images of Jupiter's north polar region is a swirling dark oval of high-atmosphere haze the size of the planet's famous Great Red Spot. "It's a phenomenon we haven't seen before, so it gives us new information about how stratospheric circulation works," said Dr. Robert West of NASA's Jet Propulsion Laboratory (JPL), Pasadena, Calif. The results show the winds and the life cycle of clouds in the stratosphere.
Also, movies of infrared images reveal persistent bands of globe-circling winds extend north of the conspicuous dark and light stripes. "The planet's appearance at high latitudes is like leopard spots, but when you see it in motion, it's interesting that all the spots at one latitude move in one direction and all the spots at adjacent latitudes move the opposite direction," said Dr. Andrew Ingersoll of the California Institute of Technology (Caltech), Pasadena.
Other discoveries reported include atmospheric glows of the large moons Io and Europa during eclipses, a volcanic plume over Io's north polar region, and the irregular shape of a small outer moon, Himalia.
"The Jupiter results provide some hints of the spectacular new findings that await Cassini when it reaches Saturn," Dr. Larry Esposito of the University of Colorado, Boulder, principal investigator for Cassini's ultraviolet-imaging spectrograph instrument, predicts in a separate commentary in Science about the Cassini camera results at Jupiter. Cassini will begin orbiting Saturn July 1, 2004, and will release its piggybacked Huygens probe about six months later for descent through the atmosphere of the moon Titan.
Cassini is a cooperative venture of NASA, the European and Italian Space Agencies. JPL manages it for NASA's Office of Space Science, Washington. Other co-authors include scientists from Cornell University, Ithaca, N.Y.; Free University of Berlin, Germany; Queen Mary, University of London, United Kingdom; University of Arizona, Tucson; University of Paris, France; German Aerospace Center, Berlin; and University of California, Los Angeles.
Images and mission information are available on the Internet at:
http://saturn.jpl.nasa.gov/gallery/jupiter-flyby/index.cfm,
and
http://ciclops.lpl.arizona.edu/ciclops/images_jupiter.html
In early November, the Galileo mission passed Jupiters small, inner moon Amalthea, at a distance of only 100 miles. It then went as close to Jupiter itself as ever, within 45,000 miles. The Amalthea pass was Galileo's last task before it plunges into Jupiter's atmosphere in September 2003. From Amalthea's newly measured mass, we should have an improved density for it, which will bear on our understanding of its composition.
PASADENA, Calif., May 28, 2002 (AScribe Newswire) -- The final images are in, and the resulting portrait of Jupiter's moon Io, after a challenging series of observations by NASA's Galileo spacecraft, is a peppery world of even more plentiful and diverse volcanoes than scientists imagined before Galileo began orbiting Jupiter in 1995. Now that Galileo's observations of Io have ended, scientists are focusing on trying to understand the big picture of how Io works by examining details.
Thirteen previously unknown active volcanoes dot infrared images from Galileo's final successful flyby of Io, volcanologist Dr. Rosaly Lopes of NASA's Jet Propulsion Laboratory reported today at the spring meeting of the American Geophysical Union in Washington, D.C. That brings the total number of known Ionian hot spots to 120. Galileo images revealed 74 of them.
"We expected maybe a dozen or two," said Dr. Torrence Johnson, Galileo project scientist at JPL in Pasadena, Calif. That expectation was based on discoveries by NASA's Voyager spacecraft in 1979 and 1980, and subsequent ground-based observations.
"The volcanoes on Io have displayed an assortment of eruption styles, but recent observations have surprised us with the frequency of both giant plumes and crusted-over lakes of molten lava," said planetary scientist Dr. Alfred McEwen of the University of Arizona, Tucson.
Galileo's latest images, which also show tall slopes
crumbling and surface deposits from two eruptions' recent
giant plumes, are available online from JPL at
http://www.jpl.nasa.gov/images/io and from the University of
Arizona Lunar and Planetary Laboratory at
http://pirlwww.lpl.arizona.edu/Galileo/Releases.
Some high-resolution views taken as Galileo skimmed past Io on Oct. 16, 2001, are aiding analysis of the connection between volcanism and the rise and fall of mountains on Io. Few of Io's volcanoes resemble the crater-topped volcanic peaks seen on Earth and Mars, said planetary scientist Dr. Elizabeth Turtle of the University of Arizona. Most of Io's volcanic craters are in relatively flat regions, not near mountains, but nearly half of the mountains Io does have sit right beside volcanic craters.
