Cassini flew by Enceladus at an altitude of about 46 miles (74 kilometers). This flyby was designed primarily for the radio science sub-system to measure variations in Enceladus' gravity field.

On approach to Enceladus, Cassini's cameras imaged the icy satellite's south polar plume, which consists of jets of water ice, water vapor and organic compounds sprayed into space from the moon's famed "tiger stripe" fractures.

The plume images were captured at distances ranging from 259,000 miles (416,000 kilometers) down to 66,000 miles (106,000 kilometers) when Enceladus was just a thin crescent and the plume was backlit.

During closest approach, the radio science team looked for a concentration of mass at the south pole that could indicate sub-surface liquid water or an intrusion of warmer-than-average ice that might explain the intriguing geologic activity at the south pole.

After the closest approach, the composite infrared spectrometer obtained a map of Enceladus' sun-lit side while Cassini's visible light cameras rode along and captured several images of the moon's leading hemisphere at resolutions of about 1,500 feet (450 meters) per pixel.

Later this month, a close encounter with Titan on May 22 will pitch the spacecraft up out of the equatorial plane and into a nearly three-year-long phase of inclined orbits that will showcase the northern and southern reaches of Saturn.

On March 9, 2013, Cassini will make a close pass by Rhea, but the spacecraft won't have another close, targeted encounter with any of Saturn's other icy satellites until June 2015, when it encounters Dione. Cassini will make its next flyby of Enceladus on Oct. 14, 2015.

]]> Thu, 17 MAY 2012 17:53:23 AEST Pasadena CA (JPL) May 02, 2012 - NASA's Cassini spacecraft will be flying within about 46 miles (74 kilometers) of Saturn's moon Enceladus on Wednesday, May 2, aiming primarily to learn more about the moon's internal structure.

The flyby is the third part of a trilogy of flybys - the other two took place on April 28, 2010, and Nov. 30, 2010 - for Cassini's radio science experiment.

The radio science team is particularly interested in learning how mass is distributed under Enceladus' south polar region, which features jets of water ice, water vapor and organic compounds spraying out of long fractures.

A concentration of mass in that region could indicate subsurface liquid water or an intrusion of warmer-than-average ice that might explain the unusual plume activity.

Cassini's scientists learn about the moon's internal structure by measuring variations in the gravitational pull of Enceladus against the steady radio link to NASA's Deep Space Network on Earth.

Cassini's composite infrared spectrometer instrument will also be observing the side of Enceladus that always faces away from Saturn to monitor for hot spots. The imaging camera team also plans to take images of the plume to look for variability in the jets.

Cassini will also be flying by Dione at a distance of about 5,000 miles (8,000 kilometers), enabling the imaging cameras to create several mosaic images of the icy moon, and the composite infrared spectrometer to monitor heat emission.

]]> Thu, 17 MAY 2012 17:53:23 AEST Pasadena CA (JPL) Apr 27, 2012 - Data from NASA's Cassini mission reveal Saturn's moon Phoebe has more planet-like qualities than previously thought. Scientists had their first close-up look at Phoebe when Cassini began exploring the Saturn system in 2004. Using data from multiple spacecraft instruments and a computer model of the moon's chemistry, geophysics and geology, scientists found Phoebe was a so-called planetesimal, or remnant planetary building block. The findings appear in the April issue of the Journal Icarus.

"Unlike primitive bodies such as comets, Phoebe appears to have actively evolved for a time before it stalled out," said Julie Castillo-Rogez, a planetary scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

"Objects like Phoebe are thought to have condensed very quickly. Hence, they represent building blocks of planets. They give scientists clues about what conditions were like around the time of the birth of planets and their moons."

Cassini images suggest Phoebe originated in the far-off Kuiper Belt, the region of ancient, icy, rocky bodies beyond Neptune's orbit. Data show Phoebe was spherical and hot early in its history, and has denser rock-rich material concentrated near its center.

Its average density is about the same as Pluto, another object in the Kuiper Belt. Phoebe likely was captured by Saturn's gravity when it somehow got close to the giant planet.

Saturn is surrounded by a cloud of irregular moons that circle the planet in orbits tilted from Saturn's orbit around the sun, the so-called equatorial plane. Phoebe is the largest of these irregular moons and also has the distinction of orbiting backward in relation to the other moons.

