Why Doesn’t Earth Have Rings?

Why Doesn't Earth Have Rings?

Before we really get started on today’s episode, I’d like to share a bunch of really cool pictures created by my friend Kevin Gill. Kevin’s a computer programmer, 3-D animator and works on climate science data for NASA.

And in his spare time, he uses his skills to help him imagine what the Universe could look like. For example, he’s mapped out what a future terraformed Mars might look like based on elevation maps, or rendered moons disturbing Saturn’s rings with their gravity.

Earth’s Rings over San Bernadino. Credit: Kevin Gill (CC BY-SA 2.0)

But one of my favorite sets of images that Kevin did were these. What would it look like if Earth had rings? Kevin and his wife went to a few cool locations, took some landscape pictures, and then Kevin did the calculations for what it would look like if Earth had a set of rings like Saturn.

And let me tell you, Earth would be so much better. At least you’d think so, but actually, it might also suck.

Last time I checked, we don’t have rings like this. In fact, we don’t have any rings at all.

Why not? Considering the fact that Saturn, Jupiter, Uranus and Neptune all have rings, don’t we deserve at least something?

Did we ever have rings in the past, or will we in the future? What’s it going to take for us to join the ring club? Short answer, an apocalypse.

Before we get into the inevitable discussion of death and devastation, let’s talk a bit about rings.

A lovely view of Saturn and its rings as seen by the Cassini spacecraft on Aug. 12, 2009. Credit: NASA/JPL-Caltech/Space Science Institute.

Saturn is the big showboat, with its fancy rings. They’re made of water ice, with chunks as big as a mountain, or as small as a piece of sand. Astronomers have been arguing about where they came from and how old they are, but the current consensus – sort of – is that the rings are almost as ancient as Saturn itself: billions of years old. And yet, some process is weathering the rings, grinding the particles so they appear much younger.

Jupiter’s rings. Image Credit: University of Maryland

Jupiter’s rings are much fainter, and we didn’t even know about them until 1979, when the Voyager spacecraft made their flybys. The rings seem to be created by dust blown off into space by impacts on the planet’s moons.

Hey, we’ve got a moon, that’s a sign.

Uranus imaged by Voyager 2 in 1986. Credit: NASA

The rings around Uranus are bigger and more complex than Jupiter’s rings, but not as substantial as Saturn’s. They’re much younger, perhaps only 600 million years old, and appear to have been caused by two moons crashing into each other, long ago.

Again, another sign. We still have the potential for stuff to crash around us.

The labeled ring arcs of Neptune as seen in newly processed data. The image spans 26 exposures combined into a equivalent 95 minute exposure, and the ring trace and an image of the occulted planet Neptune is added for reference. Credit: M. Showalter/SETI Institute

The rings around Neptune are far dustier than any of the other ring systems, and much younger than the Solar System. And like the rings around Uranus, they were probably formed when two or more of its moons collided together.

Now what about our own prospects for rings?

The problem with icy rings is that the Earth orbits too closely to the Sun. There’s a specific point in the Solar System known as the “frost line” or “snow line”. This is the point in the Solar System where deposits of ice could have survived for long periods of time. Any closer and the radiation from the Sun sublimates the ice away.

This point is actually located about 5 astronomical units away from the Sun, in the asteroid belt. Mars is much closer, so it’s very dry, while Jupiter is beyond the frost line, and its moons have plenty of water ice.

The Earth is a mere 1 AU from the Sun. That’s the very definition of an astronomical unit, which means it’s well within the frost line. The Earth itself can maintain water because the planet’s magnetosphere acts like a shield against the solar wind. But the Moon is bone dry (except for the permanently shadowed craters at its poles).

And if there was an icy ring system around the Earth, the solar wind would have blasted it away long ago.

Instead, let’s look at another kind of ring we can have. One made of rock and dust, containing death and sorrow, from a pulverized asteroid or moon. In fact, billions of years ago, we definitely had a ring when a Mars-sized planet crashed into the Earth and spewed out a massive ring of debris. This debris collected together into the Moon we know today. That impact turned the Earth’s surface inside out. It was all volcanoes, everywhere, all the time.

