Saturn’s Strange Children

Research Blogging / by Dave Munger /

Spacecraft observations of giant tenuous rings, two-toned moons, and methane fogs are showing Saturn’s moons to be even more alien than previously believed.

For nearly a century after it was discovered in 1899, Phoebe was Saturn’s oddest moon, with an orbit that was misaligned from most of the others (and from Saturn’s rings) by 27 degrees. Phoebe also has a retrograde orbit, revolving around Saturn in the opposite direction from the rest of Saturn’s moons (and opposite the motion of most major bodies in the solar system). But in the last decade, many more moons and moonlets with retrograde orbits have been found. Phoebe has become less of an outlier and more an exemplar of the striking variety of satellites supported by the Saturnian system.

It seems the more we learn about Saturn’s other moons, the stranger they become—Phoebe no longer appears to be the oddest one. Iapetus resembles the Taoist symbol of yin and yang, with one hemisphere covered in dark organic material and the other sheathed in bright white ice. It also has a curious ridge of mountains more than 10 kilometers tall stretched along its equator, making it look like a giant walnut. Titan, by far the largest of Saturn’s moons, is the only body in the solar system other than Earth with a dense nitrogen-rich atmosphere. There, it’s so cold that water ice is hard as rock, and substances like ethane and methane form their own “hydrological” cycles like water does here on Earth. Scientists suspect Titan might even be capable of supporting exotic forms of carbon-based life.

But Phoebe may not be so easily dethroned. The moon was back in the news recently when an October 7 report in Nature revealed that Saturn has a previously undiscovered ring, vastly larger than all the others but invisible to most telescopes. The next day, Dave Strickland, an astronomer at Johns Hopkins University, explained the research on his blog.

Saturn’s new ring, spotted in infrared light by the Spitzer Space Telescope, was almost inconceivably large—36 million kilometers in diameter, or a quarter of the distance from the Earth to the Sun. If you could see this ring with the naked eye, it would appear twice the width of the Moon in Earth’s skies. Strickland estimates its volume as 5 x 1021 cubic kilometers, which is more than a thousand times the volume of the Sun.

Olaf Davis, a PhD student at Oxford, also discusses the new ring on his blog, pointing out that the ring supports theories that suggest Phoebe may be the source of Iapetus’ dark side. The Cassini spacecraft, which has been orbiting Saturn since 2004, observed that the chemical composition of the dark side of Iapetus matched the surface of Phoebe. Since the new ring traces Phoebe’s orbit, it’s reasonable to think it is made of debris from comets striking Phoebe. As Iapetus sweeps across the inner portion of the new ring (but in the opposite direction), the argument goes, it collects a thin layer of this debris on its leading side, taking on Phoebe’s darker appearance.

The surface imagery of Iapetus is just one of many Cassini findings. César Tomé López, associate Spanish editor of who blogs at Experientia Docet, has posted an explanation (Google translation) of a recent study of Cassini images from Titan (along with a beautiful artist’s rendering of what Titan’s surface might look like).

Mike Brown of Caltech, along with three colleagues, analyzed Titan’s atmosphere to see if the liquid methane found on the moon’s surface also formed clouds and precipitation. By looking at a series of photos and at varying wavelengths of light, Brown found that the clouds weren’t just methane gas, but also a “fog” of liquid methane droplets. It does indeed “rain” on Titan, although the rain has more in common with lighter fluid than water.

Brown keeps his own blog, where he offers a very interesting account of how his team’s paper came about. Brown says he had been working with another team on the moon’s atmospheric data but withdrew because he didn’t believe their analysis was accurate. The other team ended up publishing their results before Brown did, with some bad blood ensuing when he criticized the methods in their paper. Brown’s paper hasn’t yet been published but is available through the pre-print repository at arXiv.

Astronomy is a challenging field because the available data is constrained in ways it wouldn’t be here on Earth. It’s difficult, if not impossible, to conduct “experiments” on remote astronomical bodies. Spacecraft and telescopes are expensive and often involve competing international teams of researchers. As Titan’s fog demonstrates, scientists might agree on the general implications of an observation while disagreeing on the details. Those sometimes-destructive conflicts—and the consensus that emerges from them—are paradoxically the building blocks of science. You can watch and be a part of that process at

Originally published October 21, 2009

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