A JPL Press Release
Jupiter's icy moon Europa has a metallic core and layered internal structure similar to the Earth's, while the heavily cratered moon Callisto is a mixture of metallic rock and ice with no identifiable central core, according to new results from NASA's Galileo mission.
In addition, recent plasma wave observations from Galileo show no evidence of a magnetic field or magnetosphere around Callisto, but do hint at the prospect of a tenuous atmosphere.
These findings are based on data gathered during Galileo's Nov. 4, 1996, flyby of Callisto and its Europa encounters on Dec. 19, 1996, and Feb. 20, 1997.
"Before Galileo, we could only make educated guesses about the structure of the Jovian moons," said Dr. John Anderson, a planetary scientist at NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA. "Now, with the help of the spacecraft, we can measure the gravitational fields of the satellites and determine their interior structure and density. We can determine how the matter is distributed inside."
While scientists use seismic waves to study Earth's interior, Galileo performs remote studies of Jupiter's moons by measuring small changes in the spacecraft's trajectory as it passes each body.
"These new results from the gravity data are very consistent with the idea of subsurface oceans on Europa," Anderson said. "We know that Europa has a very deep layer of water in some form, but we don't yet know whether that water is liquid or frozen."
In an article appearing in the May 23 edition of Science, Dr. Margaret Kivelson, principal investigator for Galileo's magnetometer, reports that during its December 1996 pass by Europa, the magnetometer detected what she described as "a substantial magnetic signature," and also found that Europa's north magnetic pole is pointed in an odd direction. Based on these observations, Kivelson, a professor at the University of California at Los Angeles, said Europa may have a magnetic field about one-quarter the strength of Ganymede's magnetic field.
Although the magnetometer was malfunctioning during Galileo's Europa flyby in February 1997, Kivelson said the problem is corrected and the device is expected to return valuable data during its upcoming Europa flybys. The next Europa encounter is scheduled for November, with a series of flybys planned during a two-year Galileo extended mission.
Galileo's findings on the Jovian moon Callisto revealed a much different structure than Europa. Scientists believe that because Callisto is the Galilean moon located farthest from Jupiter, it was never subjected to the same gravitational pull as the inner moons and, therefore, never experienced enough heating to form different layers.
"Callisto had a much more sedate, predictable and peaceful history than the other Galilean moons, "Anderson explained, "and, therefore, it is a more typical solar system object." The findings indicate Callisto has no core, but instead has a homogeneous structure, with 60 percent of its ingredients being rock, including iron and iron sulfide, and 40 percent made of compressed ice.
Dr. Donald Gurnett, principal investigator for the Galileo spacecraft's plasma wave instrument, said the instrument displayed a very minor response from Callisto and, consequently, showed no evidence of a magnetic field or magnetosphere. The latest issue of Nature magazine contains these findings, as well as supportive data from magnetometer studies of Callisto, as reported by Dr. Krishan Khurana of UCLA.
However, Gurnett added, "There is some evidence of a plasma source on Callisto, which might indicate a very tenuous atmosphere." Gurnett is a professor at the University of Iowa at Iowa City.
There are three activities for students so far: Images for you and your students to use and enjoy. They are:
Study and Interpret New Images of New Worlds
Learners can simulate the work of scientists investigating geological features of the Jovian moon Europa. The students can also attempt to figure out the relative geological age of discreet features in one region of Europa. This qualitative analysis will lead student investigators to experience science using scientific tools.
Design a Spacecraft to test for Life on Europa
This activity will introduce students to the process involved with designing a spacecraft and testing for life on Europa. The main goal of this activity is to encourage student thinking and to focus on the process, not necessarily the results.
Jupiter is currently visible in the early morning sky and will remain visible for a while. However, finding Jupiter is a challenge indeed because of its relative closeness to the Sun. This activity provides information on how to find Jupiter and encourages students to work together and develop a process for observing.
Scott Coletti, a teacher working with the Galileo project at JPL, has developed an Imaging Activity.
