DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

ductile behavior
variables of space and time along the string.
It was used to reproduce the basic features of
the Witten conducting string in the Carter–Peter
model. See Carter–Peter model, cosmic string,
cosmic topological defect, current carrier (cosmic
string), elastic string model, electric regime
(cosmic string), energy per unit length (cosmic
string), equation of state (cosmic string), tension
(cosmic string).
ductile behavior The phenomenological
macroscopic nature of solid material deformation.
In general ductile behavior refers to the
capability for substantial change of shape without
gross fracturing. The deformation processes
that may enter into ductile behavior can be distinguished
as cataclastic flow, crystal plasticity,
and diffusion flow/grain-sliding. Which combination
of processes will be involved in forming
ductile behavior depends greatly on the properties
of the material itself and the deformation
conditions such as pressure, temperature, and
strain rate.
dune Any deposit of sand-sized (1/16 to
2 mm in diameter) windblown material. Dunes
are common on bodies with an atmosphere,
having been seen on Earth, Mars, and Venus.
Dunes typically form in areas with a prevailing
wind direction and an abundant source of sand.
Dunes often migrate, as wind blows the sand
from one deposit into another area to build up a
new deposit. Vegetation and other obstructions
cause dunes to stabilize and cease their migration.
There are three primary classes of dunes:
(1) longitudinal dunes, which are oriented parallel
to the prevailing wind direction; (2) transverse
dunes, which are oriented perpendicular
to the wind direction; and (3) parabolic dunes,
which are U-shaped with the dune pointing in
the downwind direction. Many subclasses of
dunes occur, depending on differences in wind,
sand supply, and vegetation.
dusk The part of morning or evening twilight
between complete darkness and civil twilight.
See twilight, civil twilight, nautical twilight, astronomical
twilight.
dust In astrophysics, grains formed in the
envelopes of cool stars, and blown outward via
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stellar winds and radiation pressure, or formed
in supernova explosions. Interstellar dust has
typical radii of order a ∼ 1 µm to 10 µm and an
exponential size distribution n(a) ∝ aδ , where
n is the number density. The exponent δ is of
order 3.5. Dust plays a substantial part in star
forming regions and in the foundation of planetary
systems in accretion onto young stars, since
the dust has a strong influence on radiative effects.
In geophysics, aerosols (or potential aerosols)
of order 1 µm in size which have a large effect in
the atmospheric energy balance, as well as have
dramatic effects on local climate.
In relativity, a continuous medium having
a well-defined mass-density and velocity, but
whose pressure is equal to zero. It follows
from Einstein’s equations that each point of dust
moves on a geodesic. Dust is an approximate
model of the matter distribution in the universe
at the current stage of its evolution (in the early
phase of the evolution, pressure cannot be neglected).
dust storm Wind and elevated suspended
dust that visibly reduces atmospheric transparency.
dust storm (Mars) In southern summer of
Mars, global dust storms break out sometimes in
mid-latitudes of the southern hemisphere. They
were photographically observed in 1956, 1971,
1973, and 1977. The dust is wafted on the easterlies,
and encircles Mars along a latitude line.
At the same time the Hadley cell carries the
dust to the northern hemisphere. After about
two weeks the whole of Mars, except polar regions,
is obscured by the dust. Although albedo
features become visible after a month or so, it
takes a Martian year for the atmosphere to return
to the clear state of before the dust storm.
Global dust storms have not yet been observed
in the northern summer when Mars is near aphelion.
Local dust storms break out in all seasons
in both hemispheres, especially in low latitudes
and near polar caps. Dust clouds seem to be yellowish
to visual observers. However, the true
color is reddish: dust clouds are bright in red,
but not identified in blue. Some observers report
that dust clouds in their initial state, in a day or