* Astronomy

This image of Saturn's Rings was taken by the Cassini spaceprobe on July 24, 2006, when it was approximately 472,721 kilometres away.



The image was taken using the CL1 and CL2 filters.

The shadows of Saturn's C ring cover bright patches of clouds in the planet's atmosphere. The shadow-throwing rings stretch across the view at bottom. The dark inner edge of the B ring is visible at top.



The image was taken using a spectral filter sensitive to wavelengths of infrared light centred at 752 nanometers. The image was acquired with the Cassini spacecraft wide-angle camera on April 28, 2006 at a distance of approximately 340,000 kilometres from Saturn. Image scale is 17 kilometres per pixel.

This view of the faint E ring -- a ring feature now known to be created by Enceladus -- also shows two of Saturn's small moons that orbit within the ring, among a field of stars in the background.


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The image was taken in visible light with the Cassini spacecraft wide-angle camera on March 15, 2006, at a distance of approximately 2.4 million kilometres from Saturn. The image scale on the sky at the distance of Saturn is 142 kilometres per pixel.

The E ring extends from three to eight Saturn radii -- about 180,000 kilometres to 482,000 kilometres. Its full extent is not visible in this view.

Calypso (22 kilometres across) and Helene (32 kilometres across) orbit within the E ring's expanse. Helene skirts the outer parts of the E ring, but here it is projected in front of a region deeper within the ring.
Calypso and Helene are trojan satellites, or moons that orbit 60 degrees in front or behind a larger moon. Calypso is a Tethys trojan and Helene is a trojan of Dione.
An interesting feature of note in this image is the double-banded appearance of the E-ring, which is created because the ring is somewhat fainter in the ringplane than it is 500-1,000 kilometres above and below the ringplane. This appearance implies that the particles in this part of the ring have nonzero inclinations (a similar affect is seen in Jupiter's gossamer ring). An object with a nonzero inclination does not orbit exactly at Saturn's ringplane. Instead, its orbit takes it above and below the ringplane. Scientists are not entirely sure why the particles should have such inclinations, but they are fairly certain that the reason involves Enceladus.
One possible explanation is that all the E ring particles come from the plume of icy material that is shooting due south out of the moon's pole. This means all of the particles are created with a certain velocity out of the ringplane, and then they orbit above and below that plane.
Another possible explanation is that Enceladus produces particles with a range of speeds, but the moon gravitationally scatters any particles that lie very close to the ringplane, giving them nonzero inclinations.
Stray light within the camera system is responsible for the broad, faint "Y" shape across the image.

The Cassini mission has found evidence that a new class of small moonlets resides within Saturn's rings. There may be as many as 10 million of these objects within one of Saturn's rings alone.

The moonlets' existence could help answer the question of whether Saturn's rings were formed through the break-up of a larger body or are the remnants of the disk of material from which Saturn and its moons formed.
Careful analysis of high-resolution images taken by Cassini's cameras revealed four faint, propeller-shaped double streaks. These features were found in an otherwise bland part of the mid-A Ring, a bright section in Saturn's main rings. Cassini imaging scientists reporting in this week's edition of the journal Nature believe the "propellers" provide the first direct observation of how moonlets of this size affect nearby particles. Cassini took the images as it slipped into Saturn orbit on July 1, 2004.

Previous measurements, including those made by NASA's Voyager spacecraft in the early 1980s, have shown that Saturn's rings contain mostly small water-ice particles ranging from less than 1 centimetre across to the size of a small house. Scientists knew about two larger embedded ring moons such as 30-kilometer-wide Pan and 7-kilometer-wide Daphnis. The latest findings mark the first evidence of objects of about 100 meters in diameter. From the number of moonlets spotted in the very small fraction of the A ring seen in the images, scientists estimated the total number of moonlets to be about 10 million.

Moons as large as Pan and Daphnis clear large gaps in the ring particles as they orbit Saturn. In contrast, smaller moonlets are not strong enough to clear out the ring, resulting in a partial gap centred on the moonlet and shaped like an airplane propeller. Such features created by moonlets were predicted by computer models, which give scientists confidence in their latest findings.


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The propeller moonlets represent a hitherto unseen size-class of particles orbiting within the rings. This image provides broad context within the rings, and shows the B ring, Cassini Division, A ring and F ring.

The detection of moonlets embedded in a ring of smaller particles may provide an opportunity to observe the processes by which planets form in disks of material around young stars, including our own early solar system.

"The structures we observe with Cassini are strikingly similar to those seen in many numerical models of the early stages of planetary formation, even though the scales are dramatically different. Cassini is giving us a unique insight into the origin of planets" - Carl Murray, imaging team member at Queen Mary, University of London.

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