Glossary 931
closed elliptical orbits. Some comets and asteroids are thrown
on to hyperbolic orbits, which are not closed and which will
escape the solar system.
Orbital elements The six parameters that uniquely specify
an object’s orbit and it location within the orbit. Two
parameters, semimajor axis and eccentricity, enumerate the size
and shape of the orbit. Three angles, inclination, longitude of
the ascending node, and argument of perihelion, describe the
orbit’s orientation in space. Finally, the mean anomaly specifies
the position of the object along the orbit.
Orbital phase angle Angular position of a satellite in orbit
about its primary object, measured counterclockwise when
viewed from the north.
Orbit–orbit resonance Condition in which two objects have
orbital periods in the ratio of small integers. Orbit–orbit
resonances are commonly found between Jupiter and minor
planets in the asteroid belt, between Neptune and bodies in the
Kuiper belt, and in the satellite systems of Jupiter and Saturn.
Also known as amean-motion resonance.
Orogenic, orogeny Process of mountain building, with
uplift generally occurring as a result of tectonic plate collisions.
Orphan trail A dust trail that does not appear to be
connected to any cometary source. This might arise as a
consequence of planetary perturbations causing a shift in a
comet’s orbit and disconnecting it from a more distant portion of
its dust trail.
Outflow channels Large channels that start full size and
have few if any tributaries. They may be up to several tens of
kilometers across and thousands of kilometers long and are
believed to have been formed by large floods.
Oxidants Chemical compound that readily transfers oxygen
atoms.
Palimpsest Flattened, circular bright patches on Ganymede
and Callisto that are believed to be the remnants of ancient
large impact structures.
Panspermia A theory by which life spreads through the solar
system and the galaxy by spores carried on dust grains or small
particles.
Parallax The apparent change in the position of a nearby star
on the celestial sphere when measured from opposite sides of
the Earth’s orbit, usually given in seconds of arc.
Parautochthonous Describes a rock unit that has been
moved only slightly into its present location.
Parsec The distance at which a star would have a parallax of 1
arcsec, equal to 206,264.8 AU, or 3.261631 lightyears,
abbreviated: pc. One thousand parsecs are equal to a kiloparsec,
abbreviated: kpc.
Patera (pl., paterae) A collective term for a variety of
unusual, saucer-shaped, shallow volcanic constructs that often
have a central crater or caldera.
Periapse Point on an orbit closest to the central body, called
perihelionfor orbits about the Sun andperigeefor orbits
about the Earth.
Perigee The closest point to the Earth of the elliptical orbit of
the Moon or an artificial satellite.
Perihelion distance (q) The closest distance to the Sun an
object reaches in its orbit, given byq=a(1−e), whereais the
object’s semimajor axis andeis its eccentricity.
Perihelion Point in a heliocentric orbit when it is closest to
the Sun.
Perijove The point in an orbit around Jupiter when the
object is closest to the planet.
Period of late heavy bombardment The period of intense
bombardment of the inner solar system after planetary
formation. It may have been a catastrophic bombardment that
lasted only about 100 million years and peaked at∼3.9 billion
years ago, or a longer bombardment that ended 3.8 billion years
ago.
Periodic comet Traditionally, a comet with an orbital period
of less than 200 years; also known as a short-period comet.
Permafrost zone Near-surface zone within which
temperatures are always below 0◦C. It may or may not contain
ground ice.
Petrology The study of the nature and history of mineralogic
phases and chemical compositions of rocks, and conclusions
regarding their origins. One aim of mineralogy and petrology is
to decipher the history of igneous and metamorphic rocks.
Phase angle Angle between the Sun, a given object, and the
observer with the object at the vertex.
Phase function The curve describing the change in
brightness of a body as a function of the phase angle, the angle
between the observer, the body, and the Sun. Usually expressed
in astronomical magnitudes per degree.
Phase integral Integrated value of the function that
describes the directional scattering properties of a surface.
Phase space Multidimensional space in which the
coordinates are, for example, the positions and the velocities.
Photoautotrophy The capacity of an autotrophic
(self-feeding) organism to derive the energy required for its
growth from sunlight by means of photosynthesis; green plants
are photoautotrophic.
Photolysis Process that occurs when a molecule absorbs light
of sufficiently high energy (usually ultraviolet light) and breaks
apart.
Photosphere A thin, 300-km thick layer above the solar
surface from where most of the optical emission (white light) is
irradiated, with a temperature of∼6000 K.
Phototactic/phototaxis The movement of an organism in
response to light, either toward or away from the source; e.g.,
certain microorganisms are phototactic and will migrate in the
direction of sunlight.
Phylogenetic Refers to organisms that are related to each
other through evolution.
Physiographic Referring to the physical appearance of the
landscape.