The ghostly glow of Saturn's tenuous G ring hints at the form a ring around Pluto might take. Image credit: NASA/JPL/Space Science Institute

Jun 30, 2015 What might a Pluto ring look like?

Sen—It’s almost time! We’re just weeks away from New Horizon’s historic flyby of the dwarf planet Pluto and already the discoveries are rolling in. The questions we hope to answer are numerous: What does the surface of Pluto look like? What’s the relationship between Pluto and its largest moon Charon? How similar is it to Neptune’s enigmatic moon Triton?

But there is perhaps only one item that’s both a burning question and a terrifying danger: Does Pluto have rings? We now know of six objects in the Solar System encircled by rings large and small (Jupiter, Saturn, Uranus, Neptune and centaurs Chariklo and Chiron); a seventh in the wrong place could disrupt more than a decade of carefully-laid plans.

How could something made up largely of particles smaller than a pea damage a spaceship wrought of the most advanced materials? It all has to do with speed. When an object is moving, it has what physicists call kinetic energy. How much depends on its mass and speed, but particularly on the latter. Ever drop something on your foot? The pain you feel is kinetic energy at work.

Imagine New Horizons encounters a particle of ice with a mass of just one gram (about the mass of a paperclip). That’s almost nothing when compared to the mass of the spacecraft, but it would slam into the ship travelling at nearly 50,000 kilometers per hour. The energy imparted in the collision would be equivalent to detonating an explosive charge with 46 grams of TNT onboard—ouch!

This possibility is sufficiently troubling that astronomers have spent years using a variety of techniques to look for any possible structures around Pluto. So far the coast is clear and, even if it’s not, the sheer vastness of space offers New Horizons substantial protection. But, if we do find something, what might it look like? Let’s take a closer look.

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If Pluto has a ring, it looks nothing like the dense main rings of Saturn as seen in this mosaic of images captured by NASA's Cassini spacecraft. Image credit:NASA/JPL/Space Science Institute

Right off the bat, we can rule out one possibility. Pluto definitely doesn’t have anything resembling Saturn’s A and B rings. Massive, dense rings like these would clearly stand out in the observations we’ve made. If rings do exist, they must be exceptionally faint and tenuous.

What might a diffuse, tenuous ring look like? Saturn offers a couple of intriguing possibilities. Most famous of the planet’s outer rings is the E ring. It’s composed of a vast number of tiny ice particles ejected from the plumes emanating from Enceladus’ ocean of liquid water.

Could a similar icy ring exist in the Pluto system? There’s at least some evidence to suggest that the answer is yes! Observations made with the Gemini North telescope shortly after the launch of New Horizons suggest that Pluto’s largest moon Charon might be active in a way similar to Enceladus. Just like tidal heating from giant Saturn warms the interior of Enceladus, Charon’s close proximity to larger Pluto may cause enough internal friction to warm things up there, as well. Although most ejected material would likely fall back to the moon’s surface, some might escape and enter orbit about Charon or Pluto.

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Saturn's tenuous G ring glows faintly in this image, captured in 2011. Cassini is traveling so quickly in this image that the stars are blurred. Image credit: NASA/JPL-Caltech/SSI

But even if Charon doesn’t prove to be cryovolcanically active, there’s an even easier way to generate a ring: impacts. We think that at least two of Saturn’s rings, the G ring and the Phoebe ring, are largely sustained by tiny micrometeoroid impacts into nearby moons. Just like a tiny space rock hitting New Horizons would make for a very bad day, similar impacts into a moon release a shower of debris. Since these small moons have extremely limited gravitational pull, much of this material escapes into orbit and can form a ring.

Pluto is home to at least four moons similar to those found around other outer Solar System objects. In fact, two of them, Styx and Kerberos, were first noticed during one of the ring-searching campaigns surrounding New Horizons. As these are bombarded with space rocks, dust will inevitably be kicked up.

More tantalizingly, a larger collision could create a similar system in one fell swoop. That’s one idea that’s been put forth to explain the remarkable double-ring system surrounding the asteroid Chariklo. Astronomers watching it occult—or pass in front of—a background star noticed that the star’s light was dimmed where they didn’t expect it, striking evidence for a ring of material encircling this far-away object.

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This artist's conception illustrates what the ring system around Chariklo might look like. Image credit: ESO/L. Calçada/M. Kornmesser/Nick Risinger

So what happens if New Horizons does detect a ring around Pluto or one of its moons? If the team sees it in time and the material appears to endanger the mission, evasive action can be taken—but only up until July 4. After that, the spacecraft’s path is locked in.

Ultimately, though, a ring at Pluto would offer a tremendous boon to the mission’s science. Our study of the rings around Jupiter, Saturn, Uranus, and Neptune has helped us not only understand the nature of thin particle disks, but also place constraints of the size, mass, and other characteristics of nearby moons. The innermost ring of Saturn has even helped us peer inside the planet and uncover vibrations deep beneath its surface.

So, let’s all keep our fingers crossed that Pluto turns out to be a system full of rings and that, undaunted, New Horizons can fly right through unscathed.

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