August 25, 2000
Europa, the fourth largest satellite of Jupiter, has long been suspected of harboring vast quantities of water. Since life as we know it requires water, this makes the moon a prime target for the search of exobiology - life beyond Earth.
"The direction that a magnetic compass on Europa would point to flips around in a way that's best explained by the presence of a layer of electrically conducting liquid, such as saltwater, beneath the ice," explained Dr. Margaret Kivelson, one of five co- authors at the University of California, Los Angeles (UCLA).
Kivelson announced that conclusion when she first received telltale readings from the Galileo magnetometer after the veteran spacecraft flew near Europa in January. Her team details its theory about the liquid layer in this week's formal report.
"We have good reason to believe the surface layers of Europa are made up of water that is either frozen or liquid," Kivelson said, pointing out that earlier gravity measurements show a low density, such as water's, for the moon's outer portions. "But ice is not a good conductor, and therefore we infer that the conductor may be a liquid ocean."
Galileo has flown near Europa frequently since the spacecraft began orbiting Jupiter and its moons in December 1995. Pictures from those flybys show patterns that scientists see as evidence of a hidden ocean. In some, rafts of ice appear to have shifted position by floating on fluid below. In others, fluid appears to have risen to the surface and frozen.
However, those features could be explained by a past ocean that has subsequently frozen solid, said Galileo's project scientist, Dr. Torrence Johnson of NASA's Jet Propulsion Laboratory, Pasadena, CA. "This magnetometer data is the only indication we have that there's an ocean there now, rather than in the geological past," Johnson said.
Johnson said the case for liquid water on Europa is still not clinched. "The evidence is still indirect and requires several steps of inference to get to the conclusion there is really a salty ocean," he said. "A definitive answer could come from precise measurements of gravity and altitude to check for effects of tides."
NASA is planning a Europa Oribiter mission to carry instruments capable of providing that information. Magnetic evidence for an ocean is possible because Europa orbits within the magnetic field of Jupiter. That field induces electric current to flow through a conductive layer near Europa's surface, and the current creates a secondary magnetic field at Europa, the new report explains.
Key evidence that the magnetic readings near Europa result from this type of secondary effect, implying a saltwater layer, relies on timing. The direction of Jupiter's magnetic field at Europa reverses predictably as the moon's position within the field changes. During Galileo's flyby in January, the direction of Jupiter's field at Europa was the opposite of what it had been during passes in 1996 and 1998. Kivelson's team predicted how that would change the direction of Europa's magnetic polarity if Europa has a saltwater layer, and Galileo's measurements matched their prediction.
"It makes a very strong case that the source of the magnetic signature is a conducting layer near the surface," Kivelson said. Galileo's magnetometer is also expected to play an important role this fall and winter in joint studies of Jupiter while NASA's Saturn-bound Cassini spacecraft passes near Jupiter. Galileo will be inside Jupiter's magnetic field while Cassini is just outside it, in the solar wind of particles streaming away from the Sun. Scientists plan to take advantage of that positioning to learn more about how the solar wind affects the magnetic field.
Galileo completed its original mission nearly three years ago, but has been given a three-year extension and has survived three times the amount of radiation it was designed to endure.
Kivelson's UCLA co-authors are Drs. Krishan Khurana, Christopher Russell, Martin Volwerk, Raymond Walker, and Christopher Zimmer. The Galileo mission is managed for NASA's Office of Space Science, Washington, DC, by JPL, a division of the California Institute of Technology, Pasadena.
space.com
24 August 2000
It looks like it's a water world after all.
Data collected earlier this year by the Galileo spacecraft has now generated what some scientists are calling virtually undeniable evidence that Jupiter's moon Europa has a significant water ocean churning beneath its icy surface.
The data, which was collected by Galileo's magnetic-field-detecting instruments when the spacecraft flew close to the icy moon, showed that there is an electrically charged layer of some substance stirring possibly as close as 4.7 miles (7.5 kilometers) below the moon's ice crust. Planetary scientists say the most likely explanation for the data is a liquid-water ocean similar to oceans found on Earth - a possibility that has extraterrestrial life-seekers reeling with excitement.
