[X] My Assistant

[Guest_BruceMoomaw_*]

As promised: apart from the potentially gigantic biological significance of the Enceladus find, there are two big mysteries associated with it: what's driving such extraordinarily intense cryovolcanism, and why does Titan seem to have ammonia while Enceladus doesn't?

There seem to be four schools of thought about the latter, three of which look to me like serious possibilities.

(1) "Enceladus DOES have ammonia, but Cassini didn't see it because solar UV destroys it quickly." This is the theory of Baragiola et al in http://www.cosis.net/abstracts/EGU06/03150/EGU06-J-03150.pdf : "We will present results from laboratory studies on the radiation effects on ammonia–water mixtures pertaining to the environment of Saturn’s icy moon Enceladus. We show that ion irradiation destroys ammonia efficiently, and produces N2 that could be the source of N+ that has been detected in the exosphere. Warming the irradiated mixtures we observe outbursts of water and ice grains at temperatures much lower than those needed for sublimation of water ice. These radiation processes may explain the plume of water vapor and grains observed by Cassini at Enceladus." This was my own sentimental favorite when I went to the Europa meeting, and during F.J. Crary's presentation on the findings of Cassini's INMS at Enceladus I asked him about it -- but I think he disposed of it soundly in very short order by pointing out that, while this could explain the lack of frozen NH3 on the sufaces of Enceladus and the other icy moons, the gas analyzed by Cassini had been emitted from the vents only about 15 minutes earlier, which isn't time for most of the ammonia to be destroyed no matter how quickly the latter occurs.

Carolyn Porco's new "Science" article, I think hammers the last nail in the coffin by pointing out that -- since NH3 has a far lower boiling point than water -- if the liquid coming out of the vents was an NH3-water mix, most of the vapor detected by Cassini should have been NH3. Instead the article confirms that Cassini found none at all, confirming that it must at most comprise 0.5% of the plume gas. Enceladus really does lack ammonia -- which also means that the plumes really must be above 0 deg C, with all the biological implications thereof.

(2) "Contrary to initial appearances, Titan -- like Enceladus -- never had any ammonia either. instead, they both got their nitrogen, from the very start, as plain nitrogen." This actually fits better with the analyses of the heavier elements in Jupiter's air by the Galileo entry probe, whose ratios seem to indicate that the icy planetesimals that made up Jupiter were either much colder than expected, or were made of clathrates that were much more efficient than water ice at holding large amounts of molecular nitrogen and noble gases. ( http://www.lpi.usra.edu/opag/oct_05_meeting/probes.pdf , pg. 15-16)

Cassini's evidence that Titan does have large amounts of ammonia is twofold. Huygens found virtually no noble gases in the air (except for the Ar-40 produced by the decay of radioactive potassium in its rocky core), indicating that Titan was not made out of very cold ices or clathrates that could hold either those gases or N2; and Cassini's SAR images suggest large-scale cryovolcanism, which is definitely easiest to explain if it's due to a water-NH3 mixture erupting out of the ground (since this both has a very low melting point, and is less dense than solid water ice and thus capable of easily rising up through it). But Bar-Nun et al propose an alternative explanation for the lack of noble gases in Titan's air -- they claim that the rain of organic smog particles is capable of having long since scavenged all atoms of those gases out of the air and trapped them in the ground smog deposits ( http://www.aas.org/publications/baas/v37n3/dps2005/227.htm ; http://www.agu.org/cgi-bin/SFgate/SFgate?&...t;P34A-06" ). And, at the Europa meeting, Mikhail Zolotov told me that he seriously doubts Cassini's radar evidence of large-scale cryovolcanism on Titan, although he didn't say just why.

(3) "Titan, Enceladus, and Saturn's other moons acquired most of their nitrogen in organic compounds." This is Zolotov's belief. I presume he means mostly cyanide compounds, which Cassini apparently has sensed on the surfaces of the icy moons. ( http://www.aas.org/publications/baas/v37n3/dps2005/672.htm . See also Emily's log on Clark's DPS talk: http://redrover.planetary.org/news/2005/09...og_Special.html , Sept. 8 entry.) I haven't the slightest ability to judge the likelihood of this theory -- although I do note that Cassini didn't find any HCN in Enceladus' plume gas, either.

