Is Saturn's Family Showing Its Age?

A detailed color analysis suggests that the rings of moons of Saturn are ancient creations that in recent times have been coated to varying degrees by a dark, reddish patina.

Is it possible to gauge the age of Saturn's rings and major moons just by measuring how bright they look at different wavelengths? Apparently so, say a team of planetary scientists led by Gianrico Filacchione (National Institute for Astrophysics, Rome, Italy).

The researchers pulled together more than 2,200 observations of Saturn's moons and several mosaics of its rings taken by NASA's Cassini orbiter from 2004 to mid-2010. These span the spectrum from near-ultraviolet (0.35 micron) to infrared (5.1 microns) wavelengths — a range that includes a pronounced absorption by water ice at 2.0 microns.

It's been long realized that Saturn's immediate family consists almost entirely of water ice. But some parts have been darkened to varying degrees by two unknown dark materials, one neutral in color and another having a reddish tinge. The new work by Filacchione's team suggest that the distinctions in coloration form a pattern of sorts.

For example, the two small moonlets that circle just inside and outside the narrow F ring have different hues. Prometheus, the inner one, is distinctly redder than Pandora and looks to be a close color match to particles in Saturn's densely packed A and B rings. The inner C ring and Cassini Division are redder still.

Meanwhile, the moons appear progressively less contaminated moving from tiny Janus and Epimetheus outward to Enceladus, which doesn't look tainted at all. The authors attribute this to the fact that geysers near the south pole of Enceladus are constantly spouting off, creating a tenuous fog of frozen ice particles that continually coats neighboring satellites with a thin veneer of fresh frost. Then the reddening trend increases among the moons farther out, from Tethys to Dione to Rhea. Outliers Hyperion, Iapetus and Phoebe continue the trend toward darker, redder surfaces. (Titan wasn't considered, due to its dense atmosphere.)

As the researchers note in March 11th's Astrophysical Journal (you can download their article here), it's not clear what the contaminant is. Some researchers have speculated that it's a thin veneer of dark, neutral-hued dust from infalling meteorites combined with a reddish substance that could be anything from radiation-induced organic goo to iron oxide.

But the spectral signature of water ice is both clear and ubiquitous across the system, leading Filacchione and his colleagues to conclude that its presence in the rings and moons represents the "original chemistry of the circum-planetary nebula from which they condensed." Or, as a NASA press release touts, "Saturn's moons and rings are gently worn vintage goods from around the time of our solar system's birth."

Well, hold on there, NASA. As detailed in my feature article about the planet's magnificent bands in the May issue of Sky & Telescope, two aspects of the Saturnian system put serious constraints on any would-be explanation for how and when the rings and moons formed.

First, as noted earlier, the ring particles (in the main rings, at least) consist almost entirely of water ice. But it's hard to imagine forming them with a pure-ice composition. In most scenarios, Saturn's primordial surroundings should have contained roughly equal amounts of ice and rock. In addition, over time the rings should have become increasingly contaminated with rock, metal, and carbon from meteoroid strikes. Calculations suggest the accumulated debris should account for roughly 10% of the rings' mass, if they're as old as Saturn itself (4½ billion years). But observations suggest that it's no more than about 1%, which, according to ring specialist Jeffrey Cuzzi (NASA Ames Research Center) argues for rings that are no older than about 500 million years.

Second, the rings' origin must somehow be tied to that of Saturn's moons. It's hard to imagine that giant Titan and sizable Iapetus formed so far away from Saturn, or that the mid-size moons near the rings could have ended up with such a wide range of densities. Tethys, averaging 0.98 g/cm³, must be nearly pure ice, whereas Enceladus and Dione — the two moons that bracket Tethys, come in at 1.61 and 1.48 g/cm³, respectively, and therefore must contain lots of rock.

In the past few years theorists have come up with several creative schemes to make ice-dominated rings and moons. You'll have to read the May issue to get the details, but they're all messy scenarios involving lots of cosmic carnage. And calculations by Valéry Lainey (IMCCE, Paris) and others suggest that the Cassini Division might have opened up less than 10 million years ago — in which case, how did its particles become dark so fast?

One critical, but missing detail in all of these models is just exactly how much mass is stashed in Saturn's rings and, specifically, in the dense, optically opaque B ring. The more massive the rings, the less rapidly they'll be tinted by infalling meteoritic debris. Right now there's no way to know the mass for sure — it might be 10²⁰ tons (a couple of Mimas's worth), but the true total might be several times greater.

A little more clarity could come a few years from now. NASA managers have given a green light to keep Cassini going until the planet reaches its northern summer solstice in May 2017. In the final 10 months of operation, mission controllers hope to redirect the spacecraft so it repeatedly dives through the clearing between the innermost D ring and Saturn's upper atmosphere — a gap only 2,000 miles (3,000 km) wide. Careful tracking of the barnstorming spacecraft will not only reveal unprecedented details about Saturn's gravity field (and, from that, its internal structure), but also determine the rings' mass.

By the way, now's a great time to do a little inspection of the rings and moons yourself. Saturn comes to opposition in late April, so the Ringed Wonder has again positioned itself for easy viewing. But before setting up your telescope, be sure to download S&T's new Saturn app for iPhones and iPads.