Thursday, 9 May 2013

Saturn's Rings: A Guide in Pictures

Continuing the theme of the last post, which used Cassini images as an introduction to Saturn's weather, I thought I'd bring together a collection of ring science images in an attempt to learn something about these beautiful phenomena.  The water ice rings are coloured by impurities (dust, silicates, tholins) and have particles with a wide range of sizes, from a few microns across to maybe 10 metres wide.  The age and origins of the rings remain hard to assess, as the system is in a continuous state of flux - particles clump together to form larger bodies due to gravity, but are then disrupted by interactions and collisions to reform the rings and dust.  Because of this continuous recycling there's still no consensus on how the rings came to be in the first place.

Distribution of Rings

Enlarging this composite image (45 images from Cassini's Narrow Angle Camera in total, obtained in May 2007) will give you a good first guide to the ring system.  Moving from the inner edge outwards, we have the faint and innermost D ring, Colombo Gap, C ring, Maxwell Gap, the main B ring, Cassini Division, A ring with its Encke and Keeler Gaps, the Roche division and then the narrow F ring (140,220 km from Saturn).  The G ring and the diffuse E ring, due to active venting from Enceladus, are both further out and not seen in this image.  (Credit:  NASA/JPL/Space Science Institute)

Perhaps more useful is this artists concept of the structure of the rings, and where the main shepherding moons are located.  Credit: NASA/JPL,

Removing the planet entirely from this 2007 image gives us a great view of the rings in all their glory, from the outer narrow F ring, through the main A and B rings (separated by the Cassini division), and then the inner C ring.  (Credit:  NASA/JPL/Space Science Institute)

This 2008 image shows sunlight scattering off of the B ring, the brightest and most massive of all of Saturn's rings.  It was in this ring that a mysterious phenomenon known as ring spokes was observed, producing dark radial striations on the rings sunlit side, which appear to come and go with time and may be a seasonal phenomenon.

This image of the B ring in 2010 shows spoke phenomena, appearing bright when viewed at a high phase angle.  They appear dark in images taken at lower phase angles, telling us something about the nature of the particles making up the spokes.

A colour image of the Cassini division from 2005, separating the main A and B rings, possibly consisting of more contaminated ices than the fresher material comprising the two rings.

A large ring of dust was discovered in 2009 by the Spitzer Space Telescope in infrared light, possibly originating from impact events on Phoebe (a retrograde satellite with an inclined orbit).  The full story can be found here.

Probing the Ring Properties

Just as for Saturn, astronomers use images of the rings in different wavelengths to deduce the composition, sizes and structure of the various ices.  This comparison image from the Visual and Infrared Mapping Spectrometer (VIMS) in 2004 shows scattered light coming through the rings on the left (so thicker rings appear darker); then the strength of a signature of pure water ice that seems to grow strong in the A ring; and finally a signature of some unidentified 'dirty' material causing darkening of the rings.  For more details see: (Credit:  NASA/JPL/University of Arizona)

There are other ways to deduce the properties of the rings - this is a comparison of a natural-colour image from 2005 with a simulated image based on a radio occultation.  Using radio signals in the Ka, X and S bands (0.94, 3.6, and 13 cm wavelengths), the modulation of the signal strength by the rings can be used to deduce ring optical depths and particle sizes.  The colours correspond to the presence or absence of ring particles of different sizes.

Furthermore, the Cassini Ultraviolet and Imaging Spectrograph (UVIS) can measure the strength of water ice signatures.  In this 2004 image, we can see the Cassini Division in red on the left (thinner, dirtier with less of an ice signature) compared to the A ring in turquoise on the right (with a stronger water ice signature).  The redder Encke gap is also visible.  Credit:  NASA/JPL/University of Colorado.

Finally, Cassini's Composite Infrared Spectrometer (CIRS) is able to measure the thermal emission from the rings at a variety of phase angles and illuminations.  The thermal characteristics vary notably with phase angle, over a range of temperatures from 65-110 K.  The comparison between lit and unlit sides tells us how effective sunlight is at penetrating the optically thicker rings to cause heating.  For an explanation of the figure, see  Credit:  NASA/JPL/GSFC

Dynamical Phenomena

The 2009 equinox was an ideal opportunity to observe vertical structures in Saturn's rings, as they would cast long shadows across the narrow ring plane.  These structures at the edge of the main B ring (the Cassini Division is the dark expanse at the top of the image) tower 2.5 km above the plane, which is enormous compared to the expected thicknesses (tens to possibly hundreds of metres) of the rings themselves.  This pileup of material might be being caused by the gravitational effects of moonlets at the edge of the B ring.

Another example of vertical structures observed in May 2009 when tiny Daphnis, sat within the Keeler gap within Saturn's A ring, interacts with the surrounding material.  The shadows indicate structures some 1.5 km tall, compared to the expected 10-m thickness of the main rings. The continuous interaction creates an edge wave which propagates around the circumference of the Keeler gap.

The rings continuously interact with the tiny satellites such as Prometheus, seen here creating a streamer from the F ring.  This dynamic ring appears to evolve over hourly timescales, being shepherded by both Prometheus and Pandora. Here, the satellite has reached its apoapsis (furthest point from Saturn) and may be pulling material away from the ring, creating kinks, gaps and other discontinuities in the rings in a continually evolving dance.

This dance between Prometheus and the F-ring carves channels into the ring every time it pulls out a streamer of material.  As it rotates slightly faster than the F ring around Saturn, each apoapsis interaction is in front of the last, creating this wonderful sequence of striations in the F ring.

Looking even more closely at Saturn's active F ring in September 2006, it appeared that additional tiny moonlets were interacting with the ring and drawing out tiny streamers of material, a miniature version of the interaction with Prometheus observed above.

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