Category: Jupiter

Jupiter’s two red spots. Image credit: Christopher Go. Click to enlarge
Jupiter’s well known Great Red Spot storm, and its newly formed Oval BA (aka Red Jr.) are about to sweep past each other, and astronomers aren’t quite sure what’s going to happen. The storms probably won’t merge or tear each other apart, but their outer bands will rub against each other. Oval BA recently turned red, signifying its larger size and strength compared to the smaller white storms on Jupiter. Some astronomers think the encounter with the Great Red Spot will slow it down again, returning it to white.

The two biggest storms in the solar system are about to go bump in the night, in plain view of backyard telescopes.

Storm #1 is the Great Red Spot, twice as wide as Earth itself, with winds blowing 350 mph. The behemoth has been spinning around Jupiter for hundreds of years.

Storm #2 is Oval BA, also known as “Red Jr.,” a youngster of a storm only six years old. Compared to the Great Red Spot, Red Jr. is half-sized, able to swallow Earth merely once, but it blows just as hard as its older cousin.

The two are converging. Closest approach: the 4th of July, according to Amy Simon-Miller of the Goddard Space Flight Center who has been monitoring the storms using the Hubble Space Telescope.

“There won’t be a head-on collision,” she says. “The Great Red Spot is not going to ‘eat’ Oval BA or anything like that.” But the storms’ outer bands will pass quite close to one another-and no one knows exactly what will happen.

Amateur astronomers are already monitoring the event. Christopher Go of the Philippines took the picture above using his 11-inch telescope on May 28th. “The distance between the storms is shrinking visibly every night,” he says.

Similar encounters have happened before, notes JPL’s Glenn Or ton, a colleague of Simon-Miller. “Oval BA and the Great Red Spot pass each other approximately every two years.” Previous encounters in 2002 and 2004 were anti-climatic. Aside from some “roughing” around the edges, both storms survived apparently unaltered.

This time might be different. Simon-Miller and Orton think Red Jr. could lose its red color, ironically, by passing too close to the Great Red Spot.

Red Jr./Oval BA wasn’t always red. For five years, 2000 to 2005, the storm was pure white like many other small “white ovals” circling the planet. In 2006 astronomers noticed a change: a red vortex formed inside the storm, the same color as the powerful Great Red Spot. This was a sign, researchers believed, that Oval BA was intensifying.

The color of the Great Red Spot itself is a mystery. A popular theory holds that the storm dredges up material from deep inside Jupiter’s atmosphere, lifting it above the highest clouds where solar ultraviolet rays turn “chromophores” (color-changing compounds) red. A beefed-up Oval BA could suddenly do the same.

Bumping up against the Great Red Spot, however, could weaken Oval BA, turning it white again. Simon-Miller explains: “We believe the Great Red Spot will push Oval BA toward a southern jet stream, which is blowing against the oval’s counterclockwise rotation.” This would slow Oval BA’s spin, possibly reversing the process that reddened it in the first place.

What will actually happen? “We’ll see,” she says. That’s what telescopes are for.

Note to sky watchers: Jupiter is easy to find. It pops out of the evening twilight before any other star, surprisingly bright. Look for it halfway up the southeastern sky at sunset: sky map.

Original Source: NASA News Release

Jupiter’s junior red spot. Image credit: NASA/ESA. Click to enlarge
The Hubble Space Telescope has snapped a picture of “Red Spot Jr.”, the newly forming storm on Jupiter. This new spot is half the size of Jupiter’s Great Red Spot, and formed after three white storms merged together. But when viewed in near-infrared wavelengths, the spot is as prominent as the Great Red Spot, so this is a big storm too. Scientists think that Jupiter might be in the midst of a global climate change, warming up a few degrees in some latitudes.

NASA’s Hubble Space Telescope is giving astronomers their most detailed view yet of a second red spot emerging on Jupiter. For the first time in history, astronomers have witnessed the birth of a new red spot on the giant planet, which is located half a billion miles away. The storm is roughly one-half the diameter of its bigger and legendary cousin, the Great Red Spot. Researchers suggest that the new spot may be related to a possible major climate change in Jupiter’s atmosphere.