Many observations are returned this week as the Galileo spacecraft continues to play back science data acquired during its May flyby of Ganymede, Jupiter's largest moon. Playback is interrupted on Thursday so the spacecraft can perform standard maintenance on its propulsion systems.
Parts of 24 observations are returned from the Solid-State Imaging camera (SSI), Near-Infrared Mapping Spectrometer (NIMS), Photopolarimeter Radiometer (PPR), and Fields and Particles instruments. The Fields and Particles instruments are the Dust Detector, Energetic Particle Detector, Heavy Ion Counter, Magnetometer, Plasma Detector, and Plasma Wave instrument.
In addition to playback, Galileo's Dust Detector is monitoring the dust environment surrounding the spacecraft. Periodic readouts of the instrument's memory during mid- and late July showed thousands of impacts occurring on some days. Real-time data collection will allow scientists to get better information on the size, speed, and origin of these micron- and submicron-sized particles.
First on the playback schedule, SSI returns portions of two context observations that were centered at the same locations on Ganymede as some high-resolution images that were returned previously. These two observations are part of a campaign of five high-resolution and five context observations that will allow scientists to get a better idea of how different features and terrains came into existence on Ganymede's surface. In addition, combination of the high-resolution and context observations for e
Throughout the week, the Fields and Particles instruments return portions of two observations. One is a 60-minute high-resolution recording of the plasma, dust, and electric and magnetic fields surrounding Ganymede, which is the only planetary moon known to have its own magnetic field. Portions of a month-long low-resolution survey of Jupiter's magnetosphere are the second set of data returned this week. The lengthy survey not only provides context for the high-resolution recording but also provides scientists with information on both the inner and outer regions of Jupiter's magnetosphere and the transition out into the solar wind.
NIMS and SSI continue next with the return of more observations of Ganymede. NIMS returns a scan performed just off of Ganymede's limb, which is designed to give scientists more information on Ganymede's tenuous atmosphere. SSI then returns an image of enigmatic smooth dark terrain, which is characterized by a wispy appearance. NIMS returns to the playback schedule with a spectral scan of the Perrine region on Ganymede, which will provide information on the distribution and composition of ice and non-ice materials.
PPR joins the fray with the return of observations of Ganymede and Europa. First, PPR returns information describing the daytime thermal properties of Ganymede's surface. Next, and throughout the week, PPR returns nine polarimetry observations of Europa. The polarimetry measurements will allow scientists to learn about surface texture and small-scale surface properties. Each of the nine observations is taken at a different solar phase angle.
Next, NIMS returns two more observations. The first consists of a high-resolution spectral map of Ganymede's entire disk. The second observation is a distant scan of Europa while the icy moon is in Jupiter's shadow. A very low signal is expected, but detection of an elevated signal would suggest the presence of anomalously warm regions of the surface. Such regions could be caused either by unusual surface materials or by the presence of recent ice-volcanic activity.
The remaining five observations are returned by PPR, with a shift in focus to Jupiter's atmosphere. Two observations return polarimetry measurements of the atmosphere, which will provide scientists with information on the structure and temperature of its upper levels. Next, PPR returns a thermal map of recently-merged white ovals in Jupiter's atmosphere. White ovals are storms that occur between two adjacent zonal jet streams, and last for decades. Two of them have merged within the past few months to create a single storm. Finally, PPR returns two scans of Jupiter's limb. These scans are designed to detect upwellings in Jupiter's atmosphere.
For more information on the Galileo spacecraft and its mission to Jupiter, please visit the Galileo home page.
