Date: June 28, 2010
Title: Is There Life on Titan?
Podcaster: Adam Fuller
Description: Earlier this month headlines across the world were aflame with the potential discovery of evidence for life on Titan. While scientists came forward to tap down the hysteria, the real science and discoveries behind the announcement–there is an unexpectedly huge amount of hydrogen flowing towards Titan’s surface and that acetylene and hydrogen are curiously depleted at the surface–was forgotten or ignored. In today’s podcast we’ll into the work behind the hydrogen flux: how it was discovered and what it means for Titan.
Bio: Adam Fuller is a graduate student in the Planetary Sciences department at Johns Hopkins University in Baltimore, Maryland. He graduated from Columbia University in the City of New York with a B.S. in Astrophysics in 2009. He also has a B.A. in Journalism from The University of North Carolina at Chapel Hill. His research interests include planetary science, meteorology, and astrobiology. Outside of school, he is an avid marathoner and a dedicated uncle to three proto-astrophysicists.
Today’s sponsor: “Between the Hayabusa homecoming from Itokawa and the Rosetta flyby of asteroid Lutetia, 13 June until 10 July 2010, this episode of 365 Days of Astronomy is sponsored anonymously and dedicated to the memory of Annie Cameron, designer of the Tryphena Sun Wheel, Great Barrier Island, New Zealand, a project that remains to be started.”
Transcript:
Is there life on Titan? Are there tiny hydrogen-eating microbial bugs scouring Titan’s bleak surface, belching out methane like the microbes in our own guts? After a NASA press release that came out three and a half weeks ago, many media outlets screamed increasingly sensationalist headlines like: “Strange find on Titan sparks chatter about life,” “Titan’s atmosphere oddity consistent with methane-based life,” and “NASA scientists discover evidence ‘that alien life exists on Saturn’s moon.’”
Scientists and science bloggers battled back, hoping to defuse the mania. Cassini imaging team leader Carolyn Porco used twitter to say, “Everyone: Calm down!” Phil Plait, the Bad Astronomy Blogger, upbraided the media for publishing seriously misleading headlines. And one of the co-authors of a 2005 astrobiology paper about life on Titan, Chris McKay, published an essay on a Cassini website that said despite these results, “this is still a long way from evidence of life.”
Lost in all of this was the real science behind the original announcement. The June 3rd press release was about the results from two new research papers, those results being: 1) there’s a lot less acetylene on Titan’s surface than predicted, and 2) there’s an unexpectedly high flux of molecular hydrogen flowing downward through the atmosphere that isn’t seen on Titan’s surface. These are interesting results by themselves, because they point to chemical processes occurring in Titan’s atmosphere and on its surface that we don’t understand yet. Unfortunately, the press release, entitled “What is consuming hydrogen and acetylene on Titan,” focused on just one particular process, out of the many others, as a possible explanation: the consumption of acetylene and hydrogen by methane-based life on Titan’s surface. This explanation was based on that 2005 paper co-authored by McKay that predicted that depletions of acetylene, ethane, and hydrogen could be a sign of methane-based life on Titan.
Hi, my name is Adam Fuller, and I’m a grad student in the Planetary Sciences department at Johns Hopkins University in Baltimore, Maryland. In today’s 365 Days of Astronomy podcast I’m going to discuss one of the two research papers in question, namely “Molecular hydrogen in Titan’s atmosphere: implications of the measured tropospheric and thermospheric mole fractions.” It’s by my advisor, Darrell Strobel, a planetary atmospheres and astrophysics professor in our department here. He’s also an interdisciplinary scientist on the Cassini mission and a co-investigator on the New Horizons Pluto/Kuiper Belt Mission that should reach Pluto in five years. As McKay rightly points out, Strobel’s paper is really about fitting measurements of the hydrogen mole fraction in different parts of Titan’s atmosphere with chemical models that, to the best of our current knowledge, simulate Titan’s upper atmospheric chemistry.
The amount of molecular hydrogen on Titan has been calculated using direct and indirect measurements of the atmosphere. Two instruments that provide indirect remote measurements are Voyager 1’s infrared spectrometer (IRIS) and Cassini’s Composite Infrared Spectrometer (CIRS). Voyager 1 flew by Titan in 1980 while Cassini is currently in orbit around Saturn. The data gathered by these instruments allow us to determine the amount of hydrogen in Titan’s tropopause and troposphere. These regions are the part of the lower atmosphere that extends from the surface up to roughly 45 miles.
The Huygens probe, which dropped to the surface of Titan in 2005, used its Gas Chromatograph Mass Spectrometer (GCMS) to directly sample Titan’s atmosphere starting as high as 80 miles above the surface. As the probe descended, the GCMS slurped up the surrounding air, tested its composition, and then flushed it out before sampling another batch of air. Its measurements of the molecular hydrogen are complicated, however, by the fact that the gas chromatograph part came with its own supply of hydrogen as a carrier gas. This was used to flush the old air sample out before taking in a new sample. So how do we know if the hydrogen measured by the GCMS is from Titan or Earth? Five years after Huygens’s descent, scientists on the GCMS team have painstakingly sorted this out. Their results, which should be released in the coming months, corroborate the findings from IRIS and CIRS.
Data from a fourth instrument, Cassini’s Ion Neutral Mass Spectrometer (INMS), has been used to measure the molecular hydrogen in Titan’s thermosphere, 600 miles and higher above the surface. Similar to the GCMS, it directly samples the Titan atmosphere during flybys. The INMS, however, measured a higher mole fraction of hydrogen in the thermosphere than the other instruments did for the troposphere–more than three times as much!
