Galileo’s Final Study of Jupiter

Image credit: NASA/JPL

We’re only days away until Galileo’s final plunge into Jupiter on September 21. Nearly out of fuel, the spacecraft was put onto a collision course with Jupiter to prevent it from accidentally crashing into Europa and potentially contaminating it with Earth-based bacteria. The entry point on Jupiter will be 1/4 of a degree south of its equator and it will strike the planet at 174,000 km/h – obviously it’ll be destroyed almost instantly. Scientists hope to retrieve every piece of data they can, but the radiation will intensify to immense levels as the spacecraft nears the planet, so it might not be possible.

In the end, the Galileo spacecraft will get a taste of Jupiter before taking a final plunge into the planet’s crushing atmosphere, ending the mission on Sunday, Sept. 21. The team expects the spacecraft to transmit a few hours of science data in real time leading up to impact.

The spacecraft has been purposely put on a collision course with Jupiter to eliminate any chance of an unwanted impact between the spacecraft and Jupiter’s moon Europa, which Galileo discovered is likely to have a subsurface ocean. The long-planned impact is necessary now that the onboard propellant is nearly depleted.

Without propellant, the spacecraft would not be able to point its antenna toward Earth or adjust its trajectory, so controlling the spacecraft would no longer be possible.

“It has been a fabulous mission for planetary science, and it is hard to see it come to an end,” said Dr. Claudia Alexander, Galileo project manager at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “After traversing almost 3 billion miles and being our watchful eyes and ears around Jupiter, we’re keeping our fingers crossed that, even in its final hour, Galileo will still give us new information about Jupiter’s environment.”

Although scientists are hopeful to get every bit of data back for analysis, the likelihood of getting anything is unknown because the spacecraft has already endured more than four times the cumulative dose of harmful jovian radiation it was designed to withstand. The spacecraft will enter an especially high-radiation region again as it approaches Jupiter.

Launched in the cargo bay of Space Shuttle Atlantis in 1989, the mission has produced a string of discoveries while circling the solar system’s largest planet, Jupiter, 34 times. Galileo was the first mission to measure Jupiter’s atmosphere directly with a descent probe and the first to conduct long-term observations of the jovian system from orbit.

It found evidence of subsurface liquid layers of salt water on Europa, Ganymede and Callisto and it examined a diversity of volcanic activity on Io. Galileo is the first spacecraft to fly by an asteroid and the first to discover a moon of an asteroid.

The prime mission ended six years ago, after two years of orbiting Jupiter. NASA extended the mission three times to continue taking advantage of Galileo’s unique capabilities for accomplishing valuable science. The mission was possible because it drew its power from two long-lasting radioisotope thermoelectric generators provided by the Department of Energy.

From launch to impact, the spacecraft has traveled 4,631,778,000 kilometers (about 2.8 billion miles).

Its entry point into the giant planet’s atmosphere is about 1/4 degree south of Jupiter’s equator. If there were observers floating along at the cloud tops, they would see Galileo streaming in from a point about 22 degrees above the local horizon. Streaming in could also be described as screaming in, as the speed of the craft relative to those observers would be 48.2 kilometers per second (nearly 108,000 miles per hour). That is the equivalent of traveling from Los Angeles to New York City in 82 seconds. In comparison, the Galileo atmospheric probe, aerodynamically designed to slow down when entering, and parachute gently through the clouds, first reached the atmosphere at a slightly more modest 47.6 kilometers per second (106,500 miles per hour).

“This is a very exciting time for us as we draw to a close on this historic mission and look back at its science discoveries. Galileo taught us so much about Jupiter but there is still much to be learned, and for that we look with promise to future missions,” said Dr. Charles Elachi, director of JPL.

Original Source: NASA/JPL News Release

Galileo Will Plunge Into Jupiter on September 21

Image credit: NASA/JPL

Time is running out for NASA’s Galileo spacecraft. After eight years of loyal service imaging Jupiter and its moons, NASA controllers have aimed it at the gas giant. On September 21, 2003, Galileo will crash into Jupiter and be destroyed; this will prevent any chance the spacecraft will unintentionally crash into Europa and contaminate the liquid ocean. NASA is planning a series of live press conferences to explain the end of the mission and discuss Galileo’s discoveries.

Following eight years of capturing dramatic images and surprising science from Jupiter and its moons, NASA’s Galileo mission draws to a close September 21 with a plunge into Jupiter’s atmosphere. Following eight years of capturing dramatic images and surprising science from Jupiter and its moons, NASA’s Galileo mission draws to a close September 21 with a plunge into Jupiter’s atmosphere.

NASA has scheduled a Space Science Update (SSU) at 2 p.m. EDT, Wednesday., September 17, in the James E. Webb Auditorium at NASA headquarters, 300 E St. S.W., Washington. Panelists will discuss the historic mission, engineering challenges, science highlights and plans for Galileo’s impact with Jupiter’s atmosphere.