Saturn's large moons appear to have formed from gas and dust orbiting in the planet's equatorial plane. These moons currently orbit Saturn in that same plane.

"By combining Cassini data with modeling techniques previously applied to other solar system bodies, we've been able to go back in time and clarify why it is so different from the rest of the Saturn system," said Jonathan Lunine, a co-author on the study and a Cassini team member at Cornell University, Ithaca, N.Y.

analyses suggest that Phoebe was born within the first 3 million years of the birth of the solar system, which occurred 4.5 billion years ago. The moon may originally have been porous but appears to have collapsed in on itself as it warmed up. Phoebe developed a density 40 percent higher than the average inner Saturnian moon.

Objects of Phoebe's size have long been thought to form as "potato-shaped" bodies and remained that way over their lifetimes.

If such an object formed early enough in the solar system's history, it could have harbored the kinds of radioactive material that would produce substantial heat over a short timescale. This would warm the interior and reshape the moon.

"From the shape seen in Cassini images and modeling the likely cratering history, we were able to see that Phoebe started with a nearly spherical shape, rather than being an irregular shape later smoothed into a sphere by impacts," said co-author Peter Thomas, a Cassini team member at Cornell.

Phoebe likely stayed warm for tens of millions of years before freezing up. The study suggests the heat also would have enabled the moon to host liquid water at one time. This could explain the signature of water-rich material on Phoebe's surface previously detected by Cassini.

The new study also is consistent with the idea that several hundred million years after Phoebe cooled, the moon drifted toward the inner solar system in a solar-system-wide rearrangement. Phoebe was large enough to survive this turbulence.

More than 60 moons are known to orbit Saturn, varying drastically in shape, size, surface age and origin. Scientists using both ground-based observatories and Cassini's cameras continue to search for others.

]]> Thu, 17 MAY 2012 17:53:23 AEST Greenbelt, MD (SPX) Apr 27, 2012 - Saturn's giant moon Titan hides within a thick, smoggy atmosphere that's well-known to scientists as one of the most complex chemical environments in the solar system. It's a productive "factory" cranking out hydrocarbons that rain down on Titan's icy surface, cloaking it in soot and, with a brutally cold surface temperature of around minus 270 degrees Fahrenheit, forming lakes of liquid methane and ethane.

However the most important raw ingredient in this chemical factory - methane gas, a molecule made up of one carbon atom joined to four hydrogen atoms - should not last for long because it's being continuously destroyed by sunlight and converted to more complex molecules and particles.

New research from NASA-funded scientists attempts to estimate how long this factory has been operating. The results are presented as two papers appearing in the April 20 issue of the Astrophysical Journal.

These papers used data from two instruments onboard NASA's Cassini spacecraft in orbit around Saturn and one instrument on the European Space Agency's Huygens probe that landed on Titan's surface in January, 2005. All three instruments were built at NASA's Goddard Space Flight Center in Greenbelt, Md.

A paper led by Conor Nixon of the University of Maryland, College Park uses infrared signatures (spectra) of methane from Cassini's composite infrared spectrometer to estimate how much "heavy" methane containing rare isotopes is present in Titan's atmosphere.

Isotopes are versions of an element with different weights, or masses. For example, carbon 13 is a heavier (and rare) version of the most common type of carbon, called carbon 12. Occasionally, a carbon-13 atom replaces a carbon-12 atom in a methane molecule.

Because methane made with carbon 12 is slightly lighter, the chemical reactions that convert it to more complex hydrocarbons happen a bit faster. This means carbon-12 methane gets used up at a slightly faster rate than heavy carbon-13 methane, so the concentration of heavy methane in Titan's atmosphere increases slowly.

By modeling how the concentration of heavy methane changes over time, the scientists predicted how long Titan's chemical factory has been running.

"Under our baseline model assumptions, the methane age is capped at 1.6 billion years, or about a third the age of Titan itself," said Nixon, who is stationed at NASA Goddard. "However, if methane is also allowed to escape from the top of the atmosphere, as some previous work has suggested, the age must be much shorter - perhaps only 10 million years - to be compatible with observations."

