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Galileo Quick-Look Fact sheets for each orbit of Galileo's extended mission (GEM)

Galileo Europa Mission (GEM)

This Week on Galileo

May 22-28, 2000

Jupiter continues to dominate Galileo's observation schedule on Monday as instruments focus on the planet's aurora and its atmosphere, including the Great Red Spot. The spacecraft makes its closest approach to Callisto at 4:39 p.m. PDT [see Note 1] Monday at a range of 331,000 kilometers (206,000 miles).

The first three observations of the week are spectral scans of Jupiter's bright limb, performed by the Near-Infrared Mapping Spectrometer (NIMS). Like observations performed on Sunday, May 21, these scans will be used to provide scientists with data on the bulge created at Jupiter's equator as Jupiter rotates once every 10 hours. NIMS follows these observations with a series of 10 spectral scans of Jupiter's north polar region. The series consists of 10-minute samples separated by 60 minutes, and is designed to capture auroral activity.

Interspersed with the aurora observations are three global observations performed by NIMS and three Great Red Spot observations performed by the Solid-State Imaging camera (SSI). Taken together, the global observations performed by NIMS will map a substantial proportion of the complete range of longitudes of the planet. The Great Red Spot observations performed by SSI will provide scientists with high spatial and time resolution images of this storm, which is over 400 years old. Similar observations were taken at the beginning of Galileo's orbital tour in June 1996, so scientists will be able to observe long-term changes in the characteristics of the Great Red Spot. The Great Red Spot is so large that the Earth fits two times across it!

A brief interlude from Jupiter is afforded to SSI to obtain a set of global images of Europa through four different color filters. The images will fill in a gap in existing global color coverage between 120 and 230 degrees longitude. Five more Jovian equatorial bulge observations are then performed by NIMS. The first three occur on Monday night, with the remaining two performed on Tuesday morning. Next, SSI takes a set of global images through different color filters that capture Europa while eclipsed from the Sun by Jupiter. The observation should help scientists search for auroral glows in Europa's tenuous atmosphere. These glows would be produced by the interaction of atmospheric gases with Jupiter's magnetosphere, and may produce current flow between Europa and Jupiter. The geometry and timing of this observation are superior to those of similar observations taken earlier in the mission.

SSI's eclipse observation brings to a close the bulk of this encounter's observing. In previous encounters, playback of recorded data was initiated at this point. But Galileo is not yet finished collecting data. The Fields and Particles instruments will continue observing for nearly one month. This will allow them to extend their typical survey of the inner magnetosphere (performed at each encounter) through the outer magnetosphere, and through the transition from inside Jupiter's vast magnetosphere into the solar wind. Some data will be returned in realtime and some will be placed on the tape recorder for return after the conclusion of the survey.

The survey is interrupted once during the remainder of the week. On Wednesday, the spacecraft will perform a flight path adjustment, if needed.

Note 1. Pacific Daylight Time (PDT) is 7 hours behind Greenwich Mean Time (GMT). The time when an event occurs at the spacecraft is known as Spacecraft Event Time (SCET). The time at which radio signals reach Earth indicating that an event has occurred is known as Earth Received Time (ERT). Currently, it takes Galileo's radio signals 50 minutes to travel between the spacecraft and Earth.


Galileo Millennium Mission Status

May 22, 2000

NASA's Galileo spacecraft has successfully flown past the largest moon in our solar system -- Ganymede, which orbits around Jupiter.

Galileo dipped to 809 kilometers (503 miles) above the surface early Saturday, May 20. This was the spacecraft's first flyby of Ganymede since May 7, 1997.

"It's great that things went so smoothly," said Galileo Project Manager Jim Erickson of NASA's Jet Propulsion Laboratory. "The team was ready for any problems, but they got to relax on this one. We're really looking forward to the new pictures and learning more about this largest of all moons."

At 4 a.m. PDT, mission controllers at JPL received the signal indicating that the flyby took place. The spacecraft's camera and other instruments were set to capture the flyby with images and other observations. If all goes as planned, the data will be transmitted to Earth over the next several months for processing and analysis.

