Saturn Moons Continue to Shine
Saturn just passed opposition on April 15, making it a good viewing object from Earth this season. Amateur observers with telescopes may be able to make out the moons Titan, Rhea, Dione, Iapetus, Tethys, and Enceladus. They may look like beautiful little gems from Earth, but from the Cassini spacecraft in orbit at Saturn, they are no less than astonishing. Recent observations of these moons add to the astonishment.
Titan, the largest of Saturn’s family of natural satellites (larger even than the planet Mercury), yielded more secrets of its southern “great lake,” named Ontario Lacus (Lake Ontario, due to its resemblance and size to Earth’s counterpart). It turns out the lake is more like a mud flat, shallow and wide. Namibia has similar salt pans that occasionally fill in with liquid from the water table. Cassini scientists now think that liquid methane and ethane seep into the mud flat from below, instead of during downpours (see PhysOrg reprint of JPL press release). This not only makes the giant “lake” a poor splashdown site for future spaceships, but adds to the mystery of the missing ethane, once thought to have covered Titan with a global ocean several kilometers deep.
Speaking of downpours, a BBC News article reported that some Cassini scientists think they are rare. At any given location, centuries may pass between significant rainfall that is believed to carve the extensive river channels detected on the surface by radar, based on observations between 2004 and 2010. The average time between significant methane rainfall could be 1,000 years, they estimate.
Rhea is Saturn’s second largest moon, but smaller than Titan by a long shot (Rhea would cover Texas, but Titan would cover the entire United States). Cassini made another flyby of Rhea from 26,000 miles on March 10. The main thing of note in Space.com‘s coverage was an impact basin 300 miles across, nearly a third of the moon’s diameter. Rhea also retains a tenuous atmosphere of oxygen and carbon dioxide, apparently replenished by charged particles. “Researchers think the oxygen comes from Rhea’s surface ice, liberated from water molecules that get blasted apart by charged particles streaming from Saturn’s magnetosphere,” the article stated; “The source of the carbon dioxide, however, is more mysterious.”
Dione may be active like Enceladus, according to an article on the BBC News by Paul Rincon. “NASA’s Cassini spacecraft has spied possible signs of geological activity on Saturn’s icy moon Dione,” he reported. These tentative findings, presented at the Lunar and Planetary Science Conference in Texas, are based on fissures resembling the “tiger stripes” on Enceladus, a possible ice volcano, and areas that appear to have been resurfaced. So far, the suggestion has not been corroborated by observations of jets or heat signatures, although “rampart craters” hint that there may be internal heat in Dione remaining. Perhaps, though, the fissures are “fossil tiger stripes,” indicating past geological activity that ceased. It’s remarkable that little Enceladus, less than half the diameter of Dione, should be the most active moon of Saturn.
Iapetus hasn’t had any flybys as good as the best one in 2007, which led scientists to propose ice-hopping or “runaway migration” to explain the dichotomy between dark and light sides. Observations continue to be made and analyzed, though. Space.com reported how Paul Ries is using Earth radar to infer temperature changes across the dark and light parts of the moon. Unexpectedly, the measurements did not follow a Planck blackbody curve as Ries predicted, but rather showed a fairly flat spectrum across wavelengths, suggesting steep absorption. At this point Ries doesn’t have an explanation. He’s comparing radar on Earth ice to see what the signals can tell about grain size and other factors. “Iapetus certainly has some strange stuff that needs to be explained, so this is potentially very interesting for the future,” he said.
Enceladus remains the prima donna of Saturn, bursting out her fountains at the south pole like a show that never ends. On March 27, Cassini shot the plumes again from 46 miles, hoping to “taste” them and send home pretty pictures, too; dazzling geyser shots were posted by the BBC News, BBC News #2, and Live Science. Space.com reported that continued study of the “tiger stripe” cracks show them to be deformed by Saturn’s powerful gravity. On April 14, Cassini made a second pass from 46 miles. Even though the tiger stripes are in the dark now, the infrared spectrometer and cosmic dust analyzer were poised for front-seat observations. A JPL press release showed latest unprocessed images from the encounter and a follow-up flyby of Tethys that provided the highest-resolution images of a side of Tethys not seen by the ultra-close 2005 encounter. A new close-up of Connecticut-sized Janus filled in a productive month by Cassini scientists.
