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Cassini Continues Making New Discoveries

February 24, 2005

JPL — NASA’s Cassini spacecraft continues making new and exciting discoveries. New findings include wandering and rubble-pile moons; new and clumpy Saturn rings; splintering storms and a dynamic magnetosphere.

“For the last seven months it has been a nonstop, science-packed mission. It has been a whirlwind, and already we have many new results,” said Dr. Dennis Matson, Cassini project scientist at NASA’s Jet Propulsion Laboratory, Pasadena, Calif.

Weak, linear density waves caused in Saturn’s rings by the small moons Atlas and Pan have yielded more reliable calculations of their masses. The masses imply the moons are very porous, perhaps constructed like rubble piles. They are similar to the moons that shepherd Saturn’s F ring, Prometheus and Pandora.

Another discovery was a tiny moon, about 5 kilometers (3 miles) across, recently named Polydeuces. Polydeuces is a companion, or “Trojan” moon of Dione. Trojan moons are found near gravitationally stable points ahead or behind a larger moon. Saturn is the only planet known to have moons with companion Trojan moons.

The new findings, published in this week’s edition of the journal Science, include refinements in the orbits of several of Saturn’s small satellites. One intriguing result is the eccentric and slightly inclined orbit of Pan in Saturn’s A ring. The orbit’s shape is significant, as it indicates the type of interaction the moon has with the ring material surrounding it. If Pan’s orbit remains eccentric due to this interaction, then planets growing in a disc of material surrounding a star may also have eccentric orbits. This may help explain the eccentric paths of planets orbiting other stars.

Several faint Saturn rings have been discovered in Cassini images. Some lie in various gaps in the rings and may indicate the presence of tiny embedded moons acting as shepherds. Several of the rings are kinked, likely evidence of nearby moons.

Scientists also found Saturn’s winds change with altitude, and small storms emerge out of large ones. For the first time, Cassini images captured possible evidence of processes that may maintain the winds on Saturn. The observations offer a glimpse into the process which transfers energy by convection from Saturn’s interior to help sustain strong winds.

Other results improve the understanding of Saturn’s complex magnetic environment. “Saturn’s magnetosphere is truly unique. It’s dynamically similar to Jupiter’s, but in places it chemically resembles water-based plasmas surrounding comets,” said Dr. David Young. Young is Cassini principal investigator for the plasma spectrometer instrument from the Southwest Research Institute, San Antonio.

Another surprising find was made by the ion and neutral mass spectrometer instrument, which measured molecular oxygen ions above Saturn’s ring plane. “This is at first surprising since the rings are made of water ice,” said Dr. Hunter Waite, principal investigator for the spectrometer from the University of Michigan, Ann Arbor. “This may have important consequences for the identification of spectral features to use in the search for life on extrasolar terrestrial planet systems.”

The abundance of molecular oxygen on Earth is uniquely tied to biology. But these new measurements at Saturn suggest there are lifeless processes associated with cold icy surfaces that may produce an independent pathway for the formation of molecular oxygen in atmospheres.

Image Captions

Image 1: The Greatest Saturn Portrait…Yet — While cruising around Saturn in early October 2004, Cassini captured a series of images that have been composed into the largest, most detailed, global natural color view of Saturn and its rings ever made.

This grand mosaic consists of 126 images acquired in a tile-like fashion, covering one end of Saturn’s rings to the other and the entire planet in between. The images were taken over the course of two hours on Oct. 6, 2004, while Cassini was approximately 6.3 million kilometers (3.9 million miles) from Saturn. Since the view seen by Cassini during this time changed very little, no re-projection or alteration of any of the images was necessary.

Three images (red, green and blue) were taken of each of 42 locations, or “footprints”, across the planet. The full color footprints were put together to produce a mosaic that is 8,888 pixels across and 4,544 pixels tall.

The smallest features seen here are 38 kilometers (24 miles) across. Many of Saturn’s splendid features noted previously in single frames taken by Cassini are visible in this one detailed, all-encompassing view: subtle color variations across the rings, the thread-like F ring, ring shadows cast against the blue northern hemisphere, the planet’s shadow making its way across the rings to the left, and blue-grey storms in Saturn’s southern hemisphere to the right. Tiny Mimas and even smaller Janus are both faintly visible at the lower left.

The Sun-Saturn-Cassini, or phase, angle at the time was 72 degrees; hence, the partial illumination of Saturn in this portrait. Later in the mission, when the spacecraft’s trajectory takes it far from Saturn and also into the direction of the Sun, Cassini will be able to look back and view Saturn and its rings in a more fully-illuminated geometry.

Image 2: The Dragon Storm — A large, bright and complex convective storm that appeared in Saturn’s southern hemisphere in mid-September 2004 was the key in solving a long-standing mystery about the ringed planet.

