00-004

Cover Page/Proposal Summary

ROSS-99    NRA 99-OSS-01

Date Due: 5/3/99

NASA PROCEDURE FOR HANDLING PROPOSALS

This proposal shall be used and disclosed for evaluation purposes only, and a copy of this Government notice shall be applied to any reproduction or abstract thereof. Any authorized restrictive notices that the submitter places on this proposal shall also be strictly complied with. Disclosure of this proposal for any reason outside the Government evaluation purposes shall be made only to the extent authorized by the Government.

Proposal Type: New Proposal

Proposal Category: Aeromomy

Major Equipment Proposal? No

Do you intend to submit an Education/Public Outreach (E/PO) proposal? No

Proposal Title:
Photochemical Studies of Chemistry in the Outer Solar System

Abbreviated Proposal Title:
Photochemistry of Giant Planets

Principal Investigator:
Dr. Yuk Yung
Professor of Planetary Science Division of Geological & Planetary Sciences 150-21
Caltech
1200 East California Blvd.
Pasadena, CA 91125
Phone: 626-395-6940   Fax: 626-585-1917   E-mail: yly@gps.caltech.edu

SignatureDate
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Co-Investigators and Collaborators:
Type    Name    Affiliation    E-mail
Sci Co-I   Dr. Mark Allen   JPL/Caltech   maa@gps.ca ltech.edu
Sci Collab   Prof. Jonathan Lunine   LPL/University of Arizona   jlunine@lpl.arizona.edu


Proposal Summary:

The goal of the proposed science investigation is to gain a quantitative understanding of chemical processes and their coupling with atmospheric dynamics in the reducing atmospheres of the outer solar system, with a particular focus on Infrared Space Observatory (ISO) observations and future experiments such as the Cassini Mission to Saturn and Titan. The proposed work is divided into two related tasks. We propose to carry out a systematic comparison between atmospheric models for every giant planet and Titan, which employ a consistent set of photochemical reactions. Combined with recent observations of hydrocarbon species by ISO, this can provide the most rigorous test of our current understanding of the photochemistry of hydrocarbon in the outer solar system. The emphasis will be on the methyl radical (CH3), first detected by ISO in the atmospheres of Saturn and Neptune (Bézard et al., 1998). CH3 is one of the most important radicals in the hydrocarbon photochemistry because it is the primary product of methane photolysis and plays an essential role in forming C2H6, the most abundant and stable C2 species. A fundamental understanding of the distribution of CH3 provides insights into the chemistry of hydrocarbons as well as comparative planetology. We propose to investigate the deuterium to hydrogen (D/H) fractionation in the atmosphere of Titan. The fractionation occurs as a result of physical and chemical processes in the atmosphere. Quantitative estimates of chemical fractionations in reactions with the ethynyl radical will be carried out. A new theory, the photo-induced isotopic fractionation effect (PHIFE), will be applied to CH3D. Better constraints on the location of the homopause on Titan will be derived. The implications for improving our understanding of the evolution of the atmosphere of Titan, and in particular the origin and evolution of Titan's volatile inventory, will be explored in collaboration with an independently funded group.