"It appears that the process that drives mountain-building -- perhaps the tilting of blocks of crust - -- also makes it easier for magma to get to the surface," Turtle said. She showed a new image revealing that material slumping off a mountain named Tohil Mons has not piled up in a crater below, suggesting that the crater floor has been molten more recently than any landslides have occurred. Galileo's infrared-mapping instrument has detected heat from the crater, indicating an active or very recent eruption.
From the analysis of Galileo's observations, scientists are developing an understanding of how that distant world resurfaces itself differently than our world does.
"On Earth, we have large-scale lateral transport of the crust by plate tectonics," McEwen said. "Io appears to have a very different tectonic style dominated by vertical motions. Lava rises from the deep interior and spreads out over the surface. Older lavas are continuously buried and compressed until they must break, with thrust faults raising the tall mountains. These faults also open new pathways to the surface for lava to follow, so we see complex relations between mountains and volcanoes, like at Tohil."
"Io is a weird place," Johnson said. "We've known that since even before Voyager, and each time Galileo has given us a close look, we get more surprises. Galileo has vastly increased our understanding of Io even though the mission was not originally slated to study Io."
Extensions to Galileo's original two-year orbital mission included six swings close to Io, where exposure to Jupiter's intense radiation belts stresses electronic equipment on board the spacecraft. Researchers presented some results today from two Io encounters in the second half of 2001. Observations were not made successfully during Galileo's final Io flyby, in January 2002, because effects of the radiation belts put the spacecraft into a precautionary standby mode during the crucial hours of the encounter.
Galileo will make its last flyby of a moon when it passes close to Amalthea, a small inner satellite of Jupiter, on November 5. No imaging is planned for that flyby. With fuel for altering its course and pointing its antenna nearly depleted, the long-lived spacecraft will then loop one last time away from Jupiter and perish in a final plunge into Jupiter's atmosphere in September 2003.
Additional information about Galileo, Jupiter and
Jupiter's moons is available online at
http://galileo.jpl.nasa.gov.
JPL, a division of the California Institute of Technology in Pasadena, manages Galileo for NASA's Office of Space Science, Washington, D.C.
University of Hawaii press release, May 17, 2002
Web Site: http://www.ifa.hawaii.edu/~sheppard/satellites/jup.html
Summary
University of Hawaii astronomers announce the
discovery of 11 new satellites of Jupiter. These
new satellites, when added to the eleven discovered
the previous year by the Hawaii team, bring the
total of known Jupiter satellites to 39. This is
more than any other planet.
Discoveries
The new satellites were discovered during mid-December
of 2001 by a team led by Scott S. Sheppard and David
Jewitt from the University of Hawaii's Institute for
Astronomy and including Jan Kleyna of Cambridge
University, England. They used the Canada-France-Hawaii
(3.6 meter) telescope with one of the largest digital
imaging cameras in the world, the "12K", to obtain
sensitive images of a wide area around Jupiter. The
digital images were processed using high speed computers
and then searched with an efficient computer algorithm.
Candidate satellites were monitored in the succeeding
months at the University of Hawaii 2.2-meter telescope
to confirm their orbits and to reject closer asteroids
masquerading as satellites. Orbits of the new
satellites were fitted by both Robert Jacobson at the
NASA Jet Propulsion Laboratory and Brian Marsden at
the Minor Planet Center. The satellites were formally
announced by the International Astronomical Union on
Circular No. 7900 (May 16, 2002).
Properties
The 11 new objects all belong to the so-called
"irregular satellite" class, meaning that they have
large semi-major axes, eccentricities and inclinations.
All are retrograde (they orbit in the direction
opposite to the rotation of the planet), and possess
similar semi-major axes (about 300 Jupiter radii or
20 million km) The estimated diameters are between
about 2 and 4 kilometers, assuming a 4% albedo. As
yet, nothing is known about their surface properties,
compositions or densities, but they are presumed to
be rocky objects like the asteroids.
The new discoveries bring the known total of Jupiter satellites to 39, of which 31 are irregulars. (The 8 regular satellites include 4 large objects discovered by Galileo and 4 small objects on circular orbits interior to that of Io). Jupiter's nearest rival for having the largest number of known satellites is Saturn, with 30 (of which 13 are irregular).