The SSU will be carried live on NASA Television with two-way question-and-answer capability from participating agency centers. NASA TV is broadcast on AMC-9, transponder 9C, C-Band, located at 85 degrees west longitude. The frequency is 3880 MHz. Polarization is vertical, and audio is monaural at 6.80 MHz. Audio of the SSU is available on voice circuit from the Kennedy Space Center at: 321/867-1220.

SSU participants:

# Dr. Colleen Hartman, director, Solar System Exploration Division, NASA Headquarters.
# Dr. Claudia Alexander, Galileo project manager, NASA Jet Propulsion Laboratory (JPL), Pasadena, Calif.
# Dr. Michael J.S. Belton, Team Leader, Galileo Solid State Imaging Team, Emeritus Astronomer, National Optical Astronomy Observatories, Tucson, Ariz.
# Dr. Don Williams, principal investigator, Galileo heavy ion counter, The Johns Hopkins University, Applied Physics Laboratory, Laurel, Md.
# Jim Erickson, Mars Exploration Rover Mission Manager and former Galileo project manager, JPL.

The spacecraft was put on a collision course with Jupiter’s atmosphere to eliminate any chance of impact of the moon Europa, which Galileo discovered is likely to have a subsurface ocean. The team expects the spacecraft to transmit a few hours of science measurements in real time, leading up to impact on Sunday, September 21. The maneuver is necessary, since onboard propellant is nearly depleted. Without propellant, the spacecraft would not be able to point its antenna toward Earth nor adjust its flight path, so controlling the spacecraft would no longer be possible.

From 4:00 to 5:00 p.m. EDT, September 21, JPL will provide live commentary from the mission control room and footage of the countdown clock as Galileo nears its final moments. The televised special will feature two panels. One will include former project managers, and the other former project scientists.

Live satellite interview opportunities with project personnel are available Friday, September 19. To book a time, please contact Jack Dawson at: 818/354-0040.

Launched by the Space Shuttle Atlantis in 1989, the mission produced a string of discoveries while circling Jupiter, the solar system’s largest planet, 34 times. Galileo was the first spacecraft to directly measure Jupiter’s atmosphere with a probe and the first to conduct long-term observations of the Jovian system from orbit.

Galileo found evidence of subsurface liquid layers of salt water on Jupiter’s moons Europa, Ganymede and Callisto, and it detected extraordinary levels of volcanic activity on Io. Galileo was the first spacecraft to fly by an asteroid and the first to discover the moon of an asteroid. Galileo’s prime mission ended six years ago after two years orbiting Jupiter. NASA extended the mission three times to take advantage of Galileo’s unique science capabilities.

The September 17 SSU and September 21 end of mission event will be Web cast live at:

http://www.jpl.nasa.gov/webcast/galileo/
Additional information about the mission and Galileo’s discoveries is available at:

http://galileo.jpl.nasa.gov
For information about NASA on the Internet, visit:

http://www.nasa.gov

Original Source: NASA News Release

Jupiter Gets Even More Satellites

Image credit: UBC

A team of Canadian astronomers have discovered even more new satellites for Jupiter, giving the giant planet a total of 61 moons – 21 were discovered just this year. These new satellites are harder to detect because they’re only 1-5 kilometres across and have wide, irregular orbits around Jupiter. The team took a mosaic of images around the entire sky of Jupiter, and then used a computer to search for points of light that had the motion of a Jovian moon.

They were small and hard to find, but with the help of some new telescopic equipment and cameras, UBC professor Brett Gladman, UBC postdoctoral researcher Lynne Allen, and Dr. J.J. Kavelaars of the National Research Council of Canada have discovered nine previously unknown moons of Jupiter. So far this year, 21 new Jupiter moons have been identified.

The discovery of the distant satellites, announced today at the annual meeting of the Canadian Astronomical Society, boosts the number of known moons on Jupiter to 61 — more moons than any other planet in the solar system.

“The discovery of these small satellites is going to help us understand how Jupiter and the other giant planets formed,” said Gladman, a Canada Research Chair in Planetary Astronomy.

The new satellites were a challenge to detect because most are only about 1-5 kilometers in size. The feeble amounts of light they reflect back to earth must compete against the glare of brilliant Jupiter. Their small size and distance from the Sun prevent the satellites from shining any brighter than 24th magnitude, about 100 million times fainter than can be seen with the unaided eye. To locate these new moons, Gladman’s team used the new Megaprime mosaic of CCD cameras at the 3.6m Canada-France-Hawaii telescope on Mauna Kea, Hawaii.

The mosaic camera enabled the team to take three images of the entire sky around Jupiter. They used computer algorithms to search the images for the faint points of light moving across the sky as moons should.

Because moons can sometimes appear in front of distant stars or lost in the light scattered from the planet, to find them requires painstakingly repeating the search several times. The team undertook the task between February and April 2003.

International members of the jovian search team include Cornell University astronomers Phil Nicholson, Joseph A. Burns, and Valerio Carruba, Jean-Marc Petit of the Observatoire de Besancon, and Brian Marsden and Matthew Holman of the Harvard-Smithsonian Center for Astrophysics.

