Source of geysers on Saturn's moon may be underground water

This false-color view shows the extent of Enceladus’ fountain-like plumes, which emanate from the moon’s warm south polar region. NASA / JPL / SSI

December 1, 2008

Provided by University of Central Florida, Orlando

A group of scientists concluded that a reservoir deep inside Enceladus is the source of the moon's plumes of water vapor and ice particles. Scientists at Jet Propulsion Laboratory (JPL) in California, the University of Colorado, and the University of Central Florida (UCF) in Orlando teamed up to analyze the plumes. They used data collected by the Cassini spacecraft's Ultraviolet Imaging Spectrograph (UVIS).

The team, including UCF Assistant Professor Joshua Colwell, found that the source of plumes may be vents on the moon that channel water vapor from a warm, probably liquid source to the surface at supersonic speeds.

"There are only three places in the solar system we know or suspect to have liquid water near the surface," Colwell said. "Earth, Jupiter's moon Europa, and now Saturn's Enceladus. Water is a basic ingredient for life, and there are certainly implications there. If we find that the tidal heating we believe causes these geysers is a common planetary- systems phenomenon, then it gets really interesting."

The team's findings support a theory that a water source deep inside Enceladus causes the plumes. This is not a foreign concept. On Earth, liquid water exists beneath ice at Lake Vostok, Antarctica.

Scientists suggest that in Enceladus' case, the ice grains would condense from the vapor escaping from the water source and stream through the cracks in the ice crust before heading into space. That's likely what Cassini's instruments detected in 2005 and 2007, the basis for the team's investigation.

The team's work also suggests that another hypothesis is unlikely. That theory predicts that the plumes of gas and dust observed are caused by evaporation of volatile ice freshly exposed to space when Saturn's tidal forces open vents in the south pole. But the team found more water vapor coming from the vents in 2007 at a time when the theory predicted there should have been less.

"Our observations do not agree with the predicted timing of the faults opening and closing due to tidal tension and compression," said Candice Hansen, the lead author on the project. "We don't rule it out entirely... but we also definitely do not substantiate this hypothesis."

Instead, their results suggest that the geysers' behavior supports a mathematical model that treats the vents as nozzles that channel water vapor from a liquid reservoir to the moon's surface. By observing the flickering light of a star as the geysers blocked it out, the team found that the water vapor forms narrow jets. The authors theorize that only high temperatures close to the melting point of water ice could account for the water vapor jets' high speed.

Enceladus is a prime target of Cassini during its extended Equinox Mission, underway now through September 2010.

"We still have a lot to discover and learn about how this all works on Enceladus," Colwell said. "But this is a good step in figuring it all out."

Cassini was launched from the Kennedy Space Center in 1997 and has orbited Saturn since July 2004.

Saturn's hot spots

This image shows the unexpected "hot spot" at Saturn's north pole. Scientists were surprised to find that the north pole, despite being in winter darkness for more than a decade, is home to a hot, cyclonic vortex very similar to that found on Saturn's much sunnier south pole. NASA/JPL/GSFC/Oxford University

January 7, 2008

Despite more than a decade of winter darkness, Saturn's north pole is home to an unexpected hot spot remarkably similar to one at the planet's sunny south pole. The source of its heat is a mystery. Now, the first detailed views of the gas giant's high latitudes from the Cassini spacecraft reveal a matched set of hot cyclonic vortices, one at each pole.

While scientists already knew about the hot spot at Saturn's south pole from previous observations by the W. M. Keck Observatory in Hawaii, the north pole vortex was a surprise.

"We had speculated that the south pole hot spot was connected to the southern, sunlit conditions," says Glenn Orton, a senior research scientist at NASA's Jet Propulsion Laboratory, Pasadena, California, and co-investigator on Cassini's composite infrared spectrometer. "Since the north pole has been deprived of sunlight since the arrival of winter in 1995, we didn't expect to find a similar feature there."

The infrared data show that the shadowed north pole vortex shares much the same structure and temperature as the one at the sunny south pole. The cores of both show a depletion of phospine gas, an imbalance probably caused by air moving downward into the lowest part of Saturn's atmosphere, the troposphere. Both polar vortices appear to be long-lasting and intrinsic parts of Saturn and are not related to the amount of sunlight received by one pole or the other.

"The hot spots are the result of air moving polewards, being compressed and heated up as it descends over the poles into the depths of Saturn," says Leigh Fletcher, a planetary scientist from the University of Oxford, England, and lead author of the paper. "The driving forces behind the motion, and indeed the global motion of Saturn's atmosphere, still need to be understood."

