PAIR PRODUCTION - Creation of a particle and its antiparticle from some form of energy, such as photons. An example is the creation of an electron and a positron from two γ-ray photons with energies greater than the rest mass energy of the electron:
PALLASITE - One of two main classes of stony-iron meteorite, the other being mesosiderites. Pallasites are igneous in nature and characterized by crystals of olivine, often peridots (clear, green gem quality olivine crystals), embedded in a matrix of Ni-Fe metal. The type specimen, weighing 680 kg, was found in the mountains near Krasnojarsk, Siberia, and first documented by the German naturalist Peter Pallas in 1772, though it was some decades later before people recognized the extraterrestrial nature of what became known as the Pallas iron. Pallasites are believed to have come from the core/mantle boundary of differentiated asteroids that were broken apart by impact. In most cases, they have chemical, elemental, and isotopic features that link them to specific chemical groups of iron meteorites, suggesting that they come from the same parent bodies as these irons.
PANCAKE VOLCANO - Volcanic feature on Venus formed by high viscosity lava.
PARSEC (pc) - Unit of distance commonly used by astronomers. A star one parsec away has a parallax angle of one second of arc. A star with a parallax shift of 0.1 arcseconds is at a distance of 10 parsecs, and so forth. A parsec is equal to ~3.262 lightyears (3.09 x 1016 m) or 206,265 AU. Multiples of this unit are kiloparsecs (kpc) = 103 pc and megaparsec (Mpc) = 106 pc.
PASCAL - Standard unit of pressure abbreviated Pa, which is equivalent to 1 kg/m2. The pressure at the surface of the Earth is 100,000 Pa. Pressures inside planets are very large numbers, usually expressed as GPa.
PAULI EXCLUSION PRINCIPLE - First proposed by Wolfgang Pauli to explain the arrangement of electrons in atoms, the Exclusion Principle asserts that no two fermions of the same type can exist in the same state (having the same quantum numbers) at the same place and time. Bosons do not obey the Pauli Exclusion Principle.
PEAK RING - Central uplift characterized by a ring of peaks rather than a single peak. Peak rings are typical of larger terrestrial craters above ~50 km in diameter.
PENTLANDITE - Fe-Ni sulfide, (Fe,Ni)9S8, often associated with troilite, found in the matrix and chondrules of CO, CV, CK and CR chondrites.
PERIHELION - The point in an object's orbit when it is closest to the sun.
PERIOD-LUMINOSITY RELATION - Relation between the pulsation period of a variable star and its absolute brightness. Measurement of the pulsation period allows the distance of the star to be determined.
PEROVSKITE - Ca-Ti oxide found in CAIs in carbonaceous chondrites; its formula is CaTiO3. The composition (Mg,Fe)SiO3 has a perovskite structure and is a major component of Earth's lower mantle (shown).
PETROLOGY - Science dealing with the origin, history, occurrence, chemical composition, structure and classification of rocks.
PETROLOGIC TYPE - Measure of the degree of aqueous alteration (Types 1 and 2) and thermal metamorphism (Types 3-6) experienced by a chondritic meteorite. Type 3 chondrites are further subdivided into 3.0 through 3.9 subtypes.
PHONON - Quantum of vibration excited by the acoustic mode of a crystal lattice; the vibration is usually thermally excited.
PHONON EMISSION - Generation of phonons in response to electron scattering within the crystal lattice, anharmonic lattice forces due to the interaction with other phonons, or X-ray or high-energy particle bombardment.
PHOSPHOR - Substance that emits light when excited by incident radiation or particles.
PHOTODISSOCIATION - Breakup of molecules resulting from the absorption of photons, typically occurring where energetic radiation from massive stars is present. Photodissociation is not limited to visible light, but to have enough energy to breakup a molecule, the photon is likely to be an electromagnetic wave with the energy of visible light or higher, such as ultraviolet light, x-rays and γ-rays.
