Monday, October 04, 2010

Moon of the Month: Enceladus

Saturn has a lot of moons. About two-hundred have been observed, and it's suspected there are more. Most of these are tiny and orbit on very irregular orbits; astronomers only consider twenty-four of them regular satellites. Those we know much about are fascinating--each in a different way. Very little was known about any of them until the Voyager probes looked at them close-up in the early 1980's.

Before then, Saturn's moons were just dots. William Herschel--the same guy who discovered Uranus--first spotted Enceladus in 1789. Enceladus was the second moon in Saturnian system discovered despite only being the sixth-largest. Part of this was because William Herschel was a badass--he was, after all, the first guy to discover a planet that wasn't visible to the naked eye. On the other hand, Herschel had a little help from Enceladus and its albedo.

No, that has nothing to do with libido--unless you have an albino fetish. Albedo--particularly, Bond albedo--is the measure of "whiteness" of something, coming from the Latin "alba," meaning "white." Effectively, Bond albedo measures how much light a body reflects back into space divided by the amount it receives. Earth has an albedo of 0.29--that is, it reflects 29% of the light it receives back into outer space. Dark colored oceans, asphalt, and other things absorb the remaining 71%, which lingers as heat.

Most solar system objects are about where earth is, plus or minus 5%. Mars reflects about 25%; Saturn about 34%. Venus--with its notoriously thick sky that lights up evening and morning skies on Earth when its visible--reflects 75%.

Enceladus reflects 99%--nearly all light that hits it bounces off. The surface is pure white, covered completely in relatively new surface ice, laid down by active ice volcanoes--that's right, volcanoes that spray ice into space. Scientists were stunned when these were discovered, just as they were when Io's volcanoes were discovered. When Mars was found geologically dead, it was suspected most of the outer solar system was as well. Io came as a surprise; Enceladus, even more so. Io was much easier to explain; it's close to Jupiter and experiences tidal heating. Enceladus, on the other hand, was more difficult.

What fuels the volcanoes? Something has to keep the ice under Enceladus' surface a liquid. So, to look at the question differently, what is heating Enceladus? Enceladus reflects nearly all of the light that hits it back out into space, so clearly, the sun is not warming Enceladus. Tidal heating is suspected, but Enceladus' neighbors that are closer to Saturn are more geologically dead than Enceladus, so that's not the whole answer.

To make up the difference, It's suspected that the other portion of Enceladus' heat comes from one of the same sources as Earth--radioactive decay. Like on Earth, radioactive elements deep inside Enceladus break down slowly over time. These breakdowns release energy that warms Enceladus, eventually warming the thick layer of ice on surface, melting it, increasing its pressure, and forcing it to erupt. These "cryo-volcanoes" result.

The effects of Enceladus' volcanoes aren't just local. It's suspected--in conjunction with meteorite impacts--that they replenish Saturn's large, diffuse E-ring. Calculations have shown that particles in the E-ring ought to dissipate after millions of years--assuredly a long time, but short on the scale of the solar system. If they'd been around at the beginning of the solar system and not been replenished, they would have assuredly disappeared. Enceladus' eruptions--perpetually pumping particles into space--explain the continued existence of the ring.

Scientists have discussed exploring Enceladus further--however, exploration of Europa, it's been decided, is more important. As cool as Enceladus is, I agree. It's easier to get Jupiter, and Europa's about due for drilling.


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