Category: Saturn

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What lurks in the eerie fractures of Enceladus that slash across the moon’s south polar region? The Cassini spacecraft will make a Halloween flyby of Enceladus and focus its cameras and other optical remote sensing instruments on the mysterious tiger-stripe-like features seen on this strange moon of Saturn. This flyby comes just over three weeks after a previous flyby of the same moon, and then just a few days later, on November 3, Cassini will make a flyby of Titan, on the inbound leg of its orbit around the ringed world. What tricks do the Cassini scientists have up their sleeves for this flyby? Will we be treated to some spectacular images? And what about those spooky sounds from Saturn?

For the Oct. 31 flby, the spacecraft will zoom by the Enceladus at 17.7 km/sec (39,600 mph, cruising just 197 km (122 miles) from the moon’s surface. Cassini will approach the moon on a fast, inclined trajectory over the northern hemisphere and will depart over the southern hemisphere. The closest approach occurs on October 31st at 17:14:51 UT over latitude 28° S and longitude 97° W. The Optical Remote Sensing (ORS) instruments are at the focus of the science operations during this fly-by. The Imaging Science Subsystem camera will execute a sophisticated series of images starting just 2 minutes after closest-approach, obtaining images of the south polar “tiger stripes” at resolutions as high as 8.4 m/pixel. Enceladus will be in eclipse (in Saturn’s shadow) for about 2.5 hours, starting about 50 minutes after closest-approach.

In honor of the Halloween, the Cassini website has posted Spooky Sounds from Saturn. Check it out!

Sources: Cassini website, SATNews

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While Saturn and its rings are beautiful and wondrous, the sounds of Saturn are eerie and strange. Scientists have been trying to understand the bizarre radio emissions that come from the ringed planet, called the Saturn Kilometric Radiation (SKR). Scientists have used observations from NASA’s Cassini spacecraft build a 3-D picture of these intense radio emissions emanating from Saturn’s magnetic field. The SKR radio emissions are generated by high-energy electrons spiraling around magnetic field lines threaded through Saturn’s auroras.

Previous Cassini observations have shown that the SKR is closely correlated with the intensity of Saturn’s UV aurora and the pressure of the solar wind. “The animation shows radio sources clustered around curving magnetic field lines,” said Dr. Baptiste Cecconi, of LESIA, Observatoire de Paris. “Because the radio signals are beamed out from the source in a cone-shape, we can only detect the sources as Cassini flies through the cone. When Cassini flies at high altitudes over the ring planes, we see the sources clearly clustered around one or two field lines. However, at low latitudes we get more refraction and so the sources appear to be scattered.”

Link to 3-D animation.

The active area of the magnetic field matched up with near-polar latitudes degrees in both the northern and southern hemisphere, the location of Saturn’s UV aurora.

“For the purposes of the model, we’ve imagined a screen that cuts through the middle of Saturn, set up at right-angles to the line between Cassini and the centre of the planet. We’ve mapped the footprints of the radio sources projected onto the screen, which tilts as Cassini moves along its orbital path and its orientation with respect to Saturn changes. We’ve also traced the footprints of the magnetic field lines back to the cloud tops of Saturn,” said Cecconi.

Listen to the sounds of Saturn.

Although there were some minor differences between emissions in the northern and southern hemispheres, the emissions were strongest in the western part of Saturn’ss sunlit hemisphere. This area corresponds to a region of Saturn’s magnetopause where electrons are thought to be accelerated by the interaction of the solar wind and Saturn’s magnetic field.

The measurements were made using Cassini’s Radio and Plasma Wave Science (RPWS) experiment.

Cecconi presented his results at the European Planetary Science Congress on Tuesday, September 23rd.

Source: European Planetary Science Congress

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Saturn’s enigmatic rings may be much older and also much more massive than previously thought, according to a new study. Because Saturn’s rings look so clean and bright, it was thought the rings were younger than the planet itself, which is estimated to be about 4.5 billion years old. But using data from the Cassini spacecraft’s UVIS (Ultraviolet Imaging Spectrograph) instrument, Principal Investigator Dr. Larry Esposito and his team used computer simulations to study colliding particles in Saturn’s rings and their erosion by meteorites. Their results support the possibility that Saturn’s rings formed billions of years ago, perhaps at the time when giant impacts excavated the great basins on the Moon. The findings also suggest that giant exoplanets may also commonly have rings.

“Both Cassini observations and theoretical calculations can allow the rings of Saturn to be billions of years old. This means we humans are not just lucky to see rings around Saturn. This would lead us to expect massive rings also to surround giant planets circling other stars,” said Esposito.

Also, simulations run by Esposito’s colleagues Glen Stewart and Stuart Robbins from the University of Colorado showed that Saturn’s ring particles clump together, meaning previous estimates of the mass might be too low, perhaps by a factor of 3.

Recycling of ring material extends their lifetime and reduces the darkening that was expected previous to this study if the rings were older.

One problem with this proposal for more massive and ancient rings is that the Pioneer 11 space mission to Saturn in 1979 measured the ring mass indirectly by observing charged particles created by cosmic rays bombarding the rings.

“Those mass estimates were similar to the ones from Voyager star occultations, apparently confirming the previous low mass value. However, we now recognize that the charged particles are double-valued. That means they could arise from either a small or large mass. We now see that the larger mass value could be consistent with the underestimates due to ring clumpiness,” said Esposito.