Though similar, the two polar regions differ in one striking way. At the north pole, the newly discovered vortex is framed by the distinctive, long-lived and still unexplained polar hexagon. This mysterious feature encompassing the entire north pole was first spotted in the 1980s by NASA's Voyager 1 and 2 spacecraft. Cassini's infrared cameras also detected the hexagon in deep atmospheric clouds early in 2007.

In their paper, Fletcher and his colleagues report that the bright, warm hexagon is much higher than previous studies had shown. "It extends right to the top of the troposphere," says Fletcher. "It is associated with downward motion in the troposphere, though the cause of the hexagonal structure requires further study."

Winter lasts about 15 years on Saturn. Researchers anticipate that when the seasons change in the coming years and Saturn's north pole is once again in sunlight, they will be able to see a swirling vortex with high eye walls and dark central clouds like the one now visible at the south pole. "But Saturn may surprise us again," says Fletcher.

"The fact that Neptune shows a similar south polar hot spot whets our appetite for the strange dynamics of the poles of the other gas giants," Fletcher says.

More information about Jupiter's poles will come from NASA's Juno mission, currently scheduled for launch in 2011 and arrival in 2016.

Cassini Finds Mysterious New Aurora on Saturn

Wednesday, November 12, 2008

Saturn's Polar Aurora
Credit: NASA/JPL/University of Arizona

This image of the northern polar region of Saturn shows both the aurora and underlying atmosphere, seen at two different wavelengths of infrared light as captured by NASA's Cassini spacecraft.

Energetic particles, crashing into the upper atmosphere cause the aurora, shown in blue, to glow brightly at 4 microns (six times the wavelength visible to the human eye). The image shows both a bright ring, as seen from Earth, as well as an example of bright auroral emission within the polar cap that had been undetected until the advent of Cassini. This aurora, which defies past predictions of what was expected, has been observed to grow even brighter than is shown here. Silhouetted by the glow (cast here to the color red) of the hot interior of Saturn (clearly seen at a wavelength of 5 microns, or seven times the wavelength visible to the human eye) are the clouds and haze that underlie this auroral region. For a similar view of the region beneath the aurora see Saturn's North Pole Hexagon and Aurora.

This image is a composite captured with Cassini's visual and infrared mapping spectrometer.

The aurora image was taken in the near-infrared on Nov. 10, 2006, from a distance of 1,061,000 kilometers (659,000 miles), with a phase angle of 157 degrees and a sub-spacecraft planetocentric latitude of 52 degrees north. The image of the clouds was obtained by Cassini on June 15, 2008, from a distance of 602,000 kilometers (374,000 miles) and a sub-spacecraft planetocentric latitude of 73 degrees north.

Saturn has its own unique brand of aurora that lights up the polar cap, unlike any other planetary aurora known in our solar system. This odd aurora revealed itself to one of the infrared instruments on NASA's Cassini spacecraft.

"We've never seen an aurora like this elsewhere," said Tom Stallard, a scientist working with Cassini data at the University of Leicester, England. Stallard is lead author of a paper that appears in the Nov. 13 issue of the journal Nature. "It's not just a ring of auroras like those we've seen at Jupiter or Earth. This aurora covers an enormous area across the pole. Our current ideas on what forms Saturn's aurora predict that this region should be empty, so finding such a bright aurora here is a fantastic surprise."Auroras are caused by charged particles streaming along the magnetic field lines of a planet into its atmosphere. Particles from the sun cause Earth's auroras. Many, but not all, of the auroras at Jupiter and Saturn are caused by particles trapped within the magnetic environments of those planets.

Jupiter's main auroral ring, caused by interactions internal to Jupiter's magnetic environment, is constant in size. Saturn's main aurora, which is caused by the solar wind, changes size dramatically as the wind varies. The newly observed aurora at Saturn, however, doesn't fit into either category.

"Saturn's unique auroral features are telling us there is something special and unforeseen about this planet's magnetosphere and the way it interacts with the solar wind and the planet's atmosphere," said Nick Achilleos, Cassini scientist on the Cassini magnetometer team at the University College London. "Trying to explain its origin will no doubt lead us to physics which uniquely operates in the environment of Saturn."

The new infrared aurora appears in a region hidden from NASA's Hubble Space Telescope, which has provided views of Saturn's ultraviolet aurora. Cassini observed it when the spacecraft flew near Saturn's polar region. In infrared light, the aurora sometimes fills the region from around 82 degrees north all the way over the pole. This new aurora is also constantly changing, even disappearing within a 45 minute-period.

