You are here

Moon and Saturn

StarDate: 
March 10, 2018

When the Cassini spacecraft arrived at Saturn almost 14 years ago, its first target was Phoebe, a moon that’s millions of miles from Saturn. Cassini’s pictures revealed an object that’s taken a beating over the eons. It’s been hit by so many big space rocks that it’s no longer round — it has an irregular shape because chunks of its surface have been blasted into space.

Phoebe may not have been born with Saturn. Instead, it might have formed in the Kuiper Belt — a vast ring of rocky and icy bodies beyond the realm of the planets. An encounter with another object kicked it out of the Kuiper Belt. It moved toward the Sun, and was captured by Saturn.

Several bits of evidence support this idea. One is its orbit. Phoebe orbits in the opposite direction from Saturn’s major moons, for example, and the orbit it highly tilted — it carries Phoebe almost above Saturn’s poles, instead of around its equator.

Other evidence includes Phoebe’s composition. Phoebe’s density suggests it’s about half rock and half ice — the same composition as a group of objects in the Kuiper Belt. And while most of Saturn’s major moons have bright, icy surfaces, the surface of Phoebe is especially dark — also like the Kuiper Belt objects.

So one of Saturn’s most-distant moons may have originated a lot farther from the ringed planet.

And Saturn is in great view early tomorrow. The giant planet looks like a bright golden star to the right of our Moon at dawn.


Script by Damond Benningfield

Get Premium Audio

Listen to today's episode of StarDate on the web the same day it airs in high-quality streaming audio without any extra ads or announcements. Choose a $8 one-month pass, or listen every day for a year for just $30.