PHOTOELECTRIC EFFECT - Ejection of a bound electron by an incident photon (x-ray or γ-ray). The photon disappears and the incident energy is shared between the ejected electron and the atom. The photon energy must exceed the atomic binding energy. The probability for the photoelectric effect is approximately proportional to Z5 of the absorber and falls of by ~Eγ3.5.
PHOTOEVAPORATION - An interstellar cloud can be photoevaporated when an ionization front plows through and heats the gas, which expands and escapes in an ionized supersonic wind.
PHOTON - Discrete bundle of light energy. Light of a given energy (frequency) cannot be broken up indefinitely. Rather for a given frequency it comes in discrete bundles with energy (h = Planck's constant and ν= frequency):
It is often useful to think of light as a bunch of particle photons; whereas, at other times it is useful to think of light as a wave. This is termed the "wave-particle duality." The idea of the photon was advanced by Albert Einstein in his theory of the photoelectric effect.
PHOTOSPHERE - Point where a star's atmosphere appears to become completely opaque. Thus, the photosphere may be thought of as the imaginary surface from which the starlight that we see appears to be emitted. The photosphere is not a thin surface; the Sun's photosphere has a thickness of ~100 km. (Within that 100 km, the temperature drops from 6000 K at the bottom to 4000 K at the top.) Diameters quoted for stars are the diameter of the photosphere.
PHYLLOSILICATES - Class of hydroxyl-bearing silicate minerals with a sheet-like structure. They result from aqueous alteration are dominantly serpentine and smectite in meteorites; found in the matrixes of carbonaceous chondrites.
PIGEONITE - Low-Ca clinopyroxene found as a major mineral in eucrites and shergottites.
PLAGE - Bright cloud-like feature found around sunspots that represent a region of higher temperature and density within the chromosphere. Plages are particularly visible when photographed through filters passing H or Ca spectral lines.
PLAGIOCLASE - Common rock-forming series of feldspar: (Na1-x,Cax)(Alx,Si1-x)Si2O8.
PLANAR DEFORMATION FEATURES (PDFs) - Microscopic features in grains of quartz or feldspar (for example) consisting of very narrow planes of glassy material arranged in parallel sets that have distinct orientations with respect to the grain's crystal structure.
PLANET MIGRATION - Major reduction in the orbit of planet likely to take place in very young planetary systems. Favored explanation for the small orbits of most extrasolar planets found to date. Two hypotheses have been suggested to account for such drastic orbital shifts.
In the first, near encounters with other large worlds modifies the orbits. Migration requires some interaction between the planet and fairly large bodies or the gravitational forces are too weak. According to this model, early in the formation of the Solar System, there were lots of Moon-sized to Mars-sized bodies, especially in the outer Solar System. A large planetesimal that crosses near Neptune will lose some energy, fall down near Jupiter and gain energy from it to be ejected into the Oort Cloud. This decreases the size of Jupiter's orbit, and expands the orbits of Saturn, Uranus and Neptune. As Neptune moves outward, it will perturb the orbits of the trans-Neptunian objects (large ice covered bodies of which Pluto is a member). This pushes Pluto/Charon into a highly eccentric, inclined 3:2 resonant orbit that it occupies today.
In the second model, which does not exclude operation of the first, a planet experiences viscous drag as it plows through the remnants of the dusty nebula from which it formed, losing energy as it goes. Both theory and observation suggest that the migration time to a small orbit for a giant planet formed at a radius of 5 AU is <1 Ma. A migrating planet must avoid terminal orbital decay and a disastrous collision with its central star. It may be that the magnetic field of the star sweeps an inner belt or region clear of gas and dust so that the drag on the planet abruptly stops when it reaches this open zone. The planet may remain parked at the outer edge of that hole.
PLANETARY NEBULA - Emission nebula formed when a red giant star blows off its outer envelope. A planetary nebula results when the ejected material is heated by strong UV radiation from the hot central star. Planetary nebulae, so named because of their round shapes, disperse in a few tens of thousands of years into the interstellar medium.
