Friday, June 01, 2007

The hydrocarbon cycle, and uv radiation harbingers of life.

The classic experiment demonstrating the mechanisms by which inorganic elements could combine to form the precursors of organic chemicals was the 1950 experiment by Stanley Miller. He undertook experiments designed to find out how lightning--reproduced by repeated electric discharges--might have affected the primitive earth atmosphere. He discharged an electric spark into a mixture thought to resemble the primordial composition of the atmosphere.

As we saw here photo disassociation on Titan is from high energy electrons in the UV bands and cosmic radiation.

As we predicted, no lightning discharges were detected in the quiescent Titan atmosphere. Therefore, Titan's atmospheric chemistry is driven mainly by solar UV irradiation and not by electrical discharges. 4. The mixing ratios of the major gas phase species produced by UV photolysis of acetylene, as found experimentally: methylacetylene ; diacetylene ; divinyl ; and benzene were observed by the Cassini spacecraft in Titan's upper atmosphere, with an agreement within better than an order of magnitude.

Being released this week is the first results from a 10 year study and the comparisons of measurements from Cassini-Huygens.

Planetary scientists are a step closer to understanding the composition of the dust in Titan’s atmosphere. A decade-long programme of laboratory studies, aiming to reproduce Titan’s unique dust, or ‘aerosol’ population in specially constructed reactors, has proved invaluable.

Aerosols are small, solid particles that float in the air. On Earth, they are often the result of pollutants in the atmosphere. On Titan, they occur naturally and are abundant in the atmosphere, masking its surface.

Tholins are complex nitrogen-rich substances that form in the laboratory when ultraviolet radiation or electrons react with simpler molecules such as methane and ethane in a surrounding atmosphere of nitrogen. On Titan, the methane and nitrogen-rich atmosphere makes their formation easy and they drift to the surface where they continue to react with other atoms and molecules.

Faced with creating such alien molecules, the French team designed a special reaction chamber to simulate Titan’s atmosphere and produce the tholins for study. “We can generate over 200 chemical species,” says Patrice Coll, a team member at Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), Paris, “We do not yet know the detailed pathways that build the chemicals, but we believe that are very similar to those on Titan.”

The aerosols govern what you can see on Titan. They create Titan’s hazy conditions, revealed by Huygens, and give the moon its dull orange glow. If you could stand on the surface of Titan and magically tune your eyes to infrared light, the haze and the clouds would seem to disappear and Saturn would loom large in the night sky. This is because the aerosols are largely invisible at infrared wavelengths. Change your eyes to ultraviolet, however, and you would be plunged into darkness because, at these wavelengths, the tholins behaves like a thick fog that absorbs all ultraviolet radiation falling on it.

The deposits form when solar ultraviolet radiation and charged particles react at high altitudes with Titan’s abundant methane to produce carbon- and hydrogen-bearing (hydrocarbon) molecules like ethane and acetylene, and more complex nitrogen-bearing molecules generally called tholins. These products drift down to the surface as aerosols much in the same way smog particles on Earth form and coat surfaces. On Titan however these deposits may accumulate to thicknesses of hundreds of metres deep.


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