This activity will let students study the most recent pictures of Europa taken by the Galileo spacecraft. Using a Color Macintosh or Mac clone, free software, lesson, and images, students will simulate the scientific adventure of peering at new worlds. Learners will simulate the work of project scientists, investigating geological features of the Jovian moon Europa. Science students will evaluate one image to determine the relative geological age of discreet features in one region of Europa. This qualitative analysis will lead student investigators to experience science using scientific tools.
Scientists and engineers working on NASA's Galileo mission to Jupiter are sharing their diaries and notebooks on the Internet to show students and teachers what it's like to work day today on a planetary exploration project.
NASA's "Online from Jupiter" program, being presented for the second time on the Internet through the end of March, features journal entries from Galileo flight team members and scientists analyzing data returned daily from the Jupiter orbiter. The program, which encourages participants to "tell it like it is," has been well-received by educators and students who log on for a glimpse at the inner workings of the Galileo mission.
"Online from Jupiter has drawn a tremendous response from teachers," said Dr. Jo Pitesky, a member of the Galileo project's outreach office, who has recruited participants for the "Online" program. "The journals let the readers share the tribulations and triumphs they experience in flying the mission. The entries tend to dispel the nerdy stereotype of aerospace engineers and scientists, and de-mystifies their work," she added.
Following is the schedule for encounters of the Galileo Orbiter with the satellites of Jupiter:
Orbit Satellite Date Distance
G1 Ganymede June 27, 1996 835
G2 Ganymede Sept 6, 1996 260
C3 Callisto Nov 4, 1996 1118
E4 Europa Dec 19, 1996 692
J5 solar conjunction
E6 Europa Feb 20, 1997 587
G7 Ganymede Apr 5, 1997 3059
G8 Ganymede May 7, 1997 1585
C9 Callisto June 25, 1997 416
magnetotail Aug 8, 1997
C10 Callisto Sept 17, 1997 524
E11 Europa Nov 6, 1997 1125
It is hoped that enough fuel and funding will remain at the end of 1997 to allow an extended mission. It would concentrate on Europa and end with a close encounter in 1999 with Io.
A set of Galileo results were published in SCIENCE magazine for October 18, 1996, and are available on-line. New results and confirmations of already-known results indicate that the Great Red Spot is a shallow storm in Jupiter's atmosphere, show present eruptions of Io, show that Ganymede is covered by a thin layer of ice and that fractures in the ice may demonstrate that there is a fluid layer beneath, provide high-resolution images of fractures in Europa's ice covering, and show results from the magnetometer, particle and plasma wave, and dust-detector experiments.
Scientists participating in NASA's Galileo mission have discovered that the Galileo spacecraft may have flown though a dense, high-altitude ionosphere during its encounter with Jupiter's volcanic moon Io last December. The discovery suggests that Io's atmosphere is time variable and is made of volcanic gas lofted to very high altitudes.
An ionosphere is a region of electrically charged gas that exists at the top of some planetary atmospheres. The surprising discovery is being reported by Galileo scientists this week at a meeting of the American Astronomical Society's Division of Planetary Sciences being held in Tuscon, AZ, along with other Galileo results, including remarkable new images of the planet and its moons.
Sensors on the spacecraft found a very dense region of ionized oxygen, sulfur and sulfur dioxide at 900 kilometers (550 miles) above Io that must be pumped into that region by Io's relentless volcanic activity, said Dr. Louis A. Frank of the University of Iowa and principal investigator on Galileo's plasma science experiment. Instead of being swept away by Jupiter's rotating magnetosphere as anticipated, the ionized gases surprisingly remain with Io, he said.
"Passage of the Galileo spacecraft through an ionosphere was not expected because images of the volcanic plumes previously taken with the Voyager spacecraft indicated that the plume heights extended only to a few hundred kilometers or less," said Frank. A radio occultation by NASA's Pioneer 10 spacecraft in 1973 indicated ionospheric heights only 50 to 100 kilometers (about 30 to 60 miles) above the surface, he added. "No one expected to see to see this at 900-kilometer altitude." The difference between what Pioneer saw and what Galileo has observed indicates that Io's atmosphere and ionosphere are variable and may grow and shrink with more or less volcanic activity, Frank said.
