The Lure of Europa

Europa. Image credit: NASA Click to enlarge
The discovery that Jupiter’s moon Europa most likely has a cold, salty ocean beneath its frozen icy crust has put Europa on the short list of objects in our solar system that astrobiologists would like to study further. At the Earth System Processes II conference in Calgary, Canada, Ron Greeley, planetary geologist and professor of geology at Arizona State University in Phoenix, Arizona, gave a talk summing up what is known about Jupiter and its moons, and what remains to be discovered.

There have been six spacecraft that have explored the Jupiter system. The first two were Pioneer spacecraft in the 1970s that flew by the Jupiter system and made some brief observations. Those were followed by the Voyager I and II spacecraft, which gave us our first detailed views of the Galilean satellites. But most of the information we have has come from the Galileo mission. More recently, there was a flyby of the Cassini spacecraft, that went by Jupiter and made observations on its way to Saturn, where it is currently in operation. But nearly everything we know about the geology of the Jupiter system, and in particular the Galilean satellites (Io, Europa, Ganymede and Callisto), came from the Galileo mission. Galileo gave us an incredible wealth of information that we’re still in the process of analyzing today.

There are four Galilean satellites. Io, the innermost, is volcanically the most active object in the solar system. It derives its internal energy from tidal stressing in the interior, as it is being pushed-pulled between Europa and Jupiter. The explosive volcanism we see there is very impressive. There are plumes that are ejected some 200 kilometers (124 miles) above the surface. We also see effusive volcanism in the form of lava flows erupting onto the surface. These are very high-temperature, very fluid flows. On Io we see these flows extending for hundreds of kilometers across the surface.

All of the Galilean satellites are in elliptical orbits, which means that sometimes they’re closer to Jupiter, other times they’re farther away, and they’re being pushed-pulled by their neighbors. That generates internal friction to sufficient levels, in the case of Io, to melt the interior and “drive” the volcanoes. The same processes are taking place on Europa. And there is a possibility of silicate volcanism taking place beneath the icy crust on Europa.

Ganymede is the largest satellite in the solar system. It has an outer icy shell. We think that it has a sub-ice ocean of liquid water over a silicate core and perhaps a small internal metallic core. Ganymede has been subjected to geologic processes since its formation. It has a complex history, dominated by tectonic processes. We see a combination of very old features and very young features. We can see complex facture patterns on its surface that crosscut older fracture patterns. The surface is fractured into blocks that have been shifted about on the overriding, apparently liquid, interior. We also see the impact history dating from the period of early bombardment. Unscrambling the tectonic history of Ganymede is a work in process.

Callisto is the outermost of the Galilean satellites. It, too, has been subjected to impact bombardment, reflecting the early accretion history of the solar system in general, and the Jupiter system in particular. The surface is dominated by craters of all sizes. But we were surprised by the apparent lack of very tiny impact craters. We see very tiny impact craters on its neighbor, Ganymede; we don’t see them on Callisto. There is some process, we think, that is erasing the small craters – but only in selected areas on the moon. This is a mystery that has not been resolved: What is the process that is removing the tiny craters in some areas, or alternatively, might they not have formed there for some reason to begin with? Again, this is a topic of ongoing research.

What I want to talk about primarily, though, is Europa. Europa is about the size of Earth’s moon. It is primarily a silicate object, but it has an outer shell of H2O, the surface of which is frozen. The total volume of water that covers its silicate interior exceeds all of the water on Earth. The surface of that water is frozen. The question is: What’s beneath that frozen shell? Is there solid ice all the way to the bottom, or is there a liquid ocean? We think there is liquid water beneath the icy crust, but we don’t really know that for sure. Our ideas are based on models, and like all models, they are subject to further study.

The reason we think that there is a liquid ocean on Europa is from the behavior of the induced magnetic field around Europa that was measured by the magnetometer on Galileo. Jupiter has an enormous magnetic field. It, in turn, induces a magnetic field, not only on Europa, but also on Ganymede and Callisto. The way that induced magnetic field behaves is consistent with the presence of a subsurface salty liquid ocean, not just on Europa, but also on Ganymede and Callisto.

We do know that the surface is water ice. We know that there are non-ice components present, which includes various salts. And we know that the surface has been geologically processed: it has been fractured, healed, broken up repeatedly. We also see relatively few impact craters on the surface. That indicates that the surface is geologically young. Europa could even be geologically active today. Images of one region, in particular, show a surface that has been severely broken up. The icy plates have been broken apart and shifted into new positions. Material has oozed between the cracks, then apparently frozen, and we think that this could be one of the places where there was upwelling material, perhaps driven by the tidal heating I talked about earlier.

