Images of Saturn’s main rings, the F Ring, and its shepherd moons obtained by NASA’s Cassini spacecraft. The narrow F Ring is located just outside of the outer edge of the main rings. Two satellites sandwiching the F Ring slightly above and to the left of the centre of the image are the shepherd satellites Prometheus (inner orbit) and Pandora (outer orbit). Image credit: NASA / Kobe University.Ryuki Hyodo, a second-year PhD student, and Professor Keiji Ohtsuki of the Graduate School of Science at Kobe University have revealed that Saturn’s F Ring and its shepherd satellites are a natural outcome of the final stage of formation of Saturn’s satellite system. Their finding has just been published online in Nature Geoscience.
Appearing like eyes on a potato, craters cover the dimly lit surface of the moon Prometheus, one of the shepherd moons for Saturn’s F Ring, in this high-resolution image from Cassini’s early 2010 flyby. This view looks toward the trailing hemisphere of Prometheus which measures 136 × 79 × 59 kilometres (85 x 49 x 37 miles). Image credit: NASA / JPL / Space Science Institute.Saturn, which is the second largest planet in our solar system, is known to have multiple rings and satellites. In 1979, Pioneer 11 discovered the F Ring, located 3,000 kilometres (1900 miles) beyond the outer edge of the A Ring. The F Ring is very active, with features changing on a timescale of hours.
The F Ring is also very narrow with a width of only a few hundred kilometres, and has two shepherd satellites called Prometheus and Pandora, which orbit inside and outside the ring, respectively. Although the Voyager and Cassini spacecraft later made detailed observations of the F Ring and its shepherd satellites, their origin has not been clarified.
According to the latest satellite formation theory, Saturn used to have ancient rings containing many more particles than they do today, and satellites formed from spreading and accretion of these particles. During the final stage of satellite formation, multiple small satellites tend to form near the outer edge of the ring. On the other hand, observations by Cassini indicate that the small satellites orbiting near the outer edge of the main ring system have a dense core.
While inner shepherd moon Prometheus contributes to the confinement of Saturn’s narrow F Ring, outer shepherd moon Pandora plays a lesser role. NASA’s Cassini spacecraft acquired infrared, green and ultraviolet images of Pandora at a distance of approximately 52,000 kilometres (32,000 miles) on 5 September 2005, which were combined to create this false-colour view. Pandora measures 104 × 81 × 64 kilometres (65 x 50 x 40 miles). Image credit: PD-USGOV-NASA.In their simulations using in part computer systems at the National Astronomical Observatory of Japan, Hyodo and Ohtsuki revealed that the F Ring and its shepherd satellites formed as these small satellites with a dense core collided and partially disintegrated. In other words, the system of the F Ring and its shepherd satellites is a natural outcome of the formation process of Saturn’s ring-satellite system.
This new finding is expected to help elucidate the formation of satellite systems both within and outside our solar system. For example, the above formation mechanism can also be applied to the rings and shepherd satellites of Uranus, which are similar to those of Saturn.In this computer simulation, two satellites with dense cores collide at the current location of the F Ring. The collision does not destroy the cores, which become two shepherd satellites, and the dispersed particles distributed in between the two satellites form the F Ring. Image credit: Hyodo / Ohtsuki / Kobe University.Hyodo, now visiting Institut de Physique du Globe de Paris for his research from this spring (April 2015) remarked, “Through this study, we were able to show that the current rings of Saturn reflect the formation and evolution processes of the planet’s satellite system.”
“As plans are underway in and outside of Japan to explore the satellite system of Jupiter and the satellites of Mars,” said Ohtsuki, “we will continue to unravel the origin of satellite systems, which is key to understanding the formation process of planetary systems.”
NASA’s Cassini orbiter sailed through the tenuous outermost reaches of Saturn’s atmosphere without trouble Monday, performing the first of five close swings nearer to the planet than any previous spacecraft before a final dive Sept. 15 to end the probe’s nearly 20-year mission.
There’s a lot of planetary activity in the dawn sky in mid-April. If you’re an early riser in the British Isles, let the waning crescent Moon be your guide to the naked-eye planets Jupiter, Saturn and Mars on 15 and 16 April 2020. Typical 7×50 or 10×50 binoculars will show these attractive conjunctions well, while the smallest of telescopes also reveal some of Jupiter’s bright Galilean moons.
