Crater outflow

Spectacular flows of molten rock. They mix with the ejecta of impact craters and in places form long tongues of melt. They are a striking part of Venusian craters that is not known from any other body in the Solar System.

They usually lie on the side opposite the missing segment of the crater’s ejecta blanket. So these lava-like melt flows run forwards, in the direction of travel of the impactor – even where the surface rises in that direction. In the end they lose their momentum and turn downslope (Schultz 1992).

On radar images their surface is strikingly radar-bright, but their material probably forms only a thin layer. Even so, they travel far. The outflow of the crater Addams (see the figure), for example, reached six crater diameters. They move easily even on very gentle slopes, so their material must be very fluid. In places they may become transparent to radar.

Two types of outflow are distinguished. Proximal outflows have blurred margins. They start at the crater rim and merge into the normal ejecta blanket. They have a high discharge. Observations show that they were laid down before the ejecta, so very fast: in less than a minute and a half. They resemble pyroclastic flows. They probably formed as the gases from the evaporated impactor (impact vapour) condensed. They form only where the evaporating impact gases reach the conditions for condensation near the surface, before the gases can rise into the upper atmosphere. Comet nuclei are probably less able to make outflows than iron meteoroids (Sugita and Schultz 2002).

Distal outflows resemble lava flows. They have lobate ends and advance in narrow fingers. They start from the ejecta blanket and have a lower discharge. They flow downslope, fill cracks and spread out at the foot of mountains. This type of outflow lies on top of the ejecta blanket. It is probably made of surface material that melted at impact. The melt has a viscosity like that of water. Its melting point lies close to the surface temperature, which helps it to advance.

Finally, the outflow material may be a mixture of debris fluidized in the atmosphere, melt and hot rock vapour.

Outflows are unique among the planets, but on Venus they are not rare. About half of the craters smaller than 30 km have one.