3.0 Impact Craters
There are 935 recognized impact craters on Venus (Strom et al., 1994). About
half the craters have been formally assigned names; the others remain
unnamed. All have been given names
after famous women in history, but craters with diameters less than 20
km have been given female common names. Venusian craters range in size
from 1.4 km in diameter to 280 km
. Crater Mead is the largest impact crater
identified on Venus. (See
map for the location of this crater and other information available from the crater atlas).
In contrast to Mercury, Mars or the Moon, which are covered with
thousands of craters that have accumulated over the last 4 to
4.5 billion years , Venus is scarred by curiously few. Schaber et al. (1992) and
Strom et al. (1994)
have shown that the spatial distribution of craters is
uniform (random and anticlustered) over the entire planet,
suggesting that Venus experienced complete global resurfacing in the
relatively recent (geologically speaking) past.
In the global resurfacing model , tectonic and
volcanic activity affected the entire surface of Venus which
obliterated the majority of (if not all) previous impact craters. An
observation that lends support to the sudden arrest of these events is
the fact that the majority of craters, 84% , do not
show any signs of modification
(Strom et al., 1994) . This resurfacing activity is
thought to have ceased between 300 to 800 million
years ago. The uncertainty of the timing lies in the uncertainty of
estimating the impact flux.
Types of impact craters
Craters on Venus are recognized by their expression on images and
hence classified by their morphology. The high temperture of Venus'
surface ( 470 C ) and its thick atmosphere make Venus
impact morphology unique among planetary bodies in the solar system
( Ivanov et al., 1992 ).
Based on the development of crater floor structures and degree of
circularity, Schaber et al.
(1992) classified simple craters and five
types of complex craters into a six-fold scheme:
In general, small diameter craters are flat-floored, have irregular
outlines and may be part of multiple-impact event. Complex internal
structures occur in large craters and tend to develop progressively
as: a central peak, a double-ring, or a multiple-ring, with increasing
- structureless craters are simple
craters where the internal floor is flat and featureless. The
smallest craters are generally of this type.
- central peak craters (70k gif)
have a central uplift that rises above the crater floor. These
craters range in size from 8 to 79 km, but are most commonly
16-32 km. Oulining rims are quite circular and often terraced.
- double-ring craters (20k gif) are defined by an outer rim and a circular arrangement of inner peaks and ridges. These craters are typically greater than 40 km.
- multiple-ring craters (168k gif) have
two or more concentric ridge structures that rise above the
crater floor. The largest craters on Venus, ranging from 86 to
280 km in diameter, are of this type.
- irregular craters have non-circular rim
outlines and structural disruptions to otherwise flat crater
floors. Almost 1/3 of the craters on Venus are of this type,
most of which are less than 16 km across.
- multiple crater formation occurs when a
falling body fragments into pieces. Each fragment creates a
separate impact crater whose rim may overlap with adjacently
formed craters. Individuals of this type are up to 44 km in
diameter, but most are less than 11 km.
Interactive Crater Atlas
You can investigate the relationships between crater size and type,
find more information about the names of Venusian craters and look at
their distribution on a map by searching the crater database
according to crater type and diameter
. The current status of the crater database used in making
the map can be found in what's new.
As an example of what the crater atlas displays, click on
Adivar . A Magellan radar image of this impact crater is
displayed below and its features are described.
Adivar is a
complex crater with a prominent central uplift
, and thus, classified as a central peak crater
. The bright, irregular ejecta blanket
around the crater stands out against the darker background due to the
high radar backscatter of the ejecta material (surface roughness is
greater). Following an impact, travelling crater ejecta is met with a
great resistance from Venus' very dense atmosphere - about 90
times that of Earth's. Consequently, the material does not
extend for more than one or two diameters away from the crater edge
before settling to the ground.
The crater rim is terraced and extensive
collapse of the rim outline can be seen in the SW part of the
structure. A dark, V-like indentation of the NW
part of the peripheral ejecta blanket suggests that the impactor
arrived from that direction
(Schultz, 1992) . On the full image
of Adivar (88k gif) , a wide parabolic halo of
bright material opens westward and extends for many diameters around
the impact. Campbell et al.
(1992) suggested that this secondary pattern of deposition
around the crater is a result of prevailing westward
winds at higher altitudes, which carried the fine ejecta
downwind following impact.
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© 1994 - 2005 Glen
Newton and Paul Budkewitsch