Glossary 923
it produces a change of particle momentum similar to that in a
conventional collision.
Cratering flow-field Movement of target materials in an
impact event in response to the passage of the shock and
rarefaction, or decompression, waves.
Critical point Temperature and pressure for a given material
above which there is no distinction between the liquid and gas
phases.
Crust The chemically distinct, less dense, outer shell of a
planet or satellite formed by melting of the interior.
Cryovolcanism Volcanism where the volcanic materials are
melted ices, such as water, ammonia, and methane, as
distinguished from the common high-melting point volcanic
materials of the terrestrial planets, such as basalt and rhyolite.
The melted ices freeze on the surface, forming “lava flows”
composed of ice.
Cryovolcano An icy volcano. See cryovolcanism.
Cumulate Plutonic igneous rock composed of crystals
accumulated by floating or sinking in the silicate melt, or
magma.
Current Strictly, the product of the velocity and number
density of particles. The dot product of the current and the
unit normal vector of a surface yields the net number of
particles crossing the surface per unit area. In the chapter
“Remote Chemical Sensing Using Nuclear Spectroscopy,”
current is taken to be the net number of particles crossing
out of a planetary surface per cosmic ray, which is
dimensionless.
Cycloidal Motion of a cycloid, which is the curve defined by
a fixed point on a wheel as it rolls.
Cyclostrophic balance On Earth, the surface rotation
typically surpasses the zonal winds, and the pressure gradient
force generated by the unequal solar fluxes at low and high
latitudes is balanced by the Coriolis force, in what is called a
geostrophic balance. On Titan (and Venus), the opposite is true,
and pressure gradients are balanced by strong centrifugal forces
arising from the rapid rotation of the atmosphere. This balance,
typical of cyclones, is called cyclostrophic.
Degenerate matter Matter at very high pressures where the
normal atomic structure is destroyed.
Deterministic system Dynamical system in which the
individual bodies move according to fixed laws described
mathematically in the form of equations of motion. A
deterministic system can still give rise to chaotic, unpredictable
motion because of the finite precision with which any physical
measurement or numerical computation can be made.
Deuterium/deuterated Heavy form of the hydrogen atom,
consisting of one proton and one neutron. A deuterated
molecule, such as CH 3 D, deuterated methane, has one or more
deuterium atoms in place of hydrogen.
Diapir A body of rock or ice that has moved upward due to
buoyancy, attaining an inverted teardrop or pear shape, and
piercing and displacing the overlying layers.
Diaplectic glass Glass phase produced from minerals by the
destruction of internal structural order, without melting, by the
passage of a shock wave.
Differentiation Melting and fractionation of a planet, moon
or asteroid into multiple layers or zones of different chemical
composition; e.g., core, mantle and crust. High-density
materials sink and low-density materials float.
Diffraction A physical process in which light from different
parts of a mirror or lens interfere with each other. As a result of
diffraction, the image of a star is not absolutely sharp; instead
the full width at half maximum (FWHM) of the image is given
byθ= 0. 252 ∗λ/Darcseconds, where the wavelength is given by
λin micrometers andDis the telescope diameter in meters. In
the absence of atmospheric seeing, the image of a star would be
diffraction-limited if the optics were polished sufficiently well.
Dipole magnetic field The shape of the field lines around a
short bar magnet. This field can be visualized by sprinkling iron
filings on a piece of paper on top of a bar magnet.
Direct or prograde motion Orbital or axial motion of a
body in the solar system that is counterclockwise as seen from
north of the ecliptic.
Diurnal stresses Twice-daily forces exerted on a planet,
which are derived from gravitational forces between the planet
and a satellite.
Doppler shift Difference between the frequencies of the
radar echo and the transmission, caused by the relative velocity
of the target with respect to the radar. Also, the shift in
frequency of spectral lines due to the motion of a light source
toward or away from an observer.
Drainage basin Geomorphic entity that contains a drainage
network. Typically a bowl-shaped catchment in humid areas,
drainage basins in arid regions can be quite flat. Drainage
patterns typically reflect the topography of the drainage basins
that contain them.
D-type material Primordial, low-albedo material thought to
be rich in organic compounds. It is redder than C-type material.
Ductile Pliable or elastic.
Dust tail The broad, relatively featureless tail of a comet
consisting of micron-sized dust particles being driven away from
the nucleus and coma by solar radiation pressure. The dust
particles are on independent orbits around the Sun under
reduced gravity. The dust tail appears whitish or yellowish from
sunlight scattered by the dust particles.
Dwarf planet A new term created by the IAU in 2006 to
describe bodies orbiting the Sun that are round (in hydrostatic
equilibrium) but are not massive enough to have cleared their
zones. Dwarf planets include Ceres, Pluto, and Eris (2003
UB 313 ). Other main-belt asteroids and Kuiper belt objects are
potential candidates for this classification as more is learned
about them.
Dynamical inertia The increase in the radiative time
constant due to mixing of more massive, deeper layers of an
atmosphere. The thin atmosphere at high altitudes would be