Formation and Destruction of Carbon Monoxide in Cometary Comae
Formation and Destruction of Carbon Monoxide in Cometary Comae
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Date
2006-04-18
Authors
Pierce, Donna
Advisor
A'Hearn, Michael F.
Citation
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Abstract
This dissertation examines the potential impact
of chemical reactions and outflow behavior on the
chemical development of the coma within one of
the most challenging problems in cometary
chemistry - identifying the source of extended CO
as observed in the comae of some comets. Could
chemical reactions and/or outflow behavior
contribute significantly to the CO abundance? In
these studies, the impact of multiple
photochemical processes and two-body chemical
reactions are examined within a variety of cases
designed to examine the effects of nuclear
chemical composition and coma physics, including
the dynamics of gas emanating from a region of
large-scale negative relief topography.
The results show that two-body chemical reactions
can contribute as much as 30% to the formation of
CO and as much as 60% of the loss of CO within
the inner coma, depending on the production rate.
Furthermore, the fractional contribution of
chemical reactions to CO formation and
destruction depends on the outflow behavior.
However, the overall result suggests that
chemical reactions only contribute a small net
gain of CO (~5%) over the CO produced solely by
photochemistry. The effects of chemical
reactions and outflow behavior are more
important, however, to secondary species
formed exclusively in the coma.
The results also indicate that H<sub>2</sub>CO is
the primary contributor to the extended CO source
in comet Halley. Observations of H<sub>2</sub>CO
in comet Halley suggest that it most likely comes
from an extended source as well. The extended
source problems of CO and H<sub>2</sub>CO may be
linked, in which case the precursor to
H<sub>2</sub>CO would gradually produce
H<sub>2</sub>CO after its release from the
nucleus for it to form CO on the observed spatial
scale. Such precursors could be large organic
molecules, or grains rich in formaldehyde
polymers that thermally dissociate upon heating.
More difficult to explain, however, is the CO
abundance in comet Hale-Bopp, for which the
observed H<sub>2</sub>CO abundance is
insufficient to explain the observed CO
abundance.