polycyclic aromatic hydrocarbons (PAHs)

entirely on spherical surfaces, while the poloidal potential P can only be zero on a spherical surface if the radial component of v is also zero on that surface. It can further be shown that the curl of the toroidal term is poloidal, and the curl of the poloidal term is toroidal.

polycyclic aromatic hydrocarbons (PAHs)

Flat molecules made primarily of carbon atoms arranged in the graphitic form; in hexagons so that their skeleton looks like chicken wire. Pollutants on Earth, they are commonly observed in meteorites, and are widely distributed throughout the galaxy, accounting for 15 to 20% of the galactic carbon. The UV absorption of these stable molecules may be the source of the diffuse interstellar bands, and their emission in the IR is seen towards H II and star forming regions.

polynya An area of the ocean that persists in being either partially or totally free of sea ice under surface conditions where the sea would be expected to be ice covered. Polynyas appear in winter when air temperatures are well below the freezing point of sea water and are bordered by water that is covered with ice. They are typically rectangular or elliptical in shape and tend to recur in the same regions of the Arctic and Southern Ocean. The size of polynyas can range from a few hundred meters to hundreds of kilometers.

Polynyas form via two mechanisms, which often operate simultaneously. In the first, ice is continually formed and removed by winds, ocean currents, or both. The latent heat of fusion of the ice provides the energy necessary to maintain open water. The second mechanism requires oceanic heat to enter a region in quantities sufficient to prevent local ice formation.

Polynyas are sites for active brine formation, which may affect the local water density structure and current field, which in turn modifies large-scale water masses. They are also an interface for gas exchange between the ocean and atmosphere in polar regions. The large sensible heat fluxes (along with fluxes due to evaporation and longwave radiation) tend to dominate regional heat budgets. They are also of biological interest because their regular occurrence makes them important habitats.

polytropic process In thermodynamics, any process that can be described by the statement pV n = constant (for some fixed n). Here p is the pressure in the experiment, and V is the volume of the sample. Thus n = 0 yields p = constant (constant pressure process), n = ∞ yields V = constant (constant volume process). For a process at constant temperature in an ideal gas, n = 1, and for a constant entropy process in an ideal gas, n is the ratio of specific heat at constant pressure to that at constant volume.

poorly graded sediment A sediment with grain sizes that have a small standard deviation. Most grains are very close to the median grain size.

Pop I See Population I.

Pop II See Population II.

Population I Stars having a composition similar to the sun. See also main sequence star, metallicity.

Population II Stars significantly deficient, relative to the sun, in elements beyond helium. See also metallicity, main sequence star.

pore fluid pressure ratio The ratio of pore fluid pressure to lithostatic pressure, defined as

λ = pf − ps pl − ps

where pf is the pore fluid pressure, ps is pressure at the surface, and pl is the lithostatic pressure, the weight of the rock column above containing the pore fluid.

poroelastic medium A porous medium that deforms elastically under loading. A poroelastic medium can be envisaged to consist of an elastic matrix frame hosting an interstitial fluid. The elastic moduli of the matrix frame are defined under the drained condition, a situation in which the pore fluid freely enters and exits the medium such that the pore fluid pressure does not affect matrix deformation. In a poroelastic medium, strain and effective stress of the matrix frame are governed by its constitutive law (such as Hooke’s law for infinitesimal strain),

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potassium-argon age

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 ×104 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-

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, 40K (about 1% of natural abundance), is radioactive and decays with a half life of 1.277×109 years to two different daughter products, 40Ca (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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potential

Potassium-argon ages are reliable to hundreds of millions of years.

potential In nonrelativistic conservative mechanical systems, a scalar function φ from which the mechanical forces are obtained as proportional to φ. Examples are the electric potential and the gravitational potential.

In fluid mechanics, in situations where the fluid velocity vector v is irrotational, v may be obtained as the gradient of a velocity potential.

potential density The density that a parcel of fixed composition would acquire if moved adiabatically to a given pressure level (called reference pressure).

potential energy The ability to do a specific amount of work. In conservative mechanical systems, work (energy) can be done on the system and stored there. This stored work is called potential energy (units of Joules or ergs in metric systems). A prototypical example is a system consisting of a stone moved to the top of a mountain. Work is required to move it to the top; once there, the system contains potential energy, which can be recovered (as heat or as thermal energy) by allowing the stone to roll down the mountain. A related gravitational example is in the storage of water behind a dam; potential energy is converted to work by allowing the water to flow through a turbine. Electrostatics provides another example, in which work must be done to bring a positive charge from infinity to add to a collection of positive charge. Such a collection has potential energy which can be recovered by allowing the charges to freely accelerate away from one another; the energy can then be collected as kinetic energy, as the charges move off to infinity.

