LPSC 2007: Frozen volcanoes in the Kuiper belt. Cool...
Could Charon, Pluto’s largest moon, have watery volcanos spewing forth as we speak? According to maths, yes, but according to other’s views of geology, no.
Steven Desch at Arizona State University, Tempe, suggests that crystalline water seen at Charon is evidence of cryovolcanism, and he did some mathematical modelling to work out how Charon and other large Kuiper Belt Objects (KBOs), often considered to be geologically dead, could be storing liquid water or ammonia that is able to erupt through the crust even today.
Desch claims that energy is stored at the core of the KBOs and released at a different rate than predicted, and manages to keep the tempeture just right to maintain liquid ammonia temperatures under the crust – even today. This is the hard part, he says. Getting that liquid to the surface isn’t as difficult.
Desch’s model suggested a cut off radius of 500km for KBOs that could support cryovolcanism, any smaller than that and the model didn’t hold. Corroborating his model, he says is the presence of ammonia hydrates on Charon, “They provide a nice antifreeze,” Desch says.
But all is not as it seems. Bill McKinnon, from Washington University in St Louis, told me that as far as he’s concerned, Desch’s model is “geologically impossible”. He doesn’t see how the magma could be hot enough to break throuh the surface of Charon. What is more intriguing to him is the presence of ammonia ice on the surface. “You don’t’ need vulcanism to explain crystalline ice,” McKinnon says, “we see it everywhere”.
Mike Brown, at Caltech (not tempted to attend the Texan gathering), is expert in all things Kuiper (and author of a paper on related Kuiper belt objects that come from the same massive collison - published in Nature yesterday), especially crystalline water. He remains to be convinced. “Cryovolcanism is cool (literally) so everyone wants to jump to that answer, but the real answer is probably much more dull,” he told me. Just because crystalline water ice is everywhere doesn't mean, of course, that cryvolcanism is nowhere, Brown says, but to him it’s clear that the crystalline water ice is not nearly enough evidence to prove the volcano hypothesis.
I didn’t manage to get hold of Desch in person to ask him – so if I learn the real answer I’ll update here…
Oh, and if you didn't know, Charon in Greek mythology was the Ferryman of the dead. Lovely.
Comments
The formation and retention of crystalline ice is a very mixed issue. It has been seen on many, if not all, icy satellites and a growing number of KBOs. But what separates satellites from KBOs is a combination of heliocentric distance and close proximity to other satellites of similar size. Satellites are much closer to the sun, and can become much warmer. The relationship between time to crystallize and temperature decreases exponentionally such that around 85 K ice will crystallize in on million year time scales, at 90 K it is on the order of 10^5 years, and by 110 K, we are talking time scales of a few years. These high temperatures are easily reached at Jupiter, and even Saturn. One exception is Europa, probably the iciest satellite in the solar system, shows a miture of crystalline and amorphous water ice. Why? Beause the radiation environment around Jupiter is so great that high energy particles are constantly disrupting the crystalline structure of its ice. The fact we see any crystalline on the surface might suggest it is either protected or renewed.
The other issue of the satellites, their close proximity to other similar sized satellites, is equally critical. If a satellite's orbit is in resonance with other (relatively) large satellites (this is certainly the case for the Galilean satellites) then tidal heating can play a critical role on keeping the inside of these satellites mushy. The constant pushing and pulling will likely crack the surface, like Europa, and material can reach the surface. Without this extra external force, a body of water ice should have frozen out long ago.
It is difficult to understand whether or not cryovolcanism is at work on the moons of Uranus and Neptune because we've only had the one visit but surface renewal is suggestive from the pictures. The Voyager 2 images showed surfaces that appeared relatively young, some astronomers suggested even fluid-like, and some places (Triton for example) appeared to have frozen lake beds. In all our fly-bys of Saturn's moons, it took an orbiter like Cassini to take a backlit image of Enceladus to see its plumes. The fact is that astronomers were suspect of this tiny moon even before Cassini because of linear cracks (aka Tiger stripes) in the surface toward the south pole. A similar analog to the Tiger stripes on Enceladus, where the material spouts from, have been imaged elsewhere, such as Ariel. And some of the cyclical cracks seen on Europa also appear on Triton.
It is probably a little naive to think that cryovolcanism isn't at work because Voyager didn't see it directly happening. The large plumes seen shooting off the limb of Enceladus might be unique to only itself. We know something is happening on Europa, we've just not seen plumes. Cryovolcanism may be a slow creeping process like sea floor spreading is on earth.
At the edge of the solar system, where KBOs orbit the sun, their surface temperature is around 40-50 K. As far as we know, ice will not crystallize on timescales shorter than the age of the solar system. So how do you explain its presence? Its difficult, and probably impossible without ammonia. Initally starting with an ice-rock mixture, a body the size of Charon can differentiate, if only partially. This releases a great deal of energy, which take billions of years to use up. In the meantime, an ocean can form. As it refreezes, water will freeze out first, since it freezes at 273 K. This concentrates ammonia from its initial abundance up to 33%, the eutectic value. At this concentration, the mixture will freeze at 176 K. An object the size of Charon (and even slightly smaller) can still be this warm near the core.
Next is the issue of how to get the material to the surface. This is a topic which is debated, even for Europa. Some would suggest all you need to have is a small crack and once it reaches a critical height, it will rapidly grow until it reaches the surface.
The fact is that cryovolcanism works well on paper. The proof will be when New Horizons gives us a glimps in 2015 of true KBOs in their native environment.
Posted by: Jason Cook | June 1, 2007 04:51 AM