Both of these scenarios assume that methane entered the atmosphere in one burst of outgassing, probably from the restructuring of Titan's interior as heavier materials sank towards the center and lighter ones rose toward the surface.

"However, if the methane has been continuously replenished from a source then its isotopes would always appear 'fresh' and we can't restrict the age in our model," adds Nixon. Possible sources include methane clathrates, basically a methane molecule inside a "cage" or lattice of ice molecules.

Methane clathrates are found in the frigid depths of Earth's oceans, and some scientists think there could be an ocean of liquid water mixed with ammonia (acting as antifreeze) beneath Titan's water-ice crust. If this is so, methane might be released from its clathrate cages during the eruptions of proposed 'cryovolcanoes' of water-ammonia slurry, or more simply could slowly seep out through fractures in the crust.

The second paper by Kathleen Mandt of the Southwest Research Institute, San Antonio, Texas, and colleagues also models the time-evolution of methane. In this work, the concentration of the heavy methane is determined from measurements by Cassini's ion and neutral mass spectrometer, which counts molecules in the atmosphere of different masses (weights).

Measurements made by the Huygens gas chromatograph mass spectrometer, which also counts molecules of different masses, were used to constrain the impact of escape on the heavy methane in the atmosphere.

"We compute that, even if methane has been replenished from the interior over time to match or exceed the amounts fed into the atmospheric chemical factory, the process must have been running for a maximum of one billion years," said Mandt.

"If the process had started any earlier, we would see a build-up of methane in the lakes on the surface and in the atmosphere beyond what is observed today."

Together these papers add important new perspectives and constraints on the history of Titan's methane atmosphere, confirming that it must have formed long after Titan itself.

Previous work considering the evolution of Titan's interior has predicted the last major methane eruption occurred 350 million to 1.35 billion years ago, while crater counting has put the age of the current surface at 200 million to one billion years. (Crater counting works on the principle that an older surface has more craters, just as the longer you're in a paintball game, the more hits you'll get.)

The present work for the first time estimates the methane age from the atmosphere itself, at less than one billion years, considering both papers.

This research was supported by the NASA Cassini Mission and the NASA Cassini Data Analysis Program grant NNX09AK55G.

]]> Thu, 17 MAY 2012 17:53:23 AEST Boulder CO (SPX) Apr 26, 2012 - Shrouded in a thick, complex, organic haze, Saturn's giant moon Titan is proving to be one of the most scientifically interesting destinations in the solar system. Titan's atmosphere, which is mostly molecular nitrogen with a touch of methane, produces chemically complex hydrocarbons that rain down on Titan's icy surface, forming dunes of organic material.

The greenhouse effect provided by methane keeps the surface temperature around minus 270 degrees Fahrenheit, allowing lakes of liquid methane and ethane to form on the surface.

The history of methane in Titan's atmosphere is the subject of two papers published in the Astrophysical Journal. NASA-funded scientists are isolating and measuring the isotopic composition of the methane gas, creating models to better understand how long Titan's atmosphere has been cranking out organic materials.

"Methane's role on Titan is much like the role of water in the Earth's climate," explains Dr. Kathleen Mandt, the lead author of one of the papers and a scientist at Southwest Research Institute in San Antonio, Texas.

"First, methane and water are the dominant greenhouse gases on Titan and Earth, respectively, increasing surface temperatures.

Second, methane rain falls on the surface of Titan much like water does on Earth. Because Titan is the only other known body with a hydrologic cycle similar to our own, understanding how long methane has been present in Titan's atmosphere is important for making sense of this unique environment that is so different, yet so much like, our own world."

Together, the research presented in the papers estimates the age of Titan's current nitrogen-methane atmosphere at less than one billion years, confirming that the atmosphere in its current state formed long after Titan itself.

Methane, comprised of one carbon atom joined to four hydrogen atoms, breaks down relatively quickly in the atmosphere as ultraviolet sunlight converts it into more complex molecules and particles. By modeling the concentration of isotopically heavy methane over time, scientists can predict the age of Titan's atmosphere.

Isotopes are versions of an element with different weights, or masses. For example, carbon 13 is rare, heavier version of the most common type of carbon, carbon 12. Occasionally, a carbon-13 atom replaces a carbon-12 atom in a methane molecule.