To fly by Ganymede, Galileo had to approach Jupiter's powerful radiation belts. Not surprisingly, the radiation, which can affect spacecraft instruments, components and systems, did cause two main resets of Galileo's main computer. Onboard software correctly diagnosed this as a false indication, and went ahead with the Ganymede encounter unaffected.

"It appears that this workhorse spacecraft has done it again," Erickson said. Galileo has already survived three times the radiation it was designed to withstand.

Ganymede is even larger than Mercury and Pluto. Its surface is a mixture of clean, white ice and dirty, dark ice, with varied geological formations, including craters, basins, grooves and rough mountain areas.

Additional information about the Galileo mission is available at

Galileo was launched from the Space Shuttle Atlantis on October 18, 1989. After a long journey to Jupiter, Galileo began orbiting the huge planet and its moons on December 7, 1995, and successfully completed its two-year primary mission on December 16, 1997. That was followed by a two-year extended mission which concluded in December 1999, and Galileo is now continuing its studies under yet another extension, called the Galileo Millennium Mission. JPL, a division of the California Institute of Technology in Pasadena, Calif., manages the Galileo mission for NASA's Office of Space Science, Washington, D.C.

Today on Galileo

Sunday, May 21, 2000

The focus of Galileo's encounter turns away from Ganymede and Europa today, and toward observations of Jupiter, Jupiter's rings and Io. Observing activities are interrupted once today while the spacecraft performs a standard gyroscope performance test and a test to slew its scan platform.

The Photopolarimeter Radiometer (PPR) is first to observe this morning, taking data for a thermal map of the recently-merged white ovals in Jupiter's atmosphere. White ovals are storms that occur between two adjacent zonal jet streams, and have lasted for decades. However, two of them have merged within the past few months to create a single storm. Next, PPR performs two scans of Jupiter's limb. These observations are designed to detect upwellings in Jupiter's atmosphere. The Near-Infrared Mapping Spectrometer (NIMS) continues observation of Jupiter with a spectral scan of its North Equatorial Belt. The NIMS observation is performed in realtime, which means that the data are not stored on the spacecraft's tape recorder, but rather are directly transmitted to Earth after processing and packaging.

The Plasma Wave instrument (PWS) performs the next observation in conjunction with instruments on the Cassini spacecraft. The Cassini spacecraft is approaching Jupiter enroute to arrival at Saturn in 2004. Cassini will pass closest to Jupiter in December 2000, where it will perform more coordinated observations with Galileo. The current joint observation is designed to study the properties of radio-frequency emissions from Jupiter.

NIMS returns to observing Jupiter's atmosphere with a spectral scan of the North Temperate Zone. This observation is again performed in realtime. PPR is next to observe, but this time the target is Io. In a pair of observations taken at different solar phase angles, PPR will obtain data on the texture and small-scale properties of Io's surface. The Solid-State Imaging camera (SSI) shifts attention to another target by capturing a few images of Jupiter's rings. The images will be taken at relatively high resolution, low solar phase angle, and high tilt angle. They will provide scientists with better determinations of the size and distribution of ring particles, both within Jupiter's main ring, and at its inner edge. The images will also attempt to detect wavelike features in the main ring and undulations in the ring's outer boundary, which would be important for understanding how the rings are maintained by the small inner satellites.

NIMS wraps up the observing day with six more observations. The first four are realtime scans of Jupiter's North Equatorial Belt and North Temperate Zone. For the last two observations, NIMS performs spectral scans of Jupiter's bright limb. The scans will be used to provide scientists with data on the equatorial bulge created by Jupiter's rotation.

Think the fun is over? Think again! Come back tomorrow for more observations of Jupiter's atmosphere.

