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palimpsest

A roughly circular albedo spot on icy satellites that is presumed to mark the site of a crater and its rim deposit. Little, if any, of the topographic structure exists, but visual distinction from adjacent crust remains.

palus

Literally "swamp"; really a small plain.

parsec

= 206265 AU = 3.26 light year

patera

A shallow crater; scalloped, complex edge.

penumbra

Literally, "dim light"; the outer filamentary region of a sunspot.

perihelion

The orbital point of closest approach to the Sun.

perturb

To cause a planet or satellite to deviate from a theoretically regular orbital motion.

phase angle
The angle between the Sun, target, and a spacecraft.

Low-phase is a small angle between the Sun, target, and the spacecraft.
A low phase angle provides high sun illumination, similar to taking a picture from a high altitude at noon (with the sun directly overhead). Such illumination emphasizes the brightness contrasts of light and dark areas.

High-phase is a large angle between the Sun, target, and the spacecraft.
A high phase angle provides low sun illumination, similar to taking a picture from a high altitude at sunset or sunrise. Such illumination emphasizes the topography of the terrain.

photometry

The accurate quantitative measurement of the amount of light received from an object or area.

photosphere

The visible surface of the Sun; sunspots and faculae are observed in the photosphere.

pixel

Contraction of "picture element"; the area on the ground represented by each digital number in a digitized image; an individual element in a detector.

planet

A spherical ball of rock and/or gas that orbits a star. The Earth is a planet. Our solar system has nine planets. These planets are, in order of increasing average distance from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.

planetesimals

Primordial bodies of intermediate size that accreted into planets or asteroids.

planetoid
An asteroid. A medium-sized rocky object orbiting the Sun; smaller than a planet, larger than a meteoroid. Asteroids show no evidence of an atmosphere or other types of activity associated with comets. Asteroids can be less that one mile to almost 600 miles in diameter.
More on Planetoids >>

planitia

A low plain.

planum

A plateau or high plain.

plate tectonics

A geological model in which the Earth's lithosphere (crust and uppermost mantle) is divided into a number of more-or-less rigid segments which move in relation to one another.

Pluto
Pluto is the most distant planet from the sun. It is named for the Roman god of the underworld. It was not discovered until 1930 and its status as a planet remains the source of debate.
HREF="../planets/profile.cfm?Object=Pluto">More on Pluto >>

polarimetry

The measurement and study of the polarization of light reflected off of a surface.

power

A measure of the amount of work done per second, expressed in Watts.

prominence

A strand of relatively cool gas in the solar corona which appears bright when seen at the edge of the Sun against the blackness of space.

promontorium

A cape or headland.