PLANETESIMALS - Bodies from ~1-100 km in size, formed in the early solar system by accretion of dust (rock) and ice (if present) in the central plane of the solar nebula. Most planetesimals accreted to planets, but many - such as the asteroids - never combined to form large bodies. The very largest asteroids can be considered protoplanets.
PLANCK CURVE - Characteristic intensity distribution of radiation emitted by a blackbody. The frequency at which the emitted intensity is highest is an indication of the temperature of the radiating object. Also referred to as the "blackbody curve".
PLANCK'S CONSTANT (h) - Fundamental constant of physics (h = 6.625 x 10-27 erg-sec), which sets the scale of quantum-mechanical effects.
PLASMA - Fourth state of matter: a gas in which many or most of the atoms are ionized. In the plasma state the atoms have split into positive ions and negative electrons, which can flow freely, so the gas becomes electrically conducting and a current can flow.
PLATINUM GROUP ELEMENTS (PGE) - Elements with geochemical properties similar to Pt (Ru, Rh, Pd, Os, Ir, Pt). These occur in nature in close association with one another and with Ni and Cu. They are among the least abundant of the Earth's elements.
PLESSITE - Mineral composed of a fine-grained intergrowth of kamacite and taenite that fills in the wedges between wide kamacite and taenite bands in octahedrites. The name derives from the Greek word for "filling.".
PLUTONIC - Igneous rocks that form from the cooling of magma in the interior of a planet or asteroid.
POLYMICT BRECCIA - General term for all breccias that are neither monomict nor dimict.
POLYMORPHISM - Different structural arrangements in crystalline materials of the same chemical composition. Silica and carbon show numerous polymorphs; so does water in its solid form (ice). In all cases, the equilibrium phase depends on pressure as well as temperature; over 10 types of ice structures are known.
POPULATION I STARS - Relatively young stars, containing a larger fraction of elements other than hydrogen and helium, and found mainly in the disk of the Galaxy. The Sun is a Population I type star.
POPULATION II STARS - Relatively old stars, containing a smaller fraction of elements other than hydrogen and helium, mainly found in the halos of galaxies and in globular clusters.
POSTPEROVSKITE - High-pressure form of MgSiO3 with a stacked SiO6-octahedral sheet structure that is a major component of the D" layer in Earth. Postperovskite forms from perovskite at conditions exceeding 125 gigapascals and 2500 K, which corresponds to a depth of ~2700 km (near the base of the mantle). The transition results in an increase in density of 1.0 to 1.2%. Incorporation of Fe into the structure greatly reduces the pressure needed for the transition. The perovskite-postperovskite phase transition probably produces the D" seismic discontinuity located just above the core-mantle boundary.
POTENTIAL ENERGY - Energy possessed by something by virtue of its location in a potential field. AS an example, consider gravity. The higher an object is above the ground, the greater its gravitational potential energy. This potential energy is converted into kinetic energy as it falls.
P-PROCESS - Photodisintegration (hence "p") reactions are responsible for the production of proton-rich isotopes with masses >100. The relevant reactions are (γ, n) and (γ, α). A supernova explosion produces flood of γ-rays that can disintegrate the seed nuclei produced by s- and r-processes. (The p-process is sometimes called the "γ-process.") P-process contribution to isotopic abundances of elements that are also produced by s- and r-processes is usually very small. There are p-only isotopes that cannot be produced by the s- or r-process (190Pt, 168Yb); these isotopes have very small abundances compared to neighboring nuclei.
PRESOLAR GRAINS - Tiny crystalline grains found in the fine-grained matrix of primitive meteorites. They are assumed to be older than the solar system, probably formed in supernovae or the stellar outflows of red giant (AGB) stars before incorporation in the molecular cloud from the solar system formed. Presolar grains survived the collapse of the solar nebula, and also the subsequent formation of planetesimals because they consist of refractory minerals.