Credit: Kevin Gill (CC BY-SA 2.0)

It’s also possible we had a second moon in the ancient past, which collided with our current Moon. That would have generated an all new ring of material for millions of years until it was recaptured by the Moon, kicked out of orbit, or fell down onto the Earth.

It’s that “fell down onto Earth” part that’s apocalyptic. As mountains of ring material entered the Earth’s atmosphere, it would increase the temperature, baking and boiling away any life that couldn’t burrow deep underground.

It’s kind of like the book Seveneves, which you should totally read if you haven’t already. It talks about what we would see if the Moon broke apart into a ring, and the terrible terrible thing that happens next.

Earth’s Rings from New Hampshire. Credit: Kevin Gill (CC BY-SA 2.0)

If Earth did get a set of rings, they’d be pretty, but they’d also be a huge pain for astronomers. As you saw in Kevin’s original pictures, the rings take up a huge chunk of the sky for most observers. The farther north or south you go, the more dramatically the rings will ruin your view. Only if you were right at the equator, you’d have a thin line, which would be borderline acceptable.

Furthermore, the rings themselves would be incredibly reflective, and completely ruin the whole concept of dark skies. You know how the Moon sucks for astronomy? Rings would be way way worse.

Finally, rings would interfere with our ability to launch spacecraft and maintain satellites. It depends on how far they extend, but we wouldn’t be able to have any satellites in that region or cross the ring plane. Oh, and that fiery death apocalypse I mentioned earlier.

We know that the Moon is drifting away from the Earth right now thanks to the conservation of angular momentum. But in the distant future, billions of years from now, there might be a scenario that turns everything around.

The Sun’s habitable zone in its red giant phase. Credit: NASA/Goddard Space Flight Center Conceptual Image Lab

As you know, when it runs out of fuel in its core, the Sun is going to bloat up as a red giant, consuming Mercury and Venus. Scientists are on the fence about Earth. Some think that Earth will be fine. The Sun will blast off its outer layers, but not actually envelop Earth. Others think that at the Sun’s largest point, we’ll be orbiting within the outer atmosphere of the Sun. Ouch, that’s hot.

The orbiting Moon will experience drag as it goes around the Earth, slowing down its orbital velocity, and causing it to spiral inward. Once it reaches the Roche Limit of the Earth, about 9,500 km, our planet’s gravity will tear the Moon apart into a ring. The chunks in the ring will also experience drag in the solar atmosphere and continue to spiral inward until they crash into the planet.

The Moon tearing apart to become a ring around Earth. Credit: Universe Sandbox ²

That would be considered a very bad day, if it wasn’t for the fact that we were already living inside the atmosphere of the Sun. No amount of terraforming will fix that.

Sadly, the Earth doesn’t have rings like Saturn, and it probably never did. It might have had rings of rock and dust for periods, but they weren’t that majestic to look at. In fact, seeing rings around the planet would mean we’d lost a moon, and our planet was about go through a period of bombardment. I’ll pass.

What Did Cassini Teach Us?

What Did Cassini Teach Us?


Ask me my favorite object in the Solar System, especially to see through a telescope, and my answer is always the same: Saturn.

Saturn is this crazy, ringed world, different than any other place we’ve ever seen. And in a small telescope, you can really see the ball of the planet, you can see its rings. It’s one thing to see a world like this from afar, a tiny jumping image in a telescope. To really appreciate and understand a place like Saturn, you’ve got to visit.

And thanks to NASA’s Cassini spacecraft, that’s just what we’ve been doing for the last 13 years. Take a good close look at this amazing ringed planet and its moons, and studying it from every angle.

Space Probes
Cassini orbiting Saturn. Credit: NASA

Throughout this article, I’m going to regale you with the amazing discoveries made by Cassini at Saturn. What it taught us, and what new mysteries it uncovered.

NASA’s Cassini spacecraft was launched from Earth on October 15, 1997. Instead of taking the direct route, it made multiple flybys of Venus, a flyby of Earth and a flyby of Jupiter. Each one of these close encounters boosted Cassini’s velocity, allowing it to make the journey with less escape velocity from Earth.

