Complex light-induced chemistry on Titan


Image credit: Jet Propulsion Laboratory

Scientists have known since NASA’s Voyager mission flew by the Saturn system in the early 1980s that Titan, Saturn’s largest moon, has a thick, hazy atmosphere with hydrocarbons, including methane and ethane. These simple organic molecules can develop into smog-like, airborne molecules with carbon-nitrogen-hydrogen bonds, which astronomer Carl Sagan called “tholins.”

“We’ve known that Titan’s upper atmosphere is hospitable to the formation of complex organic molecules,” said co-author Mark Allen, principal investigator of the JPL Titan team that is a part of the NASA Astrobiology Institute, headquartered at Ames Research Center, Moffett Field, Calif. “Now we know that sunlight in the Titan lower atmosphere can kick-start more complex organic chemistry in liquids and solids rather than just in gases.”

The team examined an ice form of dicyanoacetylene, a molecule detected on Titan that is related to a compound that turned brown after being exposed to ambient light in Allen’s lab 40 years ago. In this latest experiment, dicyanoacetylene was exposed to laser light at wavelengths as long as 355 nanometers. Light of that wavelength can filter down to Titan’s lower atmosphere at a modest intensity. The result was the formation of a brownish haze between the two panes of glass containing the experiment, confirming that organic-ice photochemistry at conditions like Titan’s lower atmosphere could produce tholins.

355 nanometers is near-ultraviolet (UVA) radiation, which is energetic enough to drive the photochemistry.
pia14909-jet-propulsion-laboratoryJPL News release 2 April 2013

Update: Cassini Data Confirms PAHs Play Major Role in Production of Lower Haze on TitanPAHs-Play-a-Major-Role-in-Production-of-Haze-on-Titan PAHs detected in the atmosphere of Titan are the first step in a sequence of increasingly larger compounds. Models show how PAHs can coagulate and form large aggregates, which tend to sink, due to their greater weight, into the lower atmospheric layers. The higher densities in Titan’s lower atmosphere favor the further growth of these large conglomerates of atoms and molecules. These reactions eventually lead to the production of carbon-based aerosols, large aggregates of atoms and molecules that are found in the lower layers of the haze that enshrouds Titan, well below about 300 miles (500 kilometers). Image credit: ESA/ATG medialab


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