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Science 11 May 2007:
Vol. 316. no. 5826, pp. 870 - 875
DOI: 10.1126/science.1139727
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Reports

The Process of Tholin Formation in Titan's Upper Atmosphere

J. H. Waite, Jr.,1* D. T. Young,1 T. E. Cravens,2 A. J. Coates,3 F. J. Crary,1 B. Magee,1* J. Westlake4

Titan's lower atmosphere has long been known to harbor organic aerosols (tholins) presumed to have been formed from simple molecules, such as methane and nitrogen (CH4 and N2). Up to now, it has been assumed that tholins were formed at altitudes of several hundred kilometers by processes as yet unobserved. Using measurements from a combination of mass/charge and energy/charge spectrometers on the Cassini spacecraft, we have obtained evidence for tholin formation at high altitudes (~1000 kilometers) in Titan's atmosphere. The observed chemical mix strongly implies a series of chemical reactions and physical processes that lead from simple molecules (CH4 and N2) to larger, more complex molecules (80 to 350 daltons) to negatively charged massive molecules (~8000 daltons), which we identify as tholins. That the process involves massive negatively charged molecules and aerosols is completely unexpected.

1 Space Science and Engineering Division, Southwest Research Institute (SWRI), 6220 Culebra Road, San Antonio, TX 78238, USA.
2 Department of Physics and Astronomy, University of Kansas, Lawrence, KS 66045, USA.
3 Department of Space and Climate Physics, Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking RH5 6NT, UK.
4 Department of Physics and Astronomy, University of Texas, San Antonio, TX 78249, USA.

* To whom correspondence should be addressed. E-mail: hwaite{at}swri.edu (J.H.W.); bmagee{at}swri.edu (B.M.)

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