Diving past Enceladus
Published on Mar 11, 2008 at 3:00 pm.
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Filed under extraterrestrial moons.
Enceladus is the sixth largest natural satellite of the planet Saturn. It is just large enough for gravity to begin to pull it into a roughly spherical shape, but just barely. As a consequence, Enceladus is not quite spherical, with diameters ranging from about 497 km to 513 km. That amounts to only about a 3% difference, so Enceladus is pretty close to spherical.
Enceladus was discovered in 1789 by William Hershel. Not much was known about Enceladus, though, until the Voyager missions to Saturn in 1980 and 1981. Those findings were rather surprising. They showed that Enceladus was extremely reflective, having an albedo of 0.9 (that means that it reflects 90% of the light shining on it). This is the highest albedo of any major body in the Solar System. That suggests that the surface of Enceladus may be covered in something as reflective as fresh snow. The idea that ice might really be the reason that Enceladus is so reflective was further supported by images showing smooth regions of the surface taken to be possible indications of ice volcanism. In the outer solar system, water is normally solid, so if it were to melt and move onto the surface of a world, it would resemble volcanic activity on Earth, only having a different chemical makeup and temperature. Even further support for the idea of ice volcanism was the discovery of a very tenuous ring (the E Ring) of icy particles orbiting Saturn with Enceladus near the middle of that ring. Active icy volcanism was suspected as the possible source of that ring. An alternate hypothesis was that ice volcanism provided fresh ice to the surface of Enceladus where it was blasted into space by high energy collisions. Both ideas, though, assumed an icy volcanism on Enceladus.
But, the Cassini spacecraft has shown that Enceladus is even more baffling than had been imagined. Enceladus appears to have plumes of water and ice spewing up form its south pole. I wrote about those plumes and some of the ideas surrounding them two years ago. Later studies have ruled out some ideas, and the plumes are still not completely understood. One of the problems is that these plumes seem to result from high velocity geysers. That would require liquid water under pressure. The problem is that Enceladus should be too cold for liquid water. Early ideas suggested that a combination of factors could account for this. Radioactive heating could be providing some heat to Enceladus’ interior. Tidal stresses between Enceladus, Saturn, and Saturn’s other moons might also heat Enceladus. And, if the water had salt or ammonia in it, then the melting temperature would be lower, and these heat sources may be enough to melt the water. Unfortunately, spectral analysis of the plumes does not see enough ammonia or mineral to account for much of a decrease in the freezing temperature of the water. That may mean that there is a pool of nearly pure water beneath the surface ice of Enceladus’ south pole.
This has brought forth a lot of speculation that such a water deposit, if it were stable for long enough periods of time, may harbor life. That may be a long shot, though. Still, it is an interesting idea. For a long time, the only world in the Solar System that we knew could support life was Earth. Now there is talk that Mars may have once been marginally (or better) habitable. Oceans may exist under the ice of Jupiter’s moon Europa that might be habitable. For over two decades, there has been the suggestion that life of some sort (nothing like any life on Earth) might be able to exist on Saturn’s moon Titan (again a long shot). Now, we have Enceladus.
But, we don’t really know much about those water/ice geysers on Enceladus. In particular, we don’t know about the source of the geysers. So, that is why a decision was made to fly the Cassini spacecraft close to the plumes to study them. Tomorrow, March 12, 2008, Cassini will be passing very close to Enceladus’ south pole. Despite many reports that the spacecraft will fly through the plumes, the path will skirt them. It may fly through some plume material, but my understanding is that the flight path is not planned to fly directly through the middle of the plumes, only the edge of them. This will be a tricky maneuver, with little room for error. Cassini will be passing only 50 kilometers above the surface of the moon.
Cassini’s cameras will, of course, image Enceladus as the spacecraft approaches and recedes from the moon. However, astronomers are much less interested in the pretty pictures than they are in the spectral analysis, magnetometer readings, and the data collected by Cassini’s other sensors as it passes by Enceladus. Hopefully, that data will help us to understand what is going on with that moon. And, hopefully, it will help us to find out what is causing these geysers and what is going on beneath the surface of Enceladus’ south pole. This won’t be the only passage by Enceladus this year, though. Cassini will pass Enceladus again on August 11, October 9, and October 31. The October 9, 2008, pass should be even closer than the one tomorrow. So, hopefully, we may have some answers to our questions forthcoming.
These passes are not without risk, though. Indications are that the plumes are composed of only tiny particles of ice, little more than icy dust grains. If that is the case, Cassini should not be damages by impacts with those dust grains. However, if there are larger particles mixed in, then there is a risk of damage to the spacecraft. The Cassini engineers know this, and they are no doubt going to be anxiously watching tomorrow. If everything goes well, then we will have a stream of data flowing from the spacecraft to Earth. If the spacecraft suddenly goes silent, though, then they will be fearing the worst. That is unlikely to happen, but it is always possible.
-Astroprof
Images courtesy NASA, JPL