The results may lend credence to previous theories proposed by Galileo Project Scientist Dr. Torrence Johnson of NASA's Jet Propulsion Laboratory, Pasadena, CA, that invisible "stealth plumes" deliver volcanic gases to great heights above Io. Io's weak gravity field apparently permits the invisible gases emanating from the volcanoes to reach extraordinary heights far beyond the lower altitudes achieved by the dust and other volcanic ejecta that reflects sunlight and can be seen in images, Frank said.
Io Electron Beams
In a related finding, the energetic particle detector on the spacecraft measured intense, bi-directional electron beams that are aligned with Galileo's magnetic field in Io's vicinity. The beams are similar to those that impinge on Earth's atmosphere to produce aurora and, also, positive ions and electrons in Earth's atmosphere.
Dr. Donald J. Williams of the Applied Physics Laboratory of Johns Hopkins University and principal investigator on the energetic particle experiment, said the electron beams span the energy range of 15 kiloelectron volts to 190 kiloelectron volts and represent an energy deposition into Jupiter's atmosphere of up to 1 billion watts.
"This is sufficient energy input into the Jovian atmosphere to produce visible auroral emissions," Williams said. "These beams are a signature of remarkable particle acceleration processes that occur in the vicinity of Io processes that are thought to be linked to Io's motion through Jupiter's plasma and magnetic field environment." Additional work is required to determine whether the beams play a role in producing some of the auroral emissions observed in Jupiter or if they are related to radio emissions that have been correlated with Io's orbital motion.
The electron beams must have a role in maintaining the Io torus, the doughnut-shaped cloud of ionized gases that flows between Jupiter and Io, Galileo scientists said. Auroral emissions in Io's atmosphere are one likely result of the electron beams, they reported, and the two-way electron highway that the beams produce between Jupiter and Io must contribute to some of the aurora observed in Jupiter's atmosphere as well.
Io Volcano Shifting?
Several images recently returned by Galileo show new details of surface features on the moon Ganymede and Io. One new image of the active volcano Prometheus on Io has been compared to one of the same features taken by NASA's Voyager spacecraft 17 years ago, and shows that the plume is now erupting from a position about 75 kilometers (about 46 miles) west from where the hot spot resided in 1979. It is not known if the plume source is the same or if the plume is now emanating from a new source. Overall, scientists studying Galileo images of Io are observing that a wide variety of surface changes have occurred in the nearly two decades since a spacecraft last visited Jupiter's system.
Frosted Rims on Ganymede
Bright white areas seen around the circular rims of highlatitude impact craters on Ganymede in new Galileo images of that moon are likely water-ice frosts, Galileo scientists reported. Even though the Sun is shining from the south, the north-facing walls of the ridges and craters are brighter than the walls facing the Sun. Images of regions elsewhere on Ganymede show more details of the remarkable juxtaposition of newer and older fractured and faulted terrain that characterizes so much of this big moon's surface. A stereoscopic view of Ganymede has also been produced with two images of the Galileo Reggae region (one was taken during the first Ganymede flyby in June and the second was acquired in the September flyby). The image, which was computer-reconstructed by imaging scientists at JPL, shows new topographic information about the moon.
Galileo science team members are reporting on numerous other new findings about Jupiter and its moons:
The photopolarimeter radiometer experiment produced heat maps of Jupiter's Great Red Spot, the day side of the moon Europa, the night side of Io, and both the day and night sides of Ganymede during the spacecraft's flyby of Ganymede in June. The images of the Great Red Spot show temperatures of the atmosphere at the 250 and 500 millibar pressure levels, much like terrestrial weather maps. The Great Red Spot is colder than its surroundings, consistent with earlier Voyager and Earth-based observations in which the spot is modeled as an anticyclonic vortex with central up-welling balanced by subsidence at its edges.
The radiometer also produced temperature data for Io indicating a nighttime temperature about 80 Kelvin to 85 K, or - 375 degrees Fahrenheit to -380 degrees F.
The first midday temperature for Europa, 128 Kelvin (-229 Fahrenheit), has allowed the radiometer instrument team to determine that that moon has a more porous or "fluffy" ice surface than the other moons. Researchers said that such porosity indicates Europa's surface is covered with finely powdered ice grains.