Dubbed by some astronomers as “Red Spot Jr.,” the new spot has been followed by amateur and professional astronomers for the past few months. But Hubble’s new images provide a level of detail comparable to that achieved by NASA’s Voyager 1 and 2 spacecraft as they flew by Jupiter a quarter-century ago.

Before it mysteriously changed to the same color as the Great Red Spot, the smaller spot was known as the White Oval BA. It formed after three white oval-shaped storms merged during 1998 to 2000. At least one or two of the progenitor white ovals can be traced back to 90 years ago, but they may have been present earlier. A third spot appeared in 1939. (The Great Red Spot has been visible for the past 400 years, ever since earthbound observers had telescopes to see it).

When viewed at near-infrared wavelengths (specifically 892 nanometers – a methane gas absorption band) Red Spot Jr. is about as prominent in Jupiter’s cloudy atmosphere as the Great Red Spot. This may mean that the storm rises miles above the top of the main cloud deck on Jupiter just as its larger cousin is thought to do. Some astronomers think the red hue could be produced as the spots dredge up material from deeper in Jupiter’s atmosphere, which is then chemically altered by the Sun’s ultraviolet light.

Researchers think the Hubble images may provide evidence that Jupiter is in the midst of a global climate change that will alter its average temperature at some latitudes by as much as 10 degrees Fahrenheit. The transfer of heat from the equator to the planet’s south pole is predicted to nearly shut off at 34 degrees southern latitude, the latitude where the second red spot is forming. The effects of the shut-off were predicted by Philip Marcus of the University of California, Berkeley (UCB) to become apparent approximately seven years after the White Oval collisions in 1998 to 2000.

Two teams of astronomers were given discretionary time on Hubble to observe the new red spot.

Original Source: HubbleSite News Release

Jupiter and its moon Ganymede. Image credit: Alan Friedman. Click to enlarge
Look east in the early evening and you’ll see a very bright star. Well, that’s not a star, it’s Jupiter – and now’s the best time to go take a look at it. Jupiter will reach its closest approach to the Earth on May 6th. Even in small backyard telescopes, many features of the planet are visible, including its bands and 4 larger moons. If you’ve got a larger telescope, you might be able to pick out the newly formed storm dubbed “Red Spot Jr.”.

If you feel the urge to look up at the sky this month, you might be feeling the pull of Jupiter.

The giant planet is having a close encounter with Earth all month long. On May 6th, the date of closest approach, Jupiter will be 410 million miles away, which is almost 200 million miles closer than it was just six months ago in October. This makes Jupiter unusually big and bright.

Look for it rising in the east at sunset. Jupiter is unmistakable, shining ten times brighter than any star around it. The view through a backyard telescope is dynamite. You can see Jupiter’s cloud belts, the Great Red Spot and four large moons (Io, Europa, Ganymede and Callisto) circling the planet.

When you look at Jupiter through a telescope, you might notice something odd: the planet looks squashed. Your eyes are okay. Jupiter truly bulges around the middle because it spins so fast. One complete turn of the planet takes only 10 hours. That’s more than 300 Earth masses (almost enough to make a star) spinning like a nimble asteroid.

This spinning allows you to see the entire planet in a single night. On May 6th, Jupiter is “up” for more than 10 hours, or one complete turn. Judo astronomers will attempt a Jupiter marathon: In 10 hours you can see the innermost moons of Jupiter move from one side of the planet to the other. You can watch the Great Red Spot, a hurricane twice as wide as Earth, churn across Jupiter’s cloudtops. You might even see “Red Jr.,” a baby Great Red Spot trailing the original by about 2 hours: full story.

