DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

and fluid flow obeys Darcy’s law. See effective
stress, Darcy’s law.
porosity (φ) The ratio of the volume of void
spaces in a soil or rock to the total volume of the
rock or soil: φ = (Va +Vw)/Vs, where Va is the
volume of air in the sample, Vw is the volume of
water in the sample, and Vs is the total volume
of soil or rock in the sample.
Portia Moon of Uranus also designated
UXII. Discovered by Voyager 2 in 1986, it is
a small, irregular body, approximately 55 km in
radius. Its orbit has an eccentricity of 0, an inclination
of 0 ◦ , and a semimajor axis of 6.61×10 4
km. Its surface is very dark, with a geometric
albedo of less than 0.1. Its mass has not been
measured. It orbits Uranus once every 0.513
Earth days.
position vector In a Euclidean space, the
vector pointing from the origin of the reference
frame to the location of a particle, thus specifying
the coordinates of the position of the particle.
positron The antiparticle of the electron. It
has the same mass and spin as an electron, and
an equal but opposite charge.
POSS Acronym for Palomar Observatory
Sky Survey, a survey that produced a collection
of several hundreds of wide field, deep blue and
red photographic plates originally covering the
northern sky down to declination δ ≈−24 ◦ obtained
with the Oshkin telescope at Palomar observatory,
in the years from 1950 to 1955. With
a scale of 67"/mm, and a limiting magnitude of
≈ 20 in the red, the POSS plates have been instrumental
to any source identification, including
faint galaxies and quasars, for which later
observations were being planned. The POSS
plates have been digitized and supplemented
with observations obtained in the southern hemisphere
of similar scale and limiting magnitude to
cover the entire sky. The Digitized Sky Survey
(DSS) is stored on a set of 102 commercially
available compact disks, covers the entire sky
and includes an astrometric solution for each
Schmidt plate, to readily obtain equatorial coordinates.
In more recent years a second generation
survey has been carried out at Palomar em-
© 2001 by CRC Press LLC
potassium-argon age
ploying plates with finer emulsions. This second
generation survey, known as POSS-II, was
almost completed and in large part digitized as
of 1999.
post-flare loops A loop prominence system
often seen after a major two-ribbon flare, which
bridges the ribbons. Post-flare loops are frequently
observed to emit brightly in soft X-rays,
EUV, and Hα.
postglacial rebound Because the solid
Earth’s mantle has a fluid behavior, a glacial
load depresses the Earth’s surface. The center
of Greenland is depressed below sea level due
to the load of the Greenland ice sheet. During
the last ice age, from 20,000 to 10,000 years ago,
great ice sheets covered much of North America
and Europe and depressed these regions. After
the melting of these ice sheets the regions
rebounded. Postglacial rebound continues in
northern Canada and Scandinavia at rates of approximately
1 cm per year. Evidence for this
rebound comes from dated, elevated wave cut
terraces. Postglacial rebound indicates that the
interior of the Earth deforms as a viscous or viscoelastic
material and the rate of postglacial rebound
provides a quantitative measure of the
viscosity of the Earth’s mantle.
post-seismic relaxation Gradual decrease of
earthquake-induced deviatoric stress as a result
of the viscoelastic behavior of the Earth material.
potassium-argon age A naturally occurring
isotope, 40 K (about 1% of natural abundance),
is radioactive and decays with a half life of
1.277×10 9 years to two different daughter products,
40 Ca (β decay) and argon-40 (branching
ratio 10.72%, electron capture). Argon is a gas;
whenever rock is melted to become magma or
lava, the argon tends to escape. Once the molten
material hardens, it again begins to trap the argon
produced from its potassium. In this way
the potassium-argon clock is clearly reset when
an igneous rock is formed. The main complication
to this simple determination is contamination
from included air containing argon that
was in radioactive decay since the last melting.
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