Saturday, May 20, 2000

A full day lies ahead for Galileo as its instruments perform observations of Ganymede, Europa, Jupiter and the Jovian magnetosphere. The spacecraft flies over the surface of Ganymede at 3:10 a.m PDT today [see Note 1] at an altitude of 808 kilometers (502 miles) and a speed of 11.3 kilometers per second (7.0 miles per second, or 25,200 miles per hour). Galileo also makes its closest approach to Europa, Jupiter and Io at 2:29 p.m., 9:53 p.m., and 11:40 p.m. PDT, at ranges of 595,000 kilometers (370,000 miles), 479,000 kilometers (298,000 miles), and 380,000 kilometers (236,000 miles), respectively.

During the Ganymede flyby, the spacecraft will pass behind this largest of the Galilean moons as seen from the Earth and Sun. The solar occultation is uneventful as Galileo does not use solar power to operate. However, Galileo's transmissions to Earth will pass through Ganymede's tenuous atmosphere (as they did yesterday with Jupiter's atmosphere) as the spacecraft moves behind the icy moon, until they are completely blocked from reaching Earth. About 30 minutes later, the spacecraft will emerge from behind Ganymede and communications will be restored. As with yesterday's Jupiter occultation, during this passage, Galileo's radio signal is weakened and refracted by the tenuous atmosphere. The changes in the signal are again measured by radio scientists to learn more about the structure and electron density of Ganymede's tenuous atmosphere.

The Fields and Particles instruments also take advantage of the flyby to perform a 60-minute high-resolution recording of the plasma, dust, and electric and magnetic fields surrounding Ganymede. Ganymede is the only planetary moon known to have its own internally-generated magnetic field, and thus, its own magnetosphere. During the recording, Galileo hopes to actually penetrate magnetic field lines that both originate and close on Ganymede's surface. This will allow scientists to obtain a far more complete understanding of how the magnetic fields and magnetospheres of both Ganymede and Jupiter interact with one another.

The first remote sensing observation of the day is performed by the Photopolarimeter Radiometer (PPR) as it takes high-resolution thermal measurements of Ganymede's surface. Remote sensing observations occur both during the 60-minute Fields and Particles recording, and afterwards as the spacecraft moves away from Ganymede. Next, the Plasma Wave instrument (PWS) performs an observation dedicated to the detection of chorus emissions within Ganymede's magnetosphere. A chorus signal is seen in the electromagnetic fields measured by PWS when plasma is being accelerated by a particularly efficient type of wave-particle interaction. By detecting and analyzing chorus emissions, scientists hope to understand more about how Ganymede's unique magnetosphere operates.

The PWS observation is followed by a series of five observations of Ganymede performed by the Solid-State Imaging camera (SSI). The observations are designed to provide scientists with information regarding questions of how different features and terrains come to exist on Ganymede's surface. The regions examined in these mosaics are believed to have been created by processes internal to Ganymede. However, are the processes volcanic, tectonic, or from some other mechanism? This observation set may help answer the question. The first mosaic of images captures smooth bright terrain and possibly sheltered grooved terrain. The second looks at a transition region between bright and dark terrain. Yet another mosaic contains pristine dark terrain, believed to be the oldest type of terrain on Ganymede. The fourth observation captures another region of smooth bright terrain containing bands with a smooth, 'plank-like' appearance. Finally, the last mosaic of images captures a caldera-like feature.

The Near-Infrared Mapping Spectrometer (NIMS) takes the next observation of Ganymede. In it, NIMS obtains a spectral scan of a dark crater, surrounding ice, and background dark regions. The scan will allow scientists to determine if there are any differences in the composition of these different types of terrains. Then, SSI returns to the observation schedule with five mosaics centered at the locations of the high-resolution mosaics taken earlier, but covering a much wider area of the surface. These images will provide the geologic context for the high-resolution samples. In addition, the motion of the spacecraft along its flight path will allow stereo images to be produced by combining data from the two sets of images.

