Icy activity in the E ring (of Saturn)

Tuesday, November 29th, 2005 at 01:18 PM by Juan

About a year ago now, the Cassini mission to Saturn achieved one of its mission objectives by landing a probe (called Huygens) on Saturn’s moon Titan. Titan is interesting because, among other things, it is the only moon in the solar system with a significant atmosphere. Significant enough to potentially support life, although it would be a very strange life indeed living in something we would consider a petrochemical soup.

However, while Titan got all the media attention, planetary geologists (of which I am only an amateur) have also been very interested in another moon of Saturn which goes by the moniker Enceladus. Enceladus is a little snowball of a world, only about 500 km across. It orbits Saturn once every 1.3 days, close enough to place it within Saturn’s outermost (and extremely faint) “E” ring. In fact, it appears to orbit within the densest part of the E ring, but more on that in a bit.

(Click on image for full-size version.) An illustration labeling all of Saturn’s Rings (Courtesy: NASA/JPL/David Seal)

Saturn’s E ring (unlike the cheesy NBC television series of the same name) is a faint ring of dust particles that extends out about 5 times further than the much brighter A and B rings see around Saturn through earthbound telescopes. It actually is much more similar to the rings of Jupiter, Uranus, and Neptune, in being made up of tiny particles instead of the larger icy particles that make up Saturn’s more prominent rings. One of the big problems in understanding these rings made up of small particles is that due to the interactions of these small particles with the solar wind, they should slowly lose speed and thus drift closer to the planet in their orbits, eventually falling in. As such, these dust rings should be temporary features of worlds… but we see them around all the Jovian planets. This suggested that something was “replenishing” the supply of small particles in these rings. It had been suggested that micrometeorite impacts or some other geological process, like volcanism, might eject debris off of the small moons orbiting all the Jovian worlds at a high enough rate to account for the dust rings.

(Click on image for full-size version.) A false color image of Enceladus taken July 25, 2005 by the Cassini space probe. Notice the relatively small number of craters and the long fault lines running along the entire surface. (Courtesy: NASA/JPL/David Seal)

Now back to Enceladus. The reason planetary geologists pay attention to it is because, despite its small size, it appears to be geologically active! Voyager photographs of the moon from two decades ago show a moon with a relatively small number of craters. As I tell my astronomy students, since asteroids and comets have been drifting around the solar system since its formation 4.5 billion years ago, any world that doesn’t show many impact craters on its surface has a very young surface. In the case of Earth, the reason for the lack of craters is simple, erosion due to the actions of air and water. But in worlds without atmospheres, you have to look for some other process to rework the surface of the world. Recent Cassini photographs of the moon clear signs of tectonic activity (e.g. – quake fault lines) and features that look a lot like “icy volcanoes”. This theory of a cryovolcanically active Enceladus is especially appealing because Enceladus’ placement in the densest part of the E ring suggests that icy eruptions on Enceladus could be providing small particles to the E ring.

The theory has now become an observed fact. Yesterday, the Cassini Imaging Team released several photos showing plumes of material streaming away from the surface of Enceladus. Cryovolcanism has moved from the realm of well-established theory to well-established fact. Of course, explaining why Enceladus should still have enough internal heat to drive such volcanism, well, that will require enough work to keep the theorists happy for some time to come.

(Click on images for full-size versions.) To paraphrase the press release “Cassini images of Enceladus show the fountain-like sources of the fine spray of material that towers above the south polar region of Enceladus. The color-coded image (right) was processed to enhance faint signals, making contours in the plume of material even more apparent. The greatly enhanced and colorized image shows the enormous extent of the fainter, larger-scale component of the plume. (Courtesy: Cassini Imaging Team)

Posted in Astronomy News

Comments are closed.