Don: On today's Moment of Science . . . [COUGHS LOUDLY]

Yael: Are you okay? Here, have some water.

D: [DRINKS, THEN . . .] Ahh . . . that's amazing!

Y: What, a glass of water? It's just from the tap.

D: I know, but water is pretty incredible, isn't it? I mean, oceans cover seventy percent      of the planet. And our bodies are something like sixty percent water. Fruits and      vegetables are eighty percent water. Where did all this water come from?

Y: Well, one interesting theory is that water came to Earth from space. From comets,      specifically. You know that comets are basically balls of dust and ice . . .

D: Yep. They're called "dirty snowballs."

Y: Right. So the idea is that about four billion years ago, hundreds of millions of comets      bombarded the infant Earth. Comet ice either melted or vaporized when it hit and      condensed in the earth's atmosphere.

D: Okay, but aren't comets relatively small compared to Earth? I mean, a typical comet      carries maybe a few billion tons of water. The oceans contain around a million trillion      tons.

Y: True. But scientists have discovered huge ice balls, some as large as six-hundred      miles across, in the Kuiper belt, an outer area of the solar system beyond the orbit of      Neptune. So it's possible that, billions of years ago, some of these objects were      knocked out of orbit and spiraled in toward the sun. On the way, they collided with      Earth, releasing trillions of tons of water.

D: So there's a chance that some of the water I just drank came from a comet?

Y: Yep.

D: Cool.  

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