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 astonishing.
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.”….
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