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Crater formed by an explosion or collapse of a volcanic vent. A large volcanic crater produced by the collapse of underground lava reservoirs. (Image: Tupan Patera's caldera on Io.)

Lave-filled Caldera on Jupiter's moon, Io.

A deep valley with steep sides. (Also chasma)


abundant element found in all known organic compounds (living things) and many inorganic compounds. Carbon is found in everything from graphite (pencil lead) to diamonds to gasoline.

carbon dioxide

A colorless and odorless gas. Plants on Earth need carbon dioxide to live.


A compound containing carbon and oxygen (i.e. calcium carbonate a.k.a. limestone).


Having to do with the science and art of constructing maps and charts.


The science and art of constructing maps and charts.


A chain of craters.


A hollow, irregular depression.


A distinctive area of broken terrain.

charge-coupled device (CCD)

An electronic device that consists of a regular array of light sensitive elements that emit electrons when exposed to light.


A canyon.

Chicxulub crater

a very large impact crater near the Yucatan Peninsula in Mexico. The effects of this particular impact may have been responsible for the extinction of the dinosaurs 65 million years ago.


Groups of atoms or molecules that are responsible for pigmentation (color).


The lower level of the solar atmosphere between the photosphere and the corona.


Small hills or knobs.


A spherical cloud of material surrounding the head of a comet. This material is mostly gas that the Sun has caused to boil off the comet's icy nucleus. This gas is illuminated both by reflected sunlight and light emitted by excited molecules. A cometary coma can extend up to a million miles from the nucleus.

Diagram of parts of a comet - nucleus, coma and tail.
A chunk of frozen gasses, ice, and rocky debris that orbits the Sun. A comet nucleus is about the size of a mountain on Earth. When a comet nears the Sun, heat vaporizes the icy material producing a cloud of gaseous material surrounding the nucleus, called a coma. As the nucleus begins to disintegrate, it also produces a trail of dust or dust tail in its orbital path and a gas or ion tail pointing away from the Sun. Comet comas can extend up to a million miles from the nucleus and comet tails can be millions of miles long. There are thought to be literally trillions of comets in our solar system out past Neptune and Pluto, but only once a decade or so does one become near and bright enough to see easily without binoculars or a telescope. (Photo copyright: Gerald Rhemann)
More on comets >>

Comet Ikeya-Zhang

A packing or reduction in the amount of data being transmitted, using a mathematical formula similar to averaging, in order to optimize the transmission of data from the spacecraft. Once received, these data are "unpacked" (decompressed) to reconstruct the full image. However, if noise (see Radiation) is present it may cause incorrect values for the picture elements it was averaged with, which also affects the resulting image.


An inferior planet (closer to the sun than Earth - Mercury and Venus) is said to be "in inferior conjunction" when it is directly between the Earth and the Sun. It is "in superior conjunction" when it is on the opposite side of the Sun from the Earth.
The superior planets (the six planets farther from the Sun than Earth) are "in conjunction" when it is on the opposite side of the Sun from the Earth. A superior planet obviously cannot have an inferior conjunction. When the Earth is at inferior conjunction with respect to an observer on a superior planet we say that planet is "in opposition" from Earth's perspective.


One of the stellar patterns identified by name, usually of mythological gods, people, animals and objects. Also the region of the sky containing that star pattern.


The transfer of heat from a region of high temperature to a region of lower temperature by the displacement of the cooler molecules by the warmer molecules.


the uppermost level of the solar atmosphere, characterized by low densities and high temperatures (> 1.0E+06? K).


A special telescope which blocks light from the disk of the Sun in order to study the solar atmosphere. See the image on the right for more information.

SOHO's LASCO Coronograph
cosmic ray

An extremely energetic charged particle.


A typically bowl-shaped pit, depression, cavity or hole, generally of considerable size and with steep slopes, formed on a surface or in the ground by the explosive release of chemical or kinetic energy. Also depression around the mouth of a volcano.

crater density

The number of craters on a surface per unit area.

crater size distribution

The relative numbers of craters of given sizes on a surface.


An interruption of a geologic feature by another, which can give an indication of the relative ages of these geologic features/events. (i.e. a fault cutting across an impact crater would be younger than the crater.)


The outermost layer of a planet, i.e. the surface. On Earth, we live on the crust.

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Last Updated: 25 Oct 2013