Source: European Planetary Science Congress

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The Cassini spacecraft recently flew through the plane of Saturn’s rings and took this straight-on image of the G ring, showing a bright arc of material seen here as it rounds the ring’s edge, or ansa. The spacecraft also took images of the moons Mimas and Calypso (see below). In the image here, the diffuse glow at left shows the extended nature of this faint ring. The view looks toward the sunlit side of the rings from less than a degree below the ringplane. The ring moved against the background stars during this exposure, creating the star trails seen here. Cassini scientists and engineers are preparing for an upcoming flyby of the moon Enceladus on October 9. This is the second of seven targeted Enceladus fly-bys in the Extended Mission., and the spacecraft will pass through the moon’s geyser-like plumes in an attempt to measure fields and particles.

Cassini spacecraft scientists think the bright arc in the G Ring contains relatively large, icy particles held in place by a gravitational an orbital resonance with the moon Mimas. Micrometeoroids collide with the large particles, releasing smaller, dust-sized particles that brighten the arc. The plasma in the giant planet’s magnetic field sweeps through this arc continually, dragging out the fine particles and creating the G ring. The ring arc orbits Saturn along the inner edge of the G ring. The image was taken with the Cassini spacecraft narrow-angle camera on Aug. 22, 2008, from about 1.2 million kilometers (740,000 miles) from Saturn.

Here’s the image of Mimas and the rings:

And one of Calypso, too:

Source: Cassini web page, Twitter

NASA’s Cassini spacecraft has imaged a faint, partial ring orbiting with one small moon of Saturn, and has confirmed the presence of another partial ring orbiting with a second moon. This is further evidence that most of the planet’s small, inner moons orbit within partial or complete rings. Recent Cassini images show material, called ring arcs, extending ahead of and behind the small moons Anthe and Methone in their orbits. The new findings indicate that the gravitational influence of nearby moons on ring particles might be the deciding factor in whether an arc or complete ring is formed.

Both Anthe and Methone orbit Saturn in locations, called resonances, where the gravity of the nearby larger moon Mimas disturbs their orbits. Gravitational resonances are also responsible for many of the structures in Saturn’s magnificent rings. Mimas provides a regular gravitational tug on each moon, which causes the moons to skip forward and backward within an arc-shaped region along their orbital paths, according to Nick Cooper, a Cassini imaging team associate from Queen Mary, University of London. “When we realized that the Anthe and Methone ring arcs were very similar in appearance to the region in which the moons swing back and forth in their orbits due to their resonance with Mimas, we knew we had a possible cause-and-effect relationship,” Cooper said.

Scientists believe the faint ring arcs from Anthe and Methone likely consist of material knocked off these small moons by micrometeoroid impacts. This material does not spread all the way around Saturn to form a complete ring, because of the gravitational resonance with Mimas. That interaction confines the material to a narrow region along the orbits of the moons.

This is the first detection of an arc of material near Anthe. The Methone arc was previously detected by Cassini’s Magnetospheric Imaging Instrument, and the new images confirm its presence.

Source: JPL Press Release

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Cassini scientists have been “bee-busy” poring over the images from the August 11 flyby of Saturn’s geyser-spewing moon Enceladus, says Carolyn Porco, the lead for the imaging team. And quickly, they have found exactly what they were looking for: Cassini has pinpointed precisely where the icy jets erupt from the moon’s geologically active moon surface. Enceladus. “This is the mother lode for us,” said Porco, “a place that may ultimately reveal just exactly what kind of environment — habitable or not — we have within this tortured little moon.”

New carefully targeted pictures reveal exquisite details in the prominent south polar “tiger stripe” fractures from which the jets emanate. The images show the fractures are about 300 meters (980 feet) deep, with V-shaped inner walls. The outer flanks of some of the fractures show extensive deposits of fine material. Finely fractured terrain littered with blocks of ice tens of meters in size and larger (the size of small houses) surround the fractures.

One highly anticipated result of this flyby was finding the location within the fractures from which the jets blast icy particles, water vapor and trace organics into space. Scientists are now studying the nature and intensity of this process on Enceladus, and its effects on surrounding terrain. This information, coupled with observations by Cassini’s other instruments, may answer the question of whether reservoirs of liquid water exist beneath the surface.

The high-resolution images were acquired as Cassini sped past the icy moon at 64,000 kilometers per hour (40,000 miles per hour). A special technique, dubbed “skeet shooting” by the imaging team, was developed to cancel out the high speed of the moon relative to Cassini and obtain the ultra-sharp views.

“Knowing exactly where to point, at just the right time, was critical to this event,” said Paul Helfenstein, Cassini imaging team associate at Cornell University, Ithaca, NY., who developed and used the skeet-shoot technique to design the image sequence. “The challenge is equivalent to trying to capture a sharp, unsmeared picture of a distant roadside billboard with a telephoto lens out the window of a speeding car.”

The tiger stripes, officially called sulci, have been named for characters and places from “The Arabian Nights.” The yellow circles on the images indicate the sources of the jets.

Helfenstein said that from Cassini’s point of view, “Enceladus was streaking across the sky so quickly that the spacecraft had no hope of tracking any feature on its surface. Our best option was to point the spacecraft far ahead of Enceladus, spin the spacecraft and camera as fast as possible in the direction of Enceladus’ predicted path, and let Enceladus overtake us at a time when we could match its motion across the sky, snapping images along the way.”

For scientists, having the combination of high-resolution snapshots and broader images showing the whole region is critical for understanding what may be powering the activity on Enceladus.

“There appears to have been extensive fallout of icy particles to the ground, along some of the fractures, even in areas that lie between two jet source locations, though any immediate effects of presently active jets are subtle,” said Porco.

Imaging scientists suggest that once warm vapor rises from underground to the cold surface through narrow channels, the icy particles may condense and seal off an active vent. New jets may then appear elsewhere along the same fracture.

“For the first time, we are beginning to understand how freshly erupted surface deposits differ from older deposits,” said Helfenstein, an icy moons expert. “Over geologic time, the eruptions have clearly moved up and down the lengths of the tiger stripes.”

News Source: NASA/JPL