What Struck Earth at the Younger Dryas Defies Classification

The physical evidence of the Younger Dryas impact event is no longer confined to a single region or geological layer. Randall points to research now showing that the proxies - the byproducts of whatever struck or exploded over the Earth approximately 12,800 years ago - have been identified across more than half the planet's surface. That distribution is significant on its own. However, what Randall finds equally important is the nature of the object itself, because its signature doesn't fit neatly into existing categories.

The conventional distinction between comets and asteroids has already blurred considerably - research has identified a broad continuum of hybrid objects between the two. Whatever caused the Younger Dryas boundary event appears to sit somewhere within that continuum. Its chemical and physical signature is heterogeneous, carrying attributes of both a comet and an asteroid within a single object. That complexity is not a reason to doubt the impact hypothesis. Randall argues it is precisely what makes the evidence so compelling - because a heterogeneous impactor would produce exactly the kind of confusing, wide-ranging proxy signature that researchers are now finding distributed across the globe, and that subsequent research, he suggests, will likely extend further still.