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 was designed, developed and assembled at JPL. The visual and infrared mapping spectrometer team is based at the University of Arizona, Tucson.

Giant Cyclones at Saturn's Poles Create a Swirl of Mystery

Infrared Images of Saturn’s Poles
Credit: NASA/JPL/University of Arizona

New images from NASA’s Cassini spacecraft reveal a giant cyclone at Saturn’s north pole, and show that a similarly monstrous cyclone churning at Saturn’s south pole is powered by Earth-like storm patterns.

The new-found cyclone at Saturn’s north pole is only visible in the near-infrared wavelengths because the north pole is in winter, thus in darkness to visible-light cameras. At these wavelengths, about seven times greater than light seen by the human eye, the clouds deep inside Saturn’s atmosphere are seen in silhouette against the background glow of Saturn’s internal heat.



Saturn's South Polar Region Revealed
Credit: NASA/JPL/University of Arizona

The entire north pole of Saturn is now mapped in detail in infrared, with features as small as 120 kilometers (75 miles) visible in the images. Time-lapse movies of the clouds circling the north pole show the whirlpool-like cyclone there is rotating at 530 kilometers per hour (325 miles per hour), more than twice as fast as the highest winds measured in cyclonic features on Earth. This cyclone is surrounded by an odd, honeycombed-shaped hexagon, which itself does not seem to move while the clouds within it whip around at high speeds, also greater than 500 kilometers per hour (300 miles per hour). Oddly, neither the fast-moving clouds inside the hexagon nor this new cyclone seem to disrupt the six-sided hexagon.

New Cassini imagery of Saturn’s south pole shows complementary aspects of the region through the eyes of two different instruments. Near-infrared images from the visual and infrared mapping spectrometer instrument show the whole region is pockmarked with storms, while the imaging cameras show close-up details.



Convection in Saturn's Southern Vortex
Credit: NASA/JPL/Space Science Institute

Unlike Earth-bound hurricanes, powered by the ocean’s heat and water, Saturn's cyclones have no body of water at their bases, yet the eye-walls of Saturn’s and Earth’s storms look strikingly similar. Saturn's hurricanes are locked to the planet's poles, whereas terrestrial hurricanes drift across the ocean.

"These are truly massive cyclones, hundreds of times stronger than the most giant hurricanes on Earth," said Kevin Baines, Cassini scientist on the visual and infrared mapping spectrometer at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "Dozens of puffy, convectively formed cumulus clouds swirl around both poles, betraying the presence of giant thunderstorms lurking beneath. Thunderstorms are the likely engine for these giant weather systems," said Baines.

Just as condensing water in clouds on Earth powers hurricane vortices, the heat released from the condensing water in Saturnian thunderstorms deep down in the atmosphere may be the primary power source energizing the vortex.



The Yet Yawning Gulf
Credit: NASA/JPL/Space Science Institute

In the south, the new infrared images of the pole, under the daylight conditions of southern summer, show the entire region is marked by hundreds of dark cloud spots. The clouds, like those at the north pole, are likely a manifestation of convective, thunderstorm-like processes extending some 100 kilometers (62 miles) below the clouds. They are likely composed of ammonium hydrosulfide with possibly a mixture of materials dredged up from the depths. By contrast, most of the hazes and clouds seen on Saturn are thought to be composed of ammonia, which condenses at high, visible altitudes.

Complementary images of the south pole from Cassini’s imaging cameras, obtained in mid-July, are 10 times more detailed than any seen before. "What looked like puffy clouds in lower resolution images are turning out to be deep convective structures seen through the atmospheric haze," said Cassini imaging team member Tony DelGenio of NASA’s Goddard Institute for Space Studies in New York. "One of them has punched through to a higher altitude and created its own little vortex."

The "eye" of the vortex is surrounded by an outer ring of high clouds. The new images also hint at an inner ring of clouds about half the diameter of the main ring, and so the actual clear "eye" region is smaller than it appears in earlier low-resolution images.

"It’s like seeing into the eye of a hurricane," said Andrew Ingersoll, a member of Cassini's imaging team at the California Institute of Technology, Pasadena. "It’s surprising. Convection is an important part of the planet’s energy budget because the warm upwelling air carries heat from the interior. In a terrestrial hurricane, the convection occurs in the eyewall; the eye is a region of downwelling. Here convection seems to occur in the eye as well."

Further observations are planned to see how the features at both poles evolve as the seasons change from southern summer to fall in August 2009.

