Glossary 921
(B-R) color measures the proportion of red to blue light from an
object. A red astronomical object has (B-R)∼2, the Sun has
(B-R)=1.03, and a blue astronomical object has (B-R)∼0.
Bar Unit of pressure, equal to 10^6 dyn/cm^2 or 10^5 Pa;
the standard sea level pressure of the Earth’s atmosphere is
1.013 bar. Typical planetary interior pressures are measured
in megabars (Mbar) or 10^6 bar.
Baroclinic instability A 3-dimensional process common in
the midlatitude troposphere wherein cold polar air pushes
underneath hot low-latitude air, which transports heat toward
the poles and produces complex circulation patterns that
generate much of the winter rainfall in the midlatitudes.
Baroclinic, barotropic Barotropic is a region of uniform
temperature distribution; a lack of fronts. Everyday being
similar (hot and humid with no cold fronts to cool things off)
would be a barotropic type of atmosphere, such as we find at
tropical latitudes. In a baroclinic region, distinct air mass regions
exist. Fronts separate warmer from colder air. In a synoptic scale
baroclinic environment, you will find the polar jet, troughs of
low pressure (midlatitude cyclones), and frontal boundaries.
There are clear density gradients in a baroclinic environment
caused by the fronts. Mid-latitude cyclones are found in a
baroclinic environment.
Barycenter The center of mass of a system of two or more
gravitationally bound (orbiting) bodies.
Basalt An igneous rock primarily composed of plagioclase and
pyroxene. On Earth, oceanic crust is primarily basaltic in
composition.
Beta meteoroid Small meteoroid for which the solar
radiation pressure force is comparable to solar gravitational
attraction and hence leaves the solar system on an unbound
orbit.
Blackbody radiation Continuous spectrum of
electromagnetic radiation emitted by an object that absorbs all
radiation incident on it.
Bolide A meteoric fireball.
Bond albedo Ratio of the total radiation reflected in all
directions from a solar system object to the total incidence flux.
Bound orbit Circular or elliptical orbit about a central body
(e.g., Sun, planet).
Breccia Rock composed of fragments derived from previous
generations of rocks.
Bremsstrahlung Electromagnetic radiation that is emitted
when an energetic electron is deflected by an ion. It is also
called free-free emission because both the electron and ion are
free in an ionized plasma. The term is borrowed from German
and means “braking radiation” because the deflected electron
loses energy (by the emitted photon) and is slowed down.
Brightness temperature The temperature a body would
have if it were a blackbody producing the same brightness as the
observed object at the same wavelength. It can also be defined
as the radiant intensity scaled to units of temperature byλ^2 /2k
whereλis wavelength, andkis Boltzmann’s constant.
CAI: calcium-aluminum inclusion Minor component of
primitive meteorites composed of refractory minerals.
Caldera Large volcanic crater, usually greater than 1 km in
diameter. A caldera is many times the size of any associated
vent(s). Calderas are formed either by collapse (most often) or
explosion.
Carbonaceous (C-type) material Carbon-silicate material
rich in simple organic compounds, such as that found in
carbonaceous meteorites, which are believed to be among the
most primitive (unaltered since their formation in the solar
nebula) objects found in the solar system. They contain complex
carbon compounds (hydrocarbons, amino acids), made mostly
from the elements C, H, O, and N. C-type material is low
albedo, spectrally flat and exists on the surfaces of several outer
planet moons.
Catalytic cycle Series of chemical reactions facilitated by a
substance that remains unchanged.
Cd-cutoff: The neutron capture cross section for natural
cadmium (Cd) is very high for thermal neutrons, but drops
sharply for energies greater than about 0.5 eV, which is
sometimes referred to as the “Cd cutoff” energy. Consequently,
Cd is an excellent filter that absorbs thermal neutrons, but
allows epithermal and fast neutrons to pass through.
Centaur A small body in a heliocentric orbit whose average
distance from the Sun lies between the orbits of Jupiter and
Neptune, and that has a Tisserand parameter with respect to
Jupiter greater than 3. Typically, the orbits of Centaurs also
cross one or more of the orbits of the other giant planets.
Centaurs are part of the population of ecliptic comets They are
most likely derived from the Kuiper belt and the Scattered disk.
Eventually, some Centaurs may evolve into the terrestrial
planets zone and become short-period comets.
Chaotic motion A dynamical situation in which the error in
the prediction of the long-term motion of a body grows
exponentially with time. This exponential growth leads to an
inability to predict the location of the body. Chaotic motion can
be confined to fairly narrow regions of space so that the orbit of
the body will not change much over time. This is the case for the
planets in our solar system. However, for most comets and some
asteroids that we observe, chaotic motion leads to sudden and
drastic changes in the orbit of the body.
Chaotic terrain Areas of the martian surface where the
ground has collapsed to form a surface of jostled blocks standing
1–2 km below the surrounding terrain.
Charge–coupled device (CCD) A solid-state device used to
record light electronically. A typical CCD has thousands to
millions of tiny detectors arranged in a grid pattern. Each
detector element is called a pixel. These devices record images
electronically. CCDs have completely replaced photographic
plates in astronomy due to their very high efficiency in capturing
light.
Chasmata Term used in planetary geology to refer to long,
relatively narrow, steep-sided troughs.