It arrived at Saturn on July 1st, 2004 and began its science operations shortly after that. The primary mission lasted 4 years, and then NASA extended its mission two more times. The first ending in 2010, and the second due to end in 2017. But more on that later.

Before Cassini, we only had flybys of Saturn. NASA’s Pioneer 11, and Voyagers 1 and 2 both zipped past the planet and its moons, snapping pictures as they went.

But Cassini was here to stay. To orbit around and around the planet, taking photos, measuring magnetic fields, and studying chemicals.

For Saturn itself, Cassini was able to make regular observations of the planet as it passed through entire seasons. This allowed it to watch how the weather and atmospheric patterns changed over time. The spacecraft watched lightning storms dance through the cloudtops at night.

This series of images from NASA’s Cassini spacecraft shows the development of the largest storm seen on the planet since 1990. These true-color and composite near-true-color views chronicle the storm from its start in late 2010 through mid-2011, showing how the distinct head of the storm quickly grew large but eventually became engulfed by the storm’s tail. Credit: NASA/JPL-Caltech/Space Science Institute

Two highlights. In 2010, Cassini watched a huge storm erupt in the planet’s northern hemisphere. This storm dug deep into Saturn’s lower atmosphere, dredging up ice from a layer 160 kilometers below and mixing it onto the surface. This was the first time that astronomers were able to directly study this water ice on Saturn, which is normally in a layer hidden from view.

Natural color images taken by NASA’s Cassini wide-angle camera, showing the changing appearance of Saturn’s north polar region between 2012 and 2016.. Credit: NASA/JPL-Caltech/Space Science Institute/Hampton University

The second highlight, of course, is the massive hexagonal storm churning away in Saturn’s northern pole. This storm was originally seen by Voyager, but Cassini brought its infrared and visible wavelength instruments to bear.

Why a hexagon? That’s still a little unclear, but it seems like when you rotate fluids of different speeds, you get multi-sided structures like this.

Cassini showed how the hexagonal storm has changed in color as Saturn moved through its seasons.

This is one of my favorite images sent back by Cassini. It’s the polar vortex at the heart of the hexagon. Just look at those swirling clouds.

The polar vortex, in all its glory. Credit: NASA/JPL-Caltech/Space Science Institute

Now, images of Saturn itself are great and all, but there was so much else for Cassini to discover in the region.

Cassini studied Saturn’s rings in great detail, confirming that they’re made up of ice particles, ranging in size as small a piece of dust to as large as a mountain. But the rings themselves are actually quite thin. Just 10 meters thick in some places. Not 10 kilometers, not 10 million kilometers, 10 meters, 30 feet.

The spacecraft helped scientists uncover the source of Saturn’s E-ring, which is made up of fresh icy particles blasting out of its moon Enceladus. More on that in a second too.

Vertical structures, among the tallest seen in Saturn’s main rings, rise abruptly from the edge of Saturn’s B ring to cast long shadows on the ring in this image taken by NASA’s Cassini spacecraft two weeks before the planet’s August 2009 equinox. Credit: NASA/JPL/Space Science Institute

Here’s another one of my favorite images of the mission. You’re looking at strange structures in Saturn’s B-ring. Towering pillars of ring material that rise 3.5 kilometers above the surrounding area and cast long shadows. What is going on here?

They’re waves, generated in the rings and enhanced by nearby moons. They move and change over time in ways we’ve never been able to study anywhere else in the Solar System.

Daphnis, one of Saturn’s ring-embedded moons, is featured in this view, kicking up waves as it orbits within the Keeler gap. Credit: NASA/JPL-Caltech/Space Science Institute

Cassini has showed us that Saturn’s rings are a much more dynamic place than we ever thought. Some moons are creating rings, other moons are absorbing or distorting them. The rings generate bizarre spoke patterns larger than Earth that come and go because of electrostatic charges.

Speaking of moons, I’m getting to the best part. What did Cassini find at Saturn’s moons?

Let’s start with Titan, Saturn’s largest moon. Before Cassini, we only had a few low resolution images of this fascinating world. We knew Titan had a dense atmosphere, filled with nitrogen, but little else.