The near-infrared mapping spectrometer instrument and Galileo's solid state imaging camera measured hot regions on Io including erupting volcanoes and individual volcanoes, finding temperatures off 420 Kelvin to 620 K (296 degrees Fahrenheit to 656 F).
On Callisto and Ganymede, the near-infrared mapping spectrometer found surface features indicating the presence of hydrated materials, or possibly carbon dioxide frost.
Images of Ganymede taken during the recent Galileo space probe flyby show a remarkable amount of detail on the surface of the Jovian moon. Some areas have a resolution that is 17 times better than images previously taken during the Voyager 2 flyby in 1979. The images are available on the Galileo home page.
The Galileo space probe is currently transmitting data collected during its flyby of Ganymede earlier today. At its closest approach, Galileo was 70 times closer to Ganymede than Voyager 2 and 133 times closer than Voyager 1. More information is available on the Galileo home page.
Galileo's playback of the tape-recorded science data obtained by the atmospheric probe in Jupiter's atmosphere Dec. 7 was successfully concluded. Orbiter spacecraft measurements of the Io plasma torus, made a few hours before the spacecraft arrived at Jupiter, remain to be played back starting in June when the spacecraft has been reprogrammed with its new software.
The experimenter teams have not yet fully analyzed the magnetometer and dust data collected since late March. However, an initial look indicates, as expected, very few particles detected because the dust detector was pointed away from the planet during this part of the orbit.
THE PROBE STORY: SECRETS AND SURPRISES FROM JUPITER by Larry Palkovic
The article below is from the April 1996 issue of the Galileo Messenger, the official newsletter of the Galileo mission to Jupiter. This issue covers missions activities through the exciting Jupiter orbit insertion and the successful probe mission. The entire issue is also available on the Galileo home page.
After a 6-year journey replete with nail-biting trials and eye-popping triumphs in space, and after a frustrating 6-week delay back on Earth, the Galileo Probe's bounty of scientific data was finally presented, on January 22 at the Ames Research Center, to a fascinated public by a panel of Probe science investigators headed by Probe Scientist Rich Young (see photo). Their preliminary report summarized the condensed memory readout from the Orbiter's solid-state memory downlinked in December and January. In the months since that meeting, subsequent analyses have changed some of the scientists' earliest views on their data. Probe investigators are still waiting for the complete playback from the Orbiter's tape recorder, but that won't be finished until mid-April.
While the picture of Jupiter that has emerged is, generally, similar to what was expected, the details are sufficiently different to merit some serious rethinking on the origin and structure of the planet and its atmosphere.
The Probe was certainly well prepared for its brief but celebrated exploration of the Jovian atmosphere. Radio contact with the Orbiter was solid for almost an hour, and every instrument performed perfectly through the nominal mission.
The approach and entry were not without some surprises. The space between Jupiter's rings and atmosphere was expected to be fairly quiet, but Harald Fischer's energetic particle instrument discovered a new, powerful radiation belt here---populated by high-energy helium ions and ten times stronger than Earth's Van Allen belt.
Probe deceleration in the upper atmosphere as measured by Al Seiff's atmospheric structure instrument was greater than expected, indicating a much denser (100 times) and hotter (2270C) atmosphere 340 km above the 1-bar level. Unexpected, too, was a parachute deployment 53 seconds late and 26 km below the planned 0.1-bar level.
When the heat shield dropped off and the instruments started recording, Larry Sromovsky's net flux radiometer (NFR), designed to measure the energy balance between the Sun above and the planet below, showed variations in sky brightness that indicated scattered clouds. At this 0.4- to 0.6-bar level (-1500 C to -1300 C and 20 km above the 1.0-bar level), these were likely ammonia clouds.
Boris Ragent's nephelometer, which reads a reflected laser beam for cloud particles, saw none at this altitude, suggesting the ammonia clouds were distant, or at least scattered. Maybe 45 or 50 km further down, however, at about 2 bars (-700 C), it did record substantial concentrations of what were believed to be ammonium hydrosulfide clouds. While well defined, even these clouds were not nearly as thick as postulated; the NFR did not report them. Even further down, 60 to 80 km below 1 bar, at the 5- to 8-bar level where temperatures support liquid water (0 to 400 C), the nephelometer found no evidence for water clouds, though these should have been the thickest of all.
