Something is Cooking Under Enceladus
Planetary scientists have been puzzling over Enceladus, a small moon of Saturn, since geysers were discovered erupting from its south pole three years ago. Some models suggested that eruptions could occur without liquid water, but others were not sure.
Opinion now seems to be shifting back to the necessity of a wet interior, according to a press release from JPL. The Cosmic Dust Analyzer instrument (CDA) aboard Cassini measured ice grains that seem to require a hot interior (see paper in Nature1). “The large number of ice particles observed spewing from the geysers and the steady rate at which these particles are produced require high temperatures, close to the melting point of ice, possibly resulting in an internal lake,” the article says.
The observed velocity of the particles, also, requires sufficient energy for them to be launched so far and so fast. The ice-vapor mixture jets out of the south-polar cracks at supersonic jet speeds: 500 meters per second, or 1,100 miles per hour. For the larger particles to remain accelerated to escape velocity as they ricochet off the walls, vapor must re-accelerate them on the way out, scientists figure. Only about 10% of the particles actually escape to feed the E-ring. Much of the rest falls back onto Enceladus, coating it with fresh ice and making it the brightest object in the solar system.
The influence of this little moon on the whole Saturn system is astonishing. Not only do the geysers create the vast E-ring, a torus of micron-size particles extending from the G-ring to the orbit of Rhea, the ionized gas also creates a drag on the magnetosphere and distorts our measurements of Saturn’s rotation. Another JPL press release this week announced another surprise: Enceladus is feeding the main rings, too.
This is the latest surprising phenomenon associated with the ice geysers of Enceladus to be discovered or confirmed by Cassini scientists. Earlier, the geysers were found to be responsible for the content of the E-ring. Next, the whole magnetic environment of Saturn was found to be weighed down by the material spewing from Enceladus, which becomes plasma — a gas of electrically charged particles. Now, Cassini scientists confirm that the plasma, which creates a donut-shaped cloud around Saturn, is being snatched by Saturn’s A-ring, which acts like a giant sponge where the plasma is absorbed.
How long has this been going on? How long could it go on? Neither press release discussed these questions.
The Cassini team and planetary scientists around the world are looking ahead eagerly for the closest flyby ever of Enceladus next month. On March 10, Cassini will speed by this moon just 100 miles above the surface at closest approach, and then perform a daring maneuver: flying through the outer edge of the plume. This will allow Cassini’s instruments to actually “taste” the plume material. If successful, Cassini will try even closer encounters with the plume in future passes. Come back here on March 10-11 for the latest news on this long-awaited, nail-biting, closest encounter ever.
1. Schmidt, Brilliantov, Spahn, and Kempf, “Slow dust in Enceladus’ plume from condensation and wall collisions in tiger stripe fractures,” Nature 451, 685-688 (7 February 2008) | doi:10.1038/nature06491.
How can this little moon, no bigger across than the British Isles, put out so much energy? How long can it pump out so much material? Why is it still so hot inside? The scientists never think to question the assumed age of the Saturn system (4.5 billion years), but then struggle to deal with what they cannot deny: the observations.
Most of the time, the press releases talk matter-of-factly about the assumed billions of years. They get strangely silent about time issues, though, as more detail is presented about Enceladus and Titan.
This is similar to how evolutionary biologists behave. Evolutionary theory thrives on glittering generalities. The more detail about the intricacy of the cell, DNA and other high-tech wonders in biology, the quieter the Darwin-talk gets.