Cassini was carrying a special payload to assist with its exploration of Titan: the Huygens lander. This tiny probe detached from Cassini just before its arrival at Saturn, and parachuted through the cloudtops on January 14, 2005, analyzing all the way. Huygens returned images of its descent through the atmosphere, and even images of the freezing surface of Titan.

Huygen’s view of Titan. Credit: ESA/NASA/JPL/University of Arizona

But Cassini’s own observations of Titan took the story even further. Instead of a cold, dead world, Cassini showed that it has active weather, as well as lakes, oceans and rivers of hydrocarbons. It has shifting dunes of pulverized rock hard water ice.

If there’s one place that needs exploring even further, it’s Titan. We should return with sailboats, submarines and rovers to better explore this amazing place.

A view of Mimas from the Cassini spacecraft. Credit: NASA/JPL/Space Science Institute

We learned, without a shadow of a doubt, that Mimas absolutely looks like the Death Star. No question. But instead of a megalaser, this moon has a crater a third of its own size.

Saturn’s moon Iapetus. Image credit: NASA/JPL/SSI

Cassini helped scientists understand why Saturn’s moon Iapetus has one light side and one dark side. The moon is tidally locked to Saturn, its dark side always leading the moon in orbit. It’s collecting debris from another Saturnian moon, Phoebe, like bugs hitting the windshield of a car.

Perhaps the most exciting discovery that Cassini made during its mission is the strange behavior of Saturn’s moon Enceladus. The spacecraft discovered that there are jets of water ice blasting out of the moon’s southern pole. An ocean of liquid water, heated up by tidal interactions with Saturn, is spewing out into space.

And as you know, wherever we find water on Earth, we find life. We thought that water in the icy outer Solar System would be hard to reach, but here it is, right at the surface, venting into space, and waiting for us to come back and investigate it further.

Icy water vapor geysers erupting from fissures on Enceladus. Credit: NASA/JPL

On September 15, 2017, the Cassini mission will end. How do we know it’s going to happen on this exact date? Because NASA is going to crash the spacecraft into Saturn, killing it dead.

That seems a little harsh, doesn’t it, especially for a spacecraft which has delivered so many amazing images to us over nearly two decades of space exploration? And as we’ve seen from NASA’s Opportunity rover, still going, 13 years longer than anticipated. Or the Voyagers, out in the depths of the void, helping us explore the boundary between the Solar System and interstellar space. These things are built to last.

The problem is that the Saturnian system contains some of the best environments for life in the Solar System. Saturn’s moon Enceladus, for example, has geysers of water blasting out into space.

Cassini spacecraft is covered in Earth-based bacteria and other microscopic organisms that hitched a ride to Saturn, and would be glad to take a nice hot Enceladian bath. All they need is liquid water and a few organic chemicals to get going, and Enceladus seems to have both.

NASA feels that it’s safer to end Cassini now, when they can still control it, than to wait until they lose communication or run out of propellant in the future. The chances that Cassini will actually crash into an icy moon and infect it with our Earth life are remote, but why take the risk?
For the last few months, Cassini has been taking a series of orbits to prepare itself for its final mission. Starting in April, it’ll actually cross inside the orbit of the rings, getting closer and closer to Saturn. And on September 15th, it’ll briefly become a meteor, flashing through the upper atmosphere of Saturn, gone forever.

This graphic illustrates the Cassini spacecraft’s trajectory, or flight path, during the final two phases of its mission. The view is toward Saturn as seen from Earth. The 20 ring-grazing orbits are shown in gray; the 22 grand finale orbits are shown in blue. The final partial orbit is colored orange. Image credit: NASA/JPL-Caltech/Space Science Institute

Even in its final moments, Cassini is going to be sciencing as hard as it can. We’ll learn more about the density of consistency of the rings close to the planet. We’ll learn more about the planet’s upper atmosphere, storms and clouds with the closest possible photographs you can take.

And then it’ll all be over. The perfect finale to one of the most successful space missions in human history. A mission that revealed as many new mysteries about Saturn as it helped us answer. A mission that showed us not only a distant alien world, but our own planet in perspective in this vast Solar System. I can’t wait to go back.

How have the photos from Cassini impacted your love of astronomy? Let me know your thoughts in the comments.