We tend to forget the scale of things in the planetary sciences. But these icy blocks are huge. When we think about future exploration, we would like to get down on the surface and make certain key measurements. So we have to think about spacecraft systems that could land in this kind of terrain. Because it is these places that might have material derived from below the ice, they are the highest priority for exploration. And yet, as is often the case in planetary exploration, the most interesting places are the most difficult to get to.

So what would we like to know? First and most fundamental is the “ocean notion.” Does liquid water exist or not? Is the ice shell thick or thin? If there is an ocean there, how thick is that icy crust? This is very important to know when we think about exploring a possible liquid ocean on Europa: If we want to get into the ocean, how deep must we go through the ice? What is the age of the surface? We say “young,” but that’s only a relative term. Is it thousands, hundreds of thousands, millions, or even billions of years old? The models allow for quite a spread in ages, based on the impact crater frequency. What are the environments there today that are favorable for astrobiology? And what were the environments in the past? Were they the same, or have they changed through time? The answers to these questions require new data.

Another thing that drives our interest in exploring the Galilean satellites is trying to understand their geological histories. To some extent, the diversity that we see, from Io to Europa to Ganymede and Callisto, can be linked to the amount of tidal energy that’s driving the system. Maximum tidal energy drives the volcanism that is so dominant on Io. At the other extreme, very little tidal energy on Callisto results in the preservation of the impact-cratering record. Europa and Ganymede are in between these two extreme cases.

The total surface area of the three icy moons of Jupiter (Europa, Ganymede and Callisto) is greater than the surface area of Mars, and, in fact, is about equivalent to the entire land surface of Earth. So when we discuss the exploration of the icy Galilean satellites, there is a lot of terrain to cover.

As for future exploration, let me share a little history. Three years ago, NASA established the Prometheus project. The Prometheus project involves the development of nuclear power and nuclear propulsion, something that had not been considered seriously for quite some time. The first mission to be flown in the Prometheus project was the Jupiter Icy Moons Orbiter, or JIMO. The goal was to explore the three icy moons within the context of the Jupiter system. It was a very ambitious project. Well, earlier this year JIMO was cancelled. But it looks as though this coming year there will be approval for a geophysical orbiter for Europa. The initial steps for getting that spacecraft underway are being considered now. Europa is a very high priority for exploration, and in recognition of that priority, this mission is likely to happen.

Why are we so interested in Europa? When we talk about astrobiology, we consider the three ingredients for life: water, the right chemistry, and energy. Their presence doesn’t mean that the magic spark of life ever happened, but those are the things that we think are required for life. And so, as I outlined, all three of Jupiter’s icy moons are potential targets. But Europa is the highest priority, because it seems to have the maximum internal energy.

So, of course, first we would like to know: Is there an ocean, yes or no?

Then, what’s the three-dimensional configuration of the icy crust? We know that organisms can live in fractures and cracks in Arctic ice. Such cracks are likely to be present on Europa, too, and could be niches that are of high interest for astrobiology.

Then we want to map the organic and inorganic surface compositions. We see in the data that exist today that the surface is heterogeneous. It’s not just pure ice on the surface. There are some areas that seem to be richer in non-ice components than other places. We want to map that material.

We also want to map interesting surface features and identify the places that are most important for future exploration, including landers.

Then we want to understand Europa in the context of the Jupiter environment. For example, how does the radiation environment imposed by Jupiter affect surface chemistry on Europa?

Ultimately, we want to get down on the surface, because there are a number of things that we can do only from the surface. We have a great wealth of data from the Galileo mission, and hope to have even more from the potential Europa mission, but it’s remote-sensing data. Next, we want to get a lander onto the surface that could make some critical ground-truth measurements, to place the remote-sensing data into context. And so within the scientific community, we feel that the next mission to Europa and the Jupiter system ought to have a landed package of some kind. But whether this will actually happen or not, stay tuned!

Original Source: NASA Astrobiology

New Jupiter Mission Moves Forward

Galileo’s image of Jupiter. Image credit: NASA/JPL. Click to enlarge.
NASA today announced that a mission to fly to Jupiter will proceed to a preliminary design phase. The mission is called Juno, and it is the second in NASA’s New Frontiers Program.

The mission will conduct an in-depth study of the giant planet. The mission proposes to place a spacecraft in a polar orbit around Jupiter to investigate the existence of an ice-rock core; determine the amount of global water and ammonia present in the atmosphere; study convection and deep wind profiles in the atmosphere; investigate the origin of the jovian magnetic field; and explore the polar magnetosphere.

“We are excited at the prospect of the new scientific understanding and discoveries by Juno in our continued exploration of the outer reaches of our solar system during the next decade,” said Dr. Ghassem Asrar, deputy associate administrator for NASA’s Science Mission Directorate.

At the end of the preliminary design study, the mission must pass a confirmation review that will address significant schedule, technical and cost risks before being confirmed for the development phase.