Although closest approach is May 6th, the best night to look is May 11th when the full Moon and Jupiter appear side by side. The pair will rise in tandem at sunset and remain beautifully close together all night long. With a telescope you can quickly scan back and forth: The lunar Alps. The moons of Jupiter. The Sea of Tranquillity. The Great Red Spot.

This is a sky map.

Do you feel the pull yet?

Let’s calculate: Jupiter is 318 times more massive than Earth and 410 million miles away. According to Newton’s Law of Universal Gravitation, Jupiter pulls you up 34 million times less than Earth pulls you down. Jupiter’s “pull” is utterly feeble.

So it’s all in your mind. But don’t let that stop you: give in to the pull!

Original Source: NASA News Release

Jupiter as mapped by Cassini. Image credit: NASA/JPL/SSI Click to enlarge
Cassini took many photographs of Jupiter on the way to Saturn, including this unusual montage of its southern pole. This photograph was made up of 36 separate images, stitched together on computer. The planet looks strange because the photo is a polar stereographic projections, which shows the southern pole in the middle, and the equator at the edges. The original images were captured on December 11th and 12th, 2000.

These color maps of Jupiter were constructed from images taken by the narrow-angle camera onboard NASA’s Cassini spacecraft on Dec. 11 and 12, 2000, as the spacecraft neared Jupiter during its flyby of the giant planet. Cassini was on its way to Saturn. They are the most detailed global color maps of Jupiter ever produced; the smallest visible features are about 120 kilometers (75 miles) across. For other maps see PIA07782 and PIA07783. (Related thumbnail images available here.)

The maps are composed of 36 images: a pair of images covering Jupiter’s northern and southern hemispheres was acquired in two colors every hour for nine hours as Jupiter rotated beneath the spacecraft. Although the raw images are in just two colors, 750 nanometers (near-infrared) and 451 nanometers (blue), the map’s colors are close to those the human eye would see when gazing at Jupiter.

The maps show a variety of colorful cloud features, including parallel reddish-brown and white bands, the Great Red Spot, multi-lobed chaotic regions, white ovals and many small vortices. Many clouds appear in streaks and waves due to continual stretching and folding by Jupiter’s winds and turbulence. The bluish-gray features along the north edge of the central bright band are equatorial “hot spots,” meteorological systems such as the one entered by NASA’s Galileo probe. Small bright spots within the orange band north of the equator are lightning-bearing thunderstorms. The polar regions shown here are less clearly visible because Cassini viewed them at an angle and through thicker atmospheric haze.

The round maps are polar stereographic projections that show the north or south pole in the center of the map and the equator at the edge.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov . The Cassini imaging team homepage is at http://ciclops.org .

Original Source: NASA/JPL/SSI News Release

Red spots on Jupiter. Image credit: Christopher Go. Click to enlarge
If you’re an amateur astronomer with a reasonably good telescope, you might be able to see a new red spot on Jupiter. Its official name is Oval BA, and it’s half the size the of the famous Great Red Spot. It first appeared in 2000 when three smaller storms collided and merged together. It started out white, then changed to brown, and now it’s the same colour as the Great Red Spot. It’s possible that huge storms like this dredge material from deep beneath Jupiter’s cloud tops, and then ultraviolet light from the Sun changes it red.

Backyard astronomers, grab your telescopes. Jupiter is growing a new red spot.

Christopher Go of the Philippines photographed it on February 27th using an 11-inch telescope and a CCD camera:

The official name of this storm is “Oval BA,” but “Red Jr.” might be better. It’s about half the size of the famous Great Red Spot and almost exactly the same color.

Oval BA first appeared in the year 2000 when three smaller spots collided and merged. Using Hubble and other telescopes, astronomers watched with great interest. A similar merger centuries ago may have created the original Great Red Spot, a storm twice as wide as our planet and at least 300 years old.

At first, Oval BA remained white?the same color as the storms that combined to create it. But in recent months, things began to change:

“The oval was white in November 2005, it slowly turned brown in December 2005, and red a few weeks ago,” reports Go. “Now it is the same color as the Great Red Spot!”

