Physics and Chemistry of Comets 565
FIGURE 11 Schematic of a
cometary nucleus illustrating the
physical processes at work.
(Courtesy of P. R. Weissman,
NASA-JPL.)hundreds or thousands of inner solar system passes and thus
has a surface smoothed by many sublimation episodes. The
surface of comet Wild 2 appears young in terms of sublima-
tion exposure. The steep slopes, which imply some cohesive
strength, mean that the surface does not resemble a pile of
material held together by gravity. The results fromDeep
Impacthave raised new questions and begun the process of
understanding the nucleus. The surface geology of comet
Tempel 1 shows clearly distinct layers that seem to be dis-
crete blocks like geologic strata. The surfaces of the three
Jupiter-family comets (Borrelly, Wild 2, and Tempel 1) are
quite different, and this fact challenges the notion of a typ-
ical comet. Analysis of the ejecta and its evolution yields
the following results. The fine particles seen in the ejecta
must be from a surface layer at least tens of meters deep.
The tensile strength was estimated at 65 Pa or less. This is
comparable to the strength of talcum powder. The density
of the nucleus is about 0.6 g cm−^3 meaning that the interior
must be porous with some 50–70% of the volume consisting
of empty space.
4. Coma and Hydrogen Cloud
The gas and dust liberated by the sublimation of the ice
is the origin of comet features with large dimensions. The
coma is the essentially spherical cloud around the nucleus of
neutral molecules and dust particles. It is visible in images of
comets with low gas-production rates (Fig. 12) or in short-
exposure images of comets with high production rates. The
dusty gas expands at speeds of≈1 km sec−^1 , and the flow
is transonic in that the flow begins subsonic and becomes
supersonic. This is similar to the flow of the solar wind. [See
TheSolarWind.] Because the gas is dragging the dust
along, the gas flows faster than the dust. Images and in situ
measurements show that the material emission from the
nucleus is structured into jets in the near-nucleus region.
Well away from the nucleus, this structure is not usually
important. The size of comas can range up to 10^5 –10^6 km.
Most of our observations and measurements of compo-
sition in comets refer to the coma region. For some species,
the variation with radial distance from the nucleus can be
modeled by including creation and destruction mechanisms
for parent and daughter molecules. For a molecular gas
expanding radially at constant speed, the density falls off
asr−^2 (ris the distance from the nucleus), and the surface
brightness (proportional to an integral along a line of sightFIGURE 12 Comet Giacobini–Zinner on 26 October 1959
showing the coma and a plasma tail extending some 450,000 km.
(Photograph by E. Roemer, University of Arizona: official U. S.
Navy photograph.)