Dr. Scott Bolton of Southwest Research Institute, Boulder, Colo., is the principal investigator. NASA’s Jet Propulsion Laboratory, Pasadena, Calif., will provide mission project management. Lockheed Martin Space Systems, Denver, will build the spacecraft.

NASA selected two proposed mission concepts for study in July 2004 from seven submitted in February 2004 in response to an agency Announcement of Opportunity. “This was a very tough decision given the exciting and innovative nature of the two missions,” Asrar added.

The selected New Frontiers science mission must be ready for launch no later than June 30, 2010, within a mission cost cap of $700 million.

The New Frontiers Program is designed to provide opportunities to conduct several of the medium-class missions identified as top priority objectives in the Decadal Solar System Exploration Survey, conducted by the Space Studies Board of the National Research Council.

The first NASA New Frontiers mission will fly by the Pluto-Charon system in 2014 and then target another Kuiper asteroid belt object.

For information about NASA’s science programs on the Web, visit: For information about NASA and agency programs on the Web, visit:

JPL is managed for NASA by the California Institute of Technology in Pasadena.

Original Source: NASA News Release

Amalthea is Just a Pile of Icy Rubble

Artist illustration of Galileo and Jupiter’s moon, Amalthea. Image credit: NASA/JPL. Click to enlarge.
Scientists studying data from NASA’s Galileo spacecraft have found that Jupiter’s moon Amalthea is a pile of icy rubble less dense than water. Scientists expected moons closer to the planet to be rocky and not icy. The finding shakes up long-held theories of how moons form around giant planets.

“I was expecting a body made up mostly of rock. An icy component in a body orbiting so close to Jupiter was a surprise,” said Dr. John D. Anderson, an astronomer at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. Anderson is lead author of a paper on the findings that appears in the current issue of the journal Science.

“This gives us important information on how Jupiter formed, and by implication, how the solar system formed,” Anderson said.

Current models imply that temperatures were high at Amalthea’s current position when Jupiter’s moons formed, but this is inconsistent with Amalthea being icy. The findings suggest that Amalthea formed in a colder environment. One possibility is that it formed later than the major moons. Another is that the moon formed farther from Jupiter, either beyond the orbit of Jupiter’s moon Europa or in the solar nebula at or beyond Jupiter’s position. It would have then been transported or captured in its current orbit around Jupiter. Either of these explanations challenges models of moon formation around giant planets.

“Amalthea is throwing us a curve ball,” said Dr. Torrence Johnson, co-author and project scientist for the Galileo mission at JPL. “Its density is well below that of water ice, and even with substantial porosity, Amalthea probably contains a lot of water ice, as well as rock.” Analysis of density, volume, shape and internal gravitational stresses lead the scientists to conclude that Amalthea is not only porous with internal empty spaces but also contains substantial water ice.

One model for the formation of Jupiter’s moons suggests that moons closer to the planet would be made of denser material than those farther out. That is based on a theory that early Jupiter, like a weaker version of the early Sun, would have emitted enough heat to prevent volatile, low-density material from condensing and being incorporated into the closer moons. Jupiter’s four largest moons fit this model, with the innermost of them, Io, also the densest, made mainly of rock and iron.

Amalthea is a small red-tinted moon that measures about 168 miles in length and half that in width. It orbits about 181,000 kilometers (112,468 miles) from Jupiter, considerably closer than the Moon orbits Earth. Galileo passed within about 99 miles of Amalthea on Nov. 5, 2002. Galileo’s flyby of Amalthea brought the spacecraft closer to Jupiter than at any other time since it began orbiting the giant planet on Dec. 7, 1995. After more than 30 close encounters with Jupiter’s four largest moons, the Amalthea flyby was the last moon flyby for Galileo.

The Galileo spacecraft’s 14-year odyssey came to an end on Sept. 21, 2003. JPL, a division of the California Institute of Technology in Pasadena, managed the Galileo mission for NASA.

Additional information about the mission is available online at:

Original Source: NASA/JPL News Release

Jupiter Reflects the Sun’s X-Rays

Astronomers using the European Space Agency’s XMM-Newton telescope have discovered that observing the giant planet Jupiter may actually give them an insight in to solar activity on the far side of the Sun! In research reported in the most recent edition of Geophysical Research Letters, they discovered that Jupiter’s x-ray glow is due to x-rays from the Sun being reflected back off the planet’s atmosphere.

Jupiter is an intriguing object when viewed in x-rays; it has dramatic x-ray auroras at the poles and a variable x-ray glow from near the equator. Researchers had theorised that these x-rays from the equatorial regions of Jupiter, called disk x-rays, were controlled by the Sun. In November 2003, during a period of high solar activity, they observed Jupiter.

