922 Encyclopedia of the Solar System
Chemoautotrophy The capacity of an autotrophic
(self-feeding) organism to derive the energy required for its
growth from certain chemical reactions (e.g., methanogenesis)
rather than from photosynthesis; some bacterial forms are
chemoautotrophic organisms.
Chiron-type comet A Centaur that displays cometary
activity.
Chlorofluorocarbons (CFCs) Various compounds made
with the halogens chlorine and fluorine. Their stability made
them favored refrigerants until it was discovered that this also
makes them efficient atmospheric ozone destroyers.
Chondrite: Undifferentiated stony or carbonaceous
meteorite, usually containing chondrules or their fragments.
Chondrule Approximately spherical, millimeter-sized droplet
formed by partial or complete melting and quenching prior to
incorporation into undifferentiated meteorites.
Chromophore Any coloring material.
Chromosphere Lower atmosphere of the Sun, above the
photosphere and beneath the transition region, with a vertical
height extent of about 2000 km and a temperature range of
6000–20,000 K.
Circular polarization ratio Ratio of radar echo power
received in the same sense of circular polarization as transmitted
(the SC sense) to that received in the opposite (OC) sense.
Circular restricted three-body problem A special case of
the problem of calculating the gravity-controlled motion of
three bodies. In the circular restricted three-body problem,
the two massive bodies follow circular orbits about each other,
and the mass of the third body is negligible. The motion of
comets and asteroids can often be approximated with the
circular restricted three-body problem with the Sun and
Jupiter, or sometimes the Sun and Neptune, as the two massive
bodies.
Clathrate, clathrate compound, or cage compound A
chemical substance consisting of a crystalline lattice of one type
of molecule trapping and containing a second type of molecule.
A clathrate therefore is a material that is a weak composite, with
molecules of suitable size captured in spaces left by the other
crystalline molecule. Water ice often forms clathrates with more
volatile molecules.
Column abundance The product of density (g/cm^3 ) and
geometric thickness (cm). Measures the mass per unit area of an
atmospheric or surface layer.
Column density The number of molecules above a column
of unit area in an atmosphere.
Coma The freely outflowing atmosphere of gas and dust
around the nucleus of a comet. The nucleus and coma of a
comet together are often called the head.
Comet A body containing a significant fraction of ices,
smaller than a planet or dwarf planet and orbiting the Sun,
usually in a highly eccentric orbit. Most comets are stored
beyond the planetary system in two large reservoirs: the Kuiper
belt beyond the orbit of Neptune and the Oort cloud at
near-interstellar distances. Comets become “active” when their
ices sublimate and carry gas and dust into the coma.
Comet dust trail A contrail-like structure extending behind
a comet close to its orbit, and sometimes a short distance ahead
of the comet, consisting of large particles (β<∼ 10 −^3 ) emitted at
low velocities from the nucleus. Trails are distinguished from
comet tails, which consist of much smaller particles, more
sensitive to solar radiation pressure.
Cometary mantle or crust A layer of refractory material
covering some or all of a comet nucleus’ surface. When thick
enough, a cometary mantle will choke off outgassing over that
area.
Conjunction Occurs when two or more planetary bodies
appear in the same area of the sky.
Conservation of angular momentum Fundamental
physical law requiring that the quantity of angular momentum,
p, be conserved (constant) for objects in orbit around a primary
body:p=mvr, weremis mass,vis velocity, andris the
distance from the primary body, and for rotating objects.
Angular momentum is not conserved in the presence of
modifying torques.
Contact surface In the vicinity of a comet, the surface that
separates outflowing cometary plasma from the slowed solar
wind that is approaching the comet.
Convection Transport of energy by mass motion. In
turbulent regions of planetary atmospheres and interiors, rising
parcels of hot air (or rock) and sinking parcels of cool air (or
rock) transport energy outward from the interior.
Core The central part of a differentiated planet, satellite or
asteroid. Terrestrial planets have nickel-iron cores. Jovian
planets have rocky-iron cores.
Coriolis acceleration Component of the acceleration on a
rotating planet that acts perpendicular to the motion and
balances the horizontal pressure gradient in an atmosphere or
ocean. It causes circulation around high- and low-pressure
centers. It is strongest in the polar regions and weakest in the
tropics.
Corona Upper atmosphere of the Sun, extending above
the transition region out into the heliosphere, with a
dominant temperature of 1 million to 2 million K in the lowest
100,000 km. The corona is visible during solar eclipses, and
extends outward many solar radii.
Coronae Circular to oval feature surrounded by concentric
ridges and fractures.
Coronal mass ejection (CME) Magnetic instabilities in the
solar corona that lead to eruption of filaments, prominences,
and magnetic flux ropes, which propagate as ejected mass out
into the heliosphere, often accompanied by flare phenomenon.
Known as an interplanetary coronal mass ejection (ICME) when
observed in the solar wind far from the Sun.
Coulomb interaction or collision The interaction of
charged particles at large distances through the Coulomb force.
The interaction is sometimes called a Coulomb collision because