Back in December, scientists from the Cassini team presented evidence for ice volcanoes on Titan, looking at a region called Sotra Facula, which bears some resemblance to volcanoes on Earth like Mt. Etna in Italy and Laki in Iceland. An ice volcano, also known as a cryovolcano, would draw on geological activity beneath the surface that warms and melts parts of the interior and sends icy materials through a surface opening. Sotra Facula’s two 1000-meter peaks combine what appear to be deep volcanic craters with finger-like flows of material, a kind of surface sculpting that could explain some of the processes occurring on other ice-rich moons.
But work like this is part of an ongoing dialogue testing various hypotheses, and the latest round takes us in a sharply different direction. In a new paper in Icarus, Jeff Moore (NASA Ames) and Robert Pappalardo (JPL) argue that Titan is in fact much less geologically active than some have thought. A cool and dormant interior would be incapable of producing active ice volcanoes:
“It would be fantastic to find strong evidence that clearly shows Titan has an internal heat source that causes ice volcanoes and lava flows to form,” adds Moore. “But we find that the evidence presented to date is unconvincing, and recent studies of Titan’s interior conducted by geophysicists and gravity experts also weaken the possibility of volcanoes there.”
The new work looks at Titan in light of what we see on Callisto, which Moore sees as analogous to Titan ‘if Callisto had weather.’ And indeed, the two moons are roughly the same size, with Callisto’s cratered surface solely the result of impact events rather than internal heating. In the new paper, Moore and Pappalardo see Titan’s surface as explicable entirely from external processes like wind, rain and impacts. These we see in profusion — lakes of liquid methane and ethane, valleys carved by rivers, craters — through infrared and radar instrumentation, but the debate now moves to whether all surface features can be explained in the same way.
Image: These images compare surface features observed by NASA’s Cassini spacecraft at the Xanadu region on Saturn’s moon Titan (left), and features observed by NASA’s Galileo spacecraft on Jupiter’s cratered moon Callisto (right). The Cassini radar image, obtained on a Titan flyby April 30, 2006, is centered on 10 degrees south latitude and 85 degrees west longitude. The Galileo camera image, obtained on June 25, 1997, is centered on 6 degrees south latitude and 7 degrees west longitude. Titan may originally have had a cratered landscape similar to Callisto that has since been eroded by rainfall and runoff. There are many large circular features in Titan’s Xanadu region that have some of the characteristics of impact craters — such as central peaks and inward-facing circular cliffs — which make scientists think they are, in fact, eroded impact craters. Credit: NASA/JPL.
Titan’s atmosphere may remain the focus of debate between those who believe the moon is geologically dormant and the ice volcano theorists. The atmosphere is primarily nitrogen, with a few percent methane, and the Sotra Facula analysis, presented at the American Geophysical Union meeting in San Francisco last December, focused partially on that mix. Thus Linda Spilker (JPL):
“Cryovolcanoes help explain the geological forces sculpting some of these exotic places in our solar system. At Titan, for instance, they explain how methane can be continually replenished in the atmosphere when the sun is constantly breaking that molecule down.”
But are ice volcanoes the only explanation for replenished methane in this atmosphere? Both sides in the debate would agree that there is no evidence of current activity at Sotra Facula even if the topography there is suggestive of a volcanic origin. Further Cassini studies will advance the argument, but it’s helpful that Pappalardo and Moore have injected what they call ‘a necessary level of caution into the discussion’ as we wait for more definitive results.
The paper is Moore and Pappalardo, “Titan: An exogenic world?” Icarus Volume 212, Issue 2 (April, 2011), p. 790-806 (abstract).
The only way to settle this debate is by sending an orbital “rover” (perhaps via a modified hot air balloon?) with some decent cameras.
I would not be surprised if there were cryovolcanos on the surface, although I wouldn’t rule out internal heating on Titan (as its little sister Enceladus is also a little warm underneath, with little explanation as to why).
It’s well established that Titan’s crust is decoupled from the core. Doesn’t this imply/provide a source of “warm” material (water, ice-water slush, etc.) just under the crust?
Titan has been called a “Callisto with weather” before.About one year ago I read something similar, for example, here :
Unfortunately we’ll have to wait a while for a return to Titan as NASA prioritised EJSM over TSSM.
Isn’t some kind of geologic activity needed to sustain the structures that exist on Titan considering that Titan have weather and therefore more erosion?
Considering that Enceladus and Titan both rotate around Saturn (and therefore to some degree have a similar background) would it be likely that Titan’s inner parts are a bit warmer than expected for the same reason (whatevere the reason might be) that Enceladus’ are? (as per https://www.centauri-dreams.org/?p=17089),
“But are ice volcanoes the only explanation for replenished methane in this atmosphere?”
Well, no. We know another way to replenish methane — *life*.
Of course, that’s a pretty exceptional hypothesis, so we’d have to rule out volcanic/geological sources pretty thoroughly before it became the leader. But if that does transpire, I think a biological hypothesis should seriously be considered.
As more is learned about this amazing world that is Titan I feel the less I know. As new information emerges, I am beginning to loose my grip on how the pieces of this Titanic puzzle fit to create the whole. Surely a world with such a low gravity could never retain a primordial atmosphere left over from the turmoil of Saturn’s formation. If its atmosphere is volcanic, most of that nitrogen must have been erupted as ammonia, and if so, wouldn’t it have been accompanied by at least an order of magnitude more methane? If so where are all the organic tars or deep hydrocarbon oceans?