Significance
The large, elongated and inclined orbits of the
irregular satellites strongly suggest an origin by
capture. Since no efficient contemporary capture
mechanisms are known, it is likely that the irregular
satellites were acquired when Jupiter was young,
possibly still in the process of condensing down to
its equilibrium size. The precise mechanism of
capture remains unidentified but there are two
leading theories for the capture process. In the
gas drag hypothesis, passing asteroids are slowed by
friction with proto-Jupiter's bloated atmosphere.
Those which do not burn up in the atmosphere like
meteors are trapped in looping orbits like those of
the new satellites. In the mass growth hypothesis,
the rapid growth of Jupiter leads to capture of
nearby, co-moving planetesimals. Both processes
would have operated in the first million years of
the solar system.
The irregular satellites are grouped into distinct dynamical families or clusters. This suggests that individual satellites are pieces of a few precursor bodies that have been shattered. The disruptions occurred either during the process of capture or possibly after capture due to collisions with Jupiter-crossing comets. Future measurements of the size distribution, surface properties and orbits of the satellites will help determine how they formed.
Press Release 3/13/02
PASADENA, Calif., March 13 (AScribe Newswire) -- A dark patch of hydrocarbon haze, wider than Earth, develops and swirls in a new movie clip from ultraviolet images taken by NASA's Cassini spacecraft of Jupiter's upper atmosphere, or stratosphere.
Observations in the ultraviolet part of the light spectrum reveal features in Jupiter's stratosphere that are transparent in the visible-light portion of the spectrum. One surprise is the dark vortex whose birth and migration can be seen during the 11-week span of the movie taken while Cassini was approaching Jupiter in late 2000. Development of this feature resembles development of ozone holes in Earth's stratosphere in that both processes appear to occur only within confined masses of high-altitude polar air. The similarity may help scientists understand both processes better.
The movie clip and a still image mapping all 360 degrees of Jupiter in ultraviolet are available online from NASA's Jet Propulsion Laboratory, Pasadena, Calif., at
http://www.jpl.nasa.gov/images/jupiter
and from the Cassini imaging team, based at the Boulder, Colo., campus of the Southwest Research Institute, at
http://ciclops.lpl.arizona.edu/ .
Cassini made its closest pass to Jupiter on Dec. 30, 2000, gaining a gravitational boost for reaching its main destination, Saturn, in 2004. More information about the mission is available at http://saturn.jpl.nasa.gov . Cassini is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL manages the Cassini and Galileo missions for NASA's Office of Space Science, Washington, D.C. JPL is a division of the California Institute of Technology in Pasadena.
PASADENA, Calif., Jan. 18, 2002 (AScribe News) -- NASA's Galileo spacecraft resumed gathering scientific information on Jan. 17 Pacific Time) after commands radioed from Earth took the Jupiter orbiter out of the passive standby mode it entered, causing it to miss taking photos of Io.
Galileo passed within about 102 kilometers (63 miles) of Jupiter's moon Io on January 17. Planned observations for the remainder of the spacecraft's current swing near Jupiter include a series of images of the planet's atmosphere, a farewell color study of its icy moon Europa and navigational imaging of the small moon Amalthea.
Galileo hit its target point for the Io flyby so accurately that a scheduled post-encounter firing of thrusters to fine-tune the trajectory was cancelled as unnecessary, said Dr. Eilene Theilig, Galileo project manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif. The close flyby was calculated to use Io's gravity to put Galileo on course for its next encounters. Galileo will pass near Amalthea in November 2002 and plunge to its demise in Jupiter's crushing atmosphere in September 2003.
"As expected, visiting Io has proved to be a challenging and risky endeavor," Theilig said. "It's disappointing not to get the observations of Io that were planned for this encounter, but I am very proud of the flight team that has kept Galileo functioning in orbit more than three times longer than originally planned and revived it once more yesterday."
Galileo detected a computer reset and placed itself in a standby or "safe" mode on January 17 at 13:41 Universal Time (5:41 PacificTime), about half an hour before its closest approach to Io. The reset was apparently caused by exposure to the intense radiation environment at Io's distance from Jupiter. Since the spacecraft began orbiting Jupiter in 1995, it has endured a cumulative radiation exposure about three-and-a-half times what it was originally designed to withstand.
