DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

carbonaceous chondrite
the object with that in a contemporary sample.
Estimates are fairly accurate, out to about
50,000 years.
carbonaceous chondrite A chondritic meteorite
that contains carbon and organic compounds
in addition to the rocky minerals. They
also contain water-bearing minerals, taken to be
evidence of water moving slowly through their
interiors not long after formation, and evidence
of metamorphism of the meteorite by water and
of possible cometary origin. It is believed that
carbonaceous chondrites are the most primitive
meteorites and that minerals they contain were
the first minerals to crystallize during the formation
of the solar system.
carbon burning The set of nuclear reactions
that convert carbon to oxygen, neon, and other
heavier elements. Carbon burning occurs in hydrostatic
equilibrium in the evolution of stars of
more than about 10 solar masses (between helium
burning and neon burning). It can also be
ignited in degenerate matter, in which case the
burning is explosive. This may happen in the
cores of stars of about 8 solar masses. It definitely
happens at the onset of a supernova of
type Ia. Oxygen burning can also be either hydrostatic
or explosive.
carboncycle aseriesofnuclearreactionsthat
occur in the interior of stars, and in which carbon
12 acts as a sort of catalyst, not being consumed
in the reaction. The chain is 12 C→ 13 N →
14 O→ 14 N→ 15 O→ 15 N→ 12 C+ 4 He.
The two oxygen to nitrogen steps are beta decays
and emit neutrinos. The energy release is
26.7MeV,ofwhich1.7MeVisinneutrinos. This
cycle represents about 2% of the energy production
in the sun (which is mostly the p-p cycle);
however, its temperature dependence is approximately
T 15 , so it dominates in more massive
stars.
carbon dioxide Colorless, odorless trace
component gas of the Earth’s atmosphere (approximately
370 parts per million), CO2. Over
99% of the Earth’s carbon dioxide is found
in oceans. Carbon dioxide is evolved in volcanos,
and is also perhaps carried to Earth in
cometary cores. Carbon dioxide is a major re-
© 2001 by CRC Press LLC
actionproductincombustioninvolvingfossilfuels,
all of which is hydrocarbon based. Coal, essentiallypurecarbon,
producesmuchmoreCO2
than does natural gas (CH4) per unit of energy
evolved. Carbon dioxide concentrations have
been rising in the Earth’s atmosphere throughout
the last century. Carbon dioxide is a greenhouse
gas, transparent in the visible, opaque
in infrared. It is estimated to contribute more
than half the current greenhouse warming of the
Earth. Carbondioxideisevolvedbyanimalsand
sequestered into complex organic compounds
by plants.
carbon monoxide Colorless, odorless, and
very toxic trace constituent of the air, CO.
Formed in the incomplete combustion of carbon;
also apparently produced by near surface
ocean-biologic sources. A very tightly bound
molecule, as a result of which it is the most common
(apart fromH2) in the interstellar medium
and, in the atmospheres of cool stars, it generally
uses up all of the available C or O, whichever
is less abundant, leaving only the other one to
form additional detectable molecules.
carbon star Cool, evolved star of relatively
low mass in which sufficient carbon has been
mixed to the surface for the carbon-oxygen ratio
to exceed unity. As a result, carbon monoxide
formation does not use up all the carbon, and
other molecules, including CH,CN, and C2,
will appear in the star’s spectrum. The stars are
generally on the asymptotic giant branch and are
seen at somewhat lower luminosity than might
be expected from standard theories of convection
and mixing. Other chemical peculiarities
are common, and there are many subtypes. See
asymptotic giant branch star.
Caribbean current An extension of the
North Equatorial current that flows northwestward
into the Caribbean past the Yucatan peninsula
and exits the Caribbean basin westward past
the southern tip of Florida.
Carme Moon of Jupiter, also designated JXI.
Discovered by S. Nicholson in 1938, its orbit has
an eccentricity of 0.207, an inclination of 164 ◦ ,
and a semimajor axis of 2.26×10 7 km. Its radius
is approximately 20 km, its mass 9.5 × 10 16 kg,