Moon Walker Harrison Schmit is on planetary radio.

Moon Walker, Harrison Schmidt, is on planetary radio. Hi everyone I'm Matt Kaplan.

Only 12 men walked on the moon.

More than three decades ago, he was one of the last two. He came home to a career as

scientist, entrepreneur, engineer, and US senator. Harrison Schmidt returns to

our show this week.

Later Bruce Betts treats us to another look at the night sky and a new trivia contest.

First, though, Emily tells us how craters help us learn much more about our own planet and others. I'll be right back.

Hi, I'm Emily Lochuwala with questions and answers. A listener asked,

why are most of the inner planets covered with craters while the Earth has very few relatively small craters?

The Earth has very few craters because the Earth is geologically active, which means that

craters are destroyed almost as soon as they are created. On Earth craters can

be destroyed in a number of ways. They can be worn away by

weathering the process that has reduced the Appalachian Mountains of the United

States from giant rocky peaks into low rolling hills.

Craters can be destroyed by the tectonics of folding and faulting,

as in the creation of the Himalayan mountains of Asia.

Craters can be covered up by the outpouring of volcanic lava as in the deckend traps of India.

Or if craters form on the bottom of the ocean, they can be swallowed into the earth at a

subduction zone as under the Andes or Japan. Scientists actually use the number of craters on the surface of a planet

as an index of the planet's geologic activity.

What does crater counting teach us about planets other than Earth?

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