NASA has repeatedly extended Galileo's original two-year mission in orbit. The spacecraft is now nearly out of the hydrazine propellant needed to keep its antenna pointed toward Earth. Knowing they would eventually lose contact and control of the spacecraft, the Galileo team chose the planned impact with Jupiter to ensure there was no chance the spacecraft might hit Europa. One of Galileo's important discoveries has been the likelihood of a melted saltwater ocean under Europa's icy crust, making that moon of great interest for future study of the possibility of extraterrestrial life.
Additional information about the Galileo mission is available at http://galileo.jpl.nasa.gov .
Galileo was launched from the Space Shuttle Atlantis on Oct. 18, 1989. After a long journey to Jupiter, Galileo began orbiting the huge planet on Dec. 7, 1995, and successfully completed its two-year primary mission in 1997. That has been followed by three mission extensions. 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.
NASA's Galileo orbiter will dart past Jupiter's moon Io on January 17 in the veteran spacecraft's last and closest flyby of any of the giant planet's four major moons.
Io's volcanoes have presented many surprises since they were first seen in 1979 by NASA's Voyager spacecraft and especially during the six years that Galileo has been orbiting Jupiter. Scientists hope this week's encounter will reveal how several regions of Io have changed over the years.
"Galileo's days are numbered now, so it's especially exciting to visit Io one last time," said Dr. Eilene Theilig, Galileo project manager at NASA's Jet Propulsion Laboratory (JPL), Pasadena, Calif. "An orbital mission like Galileo gives you the advantage of getting to examine interesting places repeatedly over a period of time. That's been great for studying Io, since it keeps changing so much."
The Galileo flight team at JPL aimed the orbiter to skim just 100 kilometers (62 miles) above Io's multicolored surface at 9:09 a.m. EST on Jan. 17. "The reason we're going so close is to put Galileo on a ballistic trajectory for impact into Jupiter in September 2003," Theilig said.
Galileo has operated in orbit more than three times longer than its originally planned mission. The resilient spacecraft has survived about three and a half times as much exposure to radiation from Jupiter's radiation belts as it was designed to withstand. In its 33 loops around Jupiter, it has flown near Io six times previously and near the other three of Jupiter's planet-sized moons - Europa, Ganymede and Callisto - a total of 27 times.
The tour has relied on expert navigators to calculate several moves in advance, using each moon's gravity to help adjust the spacecraft's trajectory toward its various encounters.
However, the propellant supply needed for steering the spacecraft and keeping its antenna pointed toward Earth is now nearly exhausted. To avoid even a slim chance that Galileo could crash into Europa after its mission ends, NASA has decided to send it to a controlled demise in the crushing pressure of Jupiter's dense atmosphere. Galileo had earlier found evidence that Europa has a deep ocean of melted saltwater under its frozen surface, heightening interest in keeping Europa pristine for later studies of its potential for harboring extraterrestrial life.
Before its final plunge, Galileo will make the first close flyby of Amalthea, a small, inner moon of Jupiter, in November 2002.
This week, Galileo will make direct measurements of the charged particles and magnetic environment around Io. Also, its camera and instruments for infrared and thermal imaging have been programmed to make observations during the flyby. As much of the data as possible will be transmitted to Earth from the spacecraft's tape recorder in coming months, Theilig said.
Io, like Earth's Moon, always keeps the same side facing inward toward its planet. On Thursday, Galileo will be in position for its best-ever look at the Jupiter-facing side of Io. "We're hoping to see areas we haven't seen well since Voyager imaged them back in 1979," said JPL's Dr. Torrence Johnson, Galileo project scientist. "We'd like to know more about rates of change for volcanic features on Io." New observations are also planned for a previously inactive volcano that unexpectedly lofted a tall plume last summer.
On this swing through the inner portion of the Jovian system, Galileo will also examine storms on Jupiter itself and the Io torus, a doughnut-shaped band of charged particles encircling Jupiter at Io's distance from the planet.
A sporadic malfunction has affected performance of Galileo's camera since mid-2000, apparently due to radiation damage to an electronic component. The camera worked flawlessly during the most recent Io encounter in October 2001, but each time Galileo swings as close to Jupiter as Io's orbit, odds increase for more serious damage to the spacecraft from exposure to the planet's radiation belts.
Io is the innermost of Jupiter's four large moons. Heat from tidal flexing powered by Jupiter's gravitational pull makes it the most volcanically active world in the solar system, with an estimated 200 to 300 volcanoes rapidly resurfacing it.
Galileo left Earth aboard the space shuttle Atlantis in 1989. JPL, a division of the California Institute of Technology in Pasadena, Calif., manages the Galileo mission for NASA's Office of Space Science in Washington.