Jet Propulsion Laboratory
July 11, 2000
Image Title: Europa Impact Crater Catalog #: PIA02561 Target Name: Europa Is a satellite of: Jupiter Mission: Galileo Instrument: Near Infrared Mapping Spectrometer Product Size: 950 samples x 650 lines Produced By: JPL Producer ID: MRPS96437 Creation Date: 2000-07-10 Primary Data Set: Galileo EDRs Full-Res TIFF: PIA02561.tif (883 kbytes)
Original Caption Released with Image:
A newly discovered, city-sized impact crater viewed by NASA's Galileo spacecraft may shed new light on the nature of the enigmatic icy surface of Jupiters moon Europa.
This false-color image reveals the scar of a past major impact of a comet or small asteroid on Europa'ss surface. The bright, circular feature at center right has a diameter of about 80 kilometers (50 miles), making it comparable in size to the largest cities on Earth. The area within the outer boundary of the continuous bright ring is about 5,000 square kilometers (nearly 2,000 square miles). The diameter of the darker area within the bright ring is about 29 kilometers (18 miles), which is large enough to contain both the city of San Francisco and New Yorks Manhattan Island, side by side.
The brightest reds in this image correspond to surfaces with high proportions of relatively pure water ice, while the blue colors indicate that non-ice materials are also present. The composition of the darker materials is controversial; they may consist of minerals formed by evaporation of salty brines, or they may be rich in sulfuric acid. The bright ring is a blanket of ejecta that consists of icy subsurface material that was blasted out of the crater by the impact, while the darker area in the center may retain some of the materials from the impacting body. Further study may yield new insights about both the nature of the impactor and the surface chemistry of Europa.
Europa's surface is a question of great interest at present, since an ocean of liquid water may exist beneath the icy crust, possibly providing an environment suitable for life. Geologic investigations of Europa?s surface are underway, and a new spacecraft mission, the Europa Orbiter, is planned.
Impact craters with diameters of 20 kilometers (12 miles) and larger are extremely rare on Europa; as of 1999 only 7 such features were known. The rarity of larger impact craters on Europa lends greater significance to the discovery of this one. Impact crater counts are often employed to estimate the ages of the exposed surfaces of planets and satellites, and the small number of craters found on Europa implies that the surface may be quite young in geological terms. Thus the discovery of this feature may provide additional insights into questions about the age and level of geological activity of Europas surface.
Impact craters are expected to form with greater frequency on the 'leading' sides of satellites that always turn the same face to their primary planet, in this case, Jupiter. The process is much like the effect of running through a rainstorm. The 'apex' of Europa's leading side is located on the equator at 90 degrees West longitude, only about 10 degrees removed from the feature shown. Europa's leading side does not receive a continuous bombardment by ionized particles carried along by Jupiter's rapidly rotating magnetosphere (as is the case for the trailing side), which may allow greater preservation of the chemical signatures of the impacting object.
To the east of the bright ring-like feature are two, or perhaps three, similar but less well-defined quasi-circular features, raising the possibility that this crater is one member of a catena, or chain of craters. This would lend still greater interest to this area as a potential target for focused investigations by later missions such as the Europa Orbiter.
The near-infrared mapping spectrometer on board Galileo obtained this image on May 31, 1998, during that spacecraft's 15th orbital encounter with Europa. The image data was returned to Earth in several segments during both the 15th and the 16th orbital periods. Merging and processing of the full data set was accomplished in 1999. Analysis and interpretation are ongoing.
Galileo has been orbiting Jupiter and its moons since December 1995. Its primary mission ended in December 1997, and after that Galileo successfully completed a two-year extended mission. The spacecraft is in the midst of yet another extended journey called the Galileo Millennium Mission. More information about the Galileo mission is available at: http://galileo.jpl.nasa.gov.
JPL manages Galileo for NASA' s Office of Space Science, Washington, D.C. JPL is a division of the California Institute of Technology in Pasadena.
NASA Headquarters, Washington, DC
Jet Propulsion Laboratory, Pasadena, CA
January 10, 2000
As the spacecraft flew 218 miles (351 kilometers) above the icy moon on January 3, its magnetometer instrument studied changes in the direction of Europa's magnetic field. Galileo's magnetometer observed directional changes consistent with the type that would occur if Europa contained a shell of electrically conducting material, such as a salty, liquid ocean.
