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:

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.

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 crater diameter.

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.

Crater Adivar

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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