Glossary 933
Radiometric modeling Measures the thermal emission of
an asteroid to provide an estimate of the asteroid’s surface
temperature and albedo. A dark asteroid, for example, would
absorb more of the visible sunlight because it has a low albedo,
but it would radiate that additional energy at thermal
wavelengths, showing a warmer surface temperature. Combined
data on thermal “temperature,” and visible reflectance can
provide the albedo of an object and an estimate of its size.
Radio occultation The passing of a radio beam through a
planet’s atmosphere. Attenuation and refraction (bending) of
the beam, generally by phase delay, can be used to measure the
density of electrons in the planet’s ionosphere and the density of
the gas in its atmosphere. The abrupt cutoff of the signal can also
be used to make a precise measurement of the planet’s radius.
Raman scattering Inelastic scattering of sunlight by gas
molecules in an atmosphere, such that the scattered photon is
shifted in frequency.
Random walk A series of movements in which the direction
and size of each move is randomly determined.
Rayleigh scattering The scattering of sunlight by gas
molecules in an atmosphere.
Reaction notation The notation for nuclear reactions given
byT(i,p)R, whereTis the target nucleus,iis the incident
particle,pindicates the particles(s) produced by the reaction,
andRis the product nucleus. For example, the notation for
neutron inelastic scattering with^56 Fe is^56 Fe(n,n′γ), where n′is
the scattered neutron andγis the associated gamma ray.
Similarly, neutron capture with^56 Fe is denoted^56 Fe(n,γ)^57 Fe,
where the product isotope has been appended.
Reaction rate The rate (R) at which gamma rays or neutrons
interact with nuclei is given by the product of the flux (φ), the
number density of the target nuclei (Nnuclei per cm^3 ) and the
microscopic cross section (φ) for the selected reaction:
R=φNσ(interactions per unit time).
Reconnection A process in which the magnetic configuration
changes as if two field lines were broken and reconnected in a
new configuration. This can occur when two plasmas containing
oppositely directed magnetic fields flow toward each other.
Reductants Compounds or catalysts that result in the loss of
an electron.
Reflectance spectroscopy The study of the physical and
mineralogical properties of materials over the wavelength range
of reflected electromagnetic radiation. Light interacts with the
atoms and crystal structure of materials producing a diagnostic
set of absorptions and reflectances.
Refraction Bending of a light ray as it traverses a boundary,
for example, between air and glass or between space and an
atmosphere.
Refractories Materials not deformed or damaged by high
temperatures. Classic refractories are high-melting oxides, like
silica and alumina, but also carbides, nitrides, sulfides, and pure
carbon. In our terminology, refractories are materials that are
not modified by space conditions (temperature and vacuum) in
the inner solar system. The opposite are volatile materials, e.g.,
ices that rapidly sublimate close to the Sun.
Refractory inclusion SeeCAI.
Regolith The outermost unconsolidated fragmental layer on
some airless planets, satellites and asteroids that results from the
breakup of rocks by repeated impacts of meteoroids.
Regular motion A trajectory that does not display chaos.
Regular satellite A satellite with low orbital eccentricity and
inclination.
Resonance A situation in which two orbiting bodies have
orbital frequencies (related to the time they take to complete
their orbits or for their orbits to precess) that are in a simple
integer ratio. Objects in resonance exert a regular gravitational
influence on each other. Depending upon the particular
resonance involved, resonance can either stabilize an orbit, as in
the case of Pluto, or destabilize an orbit as near the Kirkwood
gaps in the asteroid belt. Strong satellite resonances open gaps
at particular locations in broad planetary rings; weaker ones
drive radial and vertical wave trains.
Retrograde, retrograde motion Orbital or rotational
motion in the solar system that is clockwise as seen from north
of the ecliptic. Nearly isotropic comets with inclinations greater
than 90◦have retrograde orbits. Triton is in a retrograde orbit
around Neptune. Venus is in retrograde rotation.
Returning comet A nearly isotropic or long-period comet
that is returning to the planetary region for at least the second
time. Returning comets are usually taken to be nearly isotropic
comets with semimajor axes less than 10,000 AU.
Reynolds number Dimensionless number that governs the
conditions for the occurrence of turbulence in fluids.
Ring current A current carried by energetic particles that
flows at radial distances beyond a few planetary radii in the
near-equatorial regions of a planetary magnetosphere.
Roche limit, Roche zone The distance from a planet or the
Sun, within which another body will be disrupted because tidal
forces from the planet exceed the self-gravity of the smaller
body, unless the material strength of the body is strong enough
to hold it together. For nonrotating bodies of equal density and
zero strength, the Roche limit is about 2.2 planetary radii.
Rock Mixture of iron, silicon, magnesium, and other
refractory elements found in the interiors of jovian planets.
Rotational lightcurve A graph depicting the variation in
brightness of an object verus time as it rotates on its axis. This
variation can be caused by nonsphericity (i.e., shape effects) or
albedo markings; for objects as large as Pluto and Charon,
albedo markings usually dominate. See alsolightcurve.
Runaway growth Stage of planetary growth in which the
largest planetesimals grow rapidly while most others remain
small.
Satellite A body in orbit around a planet, dwarf planet, or an
asteroid. Also called amoon.
Scale height The vertical distance over which atmospheric
pressure or density falls by 1/e= 0 .368; equal tokT/mg, where