* Astronomy

Multiple faint, streamer-like objects can be seen in this high resolution Cassini spacecraft view of the F ring's bright core.


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Credit NASA

The regular spacing of some of the features extending from the core indicates that they could all be produced by the perturbing effect of a single body as it passes close by. Scientists are examining Cassini images closely in an attempt to determine whether there are tiny moonlets -- or perhaps transient clumps of material -- orbiting Saturn near the F ring core. The researchers believe the streamer features seen here could be caused by a related phenomenon to that by which Prometheus produces streamers in the F ring (see Passing Lane).

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Sept. 25, 2006 at a distance of approximately 339,000 kilometres from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 69 degrees. Image scale is 2 kilometres per pixel.

The Cassini spacecraft looks through the dense B ring toward a distant star in an image from a recent stellar occultation observation. These observations point the camera toward a star whose brightness is well known. Then, as Cassini watches the rings pass in front, the star's light fluctuates, providing information about the concentrations of ring particles within the various radial features in the rings.


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Credit NASA

This view looks toward the unlit side of the rings from about 35 degrees above the ringplane. The star's image is partly saturated, causing the vertical lines that extend up and down.
The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Sept. 26, 2006 at a distance of approximately 543,000 kilometres from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 106 degrees. Image scale is about 3 kilometres per pixel.

SATURN’S RINGS SHOW EVIDENCE OF A MODERN-DAY COLLISION

Scientists on NASA’s Cassini mission have spied a new, continuously changing feature that provides circumstantial evidence that a comet or asteroid recently collided with Saturn's innermost ring, the faint D ring.
Imaging scientists see a structure in the outer part of the D-ring that looks like a series of bright ringlets with a regularly spaced interval of about 30 kilometres. An observation made by NASA's Hubble Space Telescope in 1995 also saw a periodic structure in the outer D ring, but its interval was then 60 kilometres. Thus, unlike many features in the ring system which have not changed over the last few decades, the interval of this pattern has been decreasing over time.
These findings are being presented today at the Division for Planetary Sciences Meeting of the American Astronomical Society held in Pasadena, California.

"This structure in the D ring reminds us that Saturn's rings are not eternal, but instead are active, dynamical systems, which can change and evolve" - Dr. Matt Hedman, faint ring specialist, Cassini imaging team associate at Cornell University, Ithaca, N.Y.

When Cassini researchers viewed the D ring along a line of sight nearly parallel to the ringplane, they observed a pattern of brightness reversals: a part of the ring that appears bright on the far side of the rings appeared dark on the near side of the rings, and vice versa.
This phenomenon would occur if the region contains a sheet of fine material that is vertically corrugated, like a tin roof. In this case, variations in brightness would correspond to changing slopes in the rippled ring material.
Both the changes over time and the "corrugated" structure of this region could be explained by a collision of a comet or meteoroid into the D ring, which then kicked out a cloud of fine particles. This cloud might have inherited some of the tilt of the colliding object's path as it slammed into the rings. An alternate explanation could be that the object might have struck an already inclined moonlet, shattering it to bits and leaving its debris in an inclined orbit.
In either case, the researchers speculate the aftermath of such a collision would be a ring slightly tilted relative to Saturn's equatorial plane. Over time, as the inclined orbits of the ring particles evolve, this flat sheet of material would become a corrugated spiral that winds up like a spring over time, which is what was observed.

Based on observations between 1995 and 2006, scientists reconstructed a timeline and estimated that the collision occurred in 1984.

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Cassini scientists are on the trail of the missing moons of Saturn. A recent observation by the spacecraft leads them to believe that they will find the moons near newly discovered rings around the planet.

During an unprecedented opportunity, with the sun poised behind Saturn, Cassini scientists discovered two new rings and confirmed the presence of two others. The new rings are associated with one or more small moons and share their orbits with the moons, while scientists suspect a moon is lurking near a third ring.
Under the cover of Saturn's shadow in mid-September, the entire ring system became visible, and never-before-seen microscopic particles began to appear. A single, faint new ring at the orbits of two moonlets, Janus and Epimetheus, was discovered. A second ring was found a week later. It is narrow and overlies the orbit of the tiny moon Pallene, which Cassini discovered back in 2004. A third and fourth ring are visible in the Cassini Division, the big gap in Saturn's main ring system. Curiously, these rings were not seen in images from NASA's Voyager spacecraft.

"We are hot on the trail of these possible elusive moonlets. Finding the moons and learning about their interactions with the rings will help us understand how the moons formed and perhaps how the Saturn system formed" - Dr. Joe Burns, Cassini imaging scientist at Cornell University, Ithaca, N.Y.

When viewed by Cassini's infrared instrument, one of the rings in the Cassini Division has unusual colouring and brightening, a trait it shares with fresh, faint rings like the F ring, or those in the Encke Gap in Saturn's outer A ring.
Saturn's smallest moons have weak gravity and cannot retain any loose material on their surfaces. When these moons are struck by rapidly moving interplanetary meteoroids, this loose material is blasted off their surfaces and into Saturn orbit, creating diffuse rings along the moons' orbital paths. Collisions among several moonlets, or clumps of boulder-sized rubble, might also lead to debris trails. For instance, Saturn's G ring seems not to have any single moon large enough to see; it might have formed from a recent breakup of a moon.
The unusual viewing geometry provided other insights into the changing nature of Saturn's rings. In addition to the dazzling images, data from Cassini's visual and infrared mapping spectrometer also show distinct colour differences, indicating variations in composition and in microscopic particles in the Cassini Division, the diffuse E- and G-rings, lying outside the main rings, and the D ring, which is the ring closest to Saturn.

"These tiny grains are like spices -- even a little bit of material can alter the ring's character" - Dr. Matt Hedman, Cassini scientist at Cornell.

Colour variation in the rings might imply particles are being sorted by size.

"We expected to see things we haven't seen before, but we are really, really puzzled by these new images of Saturn's main ring system. The rings appear very different, with none of their usual calling card of water-ice features. There are hints that other material besides ice might finally be detected within the rings. The main rings show a neutral colour, while the C ring is reddish, and the D and E rings are quite blue. We don't quite understand if these variations are due to differences in particle size or composition, but it's nice to be surprised every once in a while" - Dr. Phil Nicholson, of Cornell, Cassini visual and infrared spectrometer team member.

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