Why astrobiologists look to Saturn's moon Titan
Posted: December 12, 2003

Titan, Saturn's largest moon, is the best place in the solar system to study primordial soup -- the stuff from which life emerged.

In January 2005, planetary scientists will get a closer look at Titan's version of primordial soup when the European Space Agency's Huygens probe floats to the surface. The probe currently is riding aboard NASA's Cassini spacecraft, which will reach Saturn next July.

Landing on an ocean would be better than on a solid surface. Credit: ESA
"Cassini/Huygens will tell us all about the organics in the atmosphere, the stuff that's photochemically processed by sunlight and falls to the surface," said University of Arizona planetary sciences Professor Jonathan I. Lunine. "But what it won't be quite so good at is telling us what goes on at the surface."

Lunine is one of the three interdisciplinary scientists on the Cassini/Huygens mission, and heads NASA's Astrobiology Institute focus group on Titan. This group includes scientists who want to learn more about the organic chemistry of Titan's environments.

In addition, Lunine and Mark Smith collaborate with Caltech and NASA Jet Propulsion Lab scientists and engineers in designing an organic chemistry laboratory that could be deployed to Titan's surface. Smith is head of UA's chemistry department.

"We really don't know how life formed on the Earth, or on whatever planet it formed," Lunine said. "Because we are organic, with carbon and hydrogen, we want to know more about organic molecules. How do molecules change chemically into biomolecules in an environment that is not conducive to life? There are no traces left of how it happened on Earth because all of Earth's organic molecules have been processed biochemically by now. Titan is our best chance to study organic chemistry in a planetary environment occurring in the absence of life over billions of years."

Scientists have focused much of their attention on Titan's thick atmosphere, which is four times denser than Earth's atmosphere at sea level. Like Earth, and unlike Mars and Venus, Titan has mostly a nitrogen atmosphere. But because Titan is 10 times farther from the sun than Earth, surface temperatures hover around minus 290 degrees Fahrenheit. That's too cold for water vapor, even though the planet-like moon is half rock and half water.

Titan's primarily nitrogen atmosphere contains methane. Methane reacts with ultraviolet sunlight, forming organic compounds that flake from the sky. The thick atmosphere protects the hydrocarbons and other organic solids that settle on the surface from being destroyed by damaging particle and ultraviolet radiation. Some of these organic molecules are thought to have been important in Earth's prebiotic environment.

"Cassini-Hugygens will be the stepping stone that will tell us where the organics are located on the surface, and it will tell us whether there are differences in organics that have been deposited in craters or in regions that seem to have been resurfaced by cyrovolcanism." Cryovolcanism is volcanism where the fluid is not molten rock, but liquid water that possibly includes ammonia and other antifreezes. "These are interesting places because liquid water might have been briefly present and modified the organics in some way."

Last year, Lunine and Natalia Artemieva of the Institute for Geospheres in Moscow published a paper on how much water ice in Titan would melt if the moon were hit by an asteroid or other object, and how that would affect organic materials on Titan's surface.

If astrobiologists don't get some answers from Titan, they'll have to hunt for them in other solar systems.

"Our solar system can be a frustrating place," Lunine said. "There are only nine planets -- some people say eight because they exclude Pluto -- and there are only 60 moons, not 6,000. And of those 60 moons and 8 or 9 planets, there's only one place without life that has abundant organic chemistry, energy sources, a solid surface where organic molecules can actually survive and be processed, sometimes along with liquid water. And that place is Titan."