"I think these findings tell us that there is indeed a layer of liquid water beneath Europa's surface," said Dr. Margaret Kivelson, principal investigator for the magnetometer. "I'm cautious by nature, but this new evidence certainly makes the argument for the presence of an ocean far more persuasive."
It appears that the ocean lies beneath the surface somewhere in the outer 60 miles (about 100 kilometers), the approximate thickness of the ice/water layer, according to Kivelson, a researcher at the University of California, Los Angeles (UCLA).
"Jupiter's magnetic field at Europa's position changes direction every 5-1/2 hours," Kivelson explained. "This changing magnetic field can drive electrical currents in a conductor, such as an ocean. Those currents produce a field similar to Earth's magnetic field, but with its magnetic north pole -- the location toward which a compass on Europa would point -- near Europa's equator and constantly moving. In fact, it is actually reversing direction entirely every 5-1/2 hours."
On previous Europa flybys, Galileo identified a magnetic north pole, but did not determine whether its position changes with time. "We wondered, 'Was it possible that the north pole did not move?' " Kivelson said.
The new evidence was gathered during a flyby specially planned so that the observed position of Europa's north pole would make it clear whether or not it moves. In fact, Monday's data showed that its position had moved, thus providing key evidence for the existence of an ocean.
It is not likely that the electric currents on Europa flow through solid surface ice, Kivelson explained, because ice is not a good carrier of currents. "But melted ice containing salts, like the sea water found on Earth, is a fairly good conductor," she said.
There is no other likely current-carrying material near Europa's surface, Kivelson added. "Currents could flow in partially melted ice beneath Europa's surface, but that makes little sense, since Europa is hotter toward its interior, so it's more likely the ice would melt completely. In addition, as you get deeper toward the interior, the strength of the current-generated magnetic field at the surface would decrease."
These latest findings are consistent with previous Galileo images and data showing a tortured surface seemingly formed when Europa's surface ice broke and rearranged itself while floating on a sea below. Further theoretical work is under way to analyze the fluid layer and its properties.
"It will be interesting to see whether this same type of phenomenon occurs at Jupiter's moon Ganymede," Kivelson said. Galileo is tentatively scheduled to fly by Ganymede twice this year.
Kivelson is joined in her magnetometer studies by Drs. Krishan Khurana, Christopher Russell, Raymond Walker, Christophe Zimmer, Martin Volwerk of UCLA, as well as Steven Joy and Joe Mafi, also of UCLA, and Dr. Carole Polanskey of NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA.
Additional information and pictures taken by Galileo are available at
The Galileo mission is managed for NASA's Office of Space Science, Washington, D.C. by JPL, a division of the California Institute of Technology, Pasadena, CA.
University of Arizona
September 16, 1999
Until now, there have been no good ideas as to what formed these bizarre "cycloidal" features, or "flexi," as they were officially dubbed by the International Astronomical Union.
Now, planetary scientists at the University of Arizona in Tucson provide a model for how these features are created. It is perhaps the most convincing evidence yet for a global ocean. They report on it in today's issue of Science (Sept. 17).
Scientists know that Europa has a 100-mile-thick layer of water -- 20 times thicker than the Earth's oceans -- but the visible top layer is frozen. This new strong evidence for a liquid global ocean below the surface makes Europa a prime target in the search for life beyond Earth.
Gregory V. Hoppa, B. Randall Tufts, Richard Greenberg and Paul E. Geissler of the UA Lunar and Planetary Laboratory theorize that cycloidal cracks form in Europa's solid-ice surface with the daily rise and fall of tides in the subsurface ocean. They painstaking modeled and scrutinized images of Europa taken by the Galileo spacecraft between 1996 and 1999. The new images show that cycloidal cracks and ridges are widely distributed over all of the moon.
