Titan may host prebiotic brew

Saturn’s moon Titan has an environment that resembles Earth’s at the time that life first got a foothold, new findings from the Cassini spacecraft suggest.

Two close flybys have gathered fresh evidence that ammonia, most likely mixed with water ice, has recently erupted onto the surface of the moon. The likely presence of ammonia on Titan’s icy surface, combined with the abundance of methane and nitrogen in the moon’s thick atmosphere, together suggest that Titan may host a prebiotic brew, says Cassini scientist Robert Nelson of NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

The findings, reported by Nelson August 5 in Rio de Janeiro at a meeting of the International Astronomical Union, are based on data gathered by Cassini’s Visual and Infrared Mapping Spectrometer during November and December 2008. The spectrometer records emissions from seven different infrared “windows” — wavelength bands at which radiation from material on Titan’s surface can penetrate the moon’s methane-rich shroud and reach the spacecraft.

Previously, more distant flybys of Titan had hinted at the presence of fresh ammonia deposits on the surface. In addition, radar imaging with another Cassini instrument in 2008 had revealed that regions that may harbor ammonia, particularly an area known as Hotei Arcus, have lobe-shaped deposits and flows characteristic of volcanic eruptions.

Ammonia is known to lower the melting point of water and facilitate icy volcanic activity. Such activity might be triggered by heat leftover from the formation of Titan or gravitational flexing of the moon, notes Cassini researcher Rosaly Lopes of the Jet Propulsion Laboratory.

Cassini swooped low enough during the November and December flybys for the spectrometer to obtain higher-resolution spectra of material at Hotei Arcus. The spectra resemble the pattern expected from ammonia, but because the emissions also overlap with that of frozen water, the ammonia detection is “likely” rather than definitive, Nelson said. Those two flybys also enabled the spectrometer to image Hotei Arcus closely enough to find the same lobe-shaped flow pattern, indicative of volcanic terrain, that the radar instrument had found, Nelson reported at the conference. The flows imaged in the infrared bands appear truncated compared with those in earlier radar images, hinting that newly deposited ammonia on the surface might have covered the markings, he said.

The new evidence, combined with the older data, suggests that Titan has a geologically active surface, with ongoing icy volcanic eruptions that carry new deposits of ammonia ice to the surface, Nelson said. Once there, the ammonia may mix with methane and nitrogen — the principal constituents of Titan’s dense atmosphere — to create an environment that could foster life.

Other researchers have suggested that ammonia is present in Titan’s interior and might even help sustain a liquid water ocean within the moon, comments Cassini scientist Jonathan Lunine of the University of Arizona in Tucson. Ammonia recently found on geologically active Enceladus, one of Titan’s small sister moons, argues in favor of ammonia on Titan, he adds (SN Online: 7/22/09).

In addition to lowering the freezing point of liquid water, ammonia also lowers the density of the liquid relative to that of water ice, allowing some liquid to more easily rise to the surface, Lunine says. Adding ammonia to water ice would also create a thick viscous surface flow that might explain the shape and topography of the Hotei Arcus region, he notes.

“We’re very excited by the results,” Nelson said. Titan may indeed support a prebiotic mixture of chemicals similar to those under which life on Earth evolved.


Ron Cowen

Web edition : Wednesday, August 5th, 2009


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