The question then becomes, “If these measured hydrogen mole fractions are right, what does this tell us about Titan?” Can the different data sets be reconciled with each other? Is there really more hydrogen in the thermosphere than on the surface? Answering these questions was the goal of Professor Strobel’s recent paper.
To begin, solar UV photodissociates methane directly and indirectly into hydrogen and many different hydrocarbons. Titan’s gravity is strong enough to hold onto the hydrocarbons, but some of the hydrogen escapes into space. Prior work by Strobel calculated that there’s a tremendous amount of escaping hydrogen: 10^28 hydrogen molecules per second. That’s more hydrogen than what was originally expected.
The hydrogen that doesn’t escape is retained by Titan and should flow down to the surface. Strobel calculated the rate of the flow using the hydrogen mole fractions measured by the INMS and CIRS instruments. That is, we know how much hydrogen there is 600 miles and 45 miles above the surface, so we can determine how much hydrogen is flowing in both directions: upward to space and downward to the surface. His result was that there is as much hydrogen flowing downward as there is escaping: 10^28 hydrogen molecules per second!
To account for all this hydrogen escaping into space and flowing down to the surface, roughly twice as much hydrogen must be produced as originally thought as well as twice as much methane must be destroyed. And this destruction, which is caused by photodisassociation, is irreversible. Methane can’t be broken apart this way and then reform on its own. Free hydrogen and heavier hydrocarbons like acetylene, ethylene, and ethane should be the main products of methane photolysis. According to Strobel, this result, that the methane dissociation rate is doubled to account for the high hydrogen fluxes, is more surprising than the high fluxes themselves.
In a nutshell, these are the major points of the paper. If the constraints on the hydrogen mole fraction in Titan’s atmosphere are correct, then there should be a downward flux of hydrogen to the surface that matches the escaping flux to space. And if this is the case, then there should also be a much higher amount of methane being irreversibly destroyed in the stratosphere and upper atmosphere than current models predict. So why is there less hydrogen at the surface than in the thermosphere? And if so much methane is being destroyed, where is the new methane coming from? Just like the other paper that discovered there was much less acetylene on the surface than previously thought, Strobel gives several explanations for these questions, some more likely than others. And from all of these hypotheses, the one paragraph in his nine-page research paper that mentioned the paper McKay co-authored five years ago is what, unfortunately, got all the media attention. As Phil Plait said in his post about this controversy, “when it comes to media outlets and big news like this, … don’t trust, and verify.”
So, is there life on Titan? In his response essay to the media delirium, McKay offered four explanations about what’s going on at Titan’s surface. The least likely is that life really is metabolizing acetylene, ethane, and hydrogen for energy. Another possibility is the downward flowing hydrogen is being absorbed into organic particles in the lower atmosphere that then precipitate onto the surface. There could also be unidentified compounds on the surface that are triggering chemical reactions that normally wouldn’t occur at Titan’s frigid surface temperature of -289 degrees Fahrenheit. But, according to McKay, the most likely explanation is that there really isn’t a strong downward flux of hydrogen in Titan’s atmosphere. Regardless of who’s right about the downward flux, though, it’s clear from McKay and the two new research papers that there are many much more likely reasons for the depletion of hydrogen, acetylene, and ethane on Titan’s surface.
I hope you’ve found this podcast informative. If you’d like to read more, I’ve included several references in the transcript. I’d also like to thank Professor Strobel for taking time out of his busy summer schedule to answer my questions. In the meantime, have a great day and keep listening.
References:
Clark et al. Detection and Mapping of Hydrocarbon Deposits on Titan. Journal of Geophysical Research (2010)
Kovács and Turányi. Chemical reactions in the Titan’s troposphere during lightning. Icarus (2010) vol. 207 pp. 938
McKay and Smith. Possibilities for methanogenic life in liquid methane on the surface of Titan. Icarus (2005) vol. 178 pp. 274
Niemann et al. The abundances of constituents of Titan’s atmosphere from the GCMS instrument on the Huygens probe. Nature (2005) vol. 438 pp. 779
Schulze-Makuch and Grinspoon. Biologically Enhanced Energy and Carbon Cycling on Titan?. Astrobiology (2005) vol. 5 pp. 560
Strobel. Titan’s hydrodynamically escaping atmosphere. Icarus (2008)
Strobel. Titan’s hydrodynamically escaping atmosphere: Escape rates and the structure of the exobase region. Icarus (2009)
Strobel. Molecular hydrogen in Titan’s atmosphere: Implications of the measured tropospheric and thermospheric mole fractions. Icarus (2010)
Waite et al. Ion Neutral Mass Spectrometer Results from the First Flyby of Titan. Science (2005) vol. 308 pp. 982
“Has life on Titan been discovered? No.” http://blogs.discovermagazine.com/badastronomy/2010/06/07/has-life-on-titan-been-discovered-no/
“Nasa scientists discover evidence ‘that alien life exists on Saturn’s moon.’” http://www.telegraph.co.uk/science/space/7805069/Titan-Nasa-scientists-discover-evidence-that-alien-life-exists-on-Saturns-moon.html
“Strange find on Titan sparks chatter about life.” http://www.msnbc.msn.com/id/37556728/ns/technology_and_science-space/
“Titan’s atmosphere oddity consistent with methane-based life.” http://arstechnica.com/science/news/2010/06/why-is-the-hydrogen-exiting-titans-atmosphere.ars
End of podcast:
365 Days of Astronomy
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