Glenn Orton's ground-based, infrared telescopic observations showed the Probe's entry site to sit on the edge of a prominent "hot spot." This broad patch of clearer, drier atmosphere looked to be a region of thinner, even absent clouds. This certainly confirmed the nephelometer readings and suggests that, at least in its upper atmosphere, Jupiter is a very heterogeneous planet. One of the principal tasks of the investigators will be to distinguish those data that measure local phenomena from those that measure the global.
Hasso Nieman's neutral mass spectrometer, which determines the composition of the atmosphere, also revealed the atmosphere to be drier than expected---much drier than predicted from Shoemaker-Levy 9 data, and even drier than predicted from Voyager data! Initial results suggested generally Sun-normal values for many other atmospheric constituents, but later work has changed that picture. Solar values would suggest little change in Jupiter's evolution from the original solar nebula, but increased concentrations of any element (besides hydrogen and helium) would suggest a history of cometary accretions. Concentrations of methane and hydrogen sulfide were greater than solar values. Ammonia values, even at this date, still puzzle researchers. Concentrations of the noble gases krypton and xenon were much greater than solar values, but isotopic ratios were near solar. Fewer organic molecules and substantially less neon than expected also characterized the Jovian atmosphere sample.
Ulf von Zahn's helium abundance detector measured the concentration of helium at 0.24 by mass, close to solar abundance. Low levels of helium indicate depletion in the atmosphere of Saturn, but this is not seen at Jupiter, probably because of Jupiter's larger size (three times more massive) and higher internal temperatures.
Lou Lanzerotti's lightning and radio emission detector looked for both optical flashes (from near discharges) and radio waves (from more distant ones). The expected thick cloud decks suggested lots of cloud-to-cloud bolts. In retrospect, considering the lack of water clouds, it's not surprising that no flashes were seen. On the other hand, the Probe did record the radio signatures of perhaps 50,000 strikes---up to an Earth's diameter away. These numbers translate to very powerful discharges but to only a third (or even a tenth) the occurrence rate of lightning on Earth. Fewer strikes are also consistent with fewer organic molecules (which the strikes generate).
Dave Atkinson's Doppler wind experiment tracked the Doppler shift in the Probe's radio signal to measure the speed of the Jovian winds. Wind speed at the cloud tops was thought to be 360 to 540 km/h, and this was expected to drop to zero at some point---if, as on the Earth, such winds are generated by sunlight and release of latent heat by condensation of water vapor. Not unexpectedly, Jupiter is not like the Earth. Winds are faster than expected, clocking 720 km/h below the cloud-top level, and show no tendency to slow with depth. Jovian winds are apparently generated by heat coming from below.
The helium abundance detector stopped recording at 14 bars, as designed, after 40 minutes of activity. At this time, signals from the nephelometer and net flux radiometer also became useless as they degraded to noise. After 48 minutes of recording, 110 km down, the instrument shelf temperature inside the Probe was much closer to the outside 15-bar temperatures of 1000C than the expected 500C. The lightning detector and neutral mass spectrometer stopped sometime after this point.
Only Al Seiff's atmospheric structure instrument was still operating when radio transmission stopped after 57.6 minutes at the 23-bar level (1520C, 140 km down). This instrument measured the atmospheric temperature, pressure, and densities during the entire 160 km or so (20 km above 1.0 bar to 140 km below) of descent. During the entry phase, it showed hotter temperatures and higher densities than expected in the upper atmosphere, and numbers much closer to those expected in the lower atmosphere. Also consistent with Doppler data, it showed that the Probe dropped through a very turbulent atmosphere. And consistent with the other instruments, it measured a lapse rate or change of temperature with altitude that showed a very dry atmosphere in the 6- to 15-bar range and convective transfer of heat, which powers the wind systems and keeps the deep layers well mixed.