(4) "Titan and Enceladus really did start out with ammonia, but it was all broken down inside both of them by intense internal heat resulting from an early high concentration of short-lived radioiotopes inside their rocky cores." This is the view of Jonathan Lunine, Dennis Matson and J.L. Castillo ( http://www.cosis.net/abstracts/EGU06/05276/EGU06-J-05276.pdf , http://www.cosis.net/abstracts/EGU06/09655/EGU06-J-09655.pdf , http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2219.pdf ), and they mesh it with their belief that the shape of Iapetus provides evidence that it too started out with a high concentration of such isotopes-- specifically, Al-26 -- in its rock ( http://www.aas.org/publications/baas/v37n3/dps2005/274.htm ; http://www.agu.org/cgi-bin/SFgate/SFgate?&...t;P21F-02" ). Once again, however, we have to assume that there is currently no ammonia-water volcanism occurring on Titan's surface, because "Preliminary calculation indicates that all of Titan’s initial NH3 can be processed [into nitrogen] within a few million years after formation."

As for Atreya's observations indicating that the outer planets themselves were made out of materials that never had any ammonia in them to begin with, Alibert and Mousis try to reconcile this with ammonia in Saturn's moons as follows ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1141.pdf ): "We...favor an alternative scenario where Titan may be formed from satellitesimals that would have suffered a partial vaporization during their formation and/or migration in Saturn’s subnebula. The migration of satellitesimals in a balmy subnebula (as our model shows at intermediary epochs) could allow a partial or total vaporization of most volatile species (CO, N2, Kr, Xe) whereas CH4, CO2, NH3 would remain trapped in water ice."

So: does Titan really have cryovolcanism indicating that ammonia still exists in its interior, or not? That is the question, and Cassini may not be able to answer it by itself. ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2136.pdf , unfortunately, indicates that Cassini's VIMS just does not have the ability to identify ammonia ice through the limited spectral windows allowed by Titan's atmosphere.)

Dominic Fortes ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1293.pdf ) suggests an interesting twist: since it's likely that large amounts of sulfur has been leached out of Titan's carbonaceous-chondrite core rock by the water initially trapped inside it (as with the other moons of the outer planets), it may well have reacted with the ammonia dissolved in Titan's subsurface ocean to turn it into ammonium sulfate. He also suggests that ammonium sulfate "is a credible candidate" both for the mystery substance detected on Titan's surface by Huygens' near-IR spectrometer, and for the 5-micron "bright spot" Hotei Arcus. But such a solution would have about the same melting point as plain water, and a much higher density than ice, making it a lot harder for volcanic processes to drive it to the surface. Fortes suggests some mechanisms that might conceivably do so, at least in limited amounts -- and presumably these could work for plain water, too, to explain how Titan might have at least some cryovolcanism without any ammonia.

I wonder, vaguely, about a fifth possibility: it it possible that Titan still has large amounts of ammonia inside it but Enceladus doesn't, because Enceladus has recycled its inner liquid? Porco's "Science" paper concludes that "only ~1% of the upward-moving [ice] particles [in the plumes] escape to supply the E Ring" -- the rest falls back onto the surface as snow. If so, then the mounting weight of that snow on top will force the local ice crust back down into Enceladus' warm interior to remelt its lower layers into water. This explains why Enceladus hasn't dried up completely from these eruptions over the eons (although its high density suggests that the eruptions have stripped it of much of its initial ice).

But if so, then when the eruptions started, Enceladus might have had a layer of liquid water-NH3 mixture inside it -- which, since it could stay ductile at low temperatures, would have made it easier for some kind of heating to start up the eruptions in the first place -- and then, when this mixture was expelled onto the surface, the ammonia was quickly destroyed, leaving just water snow behind on the surface. Thus, after a relatively short period of ammonia-water volcanism, Enceladus would have depleted most of its ammonia (leaving only the small amount which may have been broken down by its internal heat into the reservoir of internal nitrogen which accounts for the 4% N2 in its current vapor plumes). But tidal convection -- once some kind of heat source has started it in the crust of an icy moon -- strongly tends to amplify itself and keep itself going, because the friction of ductile material churning convectively in the core (whether it's warm ice or some liquid) generates additional heat and thus warms the material further. And so the very frictional heat of the initial eruptive venting could produce enough additional warmth to keep Enceladus' eruptions going even after the ammonia was gone and the only liquid left was plain old easily freezable water. By contrast, there aren't any processes on Titan's surface that destroy ammonia erupted onto the surface -- its air pressure keeps any frozen NH3 from vaporizing, and its organic haze is a good shield against solar UV -- so its remaining ammonia keeps on getting recycled back down into the subsurface ocean instead of being destroyed as Enceladus' ammonia was, making it possible for cryovolcanism to continue on Titan even if it isn't as powerful and thus as frictionally self-heating as Enceladus' volcanism is.