Saturn’s atmosphere and its rings are shown here in a false color composite made from Cassini images taken in near infrared light through filters that sense different amounts of methane gas. Portions of the atmosphere with a large abundance of methane above the clouds are red, indicating clouds that are deep in the atmosphere. Grey indicates high clouds, and brown indicates clouds at intermediate altitudes. The rings are bright blue because there is no methane gas between the ring particles and the camera.

The complex feature with arms and secondary extensions just above and to the right of center is called the Dragon Storm. It lies in a region of the southern hemisphere referred to as “storm alley” by imaging scientists because of the high level of storm activity observed there by Cassini in the last year.

The Dragon Storm was a powerful source of radio emissions during July and September of 2004. The radio waves from the storm resemble the short bursts of static generated by lightning on Earth. Cassini detected the bursts only when the storm was rising over the horizon on the night side of the planet as seen from the spacecraft; the bursts stopped when the storm moved into sunlight. This on/off pattern repeated for many Saturn rotations over a period of several weeks, and it was the clock-like repeatability that indicated the storm and the radio bursts are related. Scientists have concluded that the Dragon Storm is a giant thunderstorm whose precipitation generates electricity as it does on Earth. The storm may be deriving its energy from Saturn’s deep atmosphere.

One mystery is why the radio bursts start while the Dragon Storm is below the horizon on the night side and end when the storm is on the day side, still in full view of the Cassini spacecraft. A possible explanation is that the lightning source lies to the east of the visible cloud, perhaps because it is deeper where the currents are eastward relative to those at cloud top levels. If this were the case, the lightning source would come up over the night side horizon and would sink down below the day side horizon before the visible cloud. This would explain the timing of the visible storm relative to the radio bursts.

The Dragon Storm is of great interest for another reason. In examining images taken of Saturn’s atmosphere over many months, imaging scientists found that the Dragon Storm arose in the same part of Saturn’s atmosphere that had earlier produced large bright convective storms. In other words, the Dragon Storm appears to be a long-lived storm deep in the atmosphere that periodically flares up to produce dramatic bright white plumes which subside over time.

One earlier sighting, in July 2004, was also associated with strong radio bursts. And another, observed in March 2004 and captured in a movie created from images of the atmosphere (http://photojournal.jpl.nasa.gov/catalog/PIA06082 and http://photojournal.jpl.nasa.gov/catalog/PIA06083) spawned three little dark oval storms that broke off from the arms of the main storm. Two of these subsequently merged with each other; the current to the north carried the third one off to the west, and Cassini lost track of it. Small dark storms like these generally get stretched out until they merge with the opposing currents to the north and south.

These little storms are the food that sustains the larger atmospheric features, including the larger ovals and the eastward and westward currents. If the little storms come from the giant thunderstorms, then together they form a food chain that harvests the energy of the deep atmosphere and helps maintain the powerful currents.

Cassini has many more chances to observe future flare-ups of the Dragon Storm, and others like it over the course of the mission. It is likely that scientists will come to solve the mystery of the radio bursts and observe storm creation and merging in the next 2 or 3 years.

Image 3: Phoebian Explorers 1 — These two montages of images of Saturn’s moon Phoebe, taken by Cassini in June 2004, show the names provisionally assigned to 24 craters on this Saturnian satellite by the International Astronomical Union.

The craters are named for the Argonauts, explorers of Greek mythology who sought the golden fleece. Argo was the name of their ship. The largest crater, approximately 100 kilometers (62 miles) across, is named after the leading Argonaut, Jason. Phoebe is an outer moon of Saturn and is 220 kilometers (136 miles) across.

The two-image montage displays mosaics made of individual, very high resolution images: 80 meters (260 feet) per pixel on the left; 200 meters (660 feet) per pixel on the right.

The other montage (see Phoebian Explorers 2) shows eight images of much lower resolution, ranging from 0.5 to 1 kilometer (0.3 to 0.6 mile) per pixel. The images in this montage show Phoebe as it rotated, and include regions of the moon not visible in the higher resolution montage.

The images have been slightly rescaled from their original formats and contrast-enhanced.

Image 4: Phoebian Explorers 2 — These two montages of images of Saturn’s moon Phoebe, taken by Cassini in June 2004, show the names provisionally assigned to 24 craters on this Saturnian satellite by the International Astronomical Union.

The craters are named for the Argonauts, explorers of Greek mythology who sought the golden fleece. Argo was the name of their ship. The largest crater, approximately 100 kilometers (62 miles) across, is named after the leading Argonaut, Jason. Phoebe is an outer moon of Saturn and is 220 kilometers (136 miles) across.

The two-image montage (See Phoebian Explorers 1) displays mosaics made of individual, very high resolution images: 80 meters (260 feet) per pixel on the left; 200 meters (660 feet) per pixel on the right.

This montage shows eight images of much lower resolution, ranging from 0.5 to 1 kilometer (0.3 to 0.6 mile) per pixel. The images in this montage show Phoebe as it rotated, and include regions of the moon not visible in the higher resolution montage.

The images have been slightly rescaled from their original formats and contrast-enhanced.

