National Geographic News
Rhea.
Saturn's moon Rhea (file image).

Image courtesy NASA

Ker Than

for National Geographic News

Published August 6, 2010

Until this week Saturn's small moon Rhea was the only known solid space object thought to have a ring. (Other known ringed bodies, such as Saturn, are mainly gaseous.)

But a new study of optical images has failed to detect any signs of structures encircling the natural satellite.

Rhea orbits within Saturn's magnetic field, which creates a bubble of charged particles—ions and electrons—around the planet. During a 2005 flyby of Rhea, scientists working with NASA's Cassini spacecraft expected to see a dip in their readings where the moon's surface intercepted the particles.

The craft's readings did show the moon's wake, but they also revealed several unexpected dips in particle detections just outside the moon's diameter.

The best possible explanation seemed to be that something physical—a ring of debris around Rhea—was blocking the ions and electrons from reaching Cassini. (See "Saturn Moon May Have Rings—A First.")

However, analysis of images taken by Cassini between 2008 and 2009 failed to turn up any evidence for rings around the Saturn moon. (See pictures of Saturn and its moons.)

"We're pretty confident that there is no solid material orbiting the moon," said astronomer Matthew Tiscareno of Cornell University in New York.

(Related: "Saturn Moon Has Surprisingly "Slushy" Insides.")

Moon Mystery Comes to Light

Tiscareno and his team analyzed 65 Cassini images of Rhea, some of which were taken with the sun behind the craft and some with the sun more or less in front of Cassini.

"Those two geometries probe different particle sizes, because dust tends to scatter light forward whereas larger particles tend to reflect light backward," Tiscareno explained.

(Also see "Saturn Rings Surprisingly Unstable, Violent.")

Instead, the pictures showed nothing illuminated around Rhea, disproving the ring hypothesis—although the new study doesn't shed any light on what was responsible for the 2005 observations.

"It's a real mystery," Tiscareno said. "I think we make a strong case that it's not a ring of solid material around the moon.

"As for what it is, that's for the magnetospheric community to figure out."

The new findings are detailed in a recent issue of the journal Geophysical Research Letters.

0 comments

Share

Latest From Nat Geo

  • Photos: Relics of the Cold War

    Photos: Relics of the Cold War

    A photographer explores the traces of a standoff that divided Europe for four decades.

     

     

  • Delicate but Deadly Jellyfish

    Delicate but Deadly Jellyfish

    Some jellyfish are known to migrate hundreds of feet in pursuit of prey. See some of our favorite jellyfish pictures in honor of Jellyfish Day.

See more photo galleries »

Featured Article

  • The Push to Get More Women Into Science

    The Push to Get More Women Into Science

    The number of women in scientific research continues to lag behind the number of men, even though women make up half the nation's workforce. The question is, What difference does it make?

The Future of Food Series

  • Download: Free iPad App

    Download: Free iPad App

    We've made our magazine's best stories about the future of food available in a free iPad app.

  • Why Food Matters

    Why Food Matters

    How do we feed nine billion people by 2050, and how do we do so sustainably?

See more food news, photos, and videos »