“Wow!” says Dr. Glenn Orton, an astronomer at JPL who specializes in studies of storms on Jupiter and other giant planets. “This is convincing. We’ve been monitoring Jupiter for years to see if Oval BA would turn red – and it finally seems to be happening.” (Red Jr? Orton prefers “the not-so-Great Red Spot.”)

Why red?

Curiously, no one knows precisely why the Great Red Spot itself is red. A favorite idea is that the storm dredges material from deep beneath Jupiter’s cloudtops and lifts it to high altitudes where solar ultraviolet radiation–via some unknown chemical reaction?produces the familiar brick color.

“The Great Red Spot is the most powerful storm on Jupiter, indeed, in the whole solar system,” says Orton. The top of the storm rises 8 km above surrounding clouds. “It takes a powerful storm to lift material so high,” he adds.

Oval BA may have strengthened enough to do the same. Like the Great Red Spot, Red Jr. may be lifting material above the clouds where solar ultraviolet rays turn “chromophores” (color-changing compounds) red. If so, the deepening red is a sign that the storm is intensifying.

“Some of Jupiter’s white ovals have appeared slightly reddish before, for example in late 1999, but not often and not for long,” says Dr. John Rogers, author of the book “Jupiter: The Giant Planet,” which recounts telescopic observations of Jupiter for the last 100+ years. “It will indeed be interesting to see if Oval BA becomes permanently red.”

See for yourself: Jupiter is easy to find in the dawn sky. Step outside before sunrise, look south and up: sky map. Jupiter outshines everything around it. Small telescopes have no trouble making out Jupiter’s cloudbelts and its four largest moons. Telescopes 10-inches or larger with CCD cameras should be able to track Red Jr. with ease.

What’s next? Will Red Jr. remain red? Will it grow or subside? Stay tuned for updates.

Original Source: NASA News Release

Jupiter’s icy moon Callisto. Image credit: NASA Click to enlarge
As scientists learn more about our Solar System, they’ve found water ice in some unusual situations. One of the most intriguing of these environments is on icy moons, like Jupiter’s Europa, and Uranus’ Triton. Researchers at the Lawrence Livermore National Laboratory have recreated this kind of ice in their laboratory; ice that probably mimics the conditions of pressure, temperature, stress, and grain size found on these moons. This ice can slowly creep and swirl around depending on the temperature of the moons’ interiors.

That everyday ice you use to chill your glass of lemonade has helped researchers better understand the internal structure of icy moons in the far reaches of the solar system.

A research team has demonstrated a new kind of “creep,” or flow, in a high-pressure form of ice by creating in a laboratory the conditions of pressure, temperature, stress, and grain size that mimic those in the deep interiors of large icy moons.

High-pressure phases of ice are major components of the giant icy moons of the outer solar system: Jupiter’s Ganymede and Callisto, Saturn’s Titan, and Neptune’s Triton. Triton is roughly the size of our own moon; the other three giants are about 1.5 times larger in diameter. Accepted theory says that most of the icy moons condensed as “dirty snowballs” from the dust cloud around the sun (the solar nebula) about 4.5 billion years ago. The moons were warmed internally by this accretionary process and by radioactive decay of their rocky fraction.

The convective flow of ice (much like the swirls in a hot cup of coffee) in the interiors of the icy moons controlled their subsequent evolution and present-day structure. The weaker the ice, the more efficient the convection, and the cooler the interiors. Conversely, the stronger the ice, the warmer the interiors and the greater the possibility of something like a liquid internal ocean appearing.

The new research reveals in one of the high-pressure phases of ice (“ice II”) a creep mechanism that is affected by the crystallite or “grain” size of the ice. This finding implies a significantly weaker ice layer in the moons than previously thought. Ice II first appears at pressures of about 2,000 atmospheres, which corresponds to a depth of about 70 km in the largest of the icy giants. The ice II layer is roughly 100 km thick. The pressure levels at the centers of the icy giant moons eventually reach the equivalent of 20,000 to 40,000 Earth atmospheres.

