Titan Is as Titan Does
Saturn’s giant moon Titan keeps revealing mysterious dynamics in the here and now, leaving interpretation to fallible scientists who sometimes predicted otherwise.
Going the second Nile: A river valley rivaling Earth’s Nile has been found flowing into Kraken Mare, one of Titan’s large north polar seas. Live Science‘s article includes a video trip along its 250-mile length as revealed by orbital radar. Its blackness suggests it flows with liquid methane that rains down and flows through tributaries. New Scientist claims the river is the longest ever found outside Earth. Scientists believe the river tracks a fault line, even though they don’t believe Titan has plate tectonics.
Icier than thought: Thoughts can be warm or icy, but when PhysOrg says “Titan, Saturn’s largest moon, icier than thought” they undoubtedly mean scientists were surprised (again). Indeed, “A new analysis of topographic and gravity data from Titan, the largest of Saturn’s moons, indicates that Titan’s icy outer crust is twice as thick as has generally been thought.” They didn’t specify the generals that thought that. Orbital dynamics shows either that Titan is misbehaving, or its human trainers were caught unprepared:
From measurements of the observed gravitational field of Titan, one can compute what the shape of Titan ought to be. But the new data show that Titan’s shape is much more distorted than would be predicted by a simple gravitational model. That discrepancy means the internal structure of Titan isn’t quite so simple.
Crater survey: Of many Titan conundrums (if the moon is as old as claimed) is its remarkable lack of craters. Either most have been covered by geological processes or there never were many impacts—hard to believe for the moon with the most gravitational pull. A new paper on Icarus found that most craters on Titan are shallower by hundreds of meters than similar ones on Ganymede, a comparably-sized moon at Jupiter. They proposed “aeolian infilling” (wind-blown particles) as the mechanism – a hypothesis subject to critique by the rates of crater formation, wind speed, and distribution of atmospheric movement. It might seem plausible, for instance, that if large craters formed in steady-state over billions of years, some would be deep and others shallow.
Rate expectations: As Titan moves into Saturnian autumn, dramatic changes can be seen in the atmosphere. A press release from University of Bristol reported that the CIRS (Composite InfraRed Spectrometer) team “observed an enormous increase in Titan’s exotic trace gases over the south pole within a relatively short time.” The trace gases are formed by sunlight hitting Titan’s upper atmosphere, converting the methane into ions that recombine into complex gases like benzene and hydrogen cyanide. “[W]e did not expect such a large and rapid change: some gas concentrations increased more than a thousand times within only a few months,”one team member said. “Also surprising was that this was happening at altitudes above 450 km, much higher than initially anticipated.”
Circulation models did not predict activity so high. “This calls into question our current understanding of how Titan’s atmosphere works and suggests new avenues to explore.” Needless to say, understanding that is called into question ceases to qualify as understanding. Any replacement model will also have to explain how this erosive process could have been sustained over Titan’s presumed billions of years, assuming we are not observing it under unique circumstances. Is it surprising that Space.com‘s coverage of this news began with the L-word? “Saturn’s cloudy moon Titan has a middle atmosphere containing organic compounds that could hold the potential for life.”
The surface methane age problem: New papers come out occasionally trying to retrofit the data to falsified predictions that Titan would be found with a global ocean (it wasn’t). One such attempt in Icarus presented a new model for the atmospheric and surface liquid concentrations. They calculated that surface liquids should be dominated by ethane (56%) over methane (32%), with much lower concentrations at the poles. This seems to run counter to the observational evidence, where the only lakes found are at the poles, but the model appears to only discuss ratios, not bulk quantities. Another paper in Icarus proposed a wavelength-dependent model for methane photolysis, but more data is needed, they said.
The atmospheric escape age problem: Another paper in Icarus discussed the rate of thermal escape of hydrogen from Titan’s upper atmosphere, caused by solar wind bombardment of methane. They found that dissipation of hydrogen and methane is similar to Jeans escape, meaning that molecules escape the atmosphere based on their kinetic energy derived from temperature. Jeans escape is also a function of the mass of the molecule, atmospheric density and mass of the planetary body. While the summary did not appear to discuss age issues, the authors said, “A global and temporal description is required to model Titan’s upper atmosphere.”
Does Titan ice float? A curious paper in Icarus asked whether ice would float in Titan’s polar seas. The answer: maybe. Under certain assumptions of porosity and density, methane and ethane ice chunks should float, but then again, they might alternately float and sink. Maybe this will fuel dreams of an epic movie, The Titan-ic.
It glows: We close with a factoid to amuse your friends: Titan glows in the dark. That’s what Astrobiology Magazine and Live Science claim, at least. When charged particles hit the atmosphere, the nitrogen gas lights up like a fluorescent tube. At a millionth of a watt, though, you couldn’t see it, even if floating in a balloon with a very thick Christmas sweater. Contrary to expectations, some of this “airglow” comes from deep down in the atmosphere—too deep to be caused by solar radiation or magnetic fields. Cosmic rays? Chemical reactions? Imaging scientist Robert West remarked, “It tells us that we don’t know all there is to know about Titan and makes it even more mysterious.”
Titan remains a fascinating and enigmatic world, illustrating two common principles of planetary science: (1) Scientists never predict what they find, and (2) Data do not cooperate with long-age assumptions. Everyone can agree that having raw data is much better than listening to groundless speculation.