In an earlier article on these pages, the unusual concentration profile of hydrogen with altitude on Titan was featured. An experiment had been done with the assumption that hydrogen loss through the exosphere changed this profile. This would have been far easier to calculate than hydrogen loss into the ground, since Titan’s atmosphere gets complex and smoggy in its lower reaches. Thus I strongly suspect that the 30 odd kilograms calculated to be disappearing into the ground each second must have been a lower estimate (if anyone can confirm or correct that belief, I would be grateful). Either way it is obvious that hydrogen loss through the exosphere must also occur, and must be added to this figure to find the grounds true hydrogen absorption rate. If we assume that that rate was also 30 kg/s we find that it takes at least 2 billion years just to generate today’s inventory of atmospheric nitrogen – and that assumes that none of it has been lost to space.
Titan’s core does not seem to be well enough differentiated to allow any past epoch of high volcanic activity let alone allowing it today, so I suppose that that atmosphere must really somehow be primordial, yet then where are all the noble gasses? I admit that a drowning man does clutch at straws, but this situation reminds me of Lovelock’s Gaia hypothesis. Could life really have engineered the maintenance of high methane levels without use of colossal inventory of carbon? If so it is at least clear why life needs to do so by Gaian principles.
In the comments section of Paul’s original précis of the hydrogen altitude data, the consensus was that the ground absorption of hydrogen was so low when expressed by weight as to be insignificant, and preclude the possibility that our first probe to study its surface might be met by an enthusiastic reception committee of natives. This was in error since a little bit of hydrogen provides much energy (especially when reacted with acetylene). Contributors to those comments correctly calculated that the measured H adsorption rate (that I believe to be a minimum), was 0.4 mg/s/sq km, but this is about 20 Watts. By comparison, over all Earth, plants capture 175 000 W/sq km.
In that discussion it was also floated that this activity could be the seen as more similar to that of animals than plants. I have no figures for Earths animal activity, but I would guess that it would total 10 000 W/sq km, so the minimum animal biological activity given by hydrogen flow might be only a few hundred times less than Earth’s.
I agree that idea of a thriving biosphere on Titan sounds crazy, even as a possibility, and would appreciate it if someone would correct me, yet I do not see an obvious mistake, and an beginning to see signs that it exists.
I don’t see what’s so bizarre about the idea of Titanian life, really. It’d be so exciting that it would be a claim that would need to be made cautiously — there’ve been a lot of extraterrestrial-life false alarms — but Titan has lots of surface liquid and lots of organic molecules. No water, of course, but methane-ocean life has been speculated on before.
“A thriving biosphere” might be a bit strong, however. Some microorganisms in the lakes, quite possibly… but we don’t see the really radical stuff that we get on Earth – I imagine Cassini would have spotted the equivalent of forest cover. It might be the most you could get with the lower energy available on cold Titan, though…
But past the speculations… how could it be tested? How can volcanic sources be ruled out? Is there anything besides life or volcanoes that could be making methane?
I agree intercostal, “thriving biosphere” sounds too strong, but I would appreciate help on, at least, putting an upper limit on it, if not also on ruling it out. I have used this term because the figures stack up several orders of magnitude better than that of the more touted potential for a biosphere on Europa (check it out under various proposed scenarios and you will be surprised!).
Another thing to think about is that if life is confined to the lakes, and the average lake surface on Titan is 200,000 sq km then my lower bounds on this suspected biological activity there is 800 W/sq km, compared to Earths oceans’ average of 100 000 W/sq km. If Titanian pace of life is slower, and there are no plants, would a living oceans there even look less bountiful than Earth’s?
Actually there is an upper bound I can put on a Titanian biosphere if that hydrogen altitude data is inadequate for lower altitudes. The atmosphere is too smoggy for enough light to penetrate for any significant level of photosynthesis on the surface and the uv level at higher altitudes is much too high for even exotic life to live high in the atmosphere, so uv absorption by methane must completely power life on Titan. That gives the energy of every photon of light incident on it above the energy needed to photolyse methane bonds truncated to that energy then multiplied by 1 less its albedo in the uv. This gives a figure very close to the dark reaction collection of energy on Earth that I gave earlier.
After saying that, what I find most important is not the tease of this upper bounds , but that (unlike Mars) signs for life on this world are either misleading, or life on this world is so abundant that any sort of real search for it should find it instantly.
Chris McKay brought up the possibility of life on Titan:
I think that the biggest problem with a biosphere on Titan is the lack of energy to power chemical reactions on the surface. Not much happens at 70K – at least not quickly.
FrankH, that was a great article but let’s assume your above assertion is right and that biological activity must be very slow at the temperature of Titan AND that the hydrogen flow was due to it. Now calculate how many meters depth of bacteria the surface has to have to account for this activity and you will see the quandary. Remember that by some estimates that half the biomass on Earth is endolithic, which would make it the typical life at our temperature. Next look at how many more orders of magnitude of energy there must be available for Titanian life and you will see my quandary. The signs are so obvious that they must be misleading, but it would be unforgivable not to investigate that off chance.
I think the answer is that there’s an “ample” source of energy under the crust and that the article leading this blog entry is wrong – at least in comparing Titan to the geologically cold and dead Callisto.
There’s an article on ArXiv (featured in today’s ArXiv blog) about a subsurface ocean:
Yet more data showing that the crust is floating on something (probably water) and that something is kept in a liquid (or near liquid state). That should be enough heat on or near the crust to power all sorts of interesting chemistry.
Living inside Titan’s volcanoes would be really nice, at least the temperature isn’t as high as Earth’s volcanoes such that life is able to exist inside the “liquid magma”.