Central peak crater

In crater development, craters just larger than simple craters grow a central peak. The peak forms as the surface rises at the focus of the impact. This is already a complex crater.

The plate shows two central peak craters (and one with a central ring). There are many differences and similarities between them. Before reading on, try to spot a few differences between the two central peak craters.

Unlike simple, bowl-shaped craters, central peak craters have a flat floor, a kind of ring plain. The peak is usually lower than the crater rim. On Venus, central peaks become common at a diameter of about 15 km (Phillips et al. 1991).

The central peak is not made of material that fell back, as the ejecta blanket around the crater is. It consists mostly of layers that rose from depth. At impact, the cosmic body evaporates in the heat and explodes, sending out a radial shock wave. The sideways part of the wave throws out the ejecta. The downward part hits the surface directly and compresses it.

Once the shock wave has passed, the surface is released from the pressure, and a rebound force arises. It is strongest at the centre of the crater. Here the surface material springs back, almost elastically. In the very first phase of crater formation, the transient crater is still steep. Material sliding in from its wall also piles up at the central peak. So the central peak brings rocks from deep layers within reach.

In craters larger than simple, small ones, the crater wall is less stable. Landslides start from it and form terraces along the inner rim.

Back to the question. The ejecta blankets of the two craters (Aurelia, 31 km across, and Callirhoe, 33 km across) are standard Venusian flower petals. The petals of Aurelia are more pronounced – no accident, since it is the type example of the petal-shaped ejecta blanket. Both ejecta blankets have a missing segment; at Aurelia it comes closer to the crater rampart. These gaps show the direction from which the body arrived (uprange). On the opposite (downrange) side lies the ejecta thrown forward. In both cases the impact melt flows run in this direction, forwards.

The ring basins around the central peaks show the most striking difference. At Aurelia the basin is radar-bright, no different from the ejecta blanket or the crater wall. This means its surface is just as rough. So the debris that fell back into the crater is still in its original state: the crater is young. The lunar crater Tycho, about 100 million years old (that is, young), is similarly radar-bright. The ring plain of Callirhoe, by contrast, is a radar-dark, featureless area. Some material finer than a centimetre fills it. A fan-shaped debris heap was later laid on top of it on the southern side.