706 Encyclopedia of the Solar System
(a) (b)FIGURE 11 Contour plots of comet Halley, November 13–16, 1985. The image is taken at the
peak flux density of the line (0.0 km/s in the reference frame of the comet). The left side shows a
low-resolution image (3′), and the right side shows a high-resolution image (1′), after the data
for both dates were combined. Contour levels for the low-resolution image are 4.9, 7.8, 10.8,
13.7, 16.7, and 18.6 mJy/beam. For the high-resolution image, they are 4.4, 4.4, 6.0, 7.7, 9.3, and
10.4 mJy/beam. Dashed contours indicate negative values. The beam size, a linear scale, the
direction of motion, and the direction to the Sun are indicated in the figures. The cross indicates
the position of the nucleus at the time of the observations. (I. de Pater et al., 1986, The
brightness distribution of OH around comet Halley,Astrophys. J. Lett. 304 , L33–L36.)particles. These data thus hint at the presence of grains
with sizes in the (sub)millimeter range.
The most significant advances in cometary radio re-
search have been obtained from spectroscopic studies. The
cometary nucleus consists primarily of water ice, which sub-
limates off the surface when the comet approaches the Sun.
After about a day, H 2 O dissociates into OH and H. Since the
early 1970s, the 18 cm OH line has been observed and mon-
itored in many comets. Such observations are important be-
cause they provide indirect information on the production
rate, and time variability therein, of water, a molecule that
remains difficult to observe on a routine basis.
The OH line is sometimes seen in emission, and at other
times in absorption against the galactic background. The
OH emission is maser emission (i.e., stimulated emission
from molecules in which the population of the various en-
ergy levels is inverted, so that the higher energy level is
overpopulated compared to the lower energy level). This
population inversion is caused by absorption of solar pho-
tons at ultraviolet (UV) wavelengths. However, this excita-
tion process depends on the comet’s velocity with respect to
the Sun (heliocentric velocity), the so-called Swings effect.
If the heliocentric velocity is such that solar Fraunhofer (ab-
sorption) lines are Doppler shifted into the OH excitation
frequency, the molecule is not excited. In that case, OH
will absorb 18 cm photons from the galactic background
and be seen in absorption against the galactic background.
If the line is excited, background radiation at the same wave-
length (18 cm) will trigger its deexcitation, and the line is
seen in emission (maser or stimulated emission). With ra-
dio interferometers, the OH emission can be imaged. Such
images have, for example, revealed the so-called quench-
ing region directly, a region around the nucleus where col-
lisions between particles thermalize the energy levels of
OH molecules, so they no longer produce maser emission
(Fig. 11).
One of the strengths of radio astronomy is the detec-
tion of “parent” molecules in a cometary coma, molecules
that evolve directly from its icy surface. Such observations
are crucial for our understanding of a comet’s composi-
tion, and, indirectly, on the conditions in the early solar
nebula from which our planetary system formed because
cometary nuclei have presumably not been altered by ex-
cessive heating or high pressures. A growing number of
molecular species have been detected at radio wavelengths.
Figure 12 shows the time evolution of observed production
rates for a large number of gases, subliming from comet
Hale–Bopp (C/1995 O2). Only the most volatile materials
sublime at heliocentric distancesr
̃>5 AU, while OH (from
H 2 O) becomes dominant atr<3 AU.
Whereas most molecules sublime directly off the
cometary nucleus, some gases, such as carbon monox-
ide and formaldehyde, are also released from dust grains
in a comet’s coma. With the advent of new powerful
(sub)millimeter arrays, much improved images of the spatial
distribution of parent molecules in a cometary coma can be
obtained. Figure 13 shows a composite of the formaldehyde
emission from comet Hale–Bopp, as observed with the