Additional information about the mission is available online at: http://galileo.jpl.nasa.gov
A slumping cliff, migrating eruptions and churning lava lakes appear in new images of Jupiter's sizzling moon Io from NASA's Galileo spacecraft.
The images and explanatory captions are available online at
<http://www.jpl.nasa.gov/images/io> from NASA's Jet Propulsion Laboratory,
Pasadena, Calif. Images are also online at
<
http://pirlwww.lpl.arizona.edu/missions/Galileo/releases>,
the University of
Arizona Galileo Imaging Team members' web site.
A high-resolution view of a cliff named Telegonus gives information about erosion on a world that has neither surface water nor wind. The cliff is slumping outward due to gravity.
The Tvashtar area on northern Io, where no volcanic activity was seen prior to December 1999, now has a cluster of hot spots. An infrared mapping image from Galileo's Aug. 6, 2001, flyby of Io shows that the surface within the Tvashtar area is hot at the sites observed in 1999 and 2000, as well as at newly observed sites. "The most explosive phase of the Tvashtar eruption may have ceased, but these observations reveal that the area is still active," said Dr. Rosaly Lopes, a volcanologist at JPL. Tvashtar appears to be an example of a volcanic site where activity starts vigorously then gradually declines, similar to many eruptions on Earth, she said.
Io's most powerful volcano, Loki, offers a contrasting style of eruption. The Loki hot spot brightens and fades over periods of several months, possibly in periodic cycles, a pattern not known on Earth. Scientists have proposed that Loki is either an active lava lake or a caldera whose floor is flooded by frequent lava flows.
Infrared mapping images of Loki from Galileo's Oct. 16, 2001, flyby of Io weigh in favor of the lava lake interpretation. They show a concentration of high temperatures along one edge, like a glowing shoreline. This suggests that hotter lava from underneath is showing through where a cooler lava crust is breaking up as it hits the crater wall. High-resolution nighttime pictures of another of Io's hot spots, Pele, also show the apparent overturning of cooler crust on a lava lake.
NASA Press Release 01-174, August 22, 2001
A spiky landscape of bright ice and dark dust shows signs of slow but active erosion on the surface of Jupiter's moon Callisto in new images from NASA's Galileo spacecraft.
The pictures taken by Galileo's camera on May 25 from a distance of less than 138 kilometers, or about 86 miles, above Callisto's surface give the highest resolution view ever seen of any of Jupiter's moons.
"We haven't seen terrain like this before. It looks like erosion is still going on, which is pretty surprising," said James Klemaszewski of Academic Research Lab, Phoenix, AZ. Klemaszewski is processing and analyzing the Galileo Callisto imagery with Dr. David A. Williams and Dr. Ronald Greeley of Arizona State University, Tempe.
Callisto, about the same size as the planet Mercury, is the most distant of Jupiter's four large moons. Callisto's surface of ice and rock is the most heavily cratered of any moon in the solar system, signifying that it is geologically "dead." There is no clear evidence that Callisto has experienced the volcanic activity or tectonic shifting that have erased some or all of the impact craters on Jupiter's other three large moons.
The jagged hills in the new images may be icy material thrown outward from a large impact billions of years ago, or the highly eroded remains of a large impact structure, Williams said. Each bright peak of dust is surrounded by darker dust that appears to be slumping off the peak.
"They are continuing to erode and will eventually disappear," Klemaszewski said. One theory for an erosion process is that, as some of the ice turns into vapor, it leaves behind dust that was bound in the ice. The accumulating dark material may also absorb enough heat from the Sun to warm the ice adjacent to it and keep the process going. The new images show areas where the sharp knobs have apparently eroded away, leaving a plain blanketed with dark material.
The close-up images show craters as small as about 3 meters across, or about 10 feet, though not as many as some predictions anticipated. One scientific goal from the high-resolution images is to see how many small craters are crowded onto the surface. Crater counts are one way to estimate the age of a moon's surface, and since Callisto has been so undisturbed by other geological processes, its cratering density is useful in calibrating the estimates for Jupiter's other moons.
NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, CA, manages Galileo for NASA's Office of Space Science, Washington, DC.
Additional information about Galileo's mission is available online at:
The close-ups and the first complete Callisto global color picture from Galileo are available on the Internet at:
http://www.jpl.nasa.gov/images/callisto