920 Encyclopedia of the Solar System
Alfven speed ́ The speed of propagation of disturbances in a
magnetized plasma that bend a magnetic field without changing
its magnitude.
Alfven waves ́ A wave propagating in a magnetized plasma in
which the magnetic field oscillates transverse to the propagation
direction. The propagation speed is given by the Alfv ́en speed
vA=B/
√
4 πρ,whereBis the magnetic field strength andρis
the mass density.
Algae Any of a large group of mostly aquatic organisms that
contain chlorophyll and other pigments and can carry on
photosynthesis, but that lack true roots, stems, or leaves; they
range from microscopic unicellular organisms to very large
multicellular structures.
Aliasing Overlapping of radar echos at different frequencies
or at different time delays.
Alkali plagioclase An aluminum silicate mineral rich in
sodium (seeFeldspar).
Allochthonous Describes a rock unit that has been moved
into its present location.
Alpha particle Helium nucleus having mass four times and
charge twice that of a proton.
Amino acid Any organic compound containing an amino acid
(–NH 2 ) and a carboxyl (–COOH) group; specifically, one of the
so-called building blocks of life, a group of 20 such compounds
from which proteins are synthesized during ribosomal
translation of messenger RNA.
Amorphous Having no crystalline form.
Angle of repose The maximum slope at which loose material
does not fall downhill.
Angular momentum Property of orbiting or rotating
objects, usually expressed asmvr, wheremis the mass,
vis the velocity, andris the distance from the center of
rotation. The Earth and Moon have orbital angular
momentum on account of their revolution around the
Sun and spin angular momentum because of axial rotation.
Angular momentum is conserved unless forces act to
change it.
Anhydrous silicates Silicates lacking in water content.
Anomalistic month The time between successive passages of
the Moon through perigee.
Anorthosite An igneous rock formed almost exclusively of
plagioclase. It forms the outer layer of the Moon.
Antenna gain Ratio of an antenna’s sensitivity in the
direction toward which it is pointed to its average sensitivity in
all directions.
Antipodes The opposite points on the surface of a sphere,
given by a line through the center of the sphere.
Aphelion The point in the elliptical orbit of a planet, comet,
or asteroid farthest from the Sun.
Aphelion distance (Q) The farthest distance from the Sun of
an object in an elliptical orbit, given byQ=a(1+e), whereais
the object’s semimajor axis andeis its eccentricity.
Apoapse Point on an orbit farthest from the center of gravity,
calledaphelionfor orbits about the Sun andapogeefor orbits
about the Earth.
Apogee The point in the orbit of the Moon or an artificial
satellite, furthest away from the Earth.
Apojove The point in an orbit around Jupiter, farthest from
the planet.
Apse Informal synonym forLongitude of perihelion.
Arc seconds, arcsec, second of arc An angle equal to 1/3600
of a degree, or 1/60 of an arc minute. The Sun subtends an angle
of∼1919 arcsec on average when viewed from the Earth.
Arc minute, arcmin, minute of arc An angle equal to
1/60th of a degree, or 60 arc seconds.
Areocentric Sun longitude (Ls) An angular measure of the
Martian year.LS= 0 ◦corresponds to the vernal equinox,
marking the beginning of Spring in the northern hemisphere.
LS= 90 ◦, 180◦, and 270◦correspond to the summer solstice,
autumnal equinox, and winter solstice, respectively.
Argument of perihelion (ω) In an orbit around the Sun, the
angle between the ascending node and the perihelion point,
measured in the body’s orbital plane and along its direction of
motion.
Asteroid A rocky, carbonaceous, or metallic body, smaller
than a planet and orbiting the Sun. Most asteroids are in
semistable orbits between Mars and Jupiter, but others are
thrown onto orbits crossing those of the major planets. Also
called aminor planet.
Asthenosphere A low viscosity zone that lies between the
lithosphere and the mantle.
Astrology A belief system in which the future of individuals
is predicted based upon the date and location of their birth and
the positions of the moon and planets relative to the Sun or
Earth at specific times.
Astronomical unit (AU) Commonly thought of as the mean
distance of the Earth from the Sun. It is more formally the
distance at which a massless particle in a circular orbit would
have an orbital period of 1 Gaussian year, equal to
365.256898326...days. It is equal to 149.59787066× 106 km,
or about 92.955807× 106 miles.
Aurora Atmospheric emissions excited by the precipitation
of energetic magnetospheric and solar particles, most frequently
at high latitudes.
Autochthonous Describes a rock unit that has been formed
in place.
Autotrophy Literally, self-feeding; the capacity of an
organism to obtain its essential nutrients by synthesizing
nonorganic materials from the environment, rather than by
consuming organic materials; photosynthetic green plants and
chemosynthetic bacteria are examples of autotrophic organisms.
(B-R) color A color scale for astronomical objects. Light from
astronomical objects consists of all the colors of the rainbow:
red, orange, yellow, green, blue, indigo, and violet (roygbiv). The