Original Source: UBC News Release

Six New Moons Found Around Jupiter

Image credit: University of Hawaii

Astronomers from the University of Hawaii have discovered six new moons for Jupiter, pushing the planet’s satellite count to 58 – the largest group of moons in the Solar System. These aren’t terribly big moons, though, only a kilometer or so across. The moons were discovered as part of an ongoing search using the world’s largest digital cameras at the Subaru and Canada-France-Hawaii telescopes atop Mauna Kea.

The majority of the new satellites were first seen in early February 2003 by Scott S. Sheppard and David C. Jewitt from the Institute for Astronomy, University of Hawaii along with Jan Kleyna of Cambridge University. The satellites were detected using the world’s two largest digital cameras at the Subaru (8.3 meter diameter) and Canada-France-Hawaii (3.6 meter diameter) telescopes atop Mauna Kea in Hawaii. Both telescopes and their imaging cameras represent the latest technology has to offer. Recoveries were performed at the University of Hawaii 2.2 meter with help from Yanga Fernandez and Henry Hsieh also from the University of Hawaii. Brian Marsden of the Harvard-Smithsonian Center for Astrophysics performed the orbit fitting for the new satellites.

The first 7 satellites were formally announced by the International Astronomical Union on Circular No. 8087 on March 4, 2003 while the eighth was announced on Circular No. 8088 on March 6, the 9th through 12th on Circular No. 8089 on March 7, S/2003 J13 through J20 were announced in April, and S/2003 J21 in May*. Except for S/2003 J20, all the new Jupiter satellites appear to have distant retrograde orbits (ie. their orbital rotation is opposite to Jupiter’s rotation) like the majority of the known irregular satellites of Jupiter. The satellite S/2003 J20 appears to be a prograde satellite dynamically distinct from any other known Jupiter satellite.

Original Source: IFA News Release

Finding Salt on Io

Image credit: NASA

A team of French and American astronomers have discovered the presence of salt (NaCl) in Io’s atmosphere. They think that the salt was ejected into the Jovian moon’s atmosphere by the many volcanos that ceaselessly bubble across its surface. The atmosphere of Io has been studied for several years now, first observed closely by the Voyager spacecraft, but this is the first time it’s been found to contain good old “table salt”.

The atmosphere of Jupiter’s moon Io is one of the most peculiar of the Solar System. In 1979, theVoyager spacecraft revealed active volcanism (Figure 1, left) at the surface of the satellite and discovered a local, tenuous SO2 atmosphere. Since 1990, millimeter-wave observations acquired at IRAM (French-German-Spanish telescope) and UV observations with HST provided a somewhat more detailed description of this atmosphere. The typical surface pressure is about 1 nanobar, and, in a unique fashion in the Solar System, the atmosphere exhibits strong horizontal variations, being apparently concentrated in an equatorial band.The main atmospheric compounds are SO2, SO and S2. The atmosphere is probably produced, on the one hand by direct volcanic output, and on the other hand by the sublimation of SO2 ices that cover Io’s surface.

However, it has been long suspected than Io’s atmosphere must contain other chemical species. As early as 1974, visible imaging and spectroscopy revealed a “cloud” of atomic sodium (Figure 1, right), roughly centered about Io’s orbit. Detailed subsequent studies of this cloud indicated a complex structure, including notably “fast sodium” features, for the production of which the role of molecular ions (NaX+ ) was evidenced. These discoveries naturally raised the question of the origin of sodium in Io’s environment. From the brightness of the optical emissions of Na, one can estimate that about 1026-1027 sodium atoms leave Io each second.

In 1999, chlorine in atomic and ionized form was discovered around Io, with an abundance comparable to that of sodium (while the cosmochemical abundance of Na is about 15 times that of Cl). This suggests a common origin, NaCl being a natural plausible parent of both. At the same time, on the basis of thermochemical equilibrium calculations, NaCl was proposed to be an important compound of Io’s volcanic magmas, with an abudance relative to SO2 as high as several percent.

Based on these discoveries and predictions, an observing campaign was conducted by E. Lellouch, from Paris Observatory, and several French and American colleagues at the IRAM 30-m radiotelescope in January 2002. Two rotational lines of NaCl at 143 and 234 GHz were unambiguously detected (Figure 2.). Because the vapor pressure of this salt is entirely negligible, NaCl cannot be in sublimation equilibrium with Io’s surface and its presence must directly result from continuous volcanic output. It appears to be a minor armospheric species. The most plausible physical model depicts the NaCl atmosphere as more localized than SO2, due to its very short lifetime (a few hours at most), and probably restricted to the volcanic centers. The local NaCl abundance in this model is 0.3-1.3 % of SO2, significantly lower than predicted. From the line strengths, volcanic emission rates of (2-8)x1028 NaCl molecules per second can be derived. According to photochemical and escape models, only a small fraction of these molecules escape from Io (about 0.1 %). A somewhat larger amount (1-2 %) leaves Io in atomic form after being photolyzed to Na and Cl. The vast majority of the volcanically-emitted NaCl molecules fall back to the surface where they condense out, potentially contributing to the white color of some of Io’s terrains. In conclusion, it appears that NaCl provides an importante source of sodium and chlorine in Io’s environment; however the precise chemical nature of the NaX+ molecular ions remains to be elucidated.

