Second Author

The damage caused by meteorites and asteroids when they strike planets or satellites is an area of research that can attract more than its fair share of hyperbole. And when Earth is brought into the equation, disaster movies are never far from the public's thoughts.

Clark Chapman of the Southwest Research Institute in Boulder, Colorado, is interested in the real effects of these objects. On page 1125 of this issue, Chapman and his colleagues assess what caused the craters on the surface of Jupiter's moon Europa. They calculate that most of Europa's smaller craters were not caused by the direct ‘primary’ impacts of asteroids and comets, as was previously believed. Instead they are the result of ‘secondary’ impacts, which occur when smaller pieces of debris are kicked up after the initial crash of the incoming body.

As most craters on Europa now seem to have been caused by secondary impacts, does that mean that the chance of an asteroid hitting Earth is lower than previously calculated?

For the small bodies — asteroids the size of a house — yes. But this does not really apply to kilometre-size asteroids hitting Earth and wiping out the planet.

According to your work, too many craters have been counted as ‘primary’ impacts. What was responsible for the miscount?

In the past, we assumed that apart from obvious secondary impacts, such as prominent clusters of craters, all other impacts were primary craters caused by asteroids.

Are there any new techniques that bolster your theoretical predictions?

There are independent estimates of the number of craters caused by secondary impacts from sources such as telescopic surveys. We believe that our results are consistent with these data.

How dramatically do your results go against conventional wisdom?

Our study of Europa, and some recent studies of Mars, are the first to raise the possibility that there are many more secondary craters than primary ones.

Do you expect some controversy or resistance?

A lot of people have spent a lot of time counting craters. There will be some inertia.

Do films about asteroids hitting Earth bother you?

Hollywood depictions of disasters are a little over-the-top. It's good that they increase the awareness that there are asteroids and comets out there, but the scientific details are way off.