Researchers from Lawrence Livermore National Laboratory (LLNL), Kyushu University in Japan and the U.S. Geological Survey conducted creep experiments using a low-temperature testing apparatus in the Experimental Geophysics Laboratory at LLNL. They then observed and measured ice II grain size using a cryogenic scanning electron microscope. The group found a creep mechanism that dominates flow at lower stresses and finer grain sizes. Earlier experiments at higher stresses and larger grain size activated flow mechanisms that did not depend on grain size.

The experimentalists were able to prove that the new creep mechanism was indeed related to the size of the ice grains, something that previously had only been examined theoretically.

But the measurement was no easy feat. First, they had to create ice II of very fine grain size (less than 10 micrometers, or one-tenth the thickness of a human hair). A technique of rapid cycling of pressure above and below 2,000 atmospheres eventually did the trick. Adding to that, the team maintained a very steady 2,000 atmospheres of pressure within the testing apparatus to run a low-stress deformation experiment for weeks on end. Finally, to delineate the ice II grains and make them visible in the scanning electron microscope, the team developed a method of marking the grain boundaries with the common form of ice (“ice I”), which appeared different from ice II in the microscope. Once the boundaries were identified, the team could measure ice II’s grain size.

“These new results show that the viscosity of a deep icy mantle is much lower than we previously thought,” said William Durham, a geophysicist in Livermore’s Energy and Environment Directorate.

Durham said the high-quality behavior of the test apparatus at 2,000 atmospheres pressure, the collaboration with Tomoaki Kubo of Kyushu University, and success in overcoming serious technical challenges made for a fortuitous experiment.

Using the new results, the researchers conclude that it is likely the ice deforms by the grain size-sensitive creep mechanism in the interior of icy moons when the grains are up to a centimeter in size.

“This newly discovered creep mechanism will change our thinking of the thermal evolution and internal dynamics of medium- and large-size moons of the outer planets in our solar system,” Durham said. “The thermal evolution of these moons can help us explain what was happening in the early solar system.”

The research appears in the March 3 issue of the journal Science.

Founded in 1952, Lawrence Livermore National Laboratory has a mission to ensure national security and apply science and technology to the important issues of our time. Lawrence Livermore National Laboratory is managed by the University of California for the U.S. Department of Energy’s National Nuclear Security Administration.

Original Source: LLNL News Release

Computer simulation of Jupiter. Image credit: UCLA. Click to enlarge.
A new computer model indicates Jupiter’s massive winds are generated from deep within the giant planet’s interior, a UCLA scientist and international colleagues report today in the journal Nature.

Jupiter’s powerful winds are very different from those on Earth. They continually circle the planet, and have changed very little in the 300 years that scientists have studied them. Massive east-west winds in Jupiter’s equatorial region reach approximately 340 miles per hour ? twice as rapid as winds generated by strong hurricanes on Earth. At higher latitudes, the wind pattern switches to alternating jets that race around the planet.

No one has been able to explain why the winds are so constant or what generates them ? but that may change.

“Our model suggests convection driven by deep internal heat sources power Jupiter’s surface winds,” said Jonathan Aurnou, UCLA assistant professor of planetary physics. “The model provides a possible answer to why the winds are so stable for centuries. Jupiter’s surface is the tail; the dog is the hot interior of the planet.

“On Earth,” Aurnou said, “we get strong changes in wind patterns every season. On Jupiter, there is almost no variation. There are changing cloud structures, but the large-scale winds remain essentially constant.”

The researchers identified key ingredients that explain Jupiter’s “super winds” and factored those into their model. Aurnou’s colleagues are Moritz Heimpel, assistant professor of physics at the University of Alberta in Edmonton, and Johannes Wicht at the Max Planck Institute for Solar System Research in Germany.