Original Source: Paris Observatory News Release

Galileo’s Last Look at Io

Image credit: NASA

The final images that the Galileo spacecraft will take of Jupiter’s moon Io were released today. They showcase crumbling crater slopes and the surface deposits from recent eruptions. Galileo also discovered 13 previously unknown hotspots on the moon’s surface, bringing the total number to 120; many more than anticipated. Galileo will make one final pass of another moon, Amalthea, before crashing into Jupiter in September, 2003.

The final images are in, and the resulting portrait of Jupiter’s moon Io, after a challenging series of observations by NASA’s Galileo spacecraft, is a peppery world of even more plentiful and diverse volcanoes than scientists imagined before Galileo began orbiting Jupiter in 1995.

Now that Galileo’s observations of Io have ended, scientists are focusing on trying to understand the big picture of how Io works by examining details.

Thirteen previously unknown active volcanoes dot infrared images from Galileo’s final successful flyby of Io, volcanologist Dr. Rosaly Lopes of NASA’s Jet Propulsion Laboratory reported today at the spring meeting of the American Geophysical Union in Washington, D.C.

That brings the total number of known Ionian hot spots to 120. Galileo images revealed 74 of them.

“We expected maybe a dozen or two,” said Dr. Torrence Johnson, Galileo project scientist at JPL in Pasadena, Calif. That expectation was based on discoveries by NASA’s Voyager spacecraft in 1979 and 1980, and subsequent ground-based observations.

“The volcanoes on Io have displayed an assortment of eruption styles, but recent observations have surprised us with the frequency of both giant plumes and crusted-over lakes of molten lava,” said planetary scientist Dr. Alfred McEwen of the University of Arizona, Tucson.

Galileo’s latest images, which also show tall slopes crumbling and surface deposits from two eruptions’ recent giant plumes, are available online from JPL at http://www.jpl.nasa.gov/images/io and from the University of Arizona Lunar and Planetary Laboratory at http://pirlwww.lpl.arizona.edu/Galileo/Releases .

Some high-resolution views taken as Galileo skimmed past Io on Oct. 16, 2001, are aiding analysis of the connection between volcanism and the rise and fall of mountains on Io. Few of Io’s volcanoes resemble the crater-topped volcanic peaks seen on Earth and Mars, said planetary scientist Dr. Elizabeth Turtle of the University of Arizona. Most of Io’s volcanic craters are in relatively flat regions, not near mountains, but nearly half of the mountains Io does have sit right beside volcanic craters.

“It appears that the process that drives mountain-building — perhaps the tilting of blocks of crust — also makes it easier for magma to get to the surface,” Turtle said. She showed a new image revealing that material slumping off a mountain named Tohil Mons has not piled up in a crater below, suggesting that the crater floor has been molten more recently than any landslides have occurred. Galileo’s infrared-mapping instrument has detected heat from the crater, indicating an active or very recent eruption.

From the analysis of Galileo’s observations, scientists are developing an understanding of how that distant world resurfaces itself differently than our world does.

“On Earth, we have large-scale lateral transport of the crust by plate tectonics,” McEwen said. “Io appears to have a very different tectonic style dominated by vertical motions. Lava rises from the deep interior and spreads out over the surface. Older lavas are continuously buried and compressed until they must break, with thrust faults raising the tall mountains. These faults also open new pathways to the surface for lava to follow, so we see complex relations between mountains and volcanoes, like at Tohil.”

“Io is a weird place,” Johnson said. “We’ve known that since even before Voyager, and each time Galileo has given us a close look, we get more surprises. Galileo has vastly increased our understanding of Io even though the mission was not originally slated to study Io.”

Extensions to Galileo’s original two-year orbital mission included six swings close to Io, where exposure to Jupiter’s intense radiation belts stresses electronic equipment on board the spacecraft. Researchers presented some results today from two Io encounters in the second half of 2001. Observations were not made successfully during Galileo’s final Io flyby, in January 2002, because effects of the radiation belts put the spacecraft into a precautionary standby mode during the crucial hours of the encounter.

Galileo will make its last flyby of a moon when it passes close to Amalthea, a small inner satellite of Jupiter, on November 5. No imaging is planned for that flyby. With fuel for altering its course and pointing its antenna nearly depleted, the long-lived spacecraft will then loop one last time away from Jupiter and perish in a final plunge into Jupiter’s atmosphere in September 2003.

Additional information about Galileo, Jupiter and Jupiter’s moons is available online at http://galileo.jpl.nasa.gov . JPL, a division of the California Institute of Technology in Pasadena, manages Galileo for NASA’s Office of Space Science, Washington, D.C.

Original Source: NASA/JPL News Release

Europa Could be Very Thick Skinned

Image credit: NASA

The evidence is mounting that Europa, one of the moons of Jupiter, has an ocean of water covered by a sheet of ice. Scientists are now speculating about how thick that ice is by measuring the size and depth of 65 impact craters on the moon’s surface – from what they can tell, it’s 19 km. The thickness of Europa’s ice will have an impact on the possibility of finding life there: too thick and sunlight will have trouble reaching photosynthetic organisms.

