- radiation
- Energy radiated in the form of waves or particles; photons.
radiationbelt
- Regions of charged particles in a
magnetosphere.
red giant
- A star that has low surface temperature and a diameter that is
large relative to the Sun.
regio
- Region.
regolith
- The layer of rocky debris and dust made by metoritic impact that
forms the uppermost surface of planets, satellites and asteroids.
relativity, Theory of
- More accurately describes the motions of bodies in strong
gravitational fields or at near the speed of light than
newtonian mechanics. All
experiments done to date agree with relativity's predictions to a
high degree of accuracy. (Curiously,
Einstein received the Nobel prize in 1921 not for Relativity
but rather for his 1905 work on the photoelectric effect.)
resolution
- The amount of small detail visible in an image; low resolution
shows only large features, high resolution shows many small details.
resonance
- A relationship in which the orbital period of one body is related
to that of another by a simple integer fraction, such as 1/2, 2/3, 3/5.
retrograde
- The rotation or orbital motion of an object
in a clockwise direction when viewed
from the north pole of the ecliptic;
moving in the opposite sense from the great majority of solar system bodies.
rhyolite
- Fine-grained extrusive igneous rock, commonly with phenocrysts of
quartz and feldspar in a glassy groundmass.
rift
- A fracture or crack in a planet's surface caused by extension.
On some volcanoes, subsurface intrusions are concentrated in certain
directions; this causes tension at the surface and also means that
there will be more eruptions in these "rift zones."
rift valley
- An elongated valley formed by the depression of a block of the
planet's crust between two faults or groups of faults of approximately
parallel strike.
rima
- A fissure.
Roche limit
- The closest a fluid body can orbit to its parent planet without being
pulled apart by tidal forces.
rupes
- The term applied to scarps on planetary surfaces;
many scarps are thought to be the surface expression of faults within
the crust of the planetary object.