Potential energy may also be mechanical (a compressed spring) or chemical, such as the energy stored in unstable compounds (e.g., nitroglycerine) or in flammable substances (e.g., H2 and O2 gases mixed). It also exists in extractable form in the nuclei of heavy atoms (e.g., U235), which release it as kinetic energy when undergoing fission; and in a system of neutrons and protons, which will, at appropriate densities and pressures, release some potential energy as ki-

netic, in fusion to form helium and heavier nuclei.

potential height Dynamic height.

potential instability Also called “convective instability”. Stratification instability caused by convective activities, i.e., the lower layer has higher moisture and becomes saturated first when being lifted, and hence cools thereafter at a slower rate than does the upper, drier portion, until the lapse rate of the whole layer becomes equal to the saturation adiabatic and any further lifting results in instability. In

general,	use	∂θsw	< 0	or	∂θse	< 0 as	the
		∂Z			∂Z
criterion	of	convective		instability. (θsw			and

θse are pseudo-wet-bulb potential temperature and pseudo-equivalent potential temperature, respectively.)

potential temperature The temperature L that a parcel of fixed composition would acquire if moved adiabatically to a given pressure level (called reference pressure). For instance in saltwater, L is the temperature that would be measured after a water parcel, which shows the measured in situ temperature T, has been moved isentropically (without exchanging heat or salt) through the ambient water masses to a reference depth, usually taken at the surface:

dθ = dT − dT . The difference between L

dz dz dz ad

and T is given by θ(z) = T(z)− z	dz ad dz .
z0	dT

potential theory In geophysics, the area that utilizes gravitational potentials to determine the interior structure of a body. Study of a body’s gravitational potential (obtained by perturbations on an orbiting body) provides information about the distribution of interior mass, how much the primary body varies from hydrostatic equilibrium, the response of a body to tidal forces, and how much isostatic equilibrium surface features have undergone.

potential vorticity (Rossby, 1940) In a shallow homogeneous layer, the potential vorticity is a conserved dynamic quantity Q as the ratio between the absolute vorticity and the thickness of the layer, defined by Q = (f + ζ)/H , where f is the planetary vorticity (i.e., Coriolis param-

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prairie

eter) and ζ is the vertical component of relative vorticity; and H is the layer thickness.

In a stratified fluid, the potential vorticity is defined by

Q = ω + 2 · L

ρ

where ω is relative vorticity, is the Earth’s rotation, ρ is density, and L is a conserved quantity. This form is called Ertel potential vorticity (Ertel, 1942). If L is potential temperature, this is also called isentropic potential vorticity.

potentiometric surface (or piezometric surface) Water will rise to the potentiometric surface in a well or piezometer penetrating a confined aquifer. The elevation of the potentiometric surface above an arbitrary datum is the sum of the pressure head and the elevation head, or the hydraulic head h in Darcy’s law. The water table is the potentiometric surface in an unconfined aquifer.

power The rate of doing work, energy per unit time. In mechanics, power P = F · v. The units of power are erg/sec, or Joule/sec. Note that 1 Joule/sec ≡ 1 Watt.

power-law fluid A viscous fluid in which the rate of shear strain is proportional to the power of shear stress. The flow law is often

˙	=	Cσ n−1		˙
generalized to be εij				σ ij , where εij
is the rate of strain,	σ ij		is deviatoric stress,

σ = (σ ij σ ij /2)1/2 is an invariant of σ ij , and C is a quantity independent of strain rate and stress. An effective viscosity can be defined as µe = (2Cσ n−1)−1, such that the flow law takes the form of the Newtonian fluid σ ij = 2µeε˙ij . A solid such as a rock deforms like a power law fluid as a result of dislocation creep at high temperatures and stresses.

Poynting–Robertson effect A drag force arising on particles orbiting the sun, because solar radiation striking the leading surface is blueshifted compared to that striking the following surface. Thus, the particles receive a component of momentum from the radiation pressure which is opposite to the direction of motion. This drag force causes interplanetary dust particles to spiral inward towards the sun, removing such parti-

cles from the solar system. For 10 micron-sized dust particles at the Earth’s orbit, the Poynting– Robertson effect will cause these particles to spiral into the sun on a time scale of only 1 million years. The presence of interplanetary dust particles even today thus indicates that such material is continuously being resupplied by comets and collisions among asteroids. In very accurately tracked satellites (e.g., LAGEOS) the Poynting– Robertson effect is important in long-term accurate modeling of the orbit.

p process The set of nuclear reactions responsible for producing the rare nuclides of the heavy elements with proton to neutron ratio higher than the ratio in the most tightly bound nuclides of each element. It acts on products of the s process and r process either by adding protons or (more likely) removing neutrons, and probably occurs in supernovae and their environs. No element has a p-process nuclide as its most abundant isotope, and so we know nothing about the abundance of p-process products outside the solar system.

pp-waves (1923) In general relativity, a particular description of gravitational waves in a matter-free space, described by the metric

ds2	=	2 du dv	−	¯ + 2H(u, ζ, ¯
				2 dζdζ	ζ) du2

discovered by Brinkmann. The Weyl conformal curvature is Petrov type N, and the principal null direction is covariantly constant.

= + ¯ ¯

H f (ζ, u) f ζ, u .

Here ζ is a complex stereographic spherical coordinate. In another representation

ds2 = 2H(x, y, z) dt2 − dx2 − dy2 − 2 dz dt

where the function H(x, y, z) satisfies the elliptic equation

	∂2		∂2	H = 0 .
		+
	∂x2		∂y2

The normals of the wave fronts are covariantly constant vectors, hence the waves are planefronted. See Petrov types, Weyl tensor.

prairie An extensive level or rolling grassland consisting of rich soil and a variety of

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