“We found that Jupiter’s day-to-day disk x-rays were synchronised with the Sun’s emissions,” says Dr Anil Bhardwaj, from NASA Marshall Space Flight Centre and lead author on the paper. “Unfortunately, we missed a relatively large solar flare during the 3.5-days observation due to the perigee passage of the XMM-Newton”. “But, still we were lucky; particularly clear was a signature of a moderate solar flare that went off during the observing period – there was a corresponding brightening of the Jovian disk x-rays”, says Anil Bhardwaj.

In addition to supporting the researchers’ theory, this result has another application – in studying the Sun. The Sun is a very dynamic environment and processes there have an impact on human activities. For example, solar flares (the most powerful explosions in the solar system) can damage satellites or injure astronauts in space, and on Earth they can disrupt radio signals in the atmosphere, so it is important to understand as much as we can about them.

There are several dedicated spacecraft watching the Sun (such as the European Space Agency’s SOHO satellite), as well as ground-based telescopes, but there are gaps in coverage as some areas of the Sun are not visible by any of these means at some times.

“As Jupiter orbits the Sun, we hope to be able to learn more about the active areas of the Sun we can’t see from Earth by watching the Jovian x-ray emissions,” says Dr Graziella Branduardi-Raymont from the University College London’s Mullard Space Science Laboratory. “If a large solar flare occurs on an area of the Sun that is facing Jupiter, we may be able to observe it in light scattered from Jupiter, even if we cannot see that region of the Sun from around the Earth at the time.”

Jupiter’s atmosphere is not a perfect mirror of the Sunlight in X-rays – typically one in a few thousand x-ray photons (packets of light) is reflected back, but the more energetic the photons, the more are reflected into space.

UK participation in this research and the UK subscription to the European Space Agency are funded by the Particle Physics and Astronomy Research Council (PPARC).

Original Source: PPARC News Release

Jupiter’s Auroras Helped by Io

Scientists have obtained new insight into the unique power source for many of Jupiter’s auroras, the most spectacular and active auroras in the Solar System. Extended monitoring of the giant planet with NASA’s Chandra X-ray Observatory detected the presence of highly charged particles crashing into the atmosphere above its poles.

X-ray spectra measured by Chandra showed that the auroral activity was produced by ions of oxygen and other elements that were stripped of most of their electrons. This implies that these particles were accelerated to high energies in a multimillion-volt environment above the planet’s poles. The presence of these energetic ions indicates that the cause of many of Jupiter’s auroras is different from auroras produced on Earth or Saturn.

“Spacecraft have not explored the region above the poles of Jupiter, so X-ray observations provide one of the few ways to probe that environment,” said Ron Elsner of the NASA Marshall Space Center in Huntsville, Alabama, and lead author on a recently published paper describing these results in the Journal for Geophysical Research. “These results will help scientists to understand the mechanism for the power output from Jupiter’s auroras, which are a thousand times more powerful than those on Earth.”

Electric voltages of about 10 million volts, and currents of 10 million amps – a hundred times greater than the most powerful lightning bolts – are required to explain the X-ray observations. These voltages would also explain the radio emission from energetic electrons observed near Jupiter by the Ulysses spacecraft.

On Earth, auroras are triggered by solar storms of energetic particles, which disturb Earth’s magnetic field. Gusts of particles from the Sun can also produce auroras on Jupiter, but unlike Earth, Jupiter has another way of producing auroras. Jupiter’s rapid rotation, intense magnetic field, and an abundant source of particles from its volcanically active moon, Io, create a huge reservoir of electrons and ions. These charged particles, trapped in Jupiter’s magnetic field, are continually accelerated down into the atmosphere above the polar regions where they collide with gases to produce the aurora, which are almost always active on Jupiter.

If the particles responsible for the aurora came from the Sun, they should have been accompanied by large number of protons, which would have produced an intense ultraviolet aurora. Hubble ultraviolet observations made during the Chandra monitoring period showed relatively weak ultraviolet flaring. The combined Chandra and Hubble data indicate that this auroral activity was caused by the acceleration of charged ions of oxygen and other elements trapped in the polar magnetic field high above Jupiter’s atmosphere.

Chandra observed Jupiter in February 2003 for four rotations of the planet (approximately 40 hours) during intense auroral activity. These Chandra observations, taken with its Advanced CCD Imaging Spectrometer, were accompanied by one-and-a-half hours of Hubble Space Telescope observations at ultraviolet wavelengths.

The research team also included Noe Lugaz, Hunter Waite, and Tariq Majeed (University of Michigan, Ann Arbor), Thomas Cravens (University of Kansas, Lawrence), Randy Gladstone (Southwest Research Institute, San Antonio, Texas), Peter Ford (Massachusetts Institute of Technology, Cambridge), Denis Grodent (University of Liege, Belgium), Anil Bhardwaj (Marshall Space Flight Center) and Robert MacDowell and Michael Desch (Goddard Space Flight Center, Greenbelt, Md.)