Whichever of these explanations for the Great Ammonia Mystery is true, we still have the other huge Enceladan mystery: what in the world started up such a violent eruption in the first place? Porco's "Science" article says that it's unlikely that Enceladus' orbit was ever eccentric enough to produce ice-melting heating by itself, but they propose an alternative: a chance giant impact. They conclude that Cassini's data on Enceladus' shape suggest that at one point it was librating four times during each orbit -- perhaps quite violently -- and that it could have been put into that resonant cycle by a giant impact, although it has since settled back down. "An initial [libration period to spin period ratio of] 0.26, with an associated libration amplitude of 22 degrees, yields a heating rate 100 times that due to the forced eccentricity today for typical fully elastic models and the same temperature-independent Q=20." As for what's kept the eruption going since that violent libration was finally damped out and disappeared: "...[I]t is plausible that Enceladus may be sufficiently heated today by some combination of the above mechanisms [tidal heating from its current slightly eccentric orbit, in which it is kept by its 2:1 resonance with Dione; and heat from the remaining long-lived radioisotopes in its rocky core] to explain the observed south polar venting, provided it underwent an early epoch of intense heating and, once heated, retained a low Q up through the present in some portion of the interior."

Lunine, Matson and Castillo instead blame the eruption's start on a concentration of Al-26 in Enceladus' rocky core. They also think that there's no way that tidal flexing or convection in the ice alone can produce enough frictional heat to keep the ice melted, and that instead what keeps the eruption going is frictional self-heating from a reservoir of magma in the rocky core itself, whose viscosity does create enough tidally generated frictional heat in Enceladus' current orbit to keep the magma molten. "The [tidal] dissipation factor in the core [for such magma] for tidal frequencies is Q~1-10, and this can provide up to 100 gigawatts, compared to 0.5 GW provided by the decay of long-lived radionuclides." But -- in their abstracts so far -- they don't seem to explain clearly why such an initial high concentration of short-lived hot radioisopes in the moon's rock didn't also happen to Saturn's other icy moons. In fact, in http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2351.pdf , they puzzle over the fact that this hasn't happened to Mimas, since its orbit is more eccentric: "Enceladus’ present eccentricity is six times smaller than Enceladus’ and the tidal dissipation for a given Q and k2 is 40 times less at Enceladus...However, the fact that intense endogenic activity is observed at Enceladus, which contains twice as much rock as Mimas, might indicate that the difference in evolution is the result of a thermal event in the history of Enceladus that created the conditions suitable for significant tidal dissipation to start and be sustained." If I understand them correctly, they're saying that it might be simply because Enceladus has more rock in its core than the other small icy moons. ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2200.pdf : "Rock mass fraction ranges between 3% in Tethys to 57% in Enceladus... Whether conditions are intense enough that endogenic activity can start and be sustained until the present is a function of several different factors, depending on the differences in size and rock mass fraction between the satellites and their orbital evolution (especially eccentricity damping)." And http://www.agu.org/cgi-bin/SFgate/SFgate?&...t;P32A-01" : "We note that in Enceladus and Titan conditions might have been such that the boiling point of water was reached and water might have been lost very early in the history of these satellites.") But this leaves us -- once again -- with the possibility that it might instead be a chance giant impact at some point in Enceladus' history that first created its eruptive heated region, which has since sustained itself through continuing positive-feedback tidal-friction heating in its ice, its interior rocky core, or both.

In any case, the fact that the eruption site is now at the south pole is easier to explain; Bob Papplardo points out ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2113.pdf ) that, wherever the eruption started on Enceladus, the lower density of the moon's internal material in that region would likely make plain old centrifugal force and Saturn's tidal tuggings cause Enceladus' polar axis to wander until the lowest-density region was at one of the poles. Moreover, if an internal layer of liquid separates its outer crust from its mantle, the crust would tend to reorient in this way without the moon's interior doing so -- which, since Enceladus is slightly oblate, would cause the crust around the eruption region to get somewhat squeezed together as it moved to the pole, producing compressional tectonic features of exactly the sort seen around Enceladus' current south pole ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2182.pdf ).