Asteroids with Saturn-envy can deck themselves out with glittering rings. A space rock called 10199 Chariklo is the first asteroid known to have a ring system, revealing an unexpected possibility for small bodies in the solar system. Until now, rings have been found only around giant planets.
Colin Snodgrass of the Max Planck Institute for Solar System Research in Göttingen, Germany, and his colleagues found the rings in June 2013 during an asteroid occultation. This is when a space rock crosses in front of a star as seen from Earth, causing the star to briefly wink out. Seen from sites across South America, the star's brightness dipped in a distinctive pattern, indicating a set of sharply defined, thin rings.
"We were amazed to see that the star didn't just blink out and come back – there were very short blinks before and after the main dip, which could only be explained by rings," says Snodgrass.
Hidden shepherds
Usually the particles in rings would spread out, making them smoother at the edges. To look so sharp, the asteroid's rings must be constantly shaped by something, most likely unseen "shepherd" satellites that move near the rings and keep their particles confined. "Shepherds have been observed around some of Uranus and Saturn's narrow rings," says team member Bruno Sicardy at Pierre-and-Marie-Curie University in Paris, France. "So even if their origins are very different, Chariklo's rings seem to follow some common rules."
Chariklo orbits between the paths of Saturn and Uranus, and the asteroid has shown mysterious behaviour in the past. The tiny body was seen dimming and brightening, and its spectrum held hints of water ice disappearing and reappearing. A pair of icy, reflective rings seen from different angles would account for this.
The authors think the rings could have formed in a number of ways, including impacts flinging up debris, or small moons colliding or being broken up by gravity if they spiralled too close to the asteroid.
"This work is very consistent with all previous data, and provides an explanation which is quite unexpected," says Aurelie Guilbert-Lepoutre of the European Space Research and Technology Centre in the Netherlands, who did not contribute to the study. "It opens a new and unique perspective on the processes dominating the history of small bodies."
Journal reference: Nature, DOI: 10.1038/nature13155
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