[X] My Assistant

[paxdan]

Nasa Watch is reporting that we shoud expect a large announcement from the Cassini team today:

My wild speculation is that it will be a subsurface ocean confirmed on enceladus. Hence more potentially habitable real eastate in the solar system.

Heads up anyone?

[elakdawalla]

I'm getting a brief note posted based on a chat with John Spencer; thought you all would enjoy the full text of what he told me.

We've refined our numbers a bit, we've gone through our highest resolution data and we've found a few areas of somewhat higher temperatures than we'd seen then. In July we were talking 125 K, now the highest we've actually seen in our entire data set is 145 Kelvin.

We're looking at black body radiation, so the wavelength distribution of the emission tells you the temperature. The warmer it is, the shorter the wavelength of the emission. And because we're looking at solid surfaces and most of those behave like pretty respectable black bodies, just by looking at the shape of the thermal emission as a function of wavelength you can get the temperature pretty well.

In this case, the job is simpler because the background is way colder. Even though, for instance, 145 Kelvin we have one spectrum that covers an area 6 by 6 kilometers. And we have just a few percent of that area that's 145 Kelvin, and the rest of it is at 70 or something much colder. But the heat radiation we see is completely dominated by that little bit of hot stuff.

The liquid water is all coming from the imaging team and their analysis of the plume. I guess they're figuring that the density of the particles they're seeing in these plume images is just too great to be explicable by water gas entraining particles, but I haven't looked at those calculations. Even they are saying this is an inference, not a direct detection.

When we fly past again in 2008, we'd like to look down the throat of these cracks and see 273-Kelvin temperatures down there, and we'll certainly try that.

Its approach is over the north pole, and it departs over the south pole. It's convenient for CIRS and it's inconvenient for most people, the problem being that right after the flyby Enceladus goes in to Saturn's shadow. So by the time the spacecraft is turned around to look back at the south pole, we won't be able to see the south pole in sunlight, so there won't be much in the way of images. We're perfectly happy; it eliminates all of the competition! We'll be getting good stuff. And I think that VIMS, if there's really stuff at 270 Kelvin, it should be warm enough for them to see emission at shorter wavelengths as well. Certainly at 5 microns, probably around 3 microns.

People have always invoked ammonia slurries as a way of lowering the melting point way down to about 175 Kelvin and allowing melting and geologic activity and volcanism on these cold objects. And this has been a tidy theory until we got the Enceladus data and seen NO sign of ammonia. Last time I checked, the mass spectrometer people had no sign of ammonia. It doesn't mean there's none there, but it's not got much -- there's more carbon dioxide than there is ammonia, for instance. We're just not seeing ammonia anywhere. If there is ammonia inside Pluto, and there probably is because there's nitrogen and ammonia is probably where the nitrogen came from, then there could be enough internal heat on Pluto, which has no tidal heat, but there could be enough radioactive heat that you might get some activity. The fact that Triton, which also has no internal heat source, also has this very exotic surface, clearly has signs of very recent geologic activity, implies that you can do stuff with just radioactive heat that far out in the solar system for a reasonably large object like Triton or Pluto. The thing about Enceladus is that it's so tiny that the radiogenic heat, it's very unlikely that it could do much of anything, and tidal heating is the only plausible heat source.

It's a matter of getting the internal structure right and then explaining why Mimas, which should have a lot more tidal heating, has absolutely none. It's closer in, and its orbit is way more eccentric. Probably part of the explanation is that it has to be warm in order to stay warm. We need to have a fairly soft squishy dissipative interior in order to generate enough heat to keep it soft and squishy and dissipative. If you were to freeze Enceladus solid, take it out of orbit, freeze it, and put it back where it is, it would just be too cold to get that engine started again. At Mimas that engine never started. But why that engine started on Enceladus and not on Mimas we just don't know.