The Stuff of Life on Titan


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By John Ruley

For almost thirty years, scientists have known that complex carbon compounds called tholins exist on comets and in the atmospheres of the outer planets. Theoretically, tholins might interact with water in a process called hydrolysis to produce complex molecules similar to those found on the early Earth.

On the Earth, complex organic molecules are believed to have been an early step in the emergence of life; such compounds are called prebiotic.

Titan, the sixth and largest moon of the planet Saturn, is thought to be made largely of ice. Some of that ice may melt during meteor impacts or in underground processes, producing "ice volcanoes" that emit a "lava" containing ammonia mixed with water.

Could tholins formed in Titan's atmosphere react with liquid water temporarily exposed by meteor impacts or ice volcanoes to produce potentially probiotic complex organic molecules - before the water freezes? Until this year, no one knew.

Now, laboratory research by Catherine Neish, a graduate student working on her doctorate in planetary science at the University of Arizona, shows in the journal Astrobiology that, over a period of days, compounds similar to tholins can be hydrolyzed (i.e., react with water) at near-freezing temperatures.

Liquid water exposed on Titan is believed to persist for hundreds to thousands of years - plenty of time for such reactions to take place.

Tantalizingly, it has been suggested that a similar process may have happened on the early Earth.

In her lab, Neish created organic compounds similar to tholins by subjecting a mixture of 5 percent methane and 95 percent nitrogen to electrical discharge at a low temperature (-78 degrees C). She dissolved samples of the resulting material in water, and then, at a range of temperatures from freezing up to 40 degrees C, measured the rate at which the mixture hydrolyzed.

Neish found that up to 10 percent of the organic compounds she began with reacted with oxygen from the water to form complex organic molecules.

While Neish's work was judged worthy of publication in a scientific journal, she has some critics. Dr. James P. Ferris, a research professor at Rensselaer Polytechnic Institute University, who has studied the chemistry of Titan's atmosphere for many years, calls her work "flawed" because she used an electric discharge to generate tholins, while those in Titan's atmosphere are probably generated by ultraviolet (UV) light and charged-particle radiation.

Ferris has conducted experiments on a mixture of gasses similar to Titan's atmosphere using UV light and says, "The structures of the compounds made by [electric] discharge differ from those formed by UV photolysis so the hydrolysis time could be very different. Some of the photochemical products [when UV light is used] are hydrocarbons that do not react with water."

Neish responds that electric, or plasma, discharge "was meant to mimic charged particle interactions (which Dr. Ferris admits is a process at work on Titan)." She agrees that "UV light radiation produces tholins that look more like Titan's haze," but she points out that "some, if not most, of the products we make also don't react with water."

She acknowledges that her work is not an ideal representation of chemistry in Titan's atmosphere: "Tholins formed at low pressure seem to 'look' more like Titan's haze than those formed at higher pressures. You can make tholins at low pressures using UV light; you cannot make tholins at low pressure using plasma discharge. And to make the amount of tholins we needed for the experiment, we needed to use the discharge technique. UV photolysis only produces small amounts."

Ferris, who was not aware of Neish's work until we contacted him, agrees that analyzing the results of hydrolysis on samples produced by UV light would be "more difficult because of the small samples formed."

Another issue is that Neish performed hydrolysis of her tholins in pure water, while any water present on Titan is probably mixed with ammonia. She told us that she recently completed another set of hydrolysis experiments using mixtures of ammonia and water, and expects to publish those results shortly.

While Neish's work is not a perfect representation of chemistry on Saturn's largest moon, it nonetheless suggests that similar processes could produce organic compounds in significant quantities during periods when liquid water is available.

On Titan, this suggests that prebiotic molecules might exist in melt water from impact craters and ice volcanoes. And similar processes might have occurred on the early Earth, before our atmosphere contained significant quantities of free oxygen.

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