From the May 2016 issue

Are impactors more prevalent in the outer solar system than they are in the inner solar system? Does this affect how surface ages are estimated?

David Kaiser, Thornton, Colorado
By | Published: May 30, 2016
Pluto’s surface is strangely lacking in craters. These craters on the dwarf planet show signs of layering, an implication that the surface has changed since the terrain first formed.
Determining surface ages throughout the solar system helps explain the evolution of planetary bodies. To estimate the surface age, you need to know the number of craters and the number of impactors near that body. The former can be measured through spacecraft images, but what about the latter?

In the outer solar system — for example, near Pluto — the impactors are small Kuiper Belt objects. Their census is known through telescope surveys, but some uncertainty remains. In the inner solar system — for example, near Ceres or Vesta — the impactors are small main-belt asteroids. Remarkably, considering that the asteroids are much closer than objects in the outer solar system, here too the census of impactors is only approximately known. In both cases, we have estimates of bodies larger than about 0.6 mile (1 kilometer).

But even if you know the number of craters and the number of potential impactors, the last remaining factor is the impact probability. If the density of impactors is high — many impactors per cubic kilometer of space — then the cratering rate is high. If the density is low, then the cratering rate is low. Herein lies the answer to your question.

There are lots of 1-kilometer-size Kuiper Belt objects, but they are spread out over a vast volume. In the inner solar system, the potential impactors are crammed into a much smaller area. So the impact rate is much higher in the main asteroid belt than in the outer solar system. Vesta and Ceres get hit much more frequently than Pluto because they orbit in a relatively dense swarm of potential impactors, whereas Pluto orbits in a relatively impactor-free environment, and therefore has a lower impact rate.

David Trilling

Northern Arizona University

Flagstaff, Arizona