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Published April 10, 2004 | Published + Submitted
Journal Article Open

On the Insignificance of Photochemical Hydrocarbon Aerosols in the Atmospheres of Close-in Extrasolar Giant Planets


The close-in extrasolar giant planets (CEGPs) reside in irradiated environments much more intense than that of the giant planets in our solar system. The high UV irradiance strongly influences their photochemistry, and the general current view believed that this high UV flux will greatly enhance photochemical production of hydrocarbon aerosols. In this Letter, we investigate hydrocarbon aerosol formation in the atmospheres of CEGPs. We find that the abundances of hydrocarbons in the atmospheres of CEGPs are significantly less than that of Jupiter except for models in which the CH4 abundance is unreasonably high (as high as CO) for the hot (effective temperatures ≳1000 K) atmospheres. Moreover, the hydrocarbons will be condensed out to form aerosols only when the temperature-pressure profiles of the species intersect with the saturation profiles—a case almost certainly not realized in the hot CEGPs' atmospheres. Hence our models show that photochemical hydrocarbon aerosols are insignificant in the atmospheres of CEGPs. In contrast, Jupiter and Saturn have a much higher abundance of hydrocarbon aerosols in their atmospheres that are responsible for strong absorption shortward of 600 nm. Thus the insignificance of photochemical hydrocarbon aerosols in the atmospheres of CEGPs rules out one class of models with low albedos and featureless spectra shortward of 600 nm.

Additional Information

© 2004 American Astronomical Society. Received 2003 November 20; accepted 2004 February 24; published 2004 March 16. We thank M. Gerstell, J. McConnell, and R. L. Shia for helpful discussions. We thank the anonymous referee for constructive comments. The support of NASA grant NAG5-6263 to the California Institute of Technology is gratefully acknowledged. This material is also based on work supported by the National Aeronautics and Space Administration through the NASA Astrobiology Institute under Cooperative Agreement CAN-00-OSS-01 and NCC 2-1056 and issued through the Office of Space Science. S. S. is supported by the Carnegie Institution of Washington and by NASA Origins grant NAG5-13478.

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Published - 1538-4357_605_1_L61.pdf

Submitted - 0402601v1.pdf


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