DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

temperature is ∼ 10 K everything except H2,
He, and Ne freezes out and ice mantles form
on grains. In regions where H < H2 the less
abundant heavier atoms such as O, C, and N can
react with one another to form oxidized species
such as CO, CO2, and presumably O2 and N2.
Where H > H2 the heavy atoms are reduced and
the ice mantles are dominated by molecules such
as H2O, CH4, and NH3. When these ices are exposed
to cosmic rays and UV photons, bonds are
broken and the resulting unstable species react
to form more complex materials, in some cases
organic molecules of the type seen in meteorites
and interplanetary dust particles.
grain chemistry: diffuse interstellar In the
diffuse interstellar medium (i.e., visual extinction
Av ≤ 2 magnitudes, or nH ≤∼ 10 1 cm −3 ),
where ice is not stable, the dust is essentially
bare and grain surfaces are available as catalysts
and reactants. Since hydrogen is the most abundant
atom, H2 formation from hydrogen atoms
is by far the most common surface reaction.
grains: in other galaxies Extragalactic extinction
curves are comparable to those within
the Milky Way galaxy, consistent with grains in
other galaxies being of similar size and composition
to those in our own. Most spiral galaxies
seem to have similar dust-to-gas ratios as in our
own, although there are some exceptions.
grains, interstellar: destruction and formation
Grains presumably form in the outflows
from cool evolved (red giant) stars, novae and
supernovae ejecta and grow as they mix with interstellar
molecules, accreting ice mantles and
sticking to one another. Such interstellar grains
may be fragmented or destroyed by shocks, collisions,
sputtering, or incorporation into forming
stars. A typical grain lifetime is thought to be
approximately 10 8 years.
grains, interstellar: size and composition
A wide-spread interstellar grain size is inferred
from optical, IR, and UV extinction curves
throughout our galaxy. The grains are usually
deduced to have sizes in the range of 0.01
to 0.2 µm, are probably of irregular (perhaps
fractal) shape, and composed of (presumably
amorphous) silicates, carbon, SiC, or metal ox-
© 2001 by CRC Press LLC
grand unification
ides. The position and profile of the 2175
Å bump is consistent with some of the carbon
grains being graphitic, or composed from
amorphous or diamond-like carbon containing
graphitic (π-bonded) domains. In the diffuse
instersellar medium where radiation is abundant,
these grains are probably bare, but in dense
molecular clouds, where the radiation is attenuated
and the average temperature low, ice mantles
condense on the surfaces of these grains.
grain size A measure of particle size in a
sediment. Often refers to the median grain size
in a sample. Typically determined by sieving.
grain size analysis A process of determining
sediment grain size. Most commonly done
using a stack of sieves of varying size.
grand potential The thermodynamic potential
equal to
= U − TS− µN ,
with U the internal energy, S the entropy, µ the
chemical potential, and N the number of particles
in the system.
grand unification Modern particle physics
describes interactions through the exchange of
so-called gauge particles, whose existence is due
to an underlying symmetry. For instance, the
gauge particle we know as the photon exists because
of the invariance of the physics under local
phase variation, which is a U(1) symmetry.
When the universe had a temperature corresponding
to a typical photon energy of roughly
250 GeV, the symmetry of particle physics interactions
is believed to have been that of the group
product SU(3) × SU(2) × U(1) describing the
strong and electroweak interactions.
Whenever a symmetry is exact, as is the case
for electromagnetism, the corresponding gauge
particle is massless (as is the photon), while if it
is broken, the gauge particle is massive and the
interaction is much weaker (short range). This is
in particular the case for the weak interaction. It
is currently expected that the product symmetry
we have at the electroweak level is just part of
a larger (simple and compact) symmetry group
that unifies forces and interaction — hence the
name grand unification. This larger group could
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