Wait. How Thin Are Saturn's Rings?

The entire universe in blog form
May 2 2014 7:30 AM

Just How Thin Are Saturn’s Rings?

Saturn and rings
Saturn's rings are so thin that when seen edge-on they practically disappear, even to the Cassini spacecraft. They barely reveal their presence in this shot, though the shadows they cast on the planet itself makes it easier.

Photo by NASA/JPL/Space Science Institute

Note: A version of this article originally appeared on my Google Plus page, but rumor has it G+ may be going the way of phlogiston and N-rays. I didn’t want to lose this article, since it’s one of the coolest facts about our solar system, so I figured I’d update it and put it here on the blog. That way I can link to it when I talk about Saturn’s rings.

The planet Saturn is beyond question the jewel of the solar system. Its magnificent system of rings is awe-inspiring, composed of countless tiny particles of water ice, each orbiting the gas giant planet individually. Sculpted by the gravity of the planet and the orbital dances of dozens of moons, the rings are divided into a few major rings and thousands of narrower ringlets.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

Advertisement

They’re also huge: The main ring system spans 300,000 kilometers (180,000 miles), and some fainter rings are cast even wider! But they're also amazingly flat. Seriously, truly flat: Observations indicate they are only 10 meters (32 feet) thick in some places, up to as much as a kilometer (0.6 miles) in others.

That’s more than just flat. That’s practically two-dimensional! Let me show you why.

Let’s take the bigger number, and assume the rings are a kilometer thick. If the main rings are 300,000 kilometers across, that makes the thickness-to-width ratio 1 in 300,000, or 0.0000033.

How thin is that? Well, a common simile used for thinness is a sheet of paper (i.e., “paper thin”). But how thin is that? A standard sheet of US writing paper has dimensions of 8.5 x 11 inches (about 22 x 28 cm) in width and height, and (for 20 pound paper) only about 0.004 inches (0.1 mm) in thickness.

Even if we use the larger dimension of 11 inches, the thickness-to-width ratio of a piece of paper is then 0.00036, over 100 times the ratio of Saturn’s rings!

In other words, to scale, a piece of paper is 100 times thicker than Saturn’s rings. And that’s using the thicker number for the rings, 1 km instead of 10 meters. If we go with the thinner number, the ratio jumps another factor of 100, so a piece of paper is 10,000 times thicker than Saturn’s ring, to scale!

Want to think of it another way? If you wanted to make a scale model of Saturn’s rings using paper, you’d have to use a sheet 10,000 x 11 inches = 1.7 miles across!

We’re using the wrong phrase. We should say something is “Saturn’s rings thin.”

It’s truly astonishing that a structure so huge, so vast—it stretches more than three-fourths of the distance from the Earth to the Moon!—can be so ethereally thin. But that’s physics at work. The particles forming the rings may have once had all different tilts as they orbited Saturn, but if they did, they would have collided with each other. The details are a bit complicated, but collisions would steal angular momentum from the particles, and their orbits would tend to flatten out. This kept happening until the particles were spread out so thinly that collisions stopped occurring, which happens when the rings were only a few times thicker than the size of the biggest particles in them—a few meters across. That’s why the rings are so thin.

Once again, processes in physics are both vast and subtle, and what seems surprising to our intuition is actually inevitable in reality. That’s one of the many reasons I rely more on science than in our own easily fooled human brains!

Science is a way of helping you understand the Universe, but it’s also a way of helping you trust it. It always follows its own rules.

TODAY IN SLATE

Sports Nut

Grandmaster Clash

One of the most amazing feats in chess history just happened, and no one noticed.

The Extraordinary Amicus Brief That Attempts to Explain the Wu-Tang Clan to the Supreme Court Justices

Amazon Is Officially a Gadget Company. Here Are Its Six New Devices.

Uh-Oh. The World’s Oceans Have Broken Their All-Time Heat Record.

The NFL Explains How It Sees “the Role of the Female”

Future Tense

Amazon Is Now a Gadget Company

Food

How to Order Chinese Food

First, stop thinking of it as “Chinese food.”

The NFL Should Lose Its Tax-Exempt Status, Which It Never Should Have Had Anyway

The Country Where Women Aren’t Allowed to Work Once They’re 36 Weeks’ Pregnant

The XX Factor
Sept. 18 2014 11:40 AM The Country Where Women Aren’t Allowed to Work Once They’re 36 Weeks’ Pregnant
  News & Politics
Weigel
Sept. 18 2014 8:20 PM A Clever Attempt at Explaining Away a Vote Against the Farm Bill
  Business
Moneybox
Sept. 18 2014 6:02 PM A Chinese Company Just Announced the Biggest IPO in U.S. History
  Life
Outward
Sept. 18 2014 4:15 PM Reactions to a Sketch of Chelsea Manning Reveal Transmisogyny
  Double X
Doublex
Sept. 18 2014 8:07 PM Crying Rape False rape accusations exist, and they are a serious problem.
  Slate Plus
Behind the Scenes
Sept. 18 2014 1:23 PM “It’s Not Every Day That You Can Beat the World Champion” An exclusive interview with chess grandmaster Fabiano Caruana.
  Arts
Brow Beat
Sept. 18 2014 4:33 PM The Top 5 Dadsplaining Moments From The Cosby Show
  Technology
Future Tense
Sept. 18 2014 6:48 PM By 2100 the World's Population Could Be 11 Billion
  Health & Science
Science
Sept. 18 2014 3:35 PM Do People Still Die of Rabies? And how do you know if an animal is rabid?
  Sports
Sports Nut
Sept. 18 2014 11:42 AM Grandmaster Clash One of the most amazing feats in chess history just happened, and no one noticed.