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Hydrocarbon Nucleation and Aerosol Formation in Neptune's Atmosphere

Moses, Julianne I. and Allen, Mark and Yung, Yuk L. (1992) Hydrocarbon Nucleation and Aerosol Formation in Neptune's Atmosphere. Icarus, 99 (2). pp. 318-346. ISSN 0019-1035. doi:10.1016/0019-1035(92)90149-2.

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Photodissociation of methane at high altitude levels in Neptune's atmosphere leads to the production of complex hydrocarbon species such as acetylene (C_2H_2), ethane (C_2H_6), methylacetylene (CH_3C_2H), propane (C_3H_8), diacetylene (C_4H_2), and butane (C_4H_8). These gases diffuse to the lower stratosphere where temperatures are low enough to initiate condensation. Particle formation may not occur readily, however, as the vapor species become supersaturated. We present a theoretical analysis of particle formation mechanisms at conditions relevant to Neptune's troposphere and stratosphere and show that hydrocarbon nucleation is very inefficient under Neptunian conditions: saturation ratios much greater than unity are required for aerosol formation by either homogeneous, heterogeneous, or ion-induced nucleation. Homogeneous nucleation will not be important for any of the hydrocarbon species considered; however, both heterogeneous and ion-induced nucleation should be possible on Neptune for most of the above species. The relative effectiveness of heterogeneous and ion-induced nucleation depends on the physical and thermodynamic properties of the particular species, the abundance of the condensable species, the temperature at which the vapor becomes supersaturated, and the number and type of condensation nuclei or ions available. Typical saturation ratios required for observable particle formation rates on Neptune range from ∼3 for heterogeneous nucleation of methane in the upper troposphere to greater than 1000 for heterogeneous nucleation of methylacetylene, diacetylene, and butane in the lower stratosphere. Thus, methane clouds may form slightly above, and stratospheric hazes far below, their saturation levels. When used in conjunction with the results of detailed models of atmospheric photochemistry, our nucleation models place realistic constraints on the altitude levels at which we expect hydrocarbon hazes or clouds to form on Neptune.

Item Type:Article
Related URLs:
URLURL TypeDescription
Moses, Julianne I.0000-0002-8837-0035
Yung, Yuk L.0000-0002-4263-2562
Additional Information:© 1992 by Academic Press, Inc. Received January 7, 1992; revised June 8, 1992. Available online 14 October 2002. We thank the two reviewers for their constructive comments and careful evaluations of the manuscript and P. S. Romani and J. E. Allen, Jr., for useful discussions of vapor pressures. This research was supported by NASA Grants NAGW 1509 and NAGW 2362 to the California Institute of Technology. The first author gratefully acknowledges support from the NASA Graduate Student Researchers Program. This paper represents Contribution Number 4995 from the Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125.
Group:UNSPECIFIED, Division of Geological and Planetary Sciences
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NASA Graduate Student Research FellowshipUNSPECIFIED
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Other Numbering System NameOther Numbering System ID
Caltech Division of Geological and Planetary Sciences4995
Issue or Number:2
Record Number:CaltechAUTHORS:20140813-103124038
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Official Citation:Julianne I. Moses, Mark Allen, Yuk L. Yung, Hydrocarbon nucleation and aerosol formation in Neptune's atmosphere, Icarus, Volume 99, Issue 2, October 1992, Pages 318-346, ISSN 0019-1035, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:48502
Deposited By: Ruth Sustaita
Deposited On:13 Aug 2014 19:45
Last Modified:10 Nov 2021 18:32

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