New article in New Scientist

umm, sure it is First, VIMS has only seen Hotei Arcus near the limb of Titan, which would make spectral comparison difficult (as Bob Brown and Jason Barnes mention in the article). Second, the feature has not been observed to change by ISS (though maybe the material that makes up the cryoclastic deposit is invisible to ISS, since Hotei Arcus is larger in longer wavelength VIMS data than in ISS data).

Just a thought: I have been thinking about the objection to the volcano hypothesis on the gounds that no temperature rise is observed at the bright spot. We normally think of erupted material being hotter than its surroundings but does this always have to be the case? When CO2 is released from a fire extinguisher the result is cooling. Could a Titan 'volcano' work like that? Could there in fact be different types of volcano on Titan (or different stages in the history of a volcano), some releasing heat while others cause refrigeration or are at least temperature neutral?

Momentarily going off the topic of New Scientist to answer this:

One would normally expect erupting material to have to start out as being warmer, or at least less dense, than its surroundings. However, I suppose it's technically possible that a subsurface reservoir of pressurized material, in thermal quasi-equilibrium with its surroundings, could "leak" gas that would cool off enough by adiabatic expansion to get below the ambient temperature.

The only thing is, how could this happen in nature? Where could that pressurized subsurface reservoir have come from, and how could it sustain itself over long periods of time? We'd need a workable scenario before proposing something like this.

Firstly why is this off topic? Where should I have raised it if not here?

Secondly I'm worried about your last statement. There was no workable scenario for continental drift when it was first proposed. The scenario was only worked out later once observations had forced the issue. If that can happen for terrestrial studies surely it's even more likely to be the order of events for strange, complex and unfamiliar Titan? Shouldn't we just observe what's there with an open mind and worry about detailed scenarios later?

Hmm. Will Cassini ever make a direct overflight of this feature? I don't see how any of these questions can be answered unless and until we get a more-or-less nadir view of it (preferably more than one in order to investigate this possible variability).

Interesting possibilities here, though. How much do we really know about organic chemistry at Titan's surface and subsurface temperatures, to say nothing of phase change dependencies for such substances under various pressures, catalytic interactions, etc., etc.? The possibilities seem endless; even active, complex adiabatic processes do not seem to be beyond the pale.

I think it's wise to remember how exceedingly challenging Titan is to our preconceptions and inherent prejudices. This place is nothing like anywhere else in the System; analogies and assumptions are therefore implicitly suspect. Given this, constructing a working hypothesis for Hotei Arcus if it is in fact a "volcanic" feature will require considerably more data, probably more than Cassini can provide.

The implications of volcanism on a world where H2O is the dominant mantle material elude me. My thoughts jumble up... is this mess untangled for someone else:

Let's assume that Titan has a rocky core and an H2O mantle. The heat driving volcanism has some radial distribution: If it is tidally-driven, then it would originate at radii somewhat out from the center. That would seem to create a layer of liquid water BELOW the icy mantle, right? Because if liquid water were atop ice, it would want to get down below it. Whereas if it reached the boiling point, it would want to escape upwards.

Water is more malleable than ice or rock, so any water layer would be a locus of tidal deformation and therefore tidal heating. If the heating did happen to have a stable equilibrium with the water liquid but not freezing or boiling, then Titan might be inactive. But if anyplace in that liquid reservoir crossed over to boiling, then some sort of upward vent would be created, along with an increase in reservoir volume. A minor vent would run out of steam (literally) after spending itself into the cooler icy mantle. That suggests an equilibrium where that "vent" is spread out in all directions, just adding some volume to the reservoir at the expense of the mantle. Activity of that sort could take place regularly if there's any input allowing new flareups, but with gravity wanting to spread the plastic ice back into a uniform spheroid swallowing any vents.

If a vent made it all the way to the surface, however, you might get a different stable equilibrium with continual cycling of surface/mantle/reservoir over geological time, with the vent continually pulling boiled H2O up from the reservoir to the surface -- but not if the vent cools to below 100C -- then the water would want to dive back down below the ice. So we might see vents that flatten and mushroom at the radius where they run out of steam (again, literally), but maybe allow cycling of the water downwards.

Another confound to all of this is the change in melting/boiling point at high pressures.

I think it's going to be seriously difficult to model the possible scenarios.

Absolutely right. Try adding lots of ammonia and methane plus dissolved nitrogen. How many dimensions is that for the phase diagram? Effects of radiocarbon? How many more factors as yet unguessed???? My suspicion is that the place is complex enough for self-organising cycles of various kinds to get going, with inherently unguessable contingent outcomes.

In this situation I suggest the following applies: Existing knowledge of the Earth and other planets can provide clues to new lines of enquiry for Titan, but limiting assumptions (such as 'volcanoes must be heat sources') should be left behind as unwanted baggage on this new journey.

Just to throw more logs on this fire...Consider the extraordinarily complex minerology of Earth, largely as a result of interaction with a single solvent (water) in various contexts. Titan may have three or more chemically distinct fluids performing a similar function...

The PS blog reports on a report by Nelson et al at LPSC, concerning surface changes derived from VIMS measurements. This appears to be the same spot (26S 78W, just north of Hotei Arcus) as the subject of this thread.

The spectrum appears to be consistent with ammonia, and there is again speculation about cryovolcanism.

Only kinda, sorta:

In otherwords, spectral evidence eliminates H2O, CO2 and CH4 as the changing species, but does not eliminate NH3. I suspect it is also consistent with HNNH(!) and possibly HCCH. That said, there is one peak at 2.01 microns that does smell like ammonia.


If there is visible ammonia venting, we should find more of it in the atmosphere - even though it is destroyed by the sun, that takes time. Volcanic events, if they are venting NH3, should be beeping all the ammonia detectors.