DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

crater depth-diameter plots
with a surface. Small impact craters, called simple
craters, are usually bowl-shaped in appearance
with little subsequent modification. Larger
craters, called complex craters, show a range
of features, including shallow depths, central
peaks or central pits, and wall terracing. Extremely
large (generally 500 km in diameter)
impact craters are called basins. Craters on the
planets are generally attributed to the impacts
of planetary bodies, meteorites, and asteroids.
Craters are a primary surface feature on Mars,
Venus, Mercury, and the moon. Erosion has destroyed
evidence of older craters on the Earth,
but a significant number of younger craters have
been recognized. Volcanic craters, now preferentially
called calderas, are produced by the
withdrawal of magma from a volcanic region.
Removal of any underlying support, such as by
removal of groundwater or the dissolution of
rocks such as limestone, can create craters by
subsidence. Volcanic and subsidence craters are
not usually as circular as impact craters.
crater depth-diameter plots Graphical representation
of a crater’s depth and width. Comparison
of the plots from different planetary
bodies shows that gravity is the factor controlling
crater depth. They illustrate that there is
an inverse relation between the depth of complex
craters and the acceleration due to gravity;
that shallower craters form for a given diameter
when the gravity field of a planet’s surface
is higher; and that there is a smaller transition
diameter from simple to complex craters in a
higher gravity field.
cratering rates The number of impacts per
time unit on a planetary surface in the course
of its geological history. Cratering rates are
required to determine the absolute ages of planetary
bodies using crater counting studies. A
fairly accurate picture of the cratering rate on
the moon has been compiled from radiometric
dating of returned samples, but not for the other
planetary bodies, at this time. In the latter cases,
attempts have been made to determine it theoretically,
first by analyzing the distribution of
objects within the solar system and predicting
how the cratering rates at the planetary body in
question may be related to those on the moon,
and second by comparing the crater statistics for
© 2001 by CRC Press LLC
the planetary body with those of the moon and
making assumptions about how the two are related.
Both approaches cause large uncertainties
that will be best reduced through sample return
missions.
cratering record The retention of impact
craters by a planetary surface provides a record
of how the frequency and size of impact craters
have changed over time. The number of craters
on a surface is related to the age of the surface;
hence, older surfaces have greater crater frequencies
than younger surfaces. Analysis of the
number of craters in the ancient highlands of the
moon indicate that the cratering rate was much
higher early in solar system history. This period
of higher cratering rates is called the period
of heavy bombardment. The material responsible
for cratering during the heavy bombardment
period was leftover material from the formation
of the planets. The formation of large impact
basins was common during this time since the
frequency of larger debris was greater. Analysis
of the lunar cratering record indicates that
the heavy bombardment period ceased about
3.8 × 10 9 years ago in the Earth-moon system.
The cratering rate since that time has been much
lower and is due to the impacts of asteroids and
comets. Study of the cratering record can provide
important information on the sources of the
impacting material as well as the geologic evolution
of the different solid-surface bodies in the
solar system.
crater number: index of age Older surfaces
accumulate more impact craters than young
ones. On planetary surfaces where erosion rates
are low (such as the moon and Mars), crater densities
depend on the age of the surface and the
cratering rate only, such that relative ages and
chronological relations can be defined fairly reliably.
Determining absolute ages is more difficult
in the absence of age-dated samples, etc.
Absolute age determination has been attempted
using impact-rate models (all assume that the
impact rate has remained unchanged over the
last 3 billion years) along with size-distribution
curves. See cumulative size-frequency curves.
crater production function The size frequency
of craters expected if there was no crater