Stones, Wind, and Ice:
A Guide to Martian Impact Craters

Glossary

Absolute chronology - An age sequence that uses isotopic ages of rocks to determine the age of a feature in years.

Albedo - The fraction of sunlight reflected from a terrain unit.

*Aeolian - Applied to the erosive action of wind, and to deposits that are due to the transporting action of the wind.

Caldera - A large basin-shaped depression on a volcano created by collapse when underlying magma is removed.

Central peak - A mound of material found in the interior of many impact craters. It is probably formed by the inward and upward movement of material during crater formation.

Central pit - A depression in the interior of an impact crater, possibly caused by the escape of volatiles during the crater formation process.

Complex crater - An impact crater that displays such characteristics as shallow depths, central peaks, terraced walls, and floor deposits. Complex craters occur when a crater is greater than about 10 km in diameter, although the actual transition size depends inversely on gravity: The larger the planet, the greater the gravity, and the smaller the transition diameter to complex craters.

Crater - A generally circular depression formed either by a collision of a small body with a larger object or by internal processes. Most craters seen on solar system objects have formed by meteorite or comet impacts.

Crater density - The number of craters of a certain size and larger per unit area. Higher crater density reflects an older age for a geologic unit.

*Dike - A tabular body of rock, usually igneous, that cuts across the structure of adjacent rocks.

Ejecta blanket - Material deposited around an impact crater during excavation of the crater. Some of the material removed from the crater cavity forms the ejecta blanket; the rest is compressed into the bottom of the crater.

*Eskers - Serpentine ridges that mark the location of channels in a decaying ice sheet, through which streams washed much of the finer drift leaving the coarser gravel between the ice walls.

*Fluvial - Of, or pertaining to, rivers; produced by river action.

*Glacier - A mass of ice with definite lateral limits, with motion in a definite direction, and originating from the compacting of snow by pressure.

Heavy bombardment - A period of time early in solar system history characterized by high impact rates due to the sweep up of material remaining after formation of the planets. On the Moon, this period appears to have ended about 4 billion years ago. Since that time, the impact flux has been significantly lower.

Isotopic ages - Ages of rocks obtained from the radioactive decay rate of certain elemental isotopes.

Lobate ejecta - See rampart ejecta.

*Magma - Naturally occurring molten rock material generated within the planet from which igneous rocks are thought to have been derived through solidification and related processes. It may or may not contain suspended solids.

Multiple ring basin - A very large circular depression created by impact processes and characterized by more than one concentric ring, each consisting of arcuate ridges and mountainous terrain extending into lower valleys sloping downward toward the next ring.

Peak ring - A crater that displays one central ring of material rather than a central peak.

Post heavy bombardment - The period of time since the end of the heavy bombardment period. This period has been characterized by crater production rates significantly lower than those occurring during heavy bombardment.

Radial ejecta - An ejecta blanket characterized by rough material near the crater rim that grades out into radial streaks of craters formed from material jetted from the primary crater.

Rampart ejecta - An ejecta blanket commonly composed of rounded lobes of material, often layered, emplaced by fluidization processes.

Relative chronology - An age sequence that dates features relative to each other or to certain events in the planet's history. Relative chronologies use terms such as “older than” or “younger than” as opposed to actual dates.

Simple crater - A small, bowl-shaped crater showing little or no interior structure.

SNC meteorites - A group of meteorites (shergottites, nakhlites, and chassignites) that are distinguished by young isotopic ages and chemistry indicative of formation on a planet-sized body. These meteorites are believed by many scientists to have been ejected off of Mars by a large impact.

Superposition - The position of one feature on top of another feature or surface.

Tectonic - Of, or pertaining to, features formed by the deformation of a planet's crust.

Volatiles - Materials that tend to evaporate rapidly or easily.

*Volcanic - Of, pertaining to, like, or characteristic of a volcano; produced, influenced, or changed by a volcano or by volcanic agencies.

Wall terraces - The steplike nature of crater walls formed by slumping of material under the influence of gravity. Wall terraces are a characteristic of complex craters.

*Definitions marked with an asterisk are from the Dictionary of Geologic Terms, revised edition, American Geological Institute, Anchor Press, Garden City, New
York, 1976.

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