Glossary – P

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A Program Officer is sometimes referred to as Discipline Scientist, Technical Officer, Program Manager or Program Scientist; we will adopt program officer to refer to the person at NASA Headquarters who manages a science program (i.e., handles all aspects of the proposal call, sets up the review panel, submits funding decisions to the selecting official, approves annual reports etc).

See a complete list of program officers for research and data analysis programs.


In general, a charged component of an atom, that is, an ion or electron.


Planetary Data System



a branch of science dealing with the relations between climate and periodic biological phenomena (as bird migration or plant flowering)


A fact or event of scientific interest susceptible to scientific description and explanation.


A quantum of electromagnetic radiation.


The layer of the Sun from which all visible light reaches us.


Principal Investigator



A fourth state of matter (in addition to solid, liquid, and gas) that exists in space. In this state, atoms are positively charged and share space with free negatively charged electrons. Plasma can conduct electricity and interact strongly with electric and magnetic fields. The solar wind is actually hot plasma blowing from the sun. See magnetosphere.


polar cusp

Two funnel-shaped areas between the Sunward magnetic field and the tailward magnetic field where the solar wind enters and follows the magnetic field lines toward Earth.


attraction toward a particular object or in a specific direction.

Practical Salinity Unit

Used to describe the concentration of dissolved salts in water, the UNESCO Practical Salinity Scale of 1978 (PSS78) defines salinity in terms of a conductivity ratio, so it is dimensionless. Salinity was formerly expressed in terms of parts per thousand (ppt) or by weight (parts per thousand or 0/00). That is, a salinity of 35 ppt meant 35 pounds of salt per 1,000 pounds of seawater. Open ocean salinity is generally in the range from 32 to 37.


Before the start of life (as we know it).


Moisture that falls from clouds.


Existing in or persisting from the beginning.


To cause to spread out and affect a greater number or greater area
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The positively charged part of an atom. The number of protons determines which element the particle is. It is part of the nucleus.