562 Encyclopedia of the Solar System
the nucleus—80–90% H 2 O (water) ice; roughly 10% CO
(carbon monoxide) ice; and small amounts of other ices—is
presented in Section 6. The ice in a cometary interior is al-
most surely amorphous ice. This comes about because ices
formed by condensation on a surface at low temperatures
do not have energy available to change into the crystalline
forms that minimize energy.
When the water ice or snow sublimates, a water vapor is
produced, and embedded dust particles are released. The
energy sources for the sublimation are solar radiation, ice
phase transitions, and radioactive decay. Solar radiation de-
posits energy on the surface or in the near-surface layers.
This energy affects the deeper layers by producing a heat
wave that moves inward. The transition from amorphous ice
to crystalline ice releases energy. Amorphous ice undergoes
a transition to cubic ice at approximately 137 K, and cubic
ice undergoes a transition to hexagonal ice at approximately
160 K. Model calculations usually treat both transitions as a
single energy release event. Radioactive decay is primarily
from short-lived isotopes, such as^26 Al. This source is most
important in the deep interior and when the nucleus is far
from the Sun and diffusion of volatiles could result.
Insight into the production of cometary features and the
energy balance for the surface region is illustrated in Fig. 6.
Considered here is the simple case of energy input from so-
lar radiation only, with no heat wave into the interior. When
a comet is far from the Sun, the energy balance is achieved
by the solar radiant energy being reradiated by blackbody
(infrared) radiation. The temperature of the surface lay-
ers is not high enough to produce significant sublimation.
At intermediate distances, the surface temperature is high
enough for sublimation, and the solar radiant energy input
is balanced both by blackbody reradiation and by sublima-
tion. At closer distances to the Sun, the surface temperature
increases further, and essentially the entire solar radiation
input is balanced by sublimation. Of course, blackbody rera-
diation takes place, but it is small in terms of the energy bal-
ance. For water ice, sublimation becomes important around
3 AU, and the energy balance (primarily through sublima-
tion) occurs near 1 AU. This copious production of material
drives cometary activity and produces cometary features as
described later.
Naturally, there are complications to this simple picture.
When the surface layer ices are sublimated, not all of the
dust is liberated, and a porous dust mantle is formed. The
mantle insulates the ices beneath the surface. This idea has
been confirmed observationally. Infrared observations of
the surface layers indicate temperatures reasonably close
to values expected for a nonsublimating, low-albedo object
bathed by sunlight. These temperatures are much higher
than the temperatures for sublimating water ice. The ice
sublimation probably takes place a few centimeters below
the surface. Also, there is no reason to believe that subli-
mation takes place uniformly over the surface. Regions of
enhanced sublimation are expected, a view consistent with
FIGURE 6 Energy-balance regimes for different distances from
the Sun. No heat flow into the interior is considered. Only the
principal components of the energy balance are shown. Some
sublimation occurs far from the Sun and some blackbody
reradiation occurs close to the Sun. (Reprinted with permission
from John C. Brandt and Robert D. Chapman, “Introduction to
Comets,” 2nd Ed., Cambridge Univ. Press, Cambridge, United
Kingdom. Copyright©CCambridge University Press, 2004.)the images of comet nuclei that show dust and gas emis-
sion predominantly in jets. These jets can produce some of
the surface features on the nucleus, and, along with impact
craters, they can produce an irregular shape for the nucleus.
Figure 7 shows how the surface layers of a comet can
become stratified and illustrates the potential complexity of
accurate modeling. These layers include many intermediate
stages—from the pristine composition of the deep interior
to the ejected gas and dust—and these must be modeled
accurately. The details of the gas flow through the porous
dust layers are important. In recent years, the trend has
been to think of the nucleus as a fairly porous body. The
porosity is defined as the fraction of the volume occupied
by the pores, and values of roughly 0.5 are often discussed.
At present, such values can apply to some, but probably not
all, comet nuclei.
The rotation of comet nuclei provides an example of how
complex some situations can become. Given the extensive