- Paleozoic
- A geological term denoting the time in Earth history between 570
and 245 million years ago.
pahoehoe
- A type of basalt lava flow characterized by a smooth glassy skin, and
constructed of innumerable "flow units" called "toes"; pahoehoe flows
advance at rates of 1 to 10 meters (3 to 33 feet) hour and are associated with
low-effusion-rate eruptions with little to no fountaining.
palimpsest
- A circular feature on the surface of dark icy moons such as
Ganymede and
Callisto lacking
the relief associated with craters;
Pamlimpsests are thought to be impact craters where the topographic relief
of the crater has been eliminated by slow adjustment of the icy surface.
palus
- A swamp.
patera
- Shallow crater; scalloped, complex edge.
peak ring
- A central uplift characterized by a ring of peaks rather than a
single peak; peak rings are typical of larger terrestrial craters
above about 50 kilometers (30 miles) in diameter.
penumbra
- The outer filamentary region of a sunspot.
periapsis
- The point in the orbit closest to the planet.
perigee
- The point in the orbit closest to the Earth.
perihelion
- The point in its orbit where a planet is closest to the Sun.
Perrine, Charles Dillon 1867-1951
- Argentine-American astronomer. Perrine discovered Himalia
and Elara.
perturb
- To cause a planet or satellite to deviate from a theoretically
regular orbital motion.
photosphere
- The visible surface of the Sun; the upper surface of a convecting
layer of gases in the outer portion of the sun whose temperature causes
it to radiate light at visible wavelengths;
sunspots
and faculae are observed in the photosphere.
phreatic eruption
- A volcanic eruption or explosion of steam, mud or other material
that is not incandescent; this form of eruption is caused by the heating
and consequent expansion of ground water due to an adjacent igneous
heat source.
Pickering, William Henry 1858-1938
- American astronomer. His photographs of Mars, among the earliest obtained, provided a basis for his opposition to Lowell's observations of supposed canals on Mars. Discovered Saturn's moon Phoebe.
plage
- Bright regions seen in the solar chromosphere.
planar features
- Microscopic features in grains of quartz or feldspar consisting
of very narrow planes of glassy material arranged in parallel sets that
have distinct orientations with respect to the grain's crystal structure.
planitia
- Broad plains that occupy lowlands on planetary surfaces.
planum
- A plateau or high plain.
plasma
- A low-density gas in which the individual atoms are charged, even
though the total number of positive and negative charges is equal,
maintaining an overall electrical neutrality.
polarization
- A special property of light; light has three properties,
brightness, color and polarization.
Precambrian
- A geological term denoting the time in Earth history prior to
570 million years ago.
pressure ridge
- A ridge formed by the uplift of a lava flow crust due to pressure of
the flowing lava.
prominence
- An eruption of hot gases above the
photosphere of the
Sun. Prominences are most
easily visible close to the limb of the Sun, but some are also
visible as bright streamers on the photosphere.
promontorium
- A cape.
pseudocrater
- A generally circular crater produced by a phreatic eruption resulting
from emplacement of a lava flow over wet ground.
Ptolemy, 2nd century AD
- Alexandrian astronomer, mathematician, and geographer who based his
astronomy on the belief that all heavenly bodies revolve around the earth.
pyroclastic
- Pertaining to clastic (broken and fragmented) rock material formed
by volcanic explosion or aerial expulsion from a volcanic vent.
pumice
- A light vesicular form of volcanic glass with a high silica
content; it is usually light in color and will float on water.
