Date: December 9th, 2012
Title: Encore: Life on Titan
Podcaster: Chris Impey
Organization: University of Arizona
This podcast has been aired on 31st January, 2011
Description: Titan may be the most fascinating destination in the Solar System. After describing Saturn’s large moon, the evidence for liquid lakes, volcanism, and life in discussed. Life on Titan, if it exists, would probably be completely unlike terrestrial life and so be Life 2.0.
Bio: Chris Impey is a University Distinguished Professor and Deputy Head of the Department, in charge of all academic programs. His research interests are observational cosmology, gravitational lensing, and the evolution and structure of galaxies. He has 160 refereed publications and 60 conference proceedings, and his work has been supported by $20 million in grants from NASA and the NSF. As a professor, he has won eleven teaching awards, and he has been heavily involved in curriculum and instructional technology development. Impey is a past Vice President of the American Astronomical Society. He has also been an NSF Distinguished Teaching Scholar, a Phi Beta Kappa Visiting Scholar, and the Carnegie Council on Teaching’s Arizona Professor of the Year. Impey has written over thirty popular articles on cosmology and astrobiology and co-authored two introductory textbooks. His first popular book “The Living Cosmos,” was published in 2007 by Random House, and his second, called “How It Ends,” was published in 2010 by Norton. He was a co-chair of the Education and Public Outreach Study Group for the Astronomy Decadal Survey of the National Academy of Sciences. In 2009 he was elected a Fellow of the American Association for the Advancement of Science.
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365 Days – Jan 31, 2011 – Life on Titan
Hello. I want to talk about Titan today, possibly the most fascinating world in the solar system. In particular, about the possibility of life on Titan. But first a little bit about it. Titan is the largest moon of Saturn and the second largest moon in the solar system eclipsed only by Jupiter’s moon Ganymede. Before the Voyager encounters in the early 1980s, astronomers suspected that Titan might have an atmosphere. They also wondered if there would be liquid seas or pools of ethane or methane. Water would have been frozen due to the low surface temperature. Expecting an unusual and interesting world, Voyager I was programmed to take numerous close views of Titan when it flew past in November 1980. But all it could see was an impenetrable layer of atmosphere and clouds.
Although Titan is a moon, it’s larger than the planet Mercury and the former planet Pluto. It has an atmosphere that’s denser than Earth and Mars. Moons are “worlds” in their own right—just as interesting as planets. That’s one of the realizations of the past few decades in planetary science. The atmospheric pressure near the surface of Titan is 1.6 bars which is 60% greater than the Earth’s. And the air is like the air we breathe, made primarily of nitrogen with other hydrocarbon elements. The only thing missing is oxygen. Titan’s surface temperature is a frigid -180 degrees C, that’s -290 degrees F.
It was known for a long time that some complex and interesting biochemistry or photochemistry might be seen on Titan. The methane cycles through ethane, acetylene and ethylene and combines with hydrogen cyanide, the last of which is an important molecule since it’s a building block of amino acids. Proof of liquids on the surface of Titan took a while to gather. Voyager I and II data showed that there was a thick atmosphere with about right composition and temperature to support liquids but there was no direct evidence. When Cassini arrived at the Saturn system in 2004, it started looking hard for liquid bodies. At Titan’s south pole there was a very strange and mysterious dark feature called Ontario Lacus, which was later confirmed to be a lake, and a possible shore line was seen at the pole using radar imagery. In the northern latitudes, the spacecraft radar saw a number of large, smooth and dark patches dotting the surface.
Scientists in 2007 announced definitive evidence of lakes filled with methane on Titan. Cassini/Huygens concluded that the features are almost certainly hydrocarbon lakes. These are the first stable bodies of surface liquid found anywhere other than the Earth, and it was a very exciting result. In 2008, liquid ethane was proved beyond a doubt in Ontario Lacus. And actually the radar gives much more detailed information than that. Radar shows that the lake levels did not vary by more than a couple of millimeters, which implies that either the surface winds were very small or that the lake was very viscous. We get a sense of the depths of the lake from the radar imaging. Ontario Lacus you could wade into. It has a median depth of about one or two meters and a maximum depth of only three or four meters. However there are lakes in the northern hemisphere that are deeper than seven meters – the maximum depth you can measure with radar imaging.
Another aspect of Titan’s behavior that was put into focus by the Cassini mission was something called cryovolcanism. Scientists have long speculated that the conditions on Titan are just like those of the early Earth, although at a much lower temperature. The detection of Argon 40 in the atmosphere in 2004 shows that the volcanoes spew plumes of lava made of water and ammonia. And now we have global maps of the lake distribution on Titan’s surface which show there is not enough surface methane to account for its presence in the atmosphere so a portion must be added through volcanic processes.
That’s just inference; direct proof of volcanism is hard to find. In 2008, astronomers saw two transient bright spots in Titan’s atmosphere which seemed too persistent to be explained by weather patterns and that could have been volcanism. And a year later structures resembling lava flows were found in a hilly region of Titan which also changed in brightness over seven months. These lava flows rose 200 meters above the surface, consistent with eruptions from below the surface.
