moon formed in a large collision, probably only one collision, between the Earth and

a Mars-size object, very early, within 60 million years of the Earth really accreting

and differentiating into a core and a mantle.

That’s a big event in Earth history.

We know from Mercury, from the moon, that there were many large impacts in the early

solar system.

When you look at the moon and you see these big, dark patches that are roundish, those

are impact basins.

Huge impact basins that are filled with lava from inside the moon.

So, we can use the moon as a measuring rod for the impact history of the Earth

and all of the inner planets.

And, of course, we’re familiar with the asteroid that hit the Earth 65 million years ago

ago, causing a huge crater in the Caribbean.

Evidence from this impact has been found around the world as a thin, sedimentary layer called

the Cretaceous-Paleogene boundary, or K-Pg for short.

Geologists discovered that this layer is rich in metallic elements like iridium, that are

rare on the crust and mantle of the earth because they’ve been concentrated in the

core, but they’re common in asteroids and comets because those bodies

haven't formed a core.

The reason geologists made that distinction between the cretaceous period and the Paleogene

is this huge change in the nature of life itself: where mammals take over,

and most of the dinosaurs disappear.

The ammonites in the oceans are gone.

But, more importantly, at the plankton level—the small, small creatures, the small life forms,

which make very nice sediments that are dateable,

it’s like a knife edge in the record of life itself.

As a result of a cosmic collision—an accident really.

Something that happens every so often, and we hope doesn’t happen to us.