Physics and Chemistry of Comets 563
FIGURE 7 Schematic showing the layered structure of a
cometary nucleus from the pristine composition up to the porous
dust mantle. The vertical scale is arbitrary. (Courtesy of D.
Prialnik, Department of Geophysics and Planetary Science, Tel
Aviv University, Israel: from Prialnik, 1997–1999, Modeling gas
and dust release from comet Hale–Bopp,Earth, Moon, and
Planets, 77: 223–230, Figure 1. Copyright©C1999, with kind
permission of Springer Science and Business Media.)
ground-based observations of comet Halley and the close-
up images taken byVEGA 1,VEGA 2, andGiotto, the deter-
mination of the rotation was expected to be straightforward.
An initial complication was the reports of different periods
of brightness variation. Sorting things out was a major effort.
In short, the rotation was complex, and a model with five jets
was needed to reproduce the observations. Figure 8 shows
views of the rotating nucleus through an entire period. The
solution was consistent with a constant internal density.
The rotation state determined for comet Halley is inter-
esting because it is not in the lowest rotational energy state
for a given angular momentum. This would be rotation only
around the short axis. The excited rotational state is proba-
bly not primordial because estimates of the relaxation time
due to frictional dissipation in the comet’s interior are in
the range 10^6 –10^8 years. It is probably due to jet activity or
splitting of the nucleus.
The splitting of comet nuclei has been observed many
times. A recent example is the case of comet LINEAR in
early August 2000 (see Fig. 9). Large pieces and fragments
of the nucleus are visible in the images. Most of the frag-
ments have an estimated size of less than 500 m. This is
an example of “spontaneous” splitting (i.e., there is no ap-
parent correlation with orbital parameters or time in the
orbit relative to perihelion). This type of splitting occurs for
roughly 10% of dynamically new comets on the first peri-
helion passage. Splitting can also occur when the nucleus
FIGURE 8 The complex rotation of comet Halley’s nucleus
through one full sequence. The images read left to right starting
at top left. The time between images is 0.25 days and the
sequence repeats afterapproximately7.25 days. The five active
areas (jets) are marked as low-albedo features. (Courtesy of M. J.
S. Belton, Belton Space Initiatives.)passes close enough to the Sun or a planet and is tidally dis-
rupted. Comet Shoemaker–Levy 9 passed close to Jupiter
in July 1992. The disruption produced about 20 fragments
(see Fig. 10). These crashed into Jupiter over several days
in July 1994. The tide-induced splittings have been used to
estimate the tensile strength of the nuclei, and very low val-
ues were found. The units of tensile strength are force per
unit area (N m−^2 ) or the pascal (Pa). The inferred values
from splittings are in the range 10^2 –10^4 Pa. For compari-
son, rocks have values∼ 4 × 106 Pa, and the value for steel
is∼ 4 × 108 Pa.
The splittings are consistent with the view of the
cometary interior as being porous, having a weak structure,
and perhaps consisting of agglomerated building blocks
called cometesimals. Available evidence indicates that the
interior consists of volatile ices (mostly H 2 O ices, probably
amorphous) and dust. The interior does not appear to be
differentiated, the compositions are surprisingly uniform,
and the ratio of ice to dust does not vary with depth.
Cometary outbursts may be related to splittings. In a
major outburst, the brightness of a comet increases by a
factor typically of 6–100, and the outburst lasts for weeks.
The observational evidence indicates that the increase in