NIMS takes another look at Ganymede by performing a scan just off of the moon's limb. The observation should allow scientists to gain more knowledge on the characteristics of Ganymede's tenuous atmosphere. SSI then takes another image of Ganymede, this time of enigmatic smooth dark terrain with a wispy appearance to it. Then, NIMS performs a spectral scan of the Perrine region of Ganymede's surface. Again, the scan will provide scientists with much desired information about the composition of the region. PPR is next on the observation plan. In an observation of Ganymede's dayside, PPR gathers information on the thermal properties of the surface in the presence of daylight.

PPR continues in the observation spotlight by shifting attention from Ganymede and initiating a series of polarimetry observations of Europa. Polarimetry measurements allow scientists to learn about surface texture and small-scale surface properties. For the remainder of the day, PPR makes nine observations of Europa at different solar phase angles. Interspersed with these observations are two observations performed by NIMS. The first is a return to Ganymede and is designed to provide a high-resolution spectral map of Ganymede's entire disk. This global map can then be used for global scale comparisons of Ganymede to the other Galilean satellites. The second observation is a distant view of Europa while the moon is in Jupiter's shadow. A very low signal is expected, but detection of an elevated signal would suggest the presence of anomolously warm regions of the surface, due to either unusual surface materials, or the presence of recent ice-volcanic activity on Europa.

The last two observations of the day turn their attention to Jupiter's atmosphere. Both performed by PPR, they are designed to capture polarimetry measurements of the atmosphere, which will provide scientists with information on the structure and temperature of its upper levels.

The excitement is not over. Come back tomorrow for more discoveries with Galileo!

Note 1. Pacific Daylight Time (PDT) is 7 hours behind Greenwich Mean Time (GMT). The time when an event occurs at the spacecraft is known as Spacecraft Event Time (SCET). The time at which radio signals reach Earth indicating that an event has occurred is known as Earth Received Time (ERT). Currently, it takes Galileo's radio signals 50 minutes to travel between the spacecraft and Earth

NASA Science News for May 19, 2000

This weekend NASA's Galileo spacecraft will pass 808 km above the surface of our solar system's largest moon, Ganymede. The spacecraft will hunt for signs of mysterious "cryptovolcanoes" and collect new data on Ganymede's unique magnetic field. This story includes plasma wave audio sounds from Ganymede's magnetosphere recorded during a previous flyby.

Great Ganymede!

For images and Web links for these items, visit
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On February 22nd, the Galileo spacecraft made its third and closest flyby of Jupiter's volcanically active moon Io. At 14:32 Universal Time (9:32 a.m. EST), the spacecraft passed only 199 kilometers above Io's surface. Images and data will be transmitted and analyzed over the coming weeks.

Although battered by Jupiter's strong radiation, Galileo continues to provide useful imagery and other data. While it isn't completely official, the Galileo spacecraft is now operating under its second mission extension. The first extension, called the Galileo Europa Mission, ostensibly ended on January 31st, following a flyby of Europa on January 3rd. As long as Galileo works, researchers hope to continue using it. Its future schedule includes another Io flyby on February 20th, flybys of Ganymede on May 20th and December 28th, and joint observations of Jupiter with the Cassini spacecraft at the end of the year.

NASA Space Science News for Jan 3, 2000

Happy New Year, Europa: On the eve of another extended mission, NASA's Galileo spacecraft swooped past Jupiter's icy moon Europa on Monday morning at an altitude of just 351 kilometers.


Galileo Mission Status

January 3, 2000

NASA's Galileo spacecraft has kicked off the new year with a successful flyby of Jupiter's icy moon Europa. The spacecraft swooped past Europa at an altitude of 351 kilometers (218 miles) today at 10:38 a.m. Pacific Standard Time.

The spacecraft is operating normally, and it appears that its instruments have completed their observations of the magnetic fields and charged particles around Europa. These observations were designed to detect any magnetic disturbances that may occur because of electrical currents set up in an ocean that may lie beneath Europa's icy crust. The prospect of a liquid ocean on Europa is intriguing, since water is one of the ingredients essential for life.

