16 December 2008

AGU Day 1: Enceladus

Posted by Ryan Anderson

After lunch, I went to a few talks about the latest results from Enceladus, Saturn’s little moon with geysers at the south pole. The first talk was by Carolyn Porco, giving a great summary of the very close flybys from earlier this year. Most of the (very very awesome) images from her talk were released today on the imaging team’s website. I especially liked this beautiful mosaic from the October 5 flyby:

Mosaic from the Oct. 5, 2008 flyby.

Mosaic from the Oct. 5, 2008 flyby.

Also way cool was this animation, showing the location and direction of the most prominent south polar geysers (click for video):


Some of the best images from the flybys have a resolution of 8 meters per pixel, which is totally amazing. The imaging team looked for evidence in the “tiger stripe” faults of where exactly the geysers are coming out, but there was no distinct change between locations with active geysers and non-active locations. The leading hypothesis to explain this is that the geysers move around along the tiger stripes: they become active, but gradually frost builds up in the opening and it seals off only to burst out elsewhere.

One of the major questions about Enceladus is where it gets all of its energy. The south pole of Enceladus has a heat flux of 5 gigawatts; more than the entire planet Earth! The problem is that the predicted tidal heating at Enceladus is only a tiny fraction of that. The way around this is to realize that the traditional prediction is based on Enceladus being in a steady state. The instantaneous heat flux can be huge, as long as the average agrees with the predictions. D. Stevenson gave a possible model to explain the unusually hight heat flux: it starts by pumping energy into increasing the eccentricity of Enceladus’ orbit until the tidal stresses break the icy shell. When it breaks, it suddenly becomes much more effective at dissipating energy because the block sof ice can shift and grind around. This increases the heat flow and would power the geysers, but at the same time it would decrease the eccentricity of the orbit until the dissipation ceased, the ice freezes back solid, and the cycle starts again. Water plays an important role in this cycle by lubricating the slidingĀ  of ice fragments during the high heat dissipation phase, but Stevenson made it very clear that the lubricating water has nothing to do with the plume water.

The uncomfortable thing about this model is that it assumes we are observing Enceladus at a “special” time. Scientists usually like to find explanations that do not require anything special to be going on, but it’s hard to argue with the high heat flow that people are seeing, and it sounds like the only way to get it is to assume that Enceladus is dumping more heat than usual right now.