Detailed mapping and measurements of impact craters on Jupiter?s large icy satellites, reported in the May 23, 2002, issue of the journal Nature, reveal that Europa?s floating ice shell may be at least 19 kilometers thick. These measurements, by Staff Scientist and geologist Dr. Paul Schenk, at Houston?s Lunar and Planetary Institute, indicate that scientists and engineers will have to develop new and clever means of searching for life on the frozen world with a warm interior.

The Great Europa Pizza Debate: “Thin Crust or Thick Crust?”
Geologic and geophysical evidence from Galileo support the idea that a liquid water ocean exists beneath the icy surface of Europa. The debate now centers on how thick this icy shell is. An ocean could melt through a thin ice shell only a few kilometers thick exposing water and anything swimming in it to sunlight (and radiation). A thin ice shell could melt through, exposing the ocean to the surface, and granting easy access of photosynthetic organisms to sunlight. A thick ice shell tens of kilometers thick would be very unlikely to melt through.

Why is the thickness of Europa’s icy shell important?
The thickness is an indirect measure of how much tidal heating Europa is getting. Tidal heating is important for estimating how much liquid water is on Europa and whether there is volcanism on Europa?s sea floor but it must be derived; it cannot be measured. The new estimate of a 19 kilometer thickness is consistent with some models for tidal heating, but requires much additional study.

The thickness is important because it controls how and where biologically important material in Europa’s ocean can move to the surface, or back down to the ocean. Sunlight cannot penetrate more than a few meters into the icy shell, so photosynthetic organisms require easy access to Europa’s surface to survive. More on this subject later.

The thickness will also ultimately determine how we can explore Europa’s ocean and search for evidence of any life or organic chemistry on Europa. We cannot drill or sample the ocean directly through such a thick crust and must develop clever ways to search for ocean material that may have been exposed on the surface.

How do we estimate the thickness of Europa?s ice shell?
This study of impact craters on the large icy Galilean satellites of Europa is based on a comparison of the topography and morphology of impact crater on Europa with those on its sister icy satellites Ganymede and Callisto. Over 240 craters, 65 of them on Europa, have been measured by Dr. Schenk using stereo and topographic analysis of images acquired from NASA?s Voyager and Galileo spacecraft. Galileo is currently orbiting Jupiter and heading toward its final plunge into Jupiter in late 2003. Although both Ganymede and Callisto are believed to have liquid water oceans inside, they are also inferred to be rather deep (roughly 100-200 kilometers). This means that most craters will be unaffected by the oceans and can be used for comparison with Europa, where the depth to the ocean is uncertain but likely to be much shallower.

The estimate of the thickness of Europa?s ice shell is based on two key observations. The first is that the shapes of Europa?s larger craters differ significantly from similar sized craters on Ganymede and Callisto. Dr. Schenk?s measurements show that craters larger 8 kilometers across are fundamentally differ from those on Ganymede or Callisto. This is due to the warmth of the lower part of the ice shell. The strength of ice is very sensitive to temperature and warm ice is soft and flows rather quickly (think glaciers).

The second observation is that morphology and shape of craters on Europa change dramatically as crater diameters exceed ~30 kilometers. Craters smaller than 30 kilometers are several hundred meters deep and have recognizable rims and central uplifts (these are standard features of impact craters). Pwyll, a crater 27 kilometers across, is one of the largest of these craters.

Craters on Europa larger than 30 kilometers, on the other hand, have no rims or uplifts and have negligible topographic expression. Rather they are surrounded by sets of concentric troughs and ridges. These changes in morphology and topography indicate a fundamental change in the properties of the icy crust of Europa. The most logical change is from solid to liquid. The concentric rings in large Europan craters are probably due to the wholesale collapse of the crater floor. As the originally deep crater hole collapses, the material underlying the icy crust rushes in to fill in the void. This inrushing material drags on the overlying crust, fracturing it and forming the observed concentric rings.

Where does the 19 to 25 kilometer value come from?
Larger impact craters penetrate more deeply into the crust of a planet and are sensitive to the properties at those depths. Europa is no exception. The key is the radical change in morphology and shape at ~30 kilometers crater diameter. To use this, we must estimate how big the original crater was and how shallow a liquid layer must be before it can affect the final shape of the impact crater. This is derived from numerical calculations and laboratory experiments into impact mechanics. This ?crater collapse model? is then used to convert the observed transition diameter to a thickness for the layer. Hence, craters 30 kilometers wide are sensing or detecting layers 19-25 kilometers deep.

How certain are these estimates of Europa?s ice shell thickness?
There is some uncertainty in the exact thickness using these techniques. This is due mostly to uncertainties in the details of impact cratering mechanics, which are very difficult to duplicate in the laboratory. The uncertainties are probably only between 10 and 20%, however, so we can be reasonably sure that Europa’s ice shell is not a few kilometers thick.

