Physics and Chemistry of Comets 571
charge exchange with a neutral molecule to produce an
ionized molecule and a five-time ionized oxygen in an ex-
cited state. X-ray lines are produced when the excited
ions spontaneously decay. Spectroscopic X-ray observa-
tions have confirmed this mechanism. Some contribution
to the total flux may come from electron-neutral thermal
bremsstrahlung.
6. Comet Chemistry
The overall chemical composition of comets seems to be
rather uniform. Exceptions to this general statement are
discussed later. Ultraviolet spectra of comets (see Fig. 21)
are dominated by the hydrogen (H) Lyman-αline at 121.6
nm and by the hydroxyl (OH) bands at 309.0 nm. This is
certainly compatible with the conclusion that the nucleus
is composed of roughly 80–90% water ice, 10% carbon
monoxide (CO), and many minor constituents.
Table 2 lists species in comets that have been observed
spectroscopically or measured in situ by mass spectrometers
on spacecraft. The list is not exhaustive.
Providing a detailed explanation of the abundances of
these species is a formidable task and is subject to many
processes in the coma. But, as argued by W. F. Huebner,
the situation is comprehensible if we assume a condensa-
tion process in the primordial solar nebula at a temperature
of 30 K and solar abundances except for H and N. The
abundance of hydrogen is determined by the capability to
chemically bind to other species. Much is lost from the so-
lar system. Some nitrogen is also lost; for example, when
N 2 is formed, the nitrogen is in a form that is not chemi-
cally active. A gas mixture consisting of C, O, Mg, Si, S, and
Fe in solar abundances with reduced amounts of H and N
can condense into molecules at 30 K. The silicates Fe 2 SiO 4
and Mg 2 SiO 4 are formed from Fe, Mg, Si, and O. Then,
the remainder of O goes into H 2 O and into HCO and CO-
compounds. Finally, the remainder of the C, N, and S goes
into HCNS-compounds.
The result of this fairly straightforward condensation
sequence is a material that, when formed into a substan-
tial solid body, resembles comets. By mass, the relative
abundance of H 2 O:silicates:carbonaceous molecules plus
hydrocarbons is approximately 1: 1 :1. Also, by mass, the
abundances of ices:dust is about 1:1.
The temperature of 30 K used in the previous discus-
sion is not only the appropriate temperature for the con-
densation sequences, but it is also consistent with direct
determinations of the interior temperatures of cometary
nuclei using theortho-topara-hydrogen ratio (OPR). Hy-
drogen in water (and some other compounds) can have the
spin of their nuclei in the same direction (ortho-water) or
in the opposite direction (para-water). The OPR depends
on the temperature of the water molecules at the time of
formation, and the OPR can only be changed by chemical
FIGURE 21 International Ultraviolet Explorer(IUE) spectra of
comet Halley. (a) Spectrum on 9 March 1986: the very strong
line close to 1200A is the Lyman- ̊ αline of neutral hydrogen.
(b) Spectrum on 12 September 1985. (c) Spectrum on 11 March- (Courtesy of P. D. Feldman, Johns Hopkins University.)
reactions. Thus, the ice can be sublimated in a comet’s sub-
surface layers and flow through the crust into the coma
while retaining its original OPR.
Infrared measurements of the OPR for comets Halley,
Hale–Bopp, and LINEAR are all consistent with an interior