This is planetary radio.

This is planetary radio. Oh, We hope all our American listeners had a wonderful Thanksgiving and that you've almost

fully recovered from that caloric nightmare of a holiday.

For all of you, welcome to the first show of our second season.

I'm Matt Kaplan, hoping you like the new theme music.

We'll have a new Q&A segment for you in just a moment and a brand new

visit with Bruce Betts at the end of our program. In between we're bringing back

our conversation with former NASA Associate Administrator Wes Huntress.

Stick around.

Stick around.

Hi, I'm Emily Lochuwala with questions and answers. A listener asked, how did water arrive or form on the Earth, and when did it happen?

Our understanding of how the planets

formed leads us to believe that the vast bulk of water on earth is nearly as old as

the earth itself but the story is a tangled one. Earth, like the other planets,

formed through the accretion of bodies called planetes

which themselves formed out of the dust that swirled around the early sun.

But conditions at the Earth's distance from the sun were simply

too hot for water to be incorporated into the planetesimals that formed here. If this were the whole

story, the Earth would have accreted dry. However, models of planetary formation show that a

great deal of mixing of planetesemals occurred early in solar system history. Water-rich

planetesimals from farther away from the sun would have been scattered throughout the

inner solar system and the Earth could have acquired its water from this source.

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