NASA’s Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for NASA’s Office of Space Science, Washington. Northrop Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the prime development contractor for the observatory. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center in Cambridge, Mass.

Additional information and images are available at: and

Original Source: Chandra News Release

Jovian Moon Was Probably Captured

The first ground based infrared spectrum of Jupiter’s moon Amalthea reveals that it must have formed far from its current location. This new result, based on observations with the Subaru telescope and the NASA Infrared Telescope Facility by a team of researchers from the National Astronomical Observatory of Japan, the University of Hawaii, and the University of Tokyo, sheds new light on our Solar System’s turbulent past.

Planets like Earth and Jupiter formed from the disk of gas and dust swirling around the Sun at the time of its birth. Rocky planets like Earth formed in the high temperature environment close to the Sun, while large gaseous planets like Jupiter formed in the cooler regions farther away. Similarly, Jupiter, the largest planet in the solar system, probably had its own disk of gas and dust. The four moons of Jupiter discovered by Galileo (Io, Europa, Ganymede, and Callisto) are likely to have been born from this disk.

In addition to the Galilean moons, Jupiter has two other types of satellites: four small inner moons orbiting Jupiter within the orbit of Io, the inner most Galilean satellite, and at least fifty five small outer moons outside the orbit of Callisto, the outer most Galilean satellite. All the outer satellites have tell-tale orbits that reveal that they must have been captured by Jupiter during or after the formation of the planet and its larger moons.

The origin of the four small inner moons remain a mystery, however. They have orbits compatible with the hypothesis that they formed in orbit around Jupiter like the Galilean moons. On the other hand, their small irregular shapes and their comparatively low reflectivity and low densities resemble asteroids and suggest that they were captured by Jupiter’s gravitational pull just like the outer moons.

The mystery persists because of the challenge inherent in observing Jupiter’s small inner moons from Earth. The moons are small and therefore faint, and they are obscured by the bright glare from Jupiter. Although NASA’s space probes Voyager and Galileo have captured detailed images of Jupiter’s small inner moons, these data have been insufficient for resolving the question of their origin.

Naruhisa Takato from the National Astronomical Observatory of Japan and his collaborators have now had success in obtaining the first infrared spectrum of two of Jupiter’s small inner moons, Amalthea and Thebe. To obtain a spectrum over a wide range of infrared wavelengths, the group combined the strengths of two instruments on two telescopes on the summit of Mauna Kea, Hawaii. For high resolution spectroscopy at wavelengths longer than 3 ?m ,the group used the Infrared Camera and Spectrograph on the Subaru telescope. For shorter wavelengths, the group used SpeX on the NASA IRTF, which has broad wavelength coverage.

The new spectrum of Amalthea shows the characteristic signatures of water. The most likely location of this water is within water containing hydrous minerals. Such minerals typically form in low temperature environments, ruling out the possibility that Amalthea could have formed in the high temperature environment of Jupiter’s immediate neighborhood while the planet was forming and where Amalthea
now is.

If Amalthea did not form near its present location, where did it come from? The surface of Amalthea resembles regions of Callisto that are not covered by ice. This suggests that Amalthea may have been one of the many small “micro-satellites” orbiting Jupiter that was sucked into an inner orbit when the Galilean moons formed. However, the spectrum of Amalthea has similarities with asteroids orbiting the Sun, suggesting that is was a “micro-planet” that was pulled into Jupiter’s orbit when Jupiter itself was forming.

Takato says “although we think Jupiter’s moons formed as an assembly of many smaller bodies, the same way we think planets formed from ‘planetesimals’, until now we have not found any example of the original building blocks of a planet’s moon. However, our results strengthen the argument that Amalthea is one of the few remaining pieces of the material that formed the Galilean moons. Amalthea may have ended up in orbit close to Jupiter rather than get incorporated into a larger moon or Jupiter itself. If this is the case, Amalthea would be the first known example of a ‘satellitesimal.'”

Original Source: Subaru News Release

Triple Eclipse on Jupiter

At first glance, Jupiter looks like it has a mild case of the measles. Five spots – one colored white, one blue, and three black – are scattered across the upper half of the planet.

Closer inspection by NASA’s Hubble Space Telescope reveals that these spots are actually a rare alignment of three of Jupiter’s largest moons – Io, Ganymede, and Callisto – across the planet’s face.

In this image, the telltale signatures of this alignment are the shadows [the three black circles] cast by the moons. Io’s shadow is located just above center and to the left; Ganymede’s on the planet’s left edge; and Callisto’s near the right edge. Only two of the moons, however, are visible in this image. Io is the white circle in the center of the image, and Ganymede is the blue circle at upper right. Callisto is out of the image and to the right.

