Planetary Science

Simulating Titan's Atmosphere

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Science  19 Apr 2013:
Vol. 340, Issue 6130, pp. 251
DOI: 10.1126/science.340.6130.251-c

Saturn's largest moon, Titan, has a thick atmosphere dominated by nitrogen (N2) and methane (CH4); the photochemistry of these two gases in the upper part of the atmosphere, initiated by solar wind particles and solar ultraviolet radiation, produces organics, including polymeric compounds called tholins that are thought to give Titan its distinctive orange-yellow color. Laboratory work by Gudipati et al. shows that tholin formation can continue at low altitudes, down to 200 km from the surface and below, through longer-wavelength photochemistry in condensed organic aerosols. They synthetized dicyanoacetylene (C4N2), a compound that has been detected on Titan and can be used as a model system for other larger unsaturated condensing compounds, cooled it to 100 K, a temperature relevant to Titan's lower atmosphere, and irradiated it with laser-produced light at 532, 355, and 266 nm. Solar light at wavelengths longer than 200 nm is expected to penetrate through Titan's atmosphere and make it to low altitudes without much attenuation. In the simulations, irradiation of C4N2 ice with light at wavelengths as long as 355 nm induced the formation of tholins. These organic compounds, which do not occur naturally on Earth, can thus be produced on Titan by different photochemical processes at different atmospheric altitudes.

CREDIT: NASA/JPL-CALTECH/SPACE SCIENCE INSTITUTE

Nat. Comm. 4, 1648 (2013).

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