PRESSURE - Force applied per area covered:
Pressure is a scalar quantity and has no direction. The unit of pressure is the pascal (Pa), which is a newton/m2. The pascal is also a unit of stress and the topics of pressure and stress are connected. One pascal is such a small unit of pressure that the kilopascal (1 kPa = 1000 Pa) is more commonly used. One hundred thousand pascals are called a bar (1 bar = 105 Pa; 1 kbar = 108 Pa). The unit atmosphere is 101.325 Pa by definition.
PRIMORDIAL ELEMENTS - Elements and isotopes formed in the Big Bang; specifically, 1H, 3He, 4He, most D (deuterium = 2H) and 3H (tritium), and some 7Li.
PRIMITIVE ACHONDRITE - Achondrite with an almost chondritic composition with age similar to the primordial chondrites. These should be better classified as "metachondrites".
PROMINENCE - Structure that occurs above the photosphere of the Sun. Prominences may reach high into the corona, often as graceful loops that may hang suspended for many days. Prominences are usually associated with regions of sunspot activity; they tend to lie on the boundary of regions having opposite magnetic polarity. Streaming arches and their stability for days at a time are associated with magnetic forces acting on the charged particles in the loops. The violently eruptive prominences that are sometimes observed are associated with corresponding sudden changes in the magnetic field of the Sun.
PROPER MOTION (μ) - Apparent change of position of a star on the celestial sphere. Proper motion is a velocity that is usually quoted in units of seconds of arc per year.
PROPLYD - Contrived acronym for protoplanetary disk. Proplyds are found around low mass young stars close to massive OB stars, whose radiation photoevaporates the outer layer regions of their gas-dust envelopes, which are blown by impinging stellar winds into a teardrop shape.
PROTON - Particle of the hadron family which is one of the two particles that makes up an atomic nucleus. The proton has a positive electrical charge equivalent to the negative charge on the electron and a mass similar to that of a neutron.
PROTOPLANETARY DISK - Flattened disk of solids and gas orbiting a young star from which planets can form.
PROTOSTAR - Star in the process of formation, which has not yet become hot enough in the core to initiate hydrogen burning (~107 K) to halt its gravitational collapse. Its luminosity results from the release of gravitational potential energy from the infall of nebula material from its accretion disk.
PROXIMAL EJECTA - Ejecta found up to 5 crater radii from the rim of the impact crater. About 90% of all ejecta is proximal. The limit of proximal ejecta scales with the crater size. Ejecta found at greater distances is called distal ejecta.
PSEUDOTACHYLITE - Rock formed by frictional melting of rocks during faulting and impact cratering as a result of high rates of deformation and nearly total transformation of kinetic energy to heat. After deformation ceases, rapid quenching of the frictional melt leads to the formation of glass. The term was first applied to veins in the Vredefort impact structure, which are found in rocks of widely varying composition. The compositions of associated pseudotachylites are the same as those of host rocks, with some systematic variations, thus indicating in situ formation. One mystery of pseudotachylite formation is how frictional processes can form large amounts of melt, because thick masses ought to preclude melting by reducing the friction between sliding rock masses. However, J. Melosh (U. Arizona) has suggested that if melt produced by sliding friction in narrow shear zones is extruded into adjacent country rock, the shearing zone can be kept narrow and continue to produce melt.
PULSAR - Rapidly rotating neutron stars have an extremely strong magnetic field, which generates a strong electric field. Electrons are torn from the surface and accelerated along the magnetic poles, resulting in twin beams of radiation shining out of the magnetic poles. Rotation beams energy into space, creating a pulsar.
PYROXENE - Major silicate mineral group of which the end members are enstatite (MgSiO3), ferrosilite (FeSiO3), and, compositionally, wollastonite (CaSiO3). Pyroxenes do not have CaSiO3 contents exceeding 50 mol. %, so compositions are often displayed on a quadrilateral.
PYRRHOTITE - Fe sulfide found as an accessory mineral in CM chondrites, which are deficient in Fe relative to S; its formula is Fe1-xS.