National Aeronautics and Space Administration Logo
Follow this link to skip to the main content NASA Banner
Solar System Exploration
Facebook Twitter YouTube Facebook Twitter YouTube Flickr iTunes
Follow Us
Dictionary Lookup
Can't find it? Don't understand it? Ask us.

A dark spot.


The property of a material to attract iron, cobalt, or nickel.


The region of space in which a planet's magnetic field dominates that of the solar wind. The asymmetrically shaped volume around a magnetic body in which charged particles are subject to the bodies magnetic field rather than the magnetic field of another body such as the Sun or Jupiter.


The portion of a planetary magnetosphere which is pushed away from the Sun by the solar wind.


The degree of brightness of a celestial body designated on a numerical scale, on which the brightest star has magnitude -1.4 and the faintest visible star has magnitude 6, with the scale rule such that a decrease of one unit represents an increase in apparent brightness by a factor of 2.512. Also called apparent magnitude.


Literally "sea" (a very bad misnomer, still in use for historical reasons); really a large circular plain (plural: "maria").

Fourth planet from the sun. Named for the Roman god of war. Other than Earth, Mars is the most explored planet in our solar system.
HREF="../planets/profile.cfm?Object=Mars">More on Mars >>

Full Disk Image of Mars
mass wasting

The downslope movement of rock, regolith, and/or soil under the influence of gravity.


A mesa. Flat-topped elevation.

Closest planet to the sun. Named for the Roman god of trade a profit.
HREF="../planets/profile.cfm?Object=Mercury">More on Mercury >>

Mercury's South Pole

Used by astrophysicists to refer to all elements except hydrogen and helium, as in: "the universe is composed of hydrogen, helium and traces of metals". (Note: this is quite different from the usual chemistry definition.)


(also "shooting star" or "falling star") a bright streak of light in the sky caused by the entry into Earth's atmosphere of a meteoroid or a small icy particle. Very large, bright ones are called fireballs and bolides. (Photo credit: P. Spurny).

Image of Czech meteor

A rock of extra-terrestrial origin found on Earth.


A small rocky object orbiting the Sun; smaller than an asteroid.


A colorless and odorless gas that burns. It is often used as a fuel for heating homes and cooking food.


A unit of distance equivalent to one-millionth (10E-6) of a meter; a micrometer. (E. coli, a common bacteria, is 0.5 microns in length.

mid-ocean ridge

A continuous mountain range with a central valley, located on the ocean floor where two tectonic plates move away from each other allowing molten rock from the Earth's interior to move toward the surface.

Milky Way Galaxy

The spiral galaxy containing our sun. Visible on the northern hemisphere in summer in the night sky as the Milky Way, a faint band stretching over the sky.

Illustration of Milky Way Galaxy

1/1000 of a bar. Standard sea-level pressure is about 1013 millibars.

minor planets
The official term used for asteroids.
More on Asteroids >>


A mountain. Plural: montes.


A body which orbits a planet.


The scientific study of form, and of the structures and development that influence form. In geology, the external structure, form, and arrangement of rocks in relation to the development of landforms.

Awards and Recognition   Solar System Exploration Roadmap   Contact Us   Site Map   Print This Page
NASA Official: Kristen Erickson
Advisory: Dr. James Green, Director of Planetary Science
Outreach Manager: Alice Wessen
Curator/Editor: Phil Davis
Science Writers: Courtney O'Connor and Bill Dunford
Producer: Greg Baerg
Webmaster: David Martin
> NASA Science Mission Directorate
> Budgets, Strategic Plans and Accountability Reports
> Equal Employment Opportunity Data
   Posted Pursuant to the No Fear Act
> Information-Dissemination Policies and Inventories
> Freedom of Information Act
> Privacy Policy & Important Notices
> Inspector General Hotline
> Office of the Inspector General
> NASA Communications Policy
> NASA Advisory Council
> Open Government at NASA
Last Updated: 25 Oct 2013