Because methane made with carbon 12 is slightly lighter, the chemical reactions that convert it to more complex hydrocarbons happen a bit faster. This means carbon-12 methane gets used up at a slightly faster rate than heavy carbon-13 methane, so the relative concentration of heavy methane in Titan's atmosphere increases slowly.

Mandt and several SwRI colleagues modeled the time-evolution of methane using data from NASA's Cassini spacecraft, which has been orbiting Saturn since 2004, and the European Space Agency's Huygens probe that landed on Titan's surface in January 2005.

The SwRI team measured the relative concentration of the heavy methane using Cassini's Ion and Neutral Mass Spectrometer and compared the results to those measured on Titan's surface by the Huygens Gas Chromatograph Mass Spectrometer, which both identify and count molecules based on their mass.

Comparing these two sets of measurements allowed the scientists to understand the impact of escape on the amount of heavy methane present in the atmosphere.

"We compute that even if methane has been replenished from the interior over time to match or exceed the amounts fed into the atmospheric chemical factory, the process must have been running for a maximum of one billion years," said Mandt. "If the process had started any earlier, we would see a build-up of methane in the lakes on the surface and in the atmosphere beyond what is observed today."

The second paper, led by Dr. Conor Nixon of the University of Maryland, College Park, discusses the use of infrared signatures (spectra) of methane from Cassini's Composite Infrared Spectrometer to estimate the levels of "heavy" methane containing rare isotopes present in Titan's atmosphere.

Researchers also determined the role of chemical reactions in influencing the isotopic composition of methane in Titan's atmosphere and illustrated the importance of chemistry to understanding the lifetime of methane.

"It is great to experience the scientific maturity of the Cassini-Huygens program, where information from many different investigations is being combined to answer fundamental outstanding questions," said SwRI Institute Scientist Dr. Hunter Waite, the principal investigator of Cassini's Ion and Neutral Mass Spectrometer.

]]> Thu, 17 MAY 2012 17:53:23 AEST Pasadena CA (JPL) Apr 24, 2012 - Scientists working with images from NASA's Cassini spacecraft have discovered strange half-mile-sized (kilometer-sized) objects punching through parts of Saturn's F ring, leaving glittering trails behind them.

These trails in the rings, which scientists are calling "mini-jets," fill in a missing link in our story of the curious behavior of the F ring. The results will be presented tomorrow at the European Geosciences Union meeting in Vienna, Austria.

"I think the F ring is Saturn's weirdest ring, and these latest Cassini results go to show how the F ring is even more dynamic than we ever thought," said Carl Murray, a Cassini imaging team member based at Queen Mary University of London, England.

"These findings show us that the F ring region is like a bustling zoo of objects from a half mile [kilometer] to moons like Prometheus a hundred miles [kilometers] in size, creating a spectacular show."

Scientists have known that relatively large objects like Prometheus (as long as 92 miles, or 148 kilometers, across) can create channels, ripples and snowballs in the F ring. But scientists didn't know what happened to these snowballs after they were created, Murray said.

Some were surely broken up by collisions or tidal forces in their orbit around Saturn, but now scientists have evidence that some of the smaller ones survive, and their differing orbits mean they go on to strike through the F ring on their own.

These small objects appear to collide with the F ring at gentle speeds - something on the order of about 4 mph (2 meters per second). The collisions drag glittering ice particles out of the F ring with them, leaving a trail typically 20 to 110 miles (40 to 180 kilometers) long.

Murray's group happened to see a tiny trail in an image from Jan. 30, 2009 and tracked it over eight hours. The long footage confirmed the small object originated in the F ring, so they went back through the Cassini image catalog to see if the phenomenon was frequent.

"The F ring has a circumference of 550,000 miles [881,000 kilometers], and these mini-jets are so tiny they took quite a bit of time and serendipity to find," said Nick Attree, a Cassini imaging associate at Queen Mary. "We combed through 20,000 images and were delighted to find 500 examples of these rogues during just the seven years Cassini has been at Saturn."

In some cases, the objects traveled in packs, creating mini-jets that looked quite exotic, like the barb of a harpoon. Other new images show grand views of the entire F ring, showing the swirls and eddies that ripple around the ring from all the different kinds of objects moving through and around it.