Hoppa has posted images and explanatory animation of cycloidal crack formation on the web site:
http://pirlwww.lpl.arizona.edu/~hoppa/science.html
Europa is about the size of our moon. Tidal stresses on its ice-covered ocean ebb and flow as it orbits Jupiter, which is 300 times as massive as Earth. According to the UA researchers' model, Europa's ocean tides rise and fall a distance of 30 meters. By comparison, tides at most ocean beaches on Earth rise and fall 1 to 2 meters, or 4 to 6 feet.
"What causes the cycloid to form is that Europa is in a slightly eccentric orbit because of Io and Ganymede (other Jovian moons). Sometimes Europa is a little closer, other times a little farther from Jupiter. When Europa is closer to Jupiter, the tides are higher because Jupiter is pulling on it more. When Europa is farther, the tides fall because Jupiter's force falls. This causes Europa's ice shell to flex."
The UA model shows that when tidal stress reaches the tensile strength of ice, the ice begins to crack. It takes very little stress to form the initial crack -- something like the force it takes to break a saltine cracker -- because Europa's surface ice is weakened by countless linear fractures.
The crack propagates relatively slowly across the ever-changing stress field. It moves following a curving path until stress drops below the tensile strength of the ice, when it halts. A few hours later, when tidal stress again exceeds the tensile strength of ice, the crack begins a new curve in another direction.
"You could probably walk along with the advancing tip of a crack as it was forming -- if you could survive Europa's radiation environment," Hoppa said. "And while there's not enough air to carry sound, you would definitely feel vibrations as it formed."
One of their most striking conclusions is that each arc segment forms in 3.5 days -- the time it takes Europa to make one complete orbit around Jupiter. The cycloids faithfully record the 85-hour daily flexing of Europa's ice shell just as trees faithfully record each growing season in annual rings.
"We can look at a crack that has 4 or 5 cusps, each formed every 3.5 days, and know that the entire chain formed in about 2.5 weeks," Hoppa said.
Arc segments in the cycloid, each ranging from 75 km to 200 km long, form cracks stretching a thousand kilometers over the ice in a fraction of an instant in geological time. Eventually, cracks evolve into ridges, typically double ridges, according to the UA model.
The scientists also can determine which direction the cracks traveled as they formed based on the orientation of the arcs and the hemisphere in which they are found.
"What amazes me about this is just how long these features have been a mystery," Hoppa said. "We've been staring at pictures of them for 20 years, since Voyager. We didn't know what made them. And it seems what they've been telling us all along is that an ocean was there when these things formed."
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
September 16, 1999
With its blasting geysers and bubbling thermal vents, "it's almost like being there," said Leslie Lowes of JPL, lead outreach coordinator for NASA's Galileo mission studying Jupiter and its moons. "Yellowstone is the closest we can come to taking teachers to Io without actually putting them on a spacecraft."
About a dozen educators, escorted by Lowes and two JPL scientists, Drs. Rosaly Lopes-Gautier and Bill Smythe, will travel to Yellowstone from September 23 through 25 for a workshop on Io, the most volcanic body in the solar system. Once they learn about Io and volcanism in our solar system, they'll hold teacher-training workshops in their own communities.
The event is particularly timely because NASA's Galileo spacecraft is gearing up for two close flybys of Io on October 10 and November 25. During the flybys, Galileo's onboard camera will snap the closest, highest-resolution pictures ever taken of Io.
These daring flyby adventures have their risks, because Io's orbit is located in a region brimming with radiation from Jupiter. The Galileo flight team is trying to prepare for any problems that may pop up when the radiation bombards the spacecraft's instruments and computer systems.
Attending the Yellowstone workshop, coordinated by the Challenger Center for Space Science Education, Alexandria, VA, are Galileo Educator Fellows, who have spent the past year-and-a- half helping teachers understand the mission's discoveries and demonstrating related classroom activities.
In addition to the Yellowstone workshop, several other events are planned for educators who want to learn more about Io and the upcoming Galileo encounters:
JPL manages the Galileo mission for NASA's Office of Space Science, Washington, D.C. JPL is a division of the California Institute of Technology, Pasadena, CA.
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