After its last transmission, the Probe, we imagine, continued to sink into the Jovian depths. Without a surface to hit, the Probe lost its Dacron parachute, its aluminum fittings, and even (by the 5000-bar level, 17000C) its titanium shell to melting and evaporation. Ten hours after entering the atmosphere there would have been nothing left to see, and the Probe would have become a part of the planet that its sister Orbiter will be watching so closely.
Jupiter's volcano-pocked moon Io has been found by NASA's Galileo spacecraft to have a giant iron core that takes up half its diameter, scientists report in the May 3 issue of Science magazine.
The spacecraft also has detected a large "hole" in Jupiter's magnetic field near Io, leading to speculation about whether Io possesses its own magnetic field. If so, it would be the first planetary moon known to have one.
These newly identified characteristics of Io may be related to the intense heating of the moon caused by the constant squeezing and distortion of Io in Jupiter's powerful gravitational grip, according to Galileo Project Scientist Dr. Torrence Johnson of NASA's Jet Propulsion Laboratory. Io is the most geologically active body in the Solar System, and though it is less than a third of Earth's size, it generates twice as much heat as the Earth.
"Jupiter's massive gravity field distorts the shape of Io in the same way that tides are raised in Earth's oceans by the gravitational tugs of the Sun and Moon," said Johnson. As Io orbits Jupiter, these so-called "body tides" rise and fall due to subtle changes in Io's orbit which, in turn, are caused by the gravitational nudges from Europa and Ganymede, other moons of Jupiter.
As a result, Io is squeezed like a rubber ball. Friction created by this action heats and melts rock within Io to produce the volcanoes and lava flows seen all over its surface and huge geysers that spew sulfur dioxide onto Io's landscape.
The large, dense core Galileo found within Io was deduced from data taken during the spacecraft's flyby within 899 kilometers (559 miles) of the moon last Dec. 7, as Galileo passed by Io on its way to enter orbit around Jupiter. Precise measurements of the spacecraft's radio signal revealed small deviations in Galileo's trajectory caused by the effects of Io's own gravity field.
From these data, Galileo scientists have determined that Io has a two-layer structure. At the center is a metallic core, probably made of iron and iron sulfide, about 900 kilometers (560 miles) in radius, which is overlain by a mantle of partially molten rock and crust, according to JPL's Dr. John Anderson, team leader of Galileo's celestial mechanics experiment and principal author of the paper published in Science today. The core was probably formed from heating in the interior of the moon, either when it originally formed or as a result of the perpetual tidal heating driving its volcanoes.
Galileo scientists are also trying to determine the cause of the hole they found in Jupiter's magnetic field when the spacecraft was closest to Io. "Instead of increasing continuously as the spacecraft neared Jupiter, the magnetic field strength took a sudden drop of about 30 percent," said Johnson.
"It's an astonishing result and completely unexpected," said Dr. Margaret Kivelson of the University of California at Los Angeles, who heads Galileo's magnetic fields investigation team. Preliminary analyses of these data are currently being prepared for formal publication.
"The data suggest that something around Io -- possibly a magnetic field generated by Io itself -- is creating a bubble or hole in Jupiter's own powerful magnetic field," Kivelson said. "But it's not clear to us just how Io can dig such a deep and wide magnetic hole."
Possible explanations for this signature can only be sorted out using data from all the other space physics instruments onboard Galileo, said Johnson. "We're eagerly awaiting the return of data from the magnetospheric measurements taken during the Io flyby to see if we can resolve this mystery," he said. This data, recorded onboard the spacecraft, will be transmitted back to Earth in June or July.
If analysis of this data eventually proves that Io indeed has a magnetic field of its own, it would be the first moon shown to have one. Io would join the Earth, planet Mercury and the outer giant planets as bodies in the Solar System that generate their own magnetic fields.
Other studies conducted by Galileo during its December flyby of Io have provided new evidence that Io is most likely the source of high velocity dust streams littering millions of miles of space around Jupiter.
In July 1994, Galileo's dust detector began sensing dust streams more powerful than those previously discovered by the Ulysses spacecraft. Dust detectors on Galileo sensed more and more particles during its approach to Jupiter, reaching a peak of 20,000 impacts per day during the longest and most intense interplanetary dust storm ever observed.