"Beyond just showing us the strange beauty of the F ring, Cassini's studies of this ring help us understand the activity that occurs when solar systems evolve out of dusty disks that are similar to, but obviously much grander than, the disk we see around Saturn," said Linda Spilker, Cassini project scientist based at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

"We can't wait to see what else Cassini will show us in Saturn's rings."

New images and movies of the mini-jets and other peculiar F ring behavior are available here.

]]> Thu, 17 MAY 2012 17:53:23 AEST Pasadena CA (JPL) Apr 23, 2012 - A new study analyzing data from NASA's Cassini spacecraft suggests that the lake, known as Ontario Lacus, behaves most similarly to what we call a salt pan on Earth.

A group led by Thomas Cornet of the Universite de Nantes, France, a Cassini associate, found evidence for long-standing channels etched into the lake bed within the southern boundary of the depression.

This suggests that Ontario Lacus, previously thought to be completely filled with liquid hydrocarbons, could actually be a depression that drains and refills from below, exposing liquid areas ringed by materials like saturated sand or mudflats.

"We conclude that the solid floor of Ontario Lacus is most probably exposed in those areas," said Cornet, whose paper appears in a recent issue of the journal Icarus.

These characteristics make Ontario Lacus very similar to the Etosha salt pan on Earth, which is a lake bed that fills with a shallow layer of water from groundwater levels that rise during the rainy season. This layer then evaporates and leaves sediments like tide marks showing the previous extent of the water.

"Some of the things we see happening in our own backyard are right there on Titan to study and learn from," said Bonnie Buratti, a co-author and Cassini team member based at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "On Earth, salt pans tend to form in deserts where liquids can suddenly accumulate, so it appears the same thing is happening on Titan."

While the liquid on Titan is methane, ethane and propane rather than water, the cycle appears to work in a very similar fashion to the water cycle on Earth. Beyond Earth, Titan is the only other world known to bear stable liquids on its surface.

There, the full hydrocarbon cycle is based on hydrogen, carbon and nitrogen, and takes place between the atmosphere, the surface and the subsurface. Titan's lakes are an integral part of this process.

This latest paper is part of an ongoing study of Ontario Lacus, the largest lake in Titan's south polar region. Cassini has been observing the lake with multiple instruments and employing multiple methods of analysis to see if Titan, like Earth, changes with the seasons. During the time Cassini has been exploring the Saturn system, Titan's southern hemisphere has transitioned from summer to fall.

"These results emphasize the importance of comparative planetology in modern planetary sciences: finding familiar geological features on alien worlds like Titan allows us to test the theories explaining their formation," said Nicolas Altobelli, ESA's Cassini-Huygens project scientist.

]]> Thu, 17 MAY 2012 17:53:23 AEST Pasadena CA (JPL) Apr 17, 2012 - These raw, unprocessed images of Saturn's moons Enceladus and Tethys were taken on April 14, 2012, by NASA's Cassini spacecraft.

Cassini flew by Enceladus at an altitude of about 46 miles (74 kilometers). This flyby was designed primarily for the ion and neutral mass spectrometer to analyze, or "taste," the composition of the moon's south polar plume as the spacecraft flew through it.

Cassini's path took it along the length of Baghdad Sulcus, one of Enceladus' "tiger stripe" fractures from which jets of water ice, water vapor and organic compounds spray into space. At this time, Baghdad Sulcus is in darkness, but that was not an obstacle for another instrument, the composite infrared spectrometer, which can see features by their surface temperatures and which also took measurements during this flyby.

As soon as daylight passed into the spacecraft's remote sensing instruments' line of sight, Cassini's cameras acquired images of the surface. The wide-angle-camera image included in the new batch, taken from around the time of closest approach, has some smearing from the movement of the spacecraft during the exposure, but still shows the surface in vivid detail.

Cassini's cameras also imaged Enceladus' south polar plume at a high phase angle as the satellite appeared as a thin crescent and the plume was backlit.

After the Enceladus encounter, Cassini passed the moon Tethys with a closest approach distance of about 5,700 miles (9,100 kilometers). This was Cassini's best imaging encounter with Tethys since a targeted encounter in September 2005.

The 2005 encounter, with a closest approach distance of about 930 miles (1,500 kilometers), provided the images of Tethys with the best resolution and captured views of the side of Tethys that faces Saturn in its orbit.