Ringside. All that glitters at Saturn is not moons. Saturn’s sparkling F-ring was the subject of a report on New Scientist about puzzling observations of possible bursting moonlets that have on rare occasions brightened the F-ring significantly. Voyagers 1 and 2 saw sparkles in the 1980s, and Hubble saw one in 1995, but the ring has brightened overall since (implying more dust than before), except for an 84% brightening in December 2006 that took two years to fade. The F-ring is known to be perturbed by the little potato-shaped moon Prometheus, but if the sparkles are due to disruption of moonlets, there aren’t that many more moonlets around, unless Prometheus has ways of continually making and destroying giant snowballs made of ring dust. Reporter Lisa Grossman left it up as an oddity that may send scientists back to the drawing board.
Update 04/24/2012: New posts about Saturn’s dynamic F-ring were published by PhysOrg and the BBC News. Space.com posted a video of observations of 1/2-mile sized objects that apparently punch through the ring, leaving streamers or “mini-jets”. The article states that Prometheus creates these “snowball” objects but did not explain how.
In addition, an important article on PhysOrg summarized two new papers from the Astrophysical Journal that limit the age of Titan’s methane to a fraction of the moon’s assumed age. The author of the first paper, Conor Nixon of NASA-Goddard said,
Under our baseline model assumptions, the methane age is capped at 1.6 billion years, or about a third the age of Titan itself. However, if methane is also allowed to escape from the top of the atmosphere, as some previous work has suggested, the age must be much shorter – perhaps only 10 million years – to be compatible with observations.
10 million years represents just a quarter of one percent the assumed age of the solar system, even assuming all of Titan’s methane was outgassed in a single event. No source of methane replenishment has been found, although previous work suggested the last great methane “burp” occurred somewhere between 350 million and 1.35 billion years ago. The second paper by Kathleen Mandt of the Southwest Research Institute limits the age even further. Even with continual outgassing, Mandt’s team cannot account for Titan’s current methane isotope ratios beyond 1 billion years, just a fifth of its assumed age. In addition, crater counts set a maximum age of 1 billion years for Titan’s surface. The article ended optimistically, “Together these papers add important new perspectives and constraints on the history of Titan’s methane atmosphere, confirming that it must have formed long after Titan itself.” Such an explanation, though, calls for unique and special ad hoc circumstances unrelated to Titan’s formation — if indeed the consensus age, 4.5 billion years, retains any merit given these new constraints.
The dynamism found at Saturn’s rings and moons raises questions about how long these energetic, entropy-rich processes on small moons and in thin rings can be sustained. Cassini already spectacularly falsified the “global ocean on Titan” hypothesis when its Huygens probe landed on a soggy mud flat in 2005. Are we to believe the geysers on Enceladus have been going on for 4.5 billion years but stopped earlier on larger Dione? Are we incredibly lucky to be seeing the flimsy F-ring running out of moonlets right when Cassini is there to observe it?
We must keep in mind that, in science, there is a vast difference between scientific observation and scientific explanation. Unquestionably, our maps of the surfaces of Enceladus, Iapetus and Titan are vastly superior today than they were after Voyagers 1 and 2. That’s significant progress. The images of the F-ring are also orders of magnitude better than they were in the 1980s. Nevertheless, the explanations for activity on Enceladus and Titan seem strained. Are these anomalies that could lead to a scientific revolution?
It’s notable that this is the 50th anniversary of philosopher of science Thomas Kuhn’s influential book, The Structure of Scientific Revolutions (see summary online by Frank Pajares). David Kaiser in Nature took note of that, as did Evolution News & Views in several posts. The book that brought “paradigm shift” into the vernacular started a wave of thought that toppled logical positivism and helped many people realize that science is not necessarily the objective, neutral, truth-seeking, progressive enterprise students are led to believe, but in fact is governed by sociological and cultural factors that often stifle creative thinking by focusing attention on what questions are worth answering, what puzzles are worth solving, and what research is worth funding. Scientists see what the paradigm teaches them to see, until a new paradigm (not necessarily closer to the truth) asks new questions to deal with anomalies.
The planetary science community tends to be a close-knit thought collective with a web of belief that is tolerant of maverick views only to a point. If enough anomalies continue to pile up at Saturn, though, it will be interesting to watch if several long-held assumptions may have to be jettisoned to account for them. This may be a good time for new minds, uncluttered by tradition and consensus, to look at Cassini’s observations afresh.