Image 5: New Ring Phenomena — A collection of new ring phenomena, first observed in the sequence of images taken of the dark side of Saturn’s rings immediately after Cassini entered orbit, may be evidence of the clumping and aggregation of ring particles. This phenomena is caused by the combined gravitational effects of Saturn, orbiting moons, and other ring particles.

Image A displays an unusual mottled-looking narrow region, with a radial width varying with longitude from 5 to 10 kilometers (3 to 6 miles), seen for the first time about 60 kilometers (37 miles) inside the outer edge of Saturn’s A ring. The resolution of this dayside image is about 1 kilometer (0.6 miles) per pixel. Image B is a close-up of the region, mapped into a longitude-radius system and contrast enhanced. The region is characterized by blotchy light and dark areas about 30 to 40 kilometers (19 to 25 miles) in longitudinal extent. The observed longitudinal extent of this region is about 3.5 degrees.

The mottled regions also are probably caused by particle clumping brought about by gravitational disturbances. The outer A ring edge is sculpted into a seven-lobed pattern called a Lindblad resonance (a type of dynamical resonance that occurs in rings systems) with the co-orbital satellites Janus and Epimetheus. The resonant perturbations in this region are complicated by the presence of these two moons whose orbits are within 50 kilometers (31 miles) of each other.

Image C is a dark-side image of the outer edge of the Encke gap, with a resolution of about 270 meters (886 feet) pixel, taken 18 degrees upstream from the moon Pan, which inhabits the gap. The regularly spaced, narrow dark lanes observed here are the wakes caused by Pan. Rope-like features can be seen between the first two wakes nearest the gap edge. These features are unique in all Cassini images taken so far. They generally are between 10 and 20 kilometers (6 and 12 miles) long.

In their orbits around Saturn, the particles comprising the rings in this region pass through the Pan wakes. When they do so, they are forced closer together than usual. These ropy features appear to be a product of the enhanced gravitational disturbances that occur when the particles pass through the wakes caused by Pan and consequently are squeezed close together. These disturbances obviously persist even outside the wakes, as is evident here in the presence of the ropy structures in the bands in between the wakes.

Image 6: Rings and More Rings — Cassini images have revealed the presence of previously unseen faint rings in some of the gaps in Saturn’s rings — possible indicators of small yet-unseen moons.
Image A is a contrast-stretched view of the 270-kilometer-wide (170 mile) Maxwell gap in Saturn’s C ring. The right arrow points to the optically thick Maxwell ringlet; the left arrow points to the new diffuse ring seen inside it.

Image B is a view of the approximately 350-kilometer-wide (220 miles) Huygens gap, between the outer edge of Saturn’s B ring (on the left) and the dark bands (on the right) in the Cassini division. The right arrow points to the optically thick Huygens ring; the left arrow points to the new diffuse ring inside it.

Image C is a view of the ringlets inside the Encke gap. Some of these had been seen by NASA’s Voyager spacecraft, but this contrast-enhanced Cassini lit-side image shows the presence of three major ringlets and a rather tenuous one.

The center ringlet, which in this image has the highest optical depth among the ringlets, is coincident with Pan’s orbit. This finding, along with observed variations in brightness along the ringlet, implies that accumulations of particles in the ringlet are maintained in special orbits that prevent them from colliding with Pan.

In Image D, which is a composite of several wide angle images taken of the lit-side of the rings after orbit insertion, there is clear indication of material extending about 400 kilometers (250 miles) beyond the edge of Saturn’s overexposed A ring (on the right), as well as two diffuse rings: a 300-kilometer-wide (190 mile) ring of material, R/2004 S1, in the orbit of Atlas (left-most arrow) and another ring, R/2004 S2, comparable to the Atlas ring and immediately interior to Prometheus’s orbit (right-most arrow). These rings had been reported earlier and are comparable to the jovian ring. Prometheus’s orbit is elliptical, and brings the moon as close to Saturn as the outer edge of R/2004 S2 and as far away from the planet as the inner sharp boundary of Saturn’s F ring. These observations indicate that Prometheus has swept material from the region occupied by its orbit.

It is not clear yet whether the origin of all these low-optical depth ringlets is the same. The association of the Atlas ring with Atlas and the main Encke ringlet with Pan would suggest that these rings derive from their associated moon. In other cases, a ring may exist because the material (or small parent bodies within it) are shepherded by a larger moon also present in the gap. The particles in many or all of these diffuse ringlets may have substantial fractions of micrometer-sized dust, implying that non-gravitational forces also may affect the ringlets’ dynamics. In any case, the presence of narrow, diffuse ringlets in gaps like Maxwell and Huygens, along with the major Maxwell and Huygens ringlets, and the additional narrow ringlets in the Encke gap, suggests that there may be yet unseen moonlets in these gaps.

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 Cassini-Huygens mission for NASA’s Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL.

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On the Net:

NASA

Cassini-Huygens Mission

Cassini Imaging Team


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