Tuesday, October 14, 2008

A Change of Seasons on Saturn

June 7, 2001

Looming like a giant flying saucer in our outer solar system, Saturn puts on a show as the planet and its magnificent ring system nod majestically over the course of its 29-year journey around the Sun. These Hubble Space Telescope images, captured from 1996 to 2000, show Saturn's rings open up from just past edge-on to nearly fully open as it moves from autumn towards winter in its Northern Hemisphere.

Saturn's equator is tilted relative to its orbit by 27 degrees, very similar to the 23-degree tilt of the Earth. As Saturn moves along its orbit, first one hemisphere, then the other is tilted towards the Sun. This cyclical change causes seasons on Saturn, just as the changing orientation of Earth's tilt causes seasons on our planet. The first image in this sequence, on the lower left, was taken soon after the autumnal equinox in Saturn's Northern Hemisphere (which is the same as the spring equinox in its Southern Hemisphere). By the final image in the sequence, on the upper right, the tilt is nearing its extreme, or winter solstice in the Northern Hemisphere (summer solstice in the Southern Hemisphere).

Astronomers are studying this set of images to investigate the detailed variations in the color and brightness of the rings. They hope to learn more about the rings' composition, how they were formed, and how long they might last. Saturn's rings are incredibly thin, with a thickness of only about 30 feet (10 meters). The rings are made of dusty water ice, in the form of boulder-sized and smaller chunks that gently collide with each other as they orbit around Saturn. Saturn's gravitational field constantly disrupts these ice chunks, keeping them spread out and preventing them from combining to form a moon. The rings, as shown here, have a slight pale reddish color due to the presence of organic material mixed with the water ice.

Saturn is about 75,000 miles (120,000 km) across, and is flattened at the poles because of its very rapid rotation. A day is only 10 hours long on Saturn. Strong winds account for the horizontal bands in the atmosphere of this giant gas planet. The delicate color variations in the clouds are due to smog in the upper atmosphere, produced when ultraviolet radiation from the Sun shines on methane gas. Deeper in the atmosphere, the visible clouds and gases merge gradually into hotter and denser gases, with no solid surface for visiting spacecraft to land on.

The Cassini/Huygens spacecraft, launched from Earth in 1997, is well on its way to the Saturn system. It will arrive in 2004 to land a probe on Titan, Saturn's largest moon, and to orbit the planet for four years for a detailed study of the entire Saturn system.

These images of Saturn, taken with the Wide Field Planetary Camera 2 onboard Hubble, were collected by Richard French (Wellesley College), Jeff Cuzzi (NASA/Ames), Luke Dones (SwRI), and Jack Lissauer (NASA/Ames), and have been prepared for presentation by the Hubble Heritage Team.

Why are Saturn's rings at a different angle in each image?

Saturn is tilted on its axis, just like Earth. Our view of the tilted planet and its rings changes as Saturn makes its 29-year journey around the Sun. For comparison, the Earth completes a circuit around the Sun in one year. Because Saturn is tilted like Earth, it, too, has seasons.

How thick are Saturn's rings?

Saturn's rings are incredibly thin, with a thickness of only about 30 feet (10 meters). The rings are made of dusty water ice, in the form of boulder-sized and smaller chunks that gently collide with each other as they orbit around Saturn. Saturn's gravitational field constantly disrupts these ice chunks, keeping them spread out and preventing them from combining to form a moon. The rings, as shown here, have a slight pale reddish color due to the presence of organic material mixed with the water ice.

An intriguing bright clump in Saturn's F ring

Date: Monday, June 25, 2007

The Cassini spacecraft spies an intriguing bright clump in Saturn's F ring. Also of interest is the dark gash that appears to cut through the ring immediately below the clump. Scientists continue to monitor this ring for small, transient clumps of material, as well as the effects of the shepherd moon Prometheus.

This view looks toward the unilluminated side of the rings from about 28 degrees above the ringplane.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on May 5, 2007 at a distance of approximately 2.1 million kilometers (1.3 million miles) from Saturn. Image scale is 12 kilometers (8 miles) per pixel.

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.

Cassini stares into the eye of monster storm on Saturn

NASA NEWS RELEASE
Posted: November 9, 2006
Credit: NASA/JPL/Space Science Institute

NASA's Cassini spacecraft has seen something never before seen on another planet -- a hurricane-like storm at Saturn's South Pole with a well-developed eye, ringed by towering clouds.


Cassini stares deep into the swirling hurricane-like vortex at Saturn's south pole, where the vertical structure of the clouds is highlighted by shadows. Such a storm, with a well-developed eye ringed by towering clouds, is a phenomenon never before seen on another planet.


