Emily Lakdawalla • Mar 21, 2018
#LPSC2018: Titan Is Terrific!
The Lunar and Planetary Science Conference (LPSC) is always madness. Once upon a time, I was able to dash off numerous blog posts in teeny bits of time snatched between sessions, but that was a younger me. I think it was also a less busy time in planetary science, with fewer missions. Now there’s an embarrassment of riches and I hardly know where to begin. So I’ll spin the wheel of possible LPSC session topics, and: Titan! I’ll talk about Saturn’s moon Titan.
There was a full session devoted to Cassini on Monday (Lisa Grossman wrote up a nice summary here), but then on Tuesday, another morning session focused entirely on Titan, called “Titan Is Terrific”. (That link will take you to a PDF program for the session, with links to two-page abstracts on all the talks.) Project scientist Linda Spilker told me that the organizing committee received so many Titan submissions that the mission asked the meeting to create a second session devoted to the complex moon. I found three of the talks to be particularly thought-provoking.
It kicked off with Chris Glein talking about how a nasty gas – hydrogen cyanide (HCN) – behaves in Titan’s atmosphere, and what that might mean for the search for the fossil record of the beginnings of life on Earth. “Something strange and wonderful is happening with nitrogen fractionation in HCN,” he said. His standards for “wonderful” might be different from yours, but what he was talking about is that at Titan, the heavier isotope of nitrogen, nitrogen-15, is far more abundant in HCN than it is in the much more common nitrogen-carrying atmospheric gas, regular old molecular nitrogen (N2). The reasons I won’t go into because I don’t want to get sidetracked. Glein’s point is that the same process could very well have happened on the very earliest Earth’s atmosphere. And HCN is an incredibly important precursor molecule to sticking nitrogen into prebiotic compounds like amino acids. So any incredibly early life would likely have been drawing this highly fractionated nitrogen from HCN in the atmosphere. The organic materials from 4-billion-year-old organisms, if they existed, has long ago been turned into graphite or other inorganic-looking materials in rocks. But we could go looking at the nitrogen in those rocks and see what the isotopic ratios are. If there’s an excess of nitrogen-15, it could’ve come from HCN in Earth’s atmosphere, processed by early life into proteins. [Abstract 2812]
Then Morgan Cable gave a talk about “Molecular Minerals on Titan.” Titan is a place where the atmosphere creates large quantities of small organic molecules like ethane, acetylene, propane, and so on. These fall out onto the surface with methane and ethane rain, and some of them accumulate in lakes. Around the edges of lakes scientists have observed “bathtub rings”, bright-colored haloes around the lakes. We don’t know for sure but it’s possible that these are made of materials deposited as lakes evaporate, leaving behind evaporite deposits, like salts form around evaporating lakes on Earth. The materials that form evaporites will be the least soluble materials. In a Titan lake, Cable said, the first thing you’d expect to fall out of solution would be benzene. But it wouldn’t just be benzene; when benzene forms crystals, it readily incorporates ethane into its crystal structure, forming a “co-crystal”. If it’s a crystal and it forms solid deposits, what you’ve got is a mineral – and the beginning of the science of Titan petrology. Cable said they looked around for other small molecules and found that acetylene readily forms lots of different co-crystals, notably with ammonia. They formed their co-crystals in the lab and tried wetting them (raining on it) with a mix of methane and ethane, and the solids stayed – suggesting they’d persist as solid deposits on Titan. One unusual thing about these minerals is that when they warm, they experience a lot of thermal expansion. So if you formed this material on Titan’s surface, and then buried it (either by subduction or just by covering it with other stuff), it could expand as it warmed with depth, causing stresses that might produce physical evidence on the surface. [Abstract #2717]
If you like Titan petrology, how about Titan seismology? Mark Panning’s talk was about that, motivated by the Dragonfly mission concept and its plans for a seismic instrument. Seismic data (recordings of ground motion caused by earthquake waves) can tell scientists about the internal structure of a world regardless of what it’s made of. Titan would have regular icequakes caused by the flexing of its crust due to tides with every Saturn orbit. When kids are taught about seismology, they learn about P and S and surface waves, and Titan would have all of those. But because it has an ice shell over a liquid ocean, there would also be some other interesting kinds of lower-frequency waves moving through the ice shell. For example, there’s a flexural wave, in which the whole ice shell bends back and forth slowly. There are Crary waves, which are waves trapped within the ice shell whose frequency is sensitive to the thickness of the ice shell. Panning used Apollo seismic data for the Moon (which also has quakes caused mostly by tidal forces) to make some predictions about how common quakes of different sizes would be on Titan. Then he asked questions like: how sensitive does the seismic instrument need to be to detect several quakes over the course of the mission? (The answer: medium-sensitive compared to off-the-shelf Earth seismometers.) How bad is atmospheric noise? (The answer: wind will be pretty noisy, but the bigger events should still be detectable.) Would waves on lakes cause noise? (The answer: they can, but if the instrument is close to the equator, it won’t be an issue.) And: is there any spot on Titan that’s more likely than anywhere else to have large quakes? (The answer: they’re expected to be fairly uniformly distributed, except that the sub-Saturn and anti-Saturn points have less quakes than everywhere else, so aiming near the leading or trailing points would be best for seismic studies.) [Abstract #1662]
There were a lot more cool talks on Titan landscape features: dissected domes! New impact craters! Ridges around lakes! --and if you're dying to read more, check out the session abstracts.
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