Because Galileo passed behind Europa during the flyby, its radio signal to Earth was blocked for a while. Scientists took advantage of this situation by studying the way the radio signal changed as the spacecraft entered this "silent zone." These radio science experiments teach us more about Europa's ionosphere -- the region of charged particles surrounding the moon -- and any possible atmosphere.

Observations of three of Jupiter's small natural satellites -- Amalthea, Thebe, and Metis -- are planned for Galileo this evening, with observations of Jupiter's volcanic moon Io on the spacecraft's agenda for the early morning. All data gathered during this flyby are being stored on Galileo's onboard tape recorder. They will be transmitted to Earth during the coming weeks.

Galileo has been orbiting Jupiter and its moons since December 1995, beaming to Earth unprecedented images and other information. JPL, a division of the California Institute of Technology, Pasadena, CA, manages the Galileo mission for NASA's Office of Space Science, Washington, DC.

NASA Space Science News for November 28, 1999

Galileo's No Turkey" On Thanksgiving Day, NASA's Galileo spacecraft survived another daring encounter with Jupiter's fiery moon Io, but not without giving ground controllers something to worry about.

NASA Space Science News for October 24, 1999

Io Close-up: On Friday JPL released the closest-ever picture of Io obtained during a daring flyby of the volcanic moon earlier this month. In spite of severe radiation hazards, the spacecraft obtained high quality images of a lava field near the center of an erupting volcano.


October 11, 1999


NASA's Galileo spacecraft has successfully zipped past Jupiter's moon Io, the most volcanic body in our solar system.

Instruments onboard the spacecraft peered down at Io from an altitude of only 611 kilometers (380 miles) at 10:06 p.m. Pacific Daylight Time on Sunday. This was the closest look at Io by any spacecraft, and Galileo's cameras were poised to capture the brief encounter.

If all goes as planned, the data will be transmitted to Earth over the next several weeks and then will undergo processing by mission scientists. New pictures would then be released at a press briefing tentatively scheduled next month.

"We're thrilled that the spacecraft handled this flyby so well, particularly because it had to endure a strong dose of radiation from Jupiter," said Jim Erickson, Galileo project manager at NASA's Jet Propulsion Laboratory, Pasadena, CA. "It appears at this point that everything went well."

Because Io is the innermost of Jupiter's moons, it lies in a region with the highest levels of radiation from Jupiter, which can wreak havoc with spacecraft instruments.

During this Io flyby, it appears the radiation did trigger an error of the onboard computer's memory, which put the spacecraft in a "safe mode," halting all non-essential activities while awaiting further commands from the ground. That occurred Sunday morning at 3:09 a.m Pacific time. Galileo engineers scrambled to prepare new commands to help the spacecraft work around the problem. The commands were transmitted to the spacecraft late Sunday afternoon, they worked as hoped, and Galileo resumed full operations at 8 p.m. Pacific time, just two hours before the Io flyby.

"It was a heroic effort to pull this off, "Erickson said. "The team diagnosed and corrected a problem we'd never come across before, and they put things back on track."

"We look forward to seeing the closest-ever pictures of Io," said Dr. Duane Bindschadler, Galileo manager of science operations and planning. "We want to learn more about the differences and similarities between volcanoes on Io and volcanoes on Earth." During the flyby, Galileo's science instruments studied the surface chemistry, heat, gravity and magnetic properties of Io.

The flyby took place while Galileo was 598 million kilometers (372 million miles) from Earth. A second, closer flyby of Io by Galileo is planned for the evening of November 25 Pacific time (November 26 Eastern time) at an altitude of 300 kilometers (186 miles).

NASA Space Science News for October 11, 1999

Galileo Survives Io Flyby: NASA's Galileo spacecraft has successfully zipped past Jupiter's moon Io, the most volcanic body in our solar system. Instruments onboard the spacecraft peered down at Io from an altitude of only 611 kilometers (380 miles) at 10:06 p.m. Pacific Daylight Time on Sunday. This was the closest look at Io by any spacecraft, and Galileo's cameras were poised to capture the brief encounter.

