This is planetary radio. This week's show is direct from Russia with love and science.

Hi everyone, I'm Matt Kaplan.

We'll get an update on the Planetary Society's Solar Sale Project from Lou Friedman, the director

of that effort to build and fly the first ever light craft.

Bruce Betts has the latest from Mars as we close in on the red planet,

along with the new trivia contest, of course.

First up, though, is Emily, who's wondering if Earth's neighbors have dishes rushing about on their surfaces.

I'll be back with Drala with questions and answers. A listener asked, are there any other planets

in the solar system besides Earth that exhibit plate tectonics?

If not, why is Earth unique?

The tectonic evolution of planets in our solar system is a fundamental question in planetary

geology.

Thus far, none of the planets we have explored has a recognizable system of plate

tectonics like that on Earth. A planet's size is one of the key factors that shape its

tectonic evolution. The smaller a planet's size, the larger the ratio of its surface

area to its volume. Small planets such as Mercury, Mars, and the Earth's Moon cooled very quickly

because their internal heat could dissipate more quickly

through their surfaces. They have cooled so much that the outer layer of the

planet has thickened to the point where it cannot break into separate mobile plates.

So unlike Earth, which has tectonic features like mountains and volcanoes,

concentrated around the edges of its plates, the Moon, Mars, and Mercury have tectonic

features that are distributed all over the planet, and which mostly formed as a result of impact

...