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Atmospheric chemistry in the outer solar system: from 40 K to 4000 K

Citation

Lyons, James Richard (1996) Atmospheric chemistry in the outer solar system: from 40 K to 4000 K. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/D9MH-G975. https://resolver.caltech.edu/CaltechTHESIS:01252013-094647944

Abstract

This thesis consists of four papers on the general subject of atmospheric chemistry in the outer solar system. Although all are reducing environments, the atmospheres differ widely in the ranges of temperature and pressure they encompass. Individual abstracts are given below.

Photochemistry of the Atmosphere and Ionosphere of Triton

A one-dimensional photochemical model of the atmosphere and ionosphere of Triton was constructed to evaluate the significance of CO and CO_2, detected as surface ices (Cruikshank et al. 1993), to the gas phase chemistry. For a CO mixing ratio of 10(^-4), consistent with the observed fraction of CO ice, the model yields several interesting results. Gas phase production of CO_2 is slow, but may, with the help of heterogeneous reactions on aerosols, be capable of producing a detectable layer of CO_2 ice over the age of the solar system; however, we consider this unlikely. Atomic nitrogen, produced in the ionosphere, diffuses to the lower atmosphere and recombines to form N_2 in a cycle in which C acts as a catalyst. In a model with solar radiation only, the model predicts N densities a factor of two smaller than reported by Krasnopolsky et al. (1993). Atomic carbon is produced in the ionosphere primarily by dissociative recombination of CO^+. For an assumed rate coefficient for charge exchange from N_2^+ and CO^+ to C of 1x10^(-10) cm^3 sec^(-1), Triton's ionosphere is dominated by C^+ and can be accounted for entirely by solar radiation. For a rate coefficient 10 times smaller, magnetospheric electron precipitation is needed to account for the observed electron density, but C^+ is still the principal ion. Electron precipitation is necessary to explain the observed N abundances. Measurements of the rate coefficients for ion-molecule reactions involving neutral C are needed.

Metal Ions in the Atmosphere of Neptune

Microwave propagation experiments performed with Voyager 2 at Neptune revealed sharp layers of electrons with densities ∼10^4 cm^(-3) in Neptune's lower ionosphere. These layers are reminiscent of terrestrial sporadic-E layers, and, when taken together with data from the other giant planets, confirm the importance of the magnetic field in layer formation. A photochemical model which incorporates species produced by meteoroid ablation predicts that Mg^+ is the most likely metal comprising the layers, although laboratory data on the kinetics of metallic atoms and ions in reducing environments are lacking. The metal chemistry discussed here is directly relevant to the abundant metals observed at the impact site of the G fragment of Comet Shoemaker-Levy 9 on Jupiter.

Meteoroidal Influx into the Upper Atmospheres of Uranus and Neptune

Results from a recent analysis of meteoroid ablation rates in the atmosphere of Neptune have been coupled with photochemical models of the upper atmospheres of Neptune and Uranus to yield estimates of stratospheric water profiles as a function of meteoroid influx. Because water has never been detected in the upper atmospheres of the giant planets, the tangential column opacities of the model water profiles were compared with UV absorption measurements made by Voyager to determine maximum water influxes. For Uranus an upper limit is obtained which is consistent with an Oort-family particle population, but not with a large population of planet-family dust particles. For Neptune the model water profile is strongly dependent on the still uncertain eddy coefficient, making it difficult to rule out a large planet-family of IDP's. However, an IDP population sufficiently large to account for the CO observed in Neptune's atmosphere can be ruled out.

A Chemical Kinetics Model for the Comet Impact with Jupiter

A chemical kinetics code was developed for gas phase species comprised of the elements H,C,N,O,S and Si. The code is valid at high temperatures and for H-dominated compositions. The kinetics model was tested by running it to steady state equilibrium and comparing the results with a thermochemical model. Model runs for pressure-temperature histories relevant to the comet Shoemaker-Levy 9 impacts with Jupiter were made for C > O and C less than O compositions and for a variety of temperatures. Results indicate that the plume gas must have C > O, in agreement with Zahnle et al. (1995), implying a greater than 50:1 mix of Jupiter gas to vaporized comet.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Planetary Science, Environmental Engineering Science, Planetary Atmospheres
Degree Grantor:California Institute of Technology
Division:Geological and Planetary Sciences
Major Option:Planetary Sciences
Minor Option:Environmental Science and Engineering
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Yung, Yuk L.
Thesis Committee:
  • Stevenson, David John
  • Ingersoll, Andrew P.
  • Muhleman, Duane Owen
  • Yung, Yuk L.
Defense Date:27 September 1995
Non-Caltech Author Email:jimlyons (AT) asu.edu
Record Number:CaltechTHESIS:01252013-094647944
Persistent URL:https://resolver.caltech.edu/CaltechTHESIS:01252013-094647944
DOI:10.7907/D9MH-G975
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:7437
Collection:CaltechTHESIS
Deposited By: Benjamin Perez
Deposited On:25 Jan 2013 18:30
Last Modified:21 Dec 2019 01:33

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