This new encounter examined the opposite side of Tethys, providing some of the highest-resolution images of the side that faces away from Saturn. Cassini acquired a 22-frame mosaic of this side, which features the large impact basin named Odysseus.

Scientists will use these new data in conjunction with images from previous encounters to create digital elevation maps of the moon's surface.

]]> Thu, 17 MAY 2012 17:53:23 AEST Manchester, UK (SPX) Apr 05, 2012 - Since the NASA / ESA Cassini-Huygens spacecraft arrived at Saturn in 2004, astronomers and space scientists have been able to study the ringed planet and its moons in great detail. Now, for the first time, a team of planetary scientists have made simultaneous measurements of Saturn's nightside aurora, magnetic field, and associated charged particles.

Together the fields and particle data provide information on the electric currents flowing that produce the emissions. Team leader Dr Emma Bunce of the University of Leicester will present the new work at the National Astronomy Meeting in Manchester on 27 March 2012.

Generally, images of the aurora (equivalent to the terrestrial 'northern lights') provide valuable information about the electromagnetic connection between the solar wind, the planet's magnetic field (magnetosphere) and its upper atmosphere. Variations in the aurora then provide information on changes in the associated magnetosphere.

But viewing the aurora (best done at a large distance) at the same time as measuring the magnetic field and charged particles at high latitudes (where the aurora is found, best done close to the planet) is hard.

In 2009, Cassini made a crossing of the magnetic field tubes that connect to the aurora on the night side of Saturn. Because of the position of the spacecraft, Dr Bunce and her team were able to obtain ultraviolet images of the aurora (which manifests itself as a complete oval around each pole of the planet) at the same time.

This is the first time it has been possible to make a direct comparison between Cassini images of the nightside aurora and the magnetic field and particle measurements made by the spacecraft.

And because of the geometry of the orbit at Cassini, it took about 11 hours to pass through the high-latitude region or about the same time it takes Saturn to make one rotation.

This meant that the team were able to watch the auroral oval move as the planet turned. As Saturn and its magnetosphere rotated, the auroral oval was tilted back and forth across the spacecraft with a speed that is consistent with a planetary rotation effect:

Dr Bunce comments: "With these observations we can see the simultaneous motion of the electric current systems connecting the magnetosphere to the atmosphere, producing the aurora. Ultimately these observations bring us a step closer to understanding the complexities of Saturn's magnetosphere and its ever elusive rotation period".

]]> Thu, 17 MAY 2012 17:53:23 AEST Pasadena CA (JPL) Apr 02, 2012 - Saturn's moon Iapetus is one of the most unusual moons in our solar system. Perhaps the most bizarre feature of Iapetus is its equatorial ridge, a 20-km (12.4-mi) high, 200-km (124-mi) wide mountain range that runs exactly along the equator, circling more than 75 percent of the moon.

No other body in the solar system exhibits such a feature, and as Dombard et al. show, previous models have been unable to adequately explain how the ridge formed.

The authors now propose that the ridge formed from an ancient giant impact that produced a subsatellite around Iapetus.

Tidal interactions with Iapetus ultimately led to orbital decay, eventually bringing the subsatellite close enough that the same forces tore it apart, forming a debris ring around Iapetus. Material from this debris ring then rained down on Iapetus, creating the mountain ring along the equator.

Andrew J. Dombard: Department of Earth and Environmental Sciences, University of Illinois at Chicago, Chicago, Illinois, USA; Andrew F. Cheng: Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA; William B. McKinnon: Department of Earth and Planetary Sciences and McDonnell Center for Space Sciences, Washington University, St. Louis, Missouri, USA; Jonathan P. Kay: Department of Earth and Environmental Sciences, University of Illinois at Chicago, Chicago, Illinois, USA. "Delayed formation of the equatorial ridge on Iapetus from a subsatellite created in a giant impact"; Journal of Geophysical Research-Planets, doi:10.1029/2011JE004010, 2012

]]> Thu, 17 MAY 2012 17:53:23 AEST Subscribe to our daily selection of space, military, environment and energy newsletters responseText http://visitor.constantcontact.com/manage/optin/ea?v=0016gbbKsaiGSpQFojVO8ZoHw%3D%3D