NASA Space Science News for October 8, 1999

Science@NASA is pleased to announce, a new web site dedicated to the coverage of science news from Galileo's daring flybys of Jupiter's volcanic moon. also explains how to see Io from your own backyard. Details concerning the October 10 Galileo flyby and newly released images of Io are available in today's headline story: "Galileo has a hot date with Io."

Galileo has a hot date with Io

Space-pizza (Nederlandse tekst met foto's van vulkanen op Io)

NASA Space Science News for October 4, 1999

Io's Mysterious Volcanoes -- Scientists are eager for a closer look at the solar system's strangest and most active volcanoes when Galileo flys by Io on October 11. This article explores what we know about Io's volcanoes and what researchers hope to learn from next week's daring encounter.

NASA Space Science News for Sept. 16, 1999

Io or Bust -- This morning, NASA's Galileo spacecraft will fly less than 670 km above Jupiter's moon Callisto. The purpose of the encounter is to alter Galileo's orbit and send the craft hurtling toward Io for a rendevous in October. Getting there might not be easy. Galileo has to survive exposure to radiation from Jupiter's inner magnetosphere before it can reach Io for a close-up look at that moon's fiery volcanoes.

NASA Space Science News for August 27, 1999

Galileo takes a closer look at Io: NASA has released new high resolution pictures of Jupiter's volcanic moon captured during Galileo's closest flyby since 1995.

NASA Space Science News -- Feb. 2, 1999: Galileo buzzes Europa
This weekend the Galileo spacecraft passed a scant 894 miles above the surface of Jupiter's moon Europa. Scientists are intrigued by Europa because of mounting evidence that liquid water exists beneath its frozen surface. Recent images reveal a complex terrain in 3D featuring cryo-volcanoes, gigantic faults and new evidence for possible oceans.


December 7, 1998


New pictures from NASA's Galileo spacecraft show a closeup view of a fault, or fracture, on Jupiter's icy moon Europa that stretches as long as the California segment of the infamous San Andreas fault.

The Europan fault, known as Astypalaea Linea (pronounced ast-ipp-uh-LAY-uh LINN-ee-uh) was first discovered in 1996 when Dr. Randy Tufts, Galileo imaging team affiliate and research associate at the University of Arizona, Tucson, AZ, reviewed distant images taken years earlier by NASA's Voyager spacecraft. The new mosaic of Galileo images released today captures a 290- kilometer-long (180-mile) portion of the fault in Europa's icy surface. Scientists calculate its full length at about 810 kilometers (more than 500 miles), about the same distance as the part of the San Andreas fault that runs from the California-Mexico border north to the San Francisco Bay.

"Comparisons between this Europan fault and faults on Earth may generate ideas we can use in studying earth movements here on our planet," said Tufts. "In addition, Astypalaea Linea is simply a beautiful structure."

The new Galileo images show that about 50 kilometers (more than 30 miles) of movement, or "displacement," has taken place along the fault, which is located near Europa's South Pole. Bends in the fault have allowed the surface to be pulled apart as this movement took place along Astypalaea Linea, which is the largest known strike-slip fault on Europa and one of the largest strike-slip faults known to exist anywhere. A strike-slip fault is one in which two crustal blocks move horizontally past one another, somewhat like two opposing traffic lanes.

This pulling-apart along the fault's bends created openings through which warmer, softer ice from below Europa's brittle ice shell surface, or frozen water from a possible subsurface ocean, could reach the surface. This upwelling of material formed large areas of new ice within the boundaries of the original fault. A similar pulling-apart phenomenon can be observed in the geological trough surrounding California's Salton Sea, and in Death Valley and the Dead Sea. However, in those cases, the pulled-apart regions can include upwelled materials, but may be mostly composed of sedimentary and erosional material deposited from above.

