MAILBOX: Why does Titan have an atmosphere?

Time for a look in the Naked Scientists mailbox and Izzie Clarke received this question from Alistair in Gloucester,

"I'm interested in NASA's dragonfly quadcopter mission to Saturn's moon Titan. How can a moon with such a low gravity hold a thicker atmosphere than Earth? Is this the same mechanism to give Venus its thick atmosphere?"

Izzie - Alistair, this is a brilliant question. Titan is Saturn's biggest moon and it's similar in size and mass to our own Moon. The difference is that Titan is much colder, and the colder molecules move more slowly, which makes them easier to hold on to. Titan is minus 100 degrees Celsius. Our moon, however, is about plus 100 degrees Celsius when it's lit by the Sun. So Titan is able to hold onto that atmosphere which the moon can't.

Now looking at Venus, Venus is a much bigger body and has stronger gravity. So even though it's extremely hot, and it’s got these energetic molecules, it's this gravity that means it can hold on to its thick atmosphere. I hope that answers your question.