Impact basin

Large craters form several structural rings. From this size on, they are no longer called craters but basins, or multi-ring basins.

An impact basin is the largest type of structure an impact can make. It has three main parts:

1) the innermost ring, which forms inside the basin as the central peak opens and the floor rocks rise;

2) the basin rim, which forms partly by uplift, through the sideways action of the shock wave, and partly from debris that falls back;

3) one or more outer rings. These are asymmetric scarps facing inwards. They mark concentric tectonic cracks and faults made by the impact. Models show that they form when material of the asthenosphere moves towards the centre of the basin and drags the crust along with it (Melosh 1989).

Some lunar impact basins are filled with lava; these are the maria. In them the inner ring cannot be seen.

In the Meitner basin, shown on the plate, the inner ring is thought to be the basin rim, and the outer ring the tectonic fault ring. The innermost ring is buried (Alexopoulos and McKinnon 1994).

There are two databases of impact structures on Venus. The catalogue by Gerald G. Schaber is available from the USGS (https://astrogeology.usgs.gov/search/map/Venus/venuscraters). The catalogue compiled by Robert Herrick is available from the LPI (https://www.lpi.usra.edu/resources/vc/).

The largest basin is Mead, 269 km across. It is followed by Isabella (176 km) (see plate 2) and by Meitner (149 km), shown here.

The Moon and Mars have much larger impact basins. On Earth, however, even the largest known crater is only 160 km across: Vredefort, 2 billion years old. Next come Chicxulub (150 km, 65 million years old) and Sudbury (130 km, 1.8 billion years old).

It is well known that larger basins are generally older. The largest basins whose relief is still clearly visible at the surface are Hellas on Mars (2300 km) and the South Pole–Aitken basin on the Moon (2500 km). Both are traces of the most ancient impacts. On Venus, the record of that time, about 4 billion years ago, is missing: it is deeply buried, unless it has melted back into the mantle. So we cannot expect such large basins on the surface of Venus.

It seems that after the Late Heavy Bombardment, the bodies able to make such large basins moved into orbits around the Sun that do not cross those of the inner planets.