Messy Impact

The small asteroid that exploded over Russia earlier this year reminds us that Earth is a target in a shooting gallery. Our planet’s been hit many times, sometimes with devastating results. But evidence of past encounters can be difficult to pin down.

In fact, scientists are locked in a “vigorous” debate over a possible impact 12,900 years ago, known as the Younger Dryas impact.

Proponents say a small comet or asteroid - either in one chunk or several smaller chunks - hit Earth or exploded in the atmosphere. The impact led to the demise of the mastodons and other large mammals, and the end of the dominant human culture in the Americas.

The idea of the impact was first proposed several years ago. Scientists detected several possible tracers of an impact in sediments in North America and elsewhere. The evidence included tiny beads of carbon and magnetic minerals, microscopic diamonds, and a relatively thick layer of charcoal, as though there had been a continent-spanning wildfire. Sev