On Earth, we witness a solar eclipse when our Moon’s shadow sweeps across our planet’s face as it passes in front of our Sun. Jupiter, however, has four moons roughly the same size as Earth’s Moon. The shadows of three of them occasionally sweep simultaneously across Jupiter. The image was taken March 28, 2004, with Hubble’s Near Infrared Camera and Multi-Object Spectrometer.

Seeing three shadows on Jupiter happens only about once or twice a decade. Why is this triple eclipse so unique?

Io, Ganymede, and Callisto orbit Jupiter at different rates. Their shadows likewise cross Jupiter’s face at different rates. For example, the outermost moon Callisto orbits the slowest of the three satellites. Callisto’s shadow moves across the planet once for every 20 shadow crossings of Io. Add the crossing rate of Ganymede’s shadow and the possibility of a triple eclipse becomes even more rare. Viewing the triple shadows in 2004 was even more special, because two of the moons were crossing Jupiter’s face at the same time as the three shadows.

Jupiter appears in pastel colors in this photo because the observation was taken in near-infrared light. Astronomers combined images taken in three near-infrared wavelengths to make this color image. The photo shows sunlight reflected from Jupiter’s clouds. In the near infrared, methane gas in Jupiter’s atmosphere limits the penetration of sunlight, which causes clouds to appear in different colors depending on their altitude.

Studying clouds in near-infrared light is very useful for scientists studying the layers of clouds that make up Jupiter’s atmosphere. Yellow colors indicate high clouds; red colors lower clouds; and blue colors even lower clouds in Jupiter’s atmosphere. The green color near the poles comes from a thin haze very high in the atmosphere. Ganymede’s blue color comes from the absorption of water ice on its surface at longer wavelengths. Io’s white color is from light reflected off bright sulfur compounds on the satellite’s surface.

“I’m increasingly aware that some of the most interesting things in astronomy and astrophysics, for instance, can change the way people understand the universe, how it got started and where it’s going. I found those Voyager pictures of the moons of Jupiter incredibly exciting, these beautiful color pictures showing volcanoes on the surface”. -Robert C. Richardson, Nobel Laureate, Physics, Cornell, (1996)

In viewing this rare alignment, astronomers also tested a new imaging technique. To increase the sharpness of the near-infrared camera images, astronomers speeded up Hubble’s tracking system so that Jupiter traveled through the telescope’s field of view much faster than normal. This technique allowed scientists to take rapid-fire snapshots of the planet and its moons. They then combined the images into one single picture to show more details of the planet and its moons.

Original Source: NASA Astrobiology

NASA Awards Jupiter Icy Moons Mission

NASA’s Jet Propulsion Laboratory (JPL), Pasadena, Calif., selected Northrop Grumman Space Technology, Redondo Beach, Calif., as the contractor for co-designing the proposed Prometheus Jupiter Icy Moons Orbiter (JIMO) spacecraft. The contract award is for approximately $400 million, covering work through mid-2008.

The Prometheus JIMO mission is part of an ambitious mission to orbit and explore three planet-sized moons, Callisto, Ganymede and Europa, of Jupiter. The moons may have vast oceans beneath their icy surfaces. A nuclear reactor would enable the mission, which would launch in the next decade.

JIMO would be the first NASA mission using nuclear electric propulsion, which would enable the spacecraft to orbit each icy world to perform extensive investigations of their composition, history, and potential for sustaining life.

The JIMO mission, integrated with the Vision for Space Exploration, also develops and demonstrates technologies and capabilities in direct support to implement the Vision, including space nuclear electric power systems and nuclear electric propulsion systems.

“We have assembled an exceptional team of professionals to take us into the next phase of the mission. To see the mission evolve is rewarding, and I am confident a good team is in place to move us forward,” said John Casani, project manager for the JIMO mission at JPL.

Under the contract, Northrop Grumman will work with a government team to complete the preliminary design for the spacecraft. The work includes developing hardware, software and test activities for the design of the non-nuclear portion of the spacecraft. It also includes developing the interfaces for the spacecraft, space reactor, and science instruments. The contractor is responsible for the integration of government-owned and provided technologies into the spacecraft. They are also responsible for assembly, integration, and testing of the space system in accordance with applicable government requirements.

The government team will co-design the spacecraft with the contractor. NASA will supply the launch vehicle. The Department of Energy’s Office of Naval Reactors, Washington, will own and be responsible for the space reactor.

The government team includes JPL, NASA’s Ames Research Center, Moffett Field, Calif.; Glenn Research Center, Cleveland; Kennedy Space Center, Fla.; Langley Research Center, Hampton, Va.; and Marshall Space Flight Center, Huntsville, Ala. Also the Office of Naval Reactors, which includesing Knolls Atomic Power Laboratory, Schenectady, N.Y.; Bettis Laboratory, Pittsburgh; and supporting Department of Energy national laboratories.

