100-C 25-C 50-C 75-C C+M 50-C+M C+Y 50-C+Y M+Y 50-M+Y 100-M 25-M 50-M 75-M 100-Y 25-Y 50-Y 75-Y 100-K 25-K 25-19-19 50-K50-40-40 75-K 75-64-64Glossary
Ablation Removal of material. Meteors ablate during their
passage through the atmosphere.
Absolute magnitude (H) A measure of the brightness of an
object. It is defined as the brightness if the object were at 1 AU
each from the Sun and the Earth, and viewed at 0 degrees phase
angle. For a given albedo, smaller absolute magnitudes
correspond to larger objects. A difference of 5 absolute
magnitudes corresponds to a factor of 10 in radius for objects
with the same albedo.
Accretion The process of building larger bodies from smaller
ones through low velocity collisions where the particles stick to
one another, or where the gravity of a relatively large body
draws the smaller bodies to it.
Achondrite Differentiated igneous stony meteorite,
apparently solidified from a magma.
Actinic flux The solar flux used in calculating
photodissociation rates, corresponding to the mean intensity at a
given point in the atmosphere.
Active optics The controlled deformation or displacement of
optics to compensate for slowly varying effects such as flexure or
temperature changes. Typical timescale for updates is longer
than 1 second.
Adaptive optics An observational technique where the phase
perturbations induced by the Earth’s atmospheric turbulence,
responsible for the blur in the images obtained, is corrected in
real-time on the incident wavefront reaching the telescope.
These perturbations are measured by a wavefront sensor.
Opposite phase corrections are then applied using a thin
deformable mirror in the pupil plane. The timescale for updates
is typically about 1/1000 of a second.
Adiabat A process occurring without exchange of heat with
the surroundings. In an atmosphere, an adiabatic temperature
gradient (about−10 K/ km for Venus) is commonly found in
regions of rapid vertical motion.
Adiabatic compression Compression of a gas without
exchange of heat. Expansion or compression of rising or sinking
air masses in planetary atmospheres is commonly assumed to be
driven by adiabatic processes.
Adiabatic temperature lapse rate (or temperature
gradient) For an atmosphere that is marginally unstable to
convection, and where there is no heat transfer between the
rising and sinking parcels of air with the environment, the
temperature profile with altitude follows a so-called adiabat.
The dry adiabatic lapse rate on Earth is roughly−7 K/km.
Adsorption The formation of a thin layer of gas, liquid,
or solid on the surface of a solid or, more rarely, a liquid.
There are two types. A single layer of molecules, atoms, or
ions can be attached to a surface by chemical bonds.
Alternatively, molecules can be held onto a surface by weaker
physical forces.
Aerosol In atmospheric physics, aerosol is a generic name for
any particle (cloud, dust, haze) suspended in the air, although in
the Earth science community the term is usually restricted to
apply to haze rather than cloud particles.
Age Time elapsed since some event at a discrete time, to.
Agglutinate Common particle in the lunar soil, usually about
60 μm in size, consisting of rock, mineral, and glass fragments
bonded together by glass (that also contains submicron metal
droplets) produced by meteorite impact.
Airglow The emission of light by an atmosphere. Airglow may
result from resonant scattering, fluorescence, impact by charged
particles, or radiative decay of atoms, ions, or molecules left in
an excited state by some chemical reaction.
Albedo (p) A ratio of scattered to incident electromagnetic
radiation power, most commonly light. It is a unitless measure of
a surface or body’s reflectivity. The geometric albedo of an
astronomical body is the ratio of its total brightness at zero
phase angle to that of an idealized fully reflecting, diffusively
scattering (Lambertian) disk with the same cross section. The
visual geometric albedo refers to this quantity when taking into
account only electromagnetic radiation in the visual range. The
bond albedo is the fraction of total power in the electromagnetic
incident radiation that is scattered back out into space, taking
into account all wavelengths. The bond albedo (A) is related to
the geometric albedo (p) by the expressionA=pq, whereqis
the phase integral.