Wednesday, July 2, 2014

Rings of Chariklo

One of the most spectacular sights in the Solar System has got to be Saturn’s majestic rings. Over the years, NASA’s Cassini spacecraft has captured the beauty of Saturn’s ring system in breathtaking detail. Less prominent ring systems also exist around Jupiter, Uranus and Neptune. Apart from the giant planets, no rings are known to exist around any other smaller objects until the surprising discovery by F. Braga-Ribas et al. (2014) of two narrow rings around a minor planet named Chariklo. With an estimated size of ~250 km, Chariklo is the largest known Centaur - a group of objects orbiting the Sun in a region of space between Saturn and Uranus. Chariklo is believed to be a former trans-Neptunian object that was recently (i.e. less than 10 million years ago) gravitationally perturbed by Uranus into a closer orbit around the Sun.

Figure 1: An artist’s impression showing a close-up of what Chariklo and its rings might look like. Image credit: ESO/L. Calçada/M. Kornmesser/Nick Risinger.

On 3 June 2013, the occultation of a faint background star by Chariklo was predicted to be observable from South America. A number of telescopes at various sites across South America took measurements of the occultation event. A few seconds before, and again, a few seconds after the main occultation, two additional tiny dips in the star’s apparent brightness were observed. This indicates the presence of two narrow rings around Chariklo that is obscuring the star’s light. In fact, the rings around Uranus and Neptune were found in much the same manner back in 1977 and 1984.

In particular, the Danish 1.54 m telescope at La Silla in the Chilean Atacama Desert observed the occultation event with a cadence of ~10Hz. The acquisition rate was high enough to resolve the secondary events occurring before and after the main occultation as two separate rings identified as 2013C1R and 2013C2R (C1R and C2R in short). Based on the observational data, C1R and C2R have estimated respective widths of about 7 and 3 km, optical depths of 0.4 and 0.06, and orbital radii of 391 and 405 km. A gap of 9 km separates the two rings. The inner ring (C1R) is estimated to have a cumulative mass equivalent to an icy object with a diameter of roughly 1 km. The outer ring (C2R), being somewhat narrower and more tenuous, has an estimated total mass corresponding to an icy object with a size of ~500 m.

Figure 2: Light curve of the occultation event observed by the Danish 1.54 m telescope on 3 June 2013. F. Braga-Ribas et al. (2014).

 Figure 3: Chariklo and its system of rings. The dotted lines mark the trajectories of the background star relative to Chariklo in the plane of the sky as observed from the eight different sites across South America. F. Braga-Ribas et al. (2014).

The rings around Chariklo are dynamically unstable and would dissipate in a span of a few million years. As a result, the rings are either very young or actively confined by the presence of one or more kilometre-size shepherd moons around Chariklo. Observations of Chariklo between 1997 and 2008 show a gradual disappearance of the water-ice signature from the spectrum. It implies that the rings are partly made of water-ice because during that period, the rings would have appeared edge-on, resulting in the disappearance of the water-ice signature. Observations made in 2013 show that the system has brightened and the water-ice signature is detectable again as more of the ring-plane becomes visible.

Several origins for Chariklo’s rings have been proposed. All scenarios resulted in the presence of a disk of debris around Chariklo. The debris came either from collisional events or from the tidal disruption of a pre-existing moon. In the aftermath, the largest pieces of debris probably became shepherd moons that shaped the rings into their present configuration. If the rings are older than ~10 million years, they would have formed when Chariklo was still in trans-Neptunian space and have thus survived the gravitational perturbations by Uranus which brought Chariklo to its current orbit between Saturn and Uranus. With this remarkable discovery, there are now 5 known objects in the Solar System with rings around them - Jupiter, Saturn, Uranus, Neptune and tiny Chariklo.

F. Braga-Ribas et al., “A ring system detected around the Centaur (10199) Chariklo”, Nature 508, 72-75 (03 April 2014)