Could the ice shell have been thinner in the past?
There is evidence in the crater topography that the thickness of ice on Ganymede has changed over time, and the same might be true for Europa. The estimate for ice shell thickness of 19 to 25 kilometers is relevant to the icy surface we now see on Europa. This surface has been estimated to be 30 to 50 million years or so. Most surface materials older than this have been destroyed by tectonism and resurfacing. This older icy crust could have been thinner than today?s crust, but we presently have no way of knowing.

Could the ice shell on Europa have thin spots now?
The impact craters Dr. Schenk studied were scattered across Europa?s surface. This suggests that the ice shell is thick everywhere. There could be local areas where the shell is thin due to higher heat flow. But the ice at the base of the shell is very warm and as we see in glaciers here on Earth, warm ice flows fairly rapidly. As a result, any ?holes? in Europa?s ice shell will be filled in quickly by flowing ice.

Does a thick ice shell mean there is no life on Europa?
No! Given how little we know about the origins of life and conditions inside Europa, life is still plausible. The probable presence of water under the ice is one of the key ingredients. A thick ice shell makes photosynthesis highly unlikely on Europa. Organisms would not have rapid or easy access to the surface. If organisms inside Europa can survive without sunlight, then the thickness of the shell is of only secondary importance. After all, organisms do quite well on the bottom of Earth?s oceans quite well without sunlight, surviving on chemical energy. This could be true on Europa if it is possible for living organisms to originate in this environment in the first place.

Then too, Europa’s ice shell could have been much thinner in the distant past, or perhaps it didn’t exist at some point and the ocean was exposed naked to space. If that were true, then a variety of organisms could evolve, depending on chemistry and time. If the ocean began to freeze over, the surviving organisms could then evolve to whatever environments allowed them to survive, such as volcanoes on the ocean floor (if volcanoes form at all).

Can we explore for life on Europa if the ice shell is thick?
If the crust is indeed this thick, then drilling or melting through the ice with tethered robots would be impractical! Nonetheless, we can search for organic ocean chemistry or life in other locations. The challenge will be for us to devise a clever strategy for exploring Europa that won?t contaminate what is there yet find it nonetheless. The prospect of a thick ice shell limits the number of likely sites where we might find exposed oceanic material. Most likely, ocean material will have to be embedded as small bubbles or pockets or as layers within ice that has been brought to the surface by other geologic means. Three geologic processes could do this:

1. Impact craters excavate crustal material from depth and eject it out onto the surface, where we might pick it up (50 years ago we could pick up iron meteorite fragments on the flanks of Meteor Crater in Arizona, but most have been found by now). Unfortunately, the largest known crater on Europa, Tyre, excavated material from only 3 kilometers deep, not deep enough to get near the ocean (due to geometry and mechanics, craters excavate from the upper part of the crater, not the lower). If a pocket or layer of ocean material were frozen into the crust at shallow depth, it might be sampled by an impact crater. Indeed, the floor of Tyre has a color that is slightly more orange than the original crust. However, roughly half of Europa was well seen by Galileo, so a larger crater might be present on the poorly seen side. We will have to go back to find out.

2. There is strong evidence that Europa?s icy shell is somewhat unstable and has been (or is) convecting. This means that blobs of deep crustal material rise upward toward the surface where they are sometimes exposed as domes several kilometers wide (think Lava Lamp, except that the blobs are soft solid material like Silly Putty). Any ocean material imbedded within the lower crust could then be exposed to the surface. This process could take thousands of years, and the exposure to Jupiter?s lethal radiation would be unfriendly to say the least! But at least we could investigate and sample what remains behind.

3. Resurfacing of wide areas of Europa?s surface where the icy shell has literally torn through and split apart. These areas are not empty but have been filled with new material from below. These areas do not appear to have been flooded by ocean material, but rather by soft warm ice from the bottom of the crust. Despite this it is very possible that oceanic material could be found within this new crustal material.

Our understanding of Europa’s surface and history is still very limited. Unknown processes could occur that bring ocean material to the surface, but only a return to Europa will tell.

What next for Europa?
With the recent cancellation of a proposed Europa Orbiter due to cost overruns, this is a good time to reexamine our strategy for exploring Europa?s ocean. Tethered submarines and deep drilling probes are rather impractical in such a deep crust, but surface landers could be very important nonetheless. Before we send a lander to the surface, we should send a reconnaissance mission, in either Jupiter or Europa orbit, to search for exposures of ocean material and thin spots in the crust, and to scout out the best landing sites. Such a mission would make use of vastly improved infrared mapping capabilities for mineral identification (after all, the Galileo instruments are nearly 25 years old). Stereo and laser instruments would be used for topographic mapping. Together with gravity studies, these data could be used to search for relatively thin regions of the icy crust. Finally, Galileo observed less than half of Europa at resolutions sufficient for mapping, including impact craters. Craters on this poorly seen hemisphere, for example, could indicate whether Europa?s ice shell was thinner in the past.