The "hurricane" spans a dark area inside a thick, brighter ring of clouds. It is approximately 5,000 miles across, or two thirds the diameter of Earth.

"It looks like a hurricane, but it doesn't behave like a hurricane," said Andrew Ingersoll, a member of Cassini's imaging team at the California Institute of Technology, Pasadena. "Whatever it is, we're going to focus on the eye of this storm and find out why it's there."

A movie taken by Cassini's camera over a three-hour period reveals winds around Saturn's South Pole blowing clockwise at 350 miles per hour. The camera also saw the shadow cast by a ring of towering clouds surrounding the pole, and two spiral arms of clouds extending from the central ring. These ring clouds, 20 to 45 miles above those in the center of the storm, are two to five times taller than the clouds of thunderstorms and hurricanes on Earth.Eye-wall clouds are a distinguishing feature of hurricanes on Earth. They form where moist air flows inward across the ocean's surface, rising vertically and releasing a heavy rain around an interior circle of descending air that is the eye of the storm itself. Though it is uncertain whether such moist convection is driving Saturn's storm, the dark "eye" at the pole, the eye-wall clouds and the spiral arms together indicate a hurricane-like system.


These images of Saturn's south pole were taken by two different instruments on Cassini. The four monochrome images displayed here were acquired by the imaging science subsystem; the blue and red images in the bottom row were taken by the visual and infrared mapping spectrometer. The images are arranged in order of increasing wavelength in nanometers as follows: (top row) 460 nm, 752 nm, 728 nm; (bottom row) 890 nm, 2,800 nm, 5,000 nm.Distinctive eye-wall clouds have not been seen on any planet other than Earth. Even Jupiter's Great Red Spot, much larger than Saturn's polar storm, has no eye or eye-wall, and is relatively calm at the center.

This giant Saturnian storm is apparently different than hurricanes on Earth because it is locked to the pole and does not drift around like terrestrial hurricanes. Also, since Saturn is a gaseous planet, the storm forms without an ocean at its base.

In the Cassini imagery the eye looks dark at light wavelengths where methane gas absorbs the light and only the highest clouds are visible.

"The clear skies over the eye appear to extend down to a level about twice as deep as the usual cloud level observed on Saturn," said Kevin H. Baines, of Cassini's visual and infrared mapping spectrometer team at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "This gives us the deepest view yet into Saturn over a wide range of wavelengths, and reveals a mysterious set of dark clouds at the bottom of the eye.


"The Cassini data presented in this view appear to confirm a region of warm atmospheric descent into the eye of a hurricane-like storm locked to Saturn's south pole. The view shows temperature data from the Cassini spacecraft composite infrared spectrometer overlaid onto an image from the imaging science subsystem wide-angle camera.Infrared images taken by the Keck I telescope in Mauna Kea, Hawaii, had previously shown Saturn's South Pole to be warm. Cassini's composite infrared spectrometer has confirmed this with higher resolution temperature maps of the area. The spectrometer observed a temperature increase of about 4 degrees Fahrenheit at the pole. The instrument measured high temperatures in the upper troposphere and stratosphere, regions higher in the atmosphere than the clouds seen by the Cassini imaging instruments.

"The winds decrease with height, and the atmosphere is sinking, compressing and heating over the South Pole," said Richard Achterberg, a member of Cassini's composite infrared spectrometer team at NASA's Goddard Spaceflight Center, Greenbelt, Md.

Observations taken over the next few years, as the South Pole season changes from summer to fall, will help scientists understand the role seasons play in driving the dramatic meteorology at the south pole of Saturn.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington.

In the Shadow of Saturn

photo: Credit: SSI, JPL, ESA, NASA

In the shadow of Saturn, unexpected wonders appear. The robotic Cassini spacecraft now orbiting Saturn recently drifted in giant planet's shadow for about 12 hours and looked back toward the eclipsed Sun. Cassini saw a view unlike any other. First, the night side of Saturn is seen to be partly lit by light reflected from its own majestic ring system. Next, the rings themselves appear dark when silhouetted against Saturn, but quite bright when viewed away from Saturn and slightly scattering sunlight, in the above exaggerated color image. Saturn's rings light up so much that new rings were discovered, although they are hard to see in the above image. Visible in spectacular detail, however, is Saturn's E ring, the ring created by the newly discovered ice-fountains of the moon Enceladus, and the outermost ring visible above. Far in the distance, visible on the image left just above the bright main rings, is the almost ignorable pale blue dot of Earth.