Impact structure / Fluidized ejecta

Impact craters seemed well understood from the Moon. On Venus they brought many surprises: the hot, dense atmosphere thoroughly changes how an impact plays out.

Impact craters are found all over Venus, and they are randomly distributed. So are the fireballs (bright meteors) seen over Earth in the past two decades. We would therefore expect impacts from space to leave a random pattern – but only if the whole surface has the same age. Every other known body has both densely cratered areas and areas without craters. The cratered areas are old; the others are young. The exception is Io, which is covered in lava like Venus. Its surface is uniform too, but it has no craters at all, because it is so young.

There are two ways to explain the crater distribution on Venus. 1) Catastrophe model: at some point in the past, a global catastrophe wiped out every earlier crater. Judging by crater density, this happened about 500 million years ago. Internal heat builds up beneath the crust for some time. Venus lacks the “valve” that releases internal heat along plate boundaries on Earth. In the end the crust melts through. Within 10–100 million years, lava 1–3 km thick buries most of the surface (Schaber et al. 1992, Herrick 1994).

2) Equilibrium model: volcanism is active all the time, at random places on the surface. Each eruption floods only a smaller area. Patch by patch, these lava floods cover the whole of Venus in about 500 million years (Phillips et al. 1992, Guest and Stofan 1999, Hansen and Young 2007).

The dense atmosphere makes smaller incoming bodies evaporate or break apart before they land. Think of rubble-pile asteroids: loose heaps of cosmic debris, barely held together in microgravity. When such a body enters the dense atmosphere, it breaks into pieces. The pieces reach the surface separately and form an irregular crater cluster. Craters smaller than a few kilometres are missing entirely. Even among the larger craters, there are no regular, simple, bowl-shaped ones.

The ejecta – the debris thrown out by the impact – also looks distinctive on Venus. The edges of the ejecta blankets are lobate. They look as if mudflows had splashed out around the crater and stopped in protruding, petal-shaped flow fronts. This is surprising, because the high surface temperature makes Venus extremely dry.

On Mars, similar craters are explained by impacts that punched through a groundwater layer. The groundwater then mixed with the debris in flight. On Venus, the explanation is fluidized ejecta. Here the fluid-like behaviour comes from mixing with the gases of the hot, dense atmosphere. Very dry sand can flow like a liquid, too. In the same way, the dry debris, partly crushed to dust by the impact, rides on the atmospheric gases and lands behaving like a fluid.