Cassini reaches the Lord of the Rings this week on planetary radio.

After a seven-year journey, the Kacini-Huegan spacecraft has successfully gone into orbit around the ring planet.

Stay with us for a special report.

Bruce Betts also gets in on the Saturnalia in this week's What's Up.

And Emily Lachto-Wala is here with a little tribute to a titanic moon of Saturn.

I'll be right back.

Hi, I'm Emily Lockwala with questions and answers. A listener asked, how can

Saturn's Moon Titan, which is less than half the size of the Earth, have a thicker

atmosphere? Titan's nitrogen-rich atmosphere is about one and a half times thicker than our own, and it's

true that smaller bodies, which have a lower escape velocity, aren't as good at holding

onto their atmospheres as larger bodies are, but size is not the only thing that matters.

The other major factor is temperature.

Gas is escape from planets when chance collisions give gas molecules enough speed to escape the planet's gravity.

The colder in atmosphere is, the slower its molecules move, and the less likely they are to escape.

Titan, which lies ten times farther from the Sun than the Earth

does, is far colder than the Earth, and so it is able to hold on to its atmosphere.

But if that's true, why aren't there other moons with thick atmospheres in the outer solar system?

Stay tuned to planetary radio to find out. That eerie music is the planet Saturn welcoming a weary traveler.

To be a bit less romantic, it is the highly compressed sound recorded by the Cassini-Huygen spacecraft as it passed through

the edge of Saturn's magnetosphere or magnetic field, right where the charged particles of the

solar wind were smacking into it.

Fortunately, the school bus-sized spacecraft got through this and much more dangerous obstacles without a scratch.

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