A Lander for Europa?
A lander with a seismometer could listen for europa-quakes generated by the daily tidal forces exerted by Jupiter and Io. Seismic waves can be used to precisely map the depth to the bottom of the ice shell, and possibly the bottom of the ocean as well. Onboard chemical analyzers would then search for organic molecules or other biologic tracers and potentially determine ocean chemistry, one of the fundamental indicators of Europa?s prospects as an ?inhabited? planet. Such a lander would probably need to drill several meters to get through the zone of radiation damage at the surface. Only after these missions are under way can we then begin the true exploration of this tantalizing planet-sized moon. To paraphrase Monty Python, ?It?s not dead yet!?

Original Source: USRA News Release

Eleven More Jupiter Moons Discovered

Image credit: NASA

Jupiter pushed past the other planets with the recent discovery of 11 new moons, bringing its total to 39. A team of US astronomers discovered the additional satellites (all 2-4 kilometres in diameter) using one of the world’s most powerful telescopes: the Canada-France-Hawaii 3.6 metre. Digital images of the space around Jupiter were processed using computers to detect objects moving in orbit, and to reject passing asteroids.

The discovery of 11 small moons orbiting Jupiter leapfrogs the number of that planet’s moons to 39, nine more than the record of the previous champ, Saturn.

A team led by astronomers from the University of Hawaii, Honolulu, made the discovery based on images taken in December 2001 and later follow-up observations. Orbits were determined by collaborators at NASA’s Jet Propulsion Laboratory, in Pasadena, Calif., and the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass.

Researchers estimate the new-found natural satellites are each about two to four kilometers (one to two miles) in diameter, and were probably passing rocks captured by Jupiter’s gravity long ago.

The discovery-team leaders, Scott Sheppard and Dr. David Jewitt of the University of Hawaii, also discovered 11 other small satellites of Jupiter in 2000.

The new moons were discovered by Sheppard, Jewitt and Jan Kleyna of Cambridge University, England. They used the Canada-France-Hawaii 3.6-meter (142-inch) telescope with one of the largest digital imaging cameras in the world to obtain sensitive images of a wide area around Jupiter.

The digital images were processed and searched using computers. Candidate satellites were monitored in the succeeding months at the University of Hawaii’s 2.2-meter (88-inch) telescope to confirm their orbits and to reject asteroids masquerading as satellites.

JPL’s Dr. Robert Jacobson and Harvard-Smithsonian’s Dr. Brian Marsden determined the satellites’ irregular — highly elongated and tilted — orbits. All 11 objects orbit in the direction opposite to the rotation of the planet.

The orbits of the irregular satellites strongly suggest an origin by capture. Since no efficient contemporary capture mechanisms are known, it is likely that the irregular satellites were acquired when Jupiter was young, possibly still in the process of condensing down to its equilibrium size. As yet, nothing is known about their surface properties, compositions or densities, but they are presumed to be rocky objects like the asteroids.

The new discoveries bring the known total of Jovian satellites to 39, of which 31 are irregulars. The eight regular satellites include four large moons discovered by the astronomer Galileo Galilei and four smaller moons on circular orbits closer to Jupiter. Jupiter’s nearest rival for having the largest number of known satellites is Saturn, with 30, of which 13 are irregular.

The satellites were formally announced by the International Astronomical Union on Circular No. 7900 (May 16, 2002). More information about them is available online from the University of Hawaii at http://www.ifa.hawaii.edu/~sheppard/satellites/jup.html. Other information about the Jupiter system is available from JPL at http://www.jpl.nasa.gov/solar_system/planets/jupiter_index.html.

The Institute for Astronomy at the University of Hawaii conducts research into galaxies, cosmology, stars, planets and the Sun. The Canada-France-Hawaii telescope is funded by the University of Hawaii and the governments of Canada and France. JPL, a division of the California Institute of Technology, Pasadena, is NASA’s lead center for robotic exploration of the solar system.

Original Source: NASA/JPL News Release

Jupiter is Buffeted by Solar Wind

Image credit: NASA
Scientists have uncovered the workings of an invisible bubble of charged particles that surround Jupiter and interact with the solar wind. This bubble is called the magnetosphere and extends to a distance of 100 times the diameter of Jupiter itself. 14 months ago, two spacecraft: Galileo and Cassini took simultaneous readings of the giant planet’s magnetosphere from different vantage points. Detailed results of their findings will be published in scientific journals in the next few days.

Scientists simultaneously using a combination of NASA spacecraft have seen into the workings of an invisible whirling bubble of charged particles surrounding Jupiter.

That bubble, Jupiter’s magnetosphere, is the biggest object with distinct boundaries within our solar system, more than 100 times wider than Jupiter itself. It contracts in response to shock waves from the Sun, according to one report appearing in the journal Nature tomorrow. In all, seven reports appearing together will detail various results from a concerted research campaign that took advantage of the Saturn-bound Cassini spacecraft’s flyby of Jupiter 14 months ago.

The campaign found extremely energetic electrons traveling near the speed of light close to Jupiter, as well as a vast nebula of neutral atoms, and triggers for glowing auroras near Jupiter’s north and south poles.

“We’re seeing results from a remarkable opportunity,” said Dr. Scott Bolton, a physicist at NASA’s Jet Propulsion Laboratory, Pasadena, Calif., and a co-author of three of the reports.

