Encyclopedia of the Solar System 2nd ed

572 Encyclopedia of the Solar System

TABLE 2 Measured and Observed Species in Comets

Atoms + Molecules Ions

H, C, O, S, Na, Fe, Ni, CO, CS, NH, C+,N+,O+,Na+,CO+,CH+,CN+,

OH, C 2 ,^12 C^13 C, CH, CN,^13 CN, S 2 , SO, OH+,NH+,H 2 O+, HCO+,CO+ 2 ,

H 2 ,CO 2 , HDO, CHO, HCN, DCN, C+ 3 ,CH+ 2 ,H 2 S+,NH+ 2 , HCN+,

H^13 CN, OCS, SO 2 ,C 3 ,NH 2 ,H 2 O, H 2 S, DCN+,CH+ 3 ,H 3 O+,H 3 S+,NH+ 3 ,

HCO, H 2 CS, C 2 H 2 , HNCO, H 2 CO, C 3 H+,CH+ 4 ,H 3 CO+,CH+ 5 ,C 3 H+ 3

CH 4 ,HC 3 N, CH 3 OH, CH 3 CN,

NH 2 CHO, C 2 H 6

temperature near 30 K. These results are important in dis-
cussing formation scenarios. The existence of S 2 in comets
may require a formation temperature as low as 15 K. While
there is some uncertainty in the exact temperature, cold
temperatures are required.
A monumental study using narrow-band photometry
with major results for the chemical compositions of comets
was led by astronomer M. F. A’Hearn. Standardized tech-
niques were used to characterize 85 comets with filters that
covered emission bands from CN, C 2 ,C 3 , OH, and NH as
well as selected continuum regions. As with the ultraviolet
results described previously, the compositions are surpris-
ingly uniform. Barring some unusual event, a comet’s pro-
duction of gases and dust from orbit to orbit (and position
in the orbit) is essentially the same. This implies a basi-
cally homogeneous interior. When the sample of comets
was divided into old and new comets based on their orbital
properties, no compositional differences were found.
Still, there were significant exceptions to the similarity
in compositions. A class of comets shows depletions in the
carbon chain molecules C 2 and C 3 relative to CN. Comet
Giacobini–Zinner is the prototype for this class. Almost all
the members of this class are Jupiter-family comets, but
not all Jupiter-family comets are members of the class. Are
the compositional differences due to formation in differ-
ent regions in the solar nebula or to some kind of physical
processing for comets with a different orbital history? Re-
cent observations show differences in the following way.
Comets Halley, Hyakutake, and Hale–Bopp were exten-
sively observed, and their compositions are similar to those
in the cores of dense interstellar clouds. The observed com-
position of comet LINEAR shows depletions in CO, CH 4 ,
C 2 H 6 , and CH 3 OH. These are highly volatile species, and
a plausible scenario could place the comet’s formation in
the warmer Jupiter–Saturn region of the solar nebula. Most
comets such as Halley, Hyakutake, and Hale–Bopp are be-
lieved to have formed in the cooler Uranus–Neptune re-
gion. An inconsistency arises with the measurement of the
interior temperature of comet LINEAR (using a variant of
the OPR discussed earlier) where a result close to 30 K
was found. Thus, comet LINEAR may have formed under
essentially the same conditions as the other comets.

Even though the gross compositions of comets are simi-

lar, chemical diversity is an established fact. Because comets

are surely formed over a range of heliocentric distances and

because they have a variety of orbital histories, diversity

could arise from formation conditions and from postforma-

tion processing. The relative importance of the two is to be

determined.

7. Formation and Ultimate Fate of Comets

The icy bodies of the solar system formed as part of the pro-

cess that produced the Sun, the terrestrial planets, and the

giant planets. The icy bodies include some of the asteroids

(including the Centaurs, which are bodies with eccentric or-

bits generally between Saturn and Neptune), comets, and

Kuiper Belt Objects (KBOs). [SeeKuiperBeltObjects:

PhysicalStudies.]

The solar system is thought to have formed from the col-

lapse of an interstellar gas cloud. The collapse process pro-

duced a newly formed star with a circumstellar disk of gas

and dust, the solar nebula. [SeeTheOrigin of theSolar

System.] As discussed in Section 6, cometary material can

condense at temperatures of roughly 30 K. Models of the

early solar nebula have temperatures of roughly 30 K in the

Uranus–Neptune region, and it is reasonable to conclude

that comets formed near there, meaning that the material

condensed and agglomerated into comet-sized (most with

radii in the range 1–10 km) bodies. Note, however, that the

uncertainty in the temperatures for models of the presolar

nebula is approximately a factor of 2.

But the story does not end there because most comets

are not in the Uranus–Neptune region today. Dynamical

processes dispersed the icy bodies. Gravitational perturba-

tions by the giant planets sent some of the comets to large

distances from the Sun and some into the inner solar sys-

tem. The latter comets faded long ago. Many of the comets

sent to large distances escaped from the solar system, but

the ones that are barely bound form a roughly spherical

cloud with dimensions of 10^4 –10^5 AU. This is the cloud of

comets, the Oort cloud, postulated by J. Oort many years

ago. It is the source of the long-period comets (P> 200