The mission instruments will be procured competitively via a NASA Announcement of Opportunity. Three crosscutting themes, identified by a NASA-chartered science definition team, drive the proposed JIMO investigations.

The themes are: evaluate the degree subsurface oceans are present on these moons; study the chemical composition of the moons, including organic materials, and the surface processes that affect them; and scrutinize the entire Jupiter system, particularly the interactions between Jupiter, the moons’ atmospheres and interiors.

JIMO is managed by JPL and is part of NASA’s Prometheus Program, a program studying a series of initiatives to develop power systems and technologies for space exploration in support of the Vision for Space Exploration.

JPL, a division of the California Institute of Technology, manages the proposed JIMO mission for NASA’s Exploration Systems Mission Directorate, Washington.

For more information about the mission or NASA, visit:

Original Source: NASA JPL News Release

Stream of Particles from Io

Jupiter’s moon Io is peppered with volcanoes, the hottest, most active volcanoes in our solar system. Sizzling vents spew plumes of gas and dust as much as 400 km high. They surge, spit, subside and surge again, non-stop.

The towering plumes, outlined by graceful arcs of rising and falling ash, are eerily beautiful. Their tops jut into space, freezing. Beneath them, scientists believe, it snows. Sulfurous flakes crystallize in the plume-tops and drift gently down to coat Io’s colorful terrain.

High above the falling snow something unexpected happens: At the apex of the plumes, some of the ash and dust that ought to turn around and fall … doesn’t. Defying gravity, it keeps going up, not slowing but accelerating, 2 times, 10 times, hundreds of times faster than a speeding bullet, away from Io and into deep space.

Passing spacecraft beware: Io is shooting at you.

The Ulysses spacecraft, a joint mission of NASA and the European Space Agency, made the discovery in 1992 when, approaching Jupiter, it was hit by a breakneck stream of volcano dust.

“What a surprise,” recalls Harold Krueger of the Max Planck Institute in Heidelberg, the principle investigator for Ulysses’ dust detector. “We expected to encounter dust,” he says. The solar system is littered with flakes from comets and asteroids. “But nothing like this.”

The dust came in a tight stream, like water from a garden hose, and it was moving extraordinarily fast, about 300 km/s (670,000 mph). “This makes it some of the fastest-moving material in the solar system,” says Krueger, “second only to the solar wind.” Fortunately the dust-bits were small, similar in size to particles in cigarette smoke, so they didn’t penetrate the ship’s hull in spite of their extreme velocity.

At first, no one suspected Io. Ulysses was 100 million kilometers from Io when the stream blew by, supposedly beyond the reach of volcanic plumes. Plus, the speed of the dust didn’t make sense. Particles emerge from Io’s vents traveling 1 or 2 km/s, not 300 km/s.

Baffled, researchers considered several possibilities: Could Jupiter’s dark rings be responsible? There’s plenty of dust there, but how could rings manufacture fast-moving jets? Comet Shoemaker-Levy 9 was another suspect. The comet flew so close to Jupiter in 1992 that it was torn apart. Comets are known to produce streams of dust, but not so fast as the stream that hit Ulysses.

NASA’s Galileo spacecraft eventually solved the puzzle. Like Ulysses, Galileo was pelted by dust when it approached Jupiter in 1995. Unlike Ulysses, which merely flew past the giant planet, Galileo settled into orbit. As data accumulated over a period of years, scientists were able to correlate volcanic activity with dust events, and they showed, furthermore, that dust streams were modulated by Io’s orbital motion.

The source was definitely Io.

Regarding the extreme velocity of the dust: “Jupiter is responsible for that,” explains Krueger.

Jupiter is not only a giant planet, but also a giant magnet, which spins once every 9 hours and 55 minutes. Spinning magnetic fields produce electric fields, and the electric fields around Jupiter are intense. Io-dust, like dust on your computer monitor, is electrically charged, so Jupiter’s electric forces naturally accelerate the grains. 300 km/s is no problem.

In 2000 when the Cassini spacecraft sailed past Jupiter en route to Saturn, it too was hit. Cassini’s dust detector is more capable than Ulysses’. In addition to mass, speed, charge and trajectory, it can also measure elemental composition. Cassini found hints of sulfur, silicon, sodium and potassium–all signs of volcanic origin.

“This raises an interesting possibility,” says Krueger. “We can analyze the hot interior of Io from a great distance.” There’s no need to get too close to the sizzling vents when you can catch the ash millions of miles away.

Io dust can even reach Earth, says Krueger, but don’t expect a meteor shower. Bright meteors such as Perseids and Leonids are caused by sand-sized comet dust. Io dust is much smaller. A typical grain is only 10 billionths of a meter wide. If a bit of it disintegrated in Earth’s atmosphere, you probably wouldn’t notice.