“We had one spacecraft, Galileo, inside the magnetosphere monitoring what was happening there at the same time another spacecraft, Cassini, was outside the magnetosphere monitoring the solar wind just upstream,” Bolton said. The solar wind is particles from the Sun flowing outward through the solar system. Jupiter’s magnetosphere, like Earth’s, deflects the solar wind but gets pushed around by its gusts.

On Jan. 10, 2001, when Cassini and Galileo were more than 20 times farther from each other than Earth is from the Moon, each spacecraft encountered the boundary of Jupiter’s magnetosphere while the bubble was contracting in response to an increase in solar-wind pressure.

“This is the first two-point measurement of the Jovian system actually responding to the solar wind,” said Dr. William Kurth, physicist at the University of Iowa, Iowa City, and lead author of the Nature report on these results. “The combined observations of Galileo and Cassini help show us the relative importance of the influence of the solar wind and the factors affecting the magnetosphere from within — primarily the energy from Jupiter’s rotation and the supply of material from volcanoes on the moon Io.” The Jupiter observations strengthen confidence in our understanding about Earth’s protective magnetosphere.

Shock waves from outbursts on the Sun, carried outward on the solar wind and detected by Cassini, also stimulated radio emissions from deep within Jupiter’s magnetosphere and brightened auroras at Jupiter’s poles, Dr. Donald Gurnett of the University of Iowa reports. Those effects suggest that electron density and electric currents in the magnetosphere increase when it is compacted by the shock wave.

Besides Galileo, which has been orbiting Jupiter since 1995, and Cassini, scientists used two Earth orbiters — the Hubble Space Telescope and Chandra X-ray Observatory ? plus radio telescopes in New Mexico and Arizona to examine Jupiter’s surroundings while Cassini was there.

Hubble images show patches of Jupiter’s aurora stimulated by an event Galileo detected within the magnetosphere, reports Dr. Barry Mauk of Johns Hopkins University’s Applied Physics Laboratory, Laurel, Md. The event is a surge of charged particles toward the planet, apparently analogous to similar aurora-triggering surges that release pent-up energy in Earth’s magnetosphere. Some other features in Jupiter’s aurora are “footprints” of currents flowing through the magnetosphere from three of the planet’s large moons, reports Dr. John Clarke of Boston University. Dr. Randall Gladstone of the Southwest Research Institute, San Antonio, Texas, describes a 45-minute rhythm in auroras at X-ray wavelengths, likely linked to a still-unidentified stimulus in the outer portion of the magnetosphere.

Cassini carries a type of magnetosphere-imaging instrument no previous interplanetary spacecraft has had. The instrument not only showed some structural detail of Jupiter’s magnetosphere, it also detected a cloud of neutral atoms stretching away from the planet as a “hot neutral wind,” reports Dr. Stamatios Krimigis of Hopkins’ Applied Physics Laboratory. The magnetic field holds charged particles in, but neutral ones escape to create a nebula of particles that extends beyond the magnetosphere.

High-energy electrons in radiation belts close to Jupiter emit radio waves that have been monitored from Earth for years. JPL’s Bolton and other scientists used Cassini while it was near Jupiter to map details never seen before in those belts. About 2,300 students at high schools and middle schools across the country participated in a program of radio-telescope observations that aided interpretation of those Cassini observations.

Cassini is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL manages Cassini and Galileo for NASA?s Office of Space Science, Washington, D.C. JPL is a division of the California Institute of Technology in Pasadena.

Original Source: NASA/JPL News Release

Jupiter’s X-Ray Hotspot Puzzles Astronomers

Image credit: Chandra
A new image taken by the Chandra X-Ray Telescope shows puzzling, pulsating hotspots at Jupiter’s north and south poles. So far, scientists have no explanation for what could be causing these X-rays; although, they do coincide with other phenomena seen on the planet, including auroras; like those at the Earth’s poles.

This image of Jupiter shows concentrations of auroral X-rays near the north and south magnetic poles. While Chandra observed Jupiter for its entire 10-hour rotation, the northern auroral X-rays were discovered to be due to a single ‘hot spot’ that pulsates with a period of 45 minutes, similar to high-latitude radio pulsations previously detected by NASA’s Galileo and Cassini spacecraft.

Although there had been prior detections of X-rays from Jupiter with other X-ray telescopes, no one expected that the sources of the X-rays would be located so near the poles. The X-rays are thought to be produced by energetic oxygen and sulfur ions that are trapped in Jupiter’s magnetic field and crash into its atmosphere. Before Chandra’s observations, the favored theory held that the ions were mostly coming from regions close to the orbit of Jupiter’s moon, Io.

Chandra’s ability to pinpoint the source of the X-rays has cast serious doubt on this model. Ions coming from near Io’s orbit cannot reach the observed high latitudes. The energetic ions responsible for the X-rays must come from much further away than previously believed.

One possibility is that particles flowing out from the Sun are captured in the outer regions of Jupiter’s magnetic field, then accelerated and directed toward its magnetic pole. Once captured, the ions would bounce back and forth in the magnetic field, from Jupiter’s north pole to south pole in an oscillating motion that could explain the pulsations.

Original Source: Chandra News Release