These fast-moving particles travel at speeds of 50 to 100 kilometers per second (30 to 60 miles per second) away from Jupiter -- fast enough to escape the Solar System. These dust impacts continued up to the time of Galileo's Io flyby and then ceased, said Dr. Eberhard Grun of Germany's Max Planck Institute in Heidelberg, who is principal investigator for Galileo's dust detector experiment.
"My preliminary interpretation of these observations is that they support the idea that Io is in some way the source of the Jupiter dust streams," Grun said.
One theory proposed after the NASA Voyager spacecraft flybys in the late 1970s is that dust particles emitted from Io's volcanoes could become electrically charged and then swept away by Jupiter's rotating magnetic field. Recent modifications to this theory suggest that the dust is subsequently accelerated in the magnetosphere and flung outward from Jupiter at high velocity, creating dust streams.
Galileo's next close encounter with a moon of Jupiter will occur June 27, when the spacecraft will pass about 850 kilometers (530 miles) above the surface of Ganymede. Larger than Mercury, Ganymede is the largest moon in the Solar System. Galileo will make repeated close flybys of Ganymede, Callisto and Europa during its two-year mission in orbit around Jupiter.
Galileo was launched aboard Space Shuttle Atlantis on Oct. 18, 1989. The mission is managed by JPL for NASA's Office of Space Science, Washington, DC.
Galileo scientists report revisions to the initial announcement that Jupiter's upper layers contain only half the amount of helium present in the primordial cloud from which the Solar System formed. Thorough reanalysis has proven that the abundance of helium in the atmosphere is 24%, not 13.7% as announced earlier.
At the rescheduled press conference about the results from the Galileo Probe, it was announced that the first results about a depleted abundance of water was incorrect. After correcting for a calibration error, scientists say that Jupiter has about the same abundance of water that is expected from the abundance of oxygen on the sun. This amount is still half as much found by Voyager and an order of magnitude less than predicted on the idea that comets have crashed into Jupiter in large quantity.
The mass spectrometer on the Probe found that the abundances of carbon and sulfur are also closer to their solar values. These measurements place in doubt the last theoretical models for Jupiter.
The determination that the helium abundance in the atmospheric layers of Jupiter through which the Probe fell is only half that of the sun can be accounted for if helium droplets rain down on Jupiter's core, an idea that had been invoked for Saturn's internal heating.
The Probe did not find the three layers of clouds that had been expected. For the moment, scientists are saying that the Probe went into an unusually cloud-free region, though such regions take up less than 2% of the atmosphere. This suggestion sounds a little suspicious to me. In any case, there is no last-minute high resolution imaging of the region where the probe entered, because the problem with the tape recorder on the Galileo orbiter that was to store the probe data led JPL engineers not to risk the loss of probe data by taking a disk image.
Though the winds had been expected to diminish as the Probe descended, the wind actually picked up from 100 m/s to 150 m/s and remained at that higher level until the end of the Probe's existence. Thus energy from Jupiter's interior instead of energy from sunlight must be driving the winds.
Finally for this brief summary, the Probe found that the rate of lightning on Jupiter is less than 1/3 that on Earth. The Probe detected about 50,000 radio signals from faraway lightning in addition to a few optical flashes.
The Probe survived for 57 minutes until the pressure reached about 23 bars, 23 times the Earth's surface pressure.
The Galileo orbiter will go so close to some of the satellites of Jupiter that resolution of 12 m (39 feet) should be obtained, 100,000 to a million times better than Galileo's original telescopic observations of these moons. But when will the photos come?
The problem with the tape recorder that occurred last October 11 led the mission to cancel plans to take photos of Io on December 7, when the Orbiter went closest to it. No closeup photos of Io will now be taken at all in the Galileo mission, though the photos that will be taken from farther away in the orbit will still surpass Voyager resolution. The tape recorder, necessary for sending photos, is full at present with the data from the Probe. These data will be sent back numerous times until scientists and engineers at JPL are content that the data are all accurately received. Then they will work on the tape recorder for a while to improve its usefulness for the orbital mission. Next, in March, a Perijove Raise maneuver will raise "perijove," the low point of the orbit, so that the spacecraft will not again traverse Jupiter's radiation belts. Thus the first time images would be taken could be April, though much of the time until mid-May will be used for loading some new software. It is therefore expected that images probably won't start being taken before mid-May 1996.
