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Dance of the Ice Crystals: What Kinds of Change Will We See in Saturn's Faint, Icy Rings?

Matt Hedman
Matt Hedman
By Matt Hedman Aug. 6, 2013

Since July 19, when Cassini positioned itself in Saturn's shadow and began gathering data from this rare vantage point, ring scientists such as myself have been poring over the streams of data coming back. We're waiting – as you all are -- to see the full mosaic of the Saturn system stitched together by the imaging team, but in the mean time, we’ve been looking at individual images and near-infrared data to see what we can learn about Saturn’s rings.

What are we looking for?

Images of Saturn and its rings backlit by the sun are not only exceptionally beautiful, they also provide us with unique information about Saturn's rings. Earlier backlit images of Saturn's rings obtained by Cassini back in 2006 and 2012 revealed previously unseen dusty rings as well as unexpected structures in already known rings. The new mosaic will allow us to look again at some of the hardest-to-see rings, to track changes in the distribution of tiny ice grains near Enceladus' orbit, and to investigate differences in the particle sizes among different dusty rings.

When Cassini looks back towards the sun during opportunities like these, the rings that are most easily seen from Earth appear fairly dark to Cassini. The reason is that the rings we Earthlings can see through telescopes are mostly formed from pebble-to boulder-sized particles, and they are getting the full blast of illumination from the sun. Cassini, however, is looking at the unlit sides of these objects. By contrast, other rings like the D, E and G rings appear exceptionally bright from Cassini's vantage point. This occurs because the tiny dust-sized grains in these so-called "dusty rings" are very good at scattering light when the sun is behind them. (Just think of how dust motes floating around in a room become visible when we look towards bright windows.)

This view, acquired with the sun almost directly behind Saturn, reveals a previously unknown faint ring of material coincident with the orbit of the small moon Pallene.
This view, acquired with the sun almost directly behind Saturn, reveals a previously unknown faint ring of material coincident with the orbit of the small moon Pallene.

Since dusty rings are so much easier to see in backlit images, extremely tenuous rings that are otherwise very difficult to detect become clearly visible. In the images Cassini took of the Saturn system in 2006, we can see faint ringlets lying along the orbits of Saturn's small moons Pallene and Janus. These rings are likely composed of material knocked off the surface of their respective moons by micrometeoroids. We are keen to get another clear look at these features to see if they have changed over the last seven years – which is about the length of a Saturn season. In particular, we are interested in seeing if changes in Janus' orbit over the last few years due to its interactions with Epimetheus have influenced the structure of its ring.

Wispy fingers of bright, icy material reach tens of thousands of kilometers outward from Saturn's moon Enceladus into the E ring, while the moon's active south polar jets continue to fire away.
Wispy fingers of bright, icy material reach tens of thousands of kilometers outward from Saturn's moon Enceladus into the E ring, while the moon's active south polar jets continue to fire away.

These large mosaics also provide a global view of Saturn's large and diffuse E ring, which surrounds the orbit of the moon Enceladus and is largely composed of particles of water ice launched from beneath that moon's surface. In the 2006 mosaic, Cassini saw tendrils of particles near Enceladus. We hope to get another look at these structures in the 2013 images.

With giant Saturn hanging in the blackness and sheltering Cassini from the sun's blinding glare, the spacecraft viewed the rings as never before, revealing previously unknown faint rings and even glimpsing its home world.
With giant Saturn hanging in the blackness and sheltering Cassini from the sun's blinding glare, the spacecraft viewed the rings as never before, revealing previously unknown faint rings and even glimpsing its home world.

On a broader scale, we saw strange asymmetries in the rings in 2006 that we still don't fully understand. If you look at the E ring in this image, you will note that it gets brighter closer to the planet. This occurs because the fine particles in this ring become brighter when you look closer to the sun (which is hidden behind Saturn). However, after correcting for brightening because of proximity to the sun, puzzling differences in the ring's brightness remain. The persistence of these differences suggests that the density of visible particles varies as one goes around the ring. In this mosaic, there seemed to be the most particles in the part of the ring situated between the sun and the planet. Other images taken around the same time showed the same trend, which was strange because the E ring particles are orbiting Saturn and it is difficult to imagine how one side of the ring could be persistently brighter than the other.

NASA's Cassini spacecraft has delivered a glorious view of Saturn, taken while the spacecraft was in Saturn's shadow.
NASA's Cassini spacecraft has delivered a glorious view of Saturn, taken while the spacecraft was in Saturn's shadow.

The 2012 observation only deepened this mystery. At this time, the brightness variations in the ring indicated the highest density of stuff fell close to Saturn's shadow, exactly opposite the case in 2006. We are not sure what forces may be causing particles to appear to congregate in different parts of the ring at different times, but we are extremely interested in seeing where the bright patch will appear in this new observation.

Finally, there will be much more color information on these dusty rings in our latest observation than in previous events, which will tell us something about the mix of particle sizes in these rings. When looking back towards the sun, the color of a dusty ring is largely determined by the size of its particles. A reddish ring will have more "large" dust grains while a blue one will have more "small" dust grains. (The difference between small or large here being between one and five microns across). Knowing the typical particle size is important because these small particles can be size-sorted by non-gravitational forces like solar radiation pressure and Saturn's electromagnetic field, so knowing the size of the grains can tell us which forces are acting on them and sculpting these rings.

In Saturn's Shadow (with labels)
In Saturn's Shadow (with labels)
For example, if you look at the right side of the 2006 enhanced color image, you see the inner part of the E ring is bluish while the outer part is reddish. However, the left side of the ring does not show this same red/blue color pattern, indicating that particles are distributed differently there. We suspect these color variations involve some sort of interaction with sunlight and electromagnetic forces, but more color information would allow us to better understand what is going on here. Due to time limitations, our previous observations were only been able to sample limited parts of the ring in more than a couple of colors. During the 2013 observation, Cassini's cameras imaged the rings with more filters, and the visual and infrared mapping spectrometer instrument obtained spectra – data in different wavelengths of radiation – of many parts of the rings. This information will allow us to better determine how tiny particles of different sizes are moving around Saturn.

We're still in the middle of the analysis, but we're really excited about the quality of the data that have come down.


Matt Hedman is a member of Cassini's imaging and visual and infrared mapping spectrometer teams, based at Cornell University, Ithaca, N.Y.