Aurnou, Heimpel and Wicht created the first three-dimensional computer model that generates both a large eastward equatorial jet and smaller alternating jets at higher latitudes. In a rapidly rotating shell of fluid, they modeled thermally driven convection, which is what drives motion in a boiling pot.

“Three critical ingredients are the correct geometry, turbulent convection and rapid rotation, and our model contains all three elements,” said Aurnou, a faculty member in UCLA’s Department of Earth and Space Sciences. “When you include all those, that gives us the right recipe. In the future, we’ll refine our model by adding even more ingredients.”

Jupiter’s radius is more than 11 times the radius of Earth. A tremendous amount of heat comes from the interior.

“The heat from Jupiter’s interior is comparable to the heat the planet receives from the sun,” Aurnou said.

The model suggests three-dimensional convection in Jupiter’s deep atmosphere is likely driving the zonal flows, Aurnou said.

Jupiter’s interior is made primarily of compressed hydrogen and helium, and a giant plasma.

Aurnou will continue to study Jupiter’s strong winds, as well as those on Saturn, Uranus and Neptune.

Original Source: UCLA News Release

Jupiter. Image credit: NASA/JPL Click to enlarge
Turbulence driven by sunlight and thunderstorm activity may explain the multiple east-west jet streams on Jupiter and Saturn and even produce strong winds extending hundreds or thousands of kilometers into the interior, far below the altitudes where the jets are driven.

Scientists have been trying to understand the mechanisms that form the jet streams and control their structure since the first high-resolution images of Jupiter were returned by the Pioneer and Voyager spacecraft in the 1970s.

On Earth, the jet streams — narrow currents of air flowing from west to east in the midlatitudes — form a major component of our planet’s global circulation, and they control much of the large-scale weather experienced by the United States and other countries outside of the tropics. Similar east-west jet streams dominate the circulation of the giant planets Jupiter, Saturn, Uranus, and Neptune, reaching up to 400 miles per hour on Jupiter and nearly 900 miles per hour on Saturn and Neptune. The question of what causes these jet streams and how deep they extend into the interior of the giant planets remain some of the most important unsolved problems in the study of planetary atmospheres.

Adam Showman and Yuan Lian of The University of Arizona in Tucson and Peter Gierasch of Cornell University in Ithaca, New York, explained how cloud-layer turbulence can drive deep jets at the 37th annual meeting of the Division of Planetary Sciences of the American Astronomical Society, held in Cambridge, England.

Lian, Showman, and Gierasch performed computer simulations showing that horizontal temperature contrasts — generated by sunlight or differences in thunderstorm activity — can produce multiple jet streams that penetrate deep into the interior of a giant planet. In the simulations, the temperature contrasts induce deep-penetrating circulation cells that in turn drive the deep jets. The study, which uses an advanced three-dimensional computer model, is among the first that allows an assessment of how jets formed near the top of the atmosphere interact with the interior.

Most planetary scientists have assumed that jets pumped near the top of the atmosphere will remain confined to those shallow layers, and we’ve shown that this is not a valid assumption,” Showman said.

NASA’s Galileo Probe, which parachuted through Jupiter’s atmosphere in 1995, was intended in part to help answer the question of how deep the jet streams extend. The probe found strong winds extending at least 150 kilometers (almost 100 miles) below the clouds. Planetary scientists have widely interpreted this measurement as evidence that the jets are driven from deep inside Jupiter’s interior. The new study challenges this interpretation.

“We still don’t know whether the jets on the giant planets are driven from the top or within the deep interior,” Showman said. “But our study shows that the deep winds measured by the Galileo probe could just as easily result from shallow cloud-layer turbulence as from turbulence deep inside Jupiter’s interior.”

“This result contradicts a long-standing assumption on the part of many planetary scientists.”

The new study also shows that, under realistic conditions, the turbulence can produce not only numerous jet streams but a strong eastward flow at the equator, as observed on Jupiter and Saturn. Such flows are notoriously difficult to produce in atmospheric models, Showman noted.

Original Source: NASA Astrobiology

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