Tufts believes Astypalaea Linea is probably no longer active, because large ridges formed more recently crosscut it without interruption. Opposite sides of the fault can be reconstructed in puzzle-like fashion, matching the shape of its sides as well as individual older lined areas that had been broken by its movements. The overall motion along the fault seems to have followed a continuous narrow break along the entire length of the feature, with a path resembling steps on a staircase crossing the pulled-apart zones. Between the zones, this break coincides with ridges that separate them.

Tufts and fellow University of Arizona researchers, in a group led by Dr. Richard Greenberg, suspect that the fault motion is induced by the pull of variable daily tides generated by Jupiter's gravitational tug on Europa's icy crust. This tidal effect produces a phenomenon they call "walking."

"In walking, tidal tension opens the fault, subsequent tidal stress causes it to move lengthwise in one direction, and then the tidal forces close the fault up again. This prevents the area from moving back to its original position; it may move forward with the next daily tidal cycle," Tufts explained. "The walking analogy describes perfectly what we think happens at the fault, resulting in a steady accumulation of these lengthwise offset motions. Walking may explain the appearance of many other faults and areas of cracks and ridges on Europa."

Unlike Europa, here on Earth, large strike-slip faults such as the San Andreas are set in motion by plate tectonic forces from the planet's mantle. Based on the Europa findings, Tufts said, "The data may teach us more about the detailed structure that develops at bends in Earth's faults, including the San Andreas."

The latest Galileo images of Astypalaea Linea are available on the Internet.

Galileo has been in orbit around Jupiter and its moons for the past three years. Its primary mission ended in December 1997, and the spacecraft is currently in the midst of a two-year extension known as the Galileo Europa Mission. Galileo is managed by JPL for NASA's Office of Space Science, Washington, DC. JPL is a division of Caltech, Pasadena, CA.

Galileo takes a close look at icy Europa -- Marshal Space Flight Center Space Science News

Galileo FAQ - Exploring Europa

Exploring Europa

What concrete evidence has Galileo found of the suspected ocean below Europa's frozen surface?

We do not have "concrete" evidence for a ocean presently existing below Europa's ice crust. However, the Galileo data have considerably strengthened the case for this. First, the images show clear evidence for near-surface melting and movements of large blocks of icy crust in ways that are similar to icebergs or ice rafts on the Earth. Second, there are very few impact craters on the surface, suggesting that this activity took place recently, geologically speaking. The problem is that we have no precise way to measure the exact age of the surface, and it is possible that we are looking at an ancient "frozen" ocean, not a current one. We feel the evidence favors an ocean now but that it is not conclusive.

What do most scientists believe about the possibility of life in Europa's oceans?

This of course depends on when there was an ocean and how long it lasted (including up to the present). When the Viking went to Mars in 1976, most life scientists felt that to have a chance for extraterrestrial life you had to have light (for photosynthesis), liquid water, and oxygen. Since then we have discovered places on the Earth (ocean floor hydrothermal vents, geothermal hotsprings, etc.) where life is currently sustained in the dark, without oxygen, using the heat and chemical energy from volcanic fluids and water. Some of these life forms such as thermophillic (or heat loving) bacteria are among the most ancient types of life on Earth. Many scientists now speculate that life may actually have arisen under such conditions here on Earth. So, of course, we are now more interested in places like Mars and Europa where there may have been liquid water and volcanic activity as a place to look for primitive life. If there is an ocean on Europa, there is no easy way to estimate the chances that life could exist there but that's exactly the question we're trying to answer by continued exploration! In other words, we have to go look. Developing experiments and missions to this is obviously challenging but we're trying.

What future missions are being considered to explore Europa?

Galileo will continue to observe Europa and we hope in the next year to have eight more close encounters. Beyond that, NASA is studying a possible Europa Orbiter mission and perhaps landers in the early part of the next century. A Europa Orbiter might be able to definitively answer the ocean question by combining a number of techniques. One approach is to use radar to penetrate the ice and perhaps measure its thickness. Another is to use very precise gravity and altimetry measurements to observe the tides raised by Jupiter on Europa (these will be much larger - as great as 40 meters or so - if there is a liquid layer than if the water is all frozen).

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