[reference: "Online from Jupiter, Update #23; to subscribe, send a message to listmanager@quest.arc.nasa.gov; the body of the message should be: subscribe updates-jup.]
The Galileo Probe plunged through the Jupiter atmosphere on December 7, 1995, sending back 57 minutes of data before it burned up in Jupiter's lower atmosphere. The first exciting result is that the abundance of water vapor is lower than expected. Since in the old of Jupiter's atmosphere, any oxygen present would have combined easily with hydrogen to form water ice, it was expected that there would be as high a percentage of water in Jupiter as there is for oxygen in the sun. And further, there is so much ice on Europa, Ganymede, and Callisto. But the Probe results show that there seems to be only about 1/10 the solar ratio of oxygen in the form of water.
A second preliminary result is an excess of krypton and xenon, both heavy inert gases. It is known from laboratory experiments that water ice traps xenon and krypton more easily than it does the lower-mass noble gases and comets are largely water ice. So the result may indicate that many comets plunged into Jupiter in its early history.
These first results still must be confirmed.
[reference: R. Cowen, "Inside Jupiter: Probe's Early Results," in Science News, vol. 148, December 23 & 30, 1995, p. 420]
The Galileo spacecraft arrived and did its job at Jupiter on December 7. On that date, the probe succesfully made its suicide plunge into Jupiter's atmosphere, sending data back to the other part of the spacecraft for about 55 minutes. Part of those data were sent to Earth in mid-December and the rest will be sent back to Earth in mid-January.
The other part of the spacecraft gathered data on the Io torus as it approached Jupiter, and then a rocket burn put it into orbit around Jupiter. The two-year orbital program were expected to bring it close to the following major moons of Jupiter at the following times, though the actual passage of the spacecraft near Io on the way into orbit gave it a little more speed than predicted, so at least the first of the encounters will be a week earlier than the time shown:
Ganymede July 4, 1996 Ganymede September 6, 1996 Callisto November 4, 1996 Europa December 19, 1996 Europa February 20, 1997 Ganymede April 5, 1997 Ganymede May 7, 1997 Callisto June 25, 1997 Callisto September 17, 1997 Europa November 6, 1997
Because of the trouble with the tape recorder, no images were recorded from the close passage of Io en route into orbit, though data about particles and fields were gathered. But even though the spacecraft will not go very close to Io in the rest of its missions, images that exceed the Voyager images in detail are expected.
The Galileo spacecraft, in August and September 1995, plowed through an intense interplanetary dust storm. The particles come from the Jupiter system, perhaps from volcanoes on Io or from Jupiter's rings. The dust particles may even be left over from last year's collision of Comet Shoemaker-Levy 9. Galileo counted up to 20,000 particles per day, compared to the normal interplanetary rate of about one particle every three days.
An infrared image taken at Mauna Kea on September 10/11, 1994, by Keith S. Noll, Diane Gilmore and David R. Soderblom at the NASA Infrared Telescope Facility. Filters between 1.2 and 2.2 microns were used. The images were taken with a 256x256 pixel infrared array called NSFCAM. The images appeared in Sky & Telescope for March 1995, p. 12.
The red band near the bottom of Jupiter's disk represents material floating high in Jupiter's atmosphere from the impact of Comet Shoemaker-Levy 9. High-level haze is also visible at the north and south poles.
photo courtesy K. Noll, D. Gillmore and D. Soderblom
Jupiter's moon Europa was found to have a thin atmosphere of oxygen. Spectra taken with the Hubble Space Telescope show barely enough molecular oxygen to be "collisionally thick," the condition where molecules collide with each other many times before escaping Europa's gravity. This condition makes it a true atmosphere. Apparently, the oxygen results from splitting of water vapor when hit by sunlight.
February 23, 1995, Nature.
Galileo images, including photos of the collision of Comet Shoemaker-Levy 9 with Jupiter, are included in the homepage of the Jet Propulsion Laboratory, a NASA laboratory in Pasadena.