End of story? Not quite.

Ulysses visited Jupiter again in early 2004 and once again the craft was pelted. Io’s volcanoes were still at work. But something was wrong: The dust was shooting in the wrong direction.

“Io dust is supposed fly out of Jupiter’s equatorial plane,” says Krueger, “because that’s the way the accelerating electric fields point.” This time Ulysses approached Jupiter’s north pole (75 degrees north latitude to be exact) where no dust should go. Yet the spacecraft was pelted anyway.

Jupiter, it seems, flings Io-dust in every direction, which is hard to understand, says Krueger. Future missions to the giant planet might unravel the mystery. Every blast of dust will remind: we’ve still got a lot to learn.

Original Source: NASA Science Article

Ganymede’s Lumpy Interior

Scientists have discovered irregular lumps beneath the icy surface of Jupiter’s largest moon, Ganymede. These irregular masses may be rock formations, supported by Ganymede’s icy shell for billions of years. This discovery comes nearly a year after the orchestrated demise of NASA’s Galileo spacecraft into Jupiter’s atmosphere and more than seven years after the data were collected.

Researchers at NASA’s Jet Propulsion Laboratory, Pasadena, Calif., and the University of California, Los Angeles, report their findings in a paper that will appear in the Aug. 13 issue of the journal Science.

The findings have caused scientists to rethink what the interior of Ganymede might contain. The reported bulges reside in the interior, and there are no visible surface features associated with them. This tells scientists that the ice is probably strong enough, at least near the surface, to support these possible rock masses from sinking to the bottom of the ice for billions of years. But this anomaly could also be caused by piles of rock at the bottom of the ice.

“The anomalies could be large concentrations of rock at or underneath the ice surface. They could also be in a layer of mixed ice and rock below the surface with variations in the amount of rock,” said Dr. John Anderson, a scientist and the paper’s lead author at JPL. “If there is a liquid water ocean inside Ganymede’s outer ice layer there might be variations in its depth with piles of rock at the ocean bottom. There could be topographic variations in a hidden rocky surface underlying a deep outer icy shell. There are many possibilities, and we need to do more studies.”

Dr. Gerald Schubert, co-author at UCLA, said “Although we don’t yet have anything definitive about the depth at this point, we did not expect Ganymede’s ice shell to be strong enough to support these lumpy mass concentrations. Thus, we expect that the irregularities would be close to the surface where the ice is coldest and strongest, or at the bottom of the thick ice shell resting on the underlying rock. It would really be a surprise if these masses were deep and in the middle of the ice shell.”

Ganymede has three main layers. A sphere of metallic iron at the center (the core), a spherical shell of rock (mantle) surrounding the core, and a spherical shell of mostly ice surrounding the rock shell and the core. The ice shell on the outside is very thick, maybe 800 kilometers (497 miles) thick. The surface is the very top of the ice shell. Though it is mostly ice, the ice shell might contain some rock mixed in. Scientists believe there must be a fair amount of rock in the ice near the surface. Variations in this amount of rock may be the source of these possible rock formations.

Scientists stumbled on the results by studying Doppler measurements of Ganymede’s gravity field during Galileo’s second flyby of the moon in 1996. Scientists were measuring the effect of the moon’s gravity on the spacecraft as it flew by. They found unexpected variations.

“Believe it or not, it took us this long to straighten out the anomaly question, mostly because we were analyzing all 31 close flybys for all four of Jupiter’s large moons,” said Anderson. “In the end, we concluded that there is only one flyby, the second flyby of Ganymede, where mass anomalies are evident.”

Scientists have seen mass concentration anomalies on one other moon before, Earth’s, during the first lunar orbiter missions in the 1960s. The lunar mass concentrations during the Apollo moon mission era were due to lava in flat basins. However, scientists cannot draw any similarities between these mass concentrations and what they see at Ganymede.

“The fact that these mass anomalies can be detected with just flybys is significant for future missions,” said Dr. Torrence Johnson, former Galileo project scientist. “With this type of information you could make detailed gravity and altitude maps that allow us to actually map structures within the ice crust or on the rocky surface. Knowing more about the interior of Ganymede raises the level of importance of looking for gravity anomalies around Jupiter’s moons and gives us something to look for. This might be something NASA’s proposed Jupiter Icy Moons Orbiter Mission could probe into deeper.”

The paper was co-authored by Dr. Robert A. Jacobson and Eunice L. Lau of JPL, with Dr. William B. Moore and Jennifer L. Palguta of UCLA. JPL is a division of the California Institute of Technology in Pasadena. JPL designed and built the Galileo orbiter, and operated the mission. For images and information about the Galileo mission, visit