Halos of Ice and Dust The Ringed Worlds of the Solar System

Saturn's main rings stretch across roughly two hundred and eighty thousand kilometers, about three quarters of the distance from Earth to the Moon. In most places they are only about ten meters thick. Shrink the system until it is as wide as a football field, and it would be thinner than a sheet of paper. It is one of the largest structures in the solar system, and it is almost entirely made of empty space and frozen water.

A ring system, in the plainest terms, is a flattened disc of countless solid chunks of ice, rock, and dust, all orbiting a single world. Every one of those chunks is a tiny moon following its own path around the planet. From far away the ring looks solid only because there are so many pieces, so evenly spread.

This episode is about those halos. We will look closely at Saturn's, which is the best studied and the strangest. Then we will visit the fainter rings around Jupiter, Uranus, and Neptune, and even a ring found around a small icy world far beyond Neptune. We will spend a few minutes imagining what it would be like to live under a ring of our own. And we will ask why Mercury, Venus, Earth, and Mars have none, a question whose answer turns out to be more tentative than you might expect.