Postcard From Cassini

PIA17157 detail
There is always this. Given the times, and the scale of science fiction, perhaps the idea of a holiday snap featuring four moons does not necessarily strike us as anything special. But then we stop to think about it:

Two pairs of moons make a rare joint appearance. The F ring’s shepherd moons, Prometheus and Pandora, appear just inside and outside of the F ring (the thin faint ring furthest from Saturn). Meanwhile, farther from Saturn the co-orbital moons Janus (near the bottom) and Epimetheus (about a third of the way down from the top) also are captured.

Prometheus (53 miles, or 86 kilometers across) and Pandora (50 miles, or 81 kilometers across) sculpt the F ring through their gravitational influences. Janus (111 miles, or 179 kilometers across) and Epimetheus (70 miles, or 113 kilometers across) are famous for their orbital dance, swapping places about every four years. They are also responsible for gravitationally shaping the outer edge of the A ring into seven scallops.


Just to go clockwise, that’s Epimetheus, Prometheus, Pandora, and Janus.

Seriously, though … anyone have a holiday snap to match? Didn’t think so.


Cassini Dances With Crystals

Cassini, ever faithful, brings out the geek in Matt Hedman. And it is to our great benefit:

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.

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.)

PIA08328Since 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.

PIA08321 detail 2