Last year, the Cassini team found the most compelling cryovolcanic yet, called Sotra Facula, part of a chain of three mountains about a thousand meters in height topped by large craters. The ground around their bases seems to be overlaid by frozen lava flows. Volcanism on Titan is different than volcanism on the Earth. It is driven by the energy release by the decay of radioactive elements, but it’s assisted by tidal flexing from the huge nearby planet Saturn. Titan is resurfaced in a process by which grain-sized ice and ash of ammonium sulfate produce a wind-shaped landscape and sand dune features.
On to life. One of the most interesting features of the last decade or so is the idea that the “habitable zone” may be much more extensive than previously thought. Traditionally the idea was of Goldilocks Zone where a planet would have to be within a range that would allow liquid water to be on its surface. Earth is the only planet or body of any kind in that habitable zone. The Moon of course has no atmosphere and can’t support life. A realization of the last decade however, is of a cryogenic habitable zone that is the region far from the Sun where water would be frozen, but it could still support life because energy sources are available. The energy comes from within planet and moon interiors and from gravitational flexing by the host planet if it’s a moon. So moons are just as interesting as planets in terms of looking for life.
An argument can be made that Titan is the most important destination to look for life. Mars has a small atmosphere is cold and arid, and it could in fact be dead. Europa, near Jupiter, is a very interesting water world. But Titan, if life exists there, might not be like life on Earth, and would be an example of Life 2.0 or independent origination of biology.
Cassini/Huygens was not equipped to look for biology or complex organics. But it did show an environment on Titan that’s very similar to the environment theorized for the primordial, with the important exception of the lack of water vapor. The Miller-Urey experiment, “Life in a Bottle”, and several following experiments have shown that an atmosphere similar to that of Titan and with the addition of UV radiation can generate complex molecules and polymer-like substances. The reaction starts with a disassociation of nitrogen and methane, forming hydrogen cyanide and acetylene. These are extremely toxic chemicals on the Earth and not consistent with life. But life on Titan, if it exists, will not be like life on Earth.
Just last year Sarah Horst, who is a colleague at the University of Arizona, found five nucleotide bases, the building blocks of RNA and DNA, amongst many compounds produced in the lab when energy was applied to a combination of gases like that in Titan’s atmosphere. She also found amino acids, the building blocks of protein. This is the first time nucleotide bases and amino acids had been found in any Miller-Urey type experiment without liquid water being present. These lab simulations have led to the suggestion that enough organic material exists on Titan to start biochemical evolution, analogous to what would have started life on Earth.
The analogy assumes the presence of liquid water for longer periods than is currently observable. So there does need to be a source of water. This could actually happen under the surface. One model suggests that a ammonia-water solution might exist 100 or 200 kilometer to two kilometers deep below a water-ice crust. In these conditions, life similar but not quite like that on Earth could persist. To detect microbial life we’ll have to actually have to look for its biogenic effects. Just as with Mars we look for methane and nitrogen that might have a biological origin.
Astrobiologist Chris McKay suggests that life could exist in the lakes of liquid methane on Titan just as organisms on Earth live in water. Such creatures would inhale and metabolize hydrogen in place of oxygen. They would react it with acetylene instead of glucose, and exhale methane instead of carbon dioxide. That’s a stunningly different form of life. Tangible but tantalizing evidence for this form of life was found last year by researchers at Johns Hopkins University, in the form of an overabundance of molecular hydrogen in the upper atmosphere of Titan leading to a downward flow towards the surface. Near the surface hydrogen apparently disappears which implies its consumption by a methanogenic life form.
Another paper released in the same month showed little evidence of acetylene on Titan’s the surface, where scientists expected the compound to accumulate. This is also consistent with McKay’s hypothesis that acetylene is being consumed by a methanogenic life form. Chris McKay however has cautioned that there are other possible explanations. So this evidence for life is not a slam-dunk and is not conclusive enough to convince most scientists. We badly need more information.
Which leads to the question of when are we going to go back? Remember that the Huygens Lander delivered a bare 30 minutes of data before giving up the ghost. NASA and ESA have a joint proposal, the Titan–Saturn Mission System (TSSM), to put a hot air balloon in the moon’s atmosphere to float for six months and sample the atmosphere and dip down into the lakes to sample them. Unfortunately, two years ago it lost a competition against the Europa-Jupiter system mission so it’s behind it in the queue and at least 15 years off. There’s also a proposal for a Titan Mare explorer, which would be a low cost Lander to splash down in a lake near Titan’s north pole and float on the surface for 16 Earth days. It could possibly launch as early as 2015 and arrive in 2022 or 2023. It would have the possibility of detecting life. For people hungry to find life in the solar system, that’s a long wait. It looks like Titan will be tantalizing us for awhile longer.
This has been Chris Impey at the University of Arizona, talking about “Life on Titan” for 365 Days of Astronomy. Goodbye.
End of podcast:
365 Days of Astronomy
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