Alien Seas by Michael Carroll & Rosaly Lopes
Author:Michael Carroll & Rosaly Lopes
Language: eng
Format: epub
Publisher: Springer New York, New York, NY
In the following years, Cassini has flown past Enceladus many more times, skimming the surface at altitudes as low as 20 km, less than twice the height reached by commercial airliners on Earth (but at 70 times the speed). The trajectory and spacecraft orientation during each flyby is optimized for specific science goals- close flybys through the plume for direct sampling, more distant flybys for mapping the surface, and close flybys on a range of trajectories to map Enceladus’ gravity field. After its first inadvertent brush through the edge of the gas and ice plume in July 2005, Cassini has ventured deeper and deeper into the heart of the plume. The plume turns out to be safe to fly through because the ice grains in it are so small: even at the high speeds of the Cassini flybys, the blizzard of tiny particles hitting the spacecraft is only a minor annoyance.
The stream of data from Cassini is gradually building a picture of how Enceladus works. For Enceladus, the ultimate engine powering its activity, tidal heating, is not just a theoretical concept- the amount of heating is so large that the heat can be measured directly by Cassini’s infrared instrument. The measurement is tricky because Enceladus also radiates heat simply by virtue of being warmed by the Sun, and the solar heating must be estimated and subtracted from the total in order to determine the internal heat. The current best estimate of the internal tidal power is about 16 Gigawatts, eight times the hydroelectric power output of the Hoover Dam- impressive for such a tiny world. In fact the observed power is a bit too impressive- calculations based on the orbits of Saturn’s moons suggest that Enceladus can only generate about one tenth the observed tidal power on average. Perhaps the power is higher than average right now because Enceladus has been storing heat for a while and is now letting it go in a sudden burst. If so, we are lucky to have arrived at Enceladus while it’s in this unusually active state.
Another puzzle is why the activity is so perfectly centered on the south pole. The most plausible explanation is that the heating started in some random location on Enceladus, and that material was transported away from the heated area, perhaps by melting of the ice or even ejection in the plumes. Once a symmetrical spinning object looses mass in one location, the physics governing rotating bodies dictates that its rotation becomes unstable until the spin axis realigns to pass through the less massive region. So maybe that’s what happened on Enceladus. The active region could have aligned itself with either the north or the south pole- heads or tails –Enceladus just happened to come up tails.
Cassini’s repeated barnstorming of Enceladus has now given us a quite detailed picture of what the plume is made of. Because the plume lets us sample stuff that was inside Enceladus only minutes earlier, it gives us a window into the moon’s interior that we don’t have for any other ice world.
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