Evolution Of Planetary Rings Research Articles

Dynamical Interactions and Mass Loss within the Uranian System

The origin and evolution of planetary rings and moons remains an active area of study, particularly as they relate to the impact history and volatile inventory of the outer solar system. The Uranian system contains a complex system of rings that are coplanar with the highly inclined planetary equator relative to the orbital plane. Uranus also harbors five primary regular moons that play an important role in the distribution of material that surrounds the planet. Here we present the results of a dynamical simulation suite for the Uranian system, intended to explore the interaction between the five primary regular moons and particles within the system. We identify regions of extreme mass loss within 40 planetary radii of Uranus, including eccentricity excitation of particle orbits at resonance locations that can promote moonlet formation within the rings. We calculate a total dynamical particle mass-loss rate of 35% within 0.5 × 106 yr and 40% mass loss within 107 yr. We discuss the implications for postimpact material, including dynamical truncation of stable ring locations and/or locations of moon formation promoted by dynamical excitation of ring material.

Size of the smallest particles in Saturn's rings

Spacecraft and ground-based observations have shown that particles in Saturn's main rings have a power-law distribution roughly between centimeter and ten meters, but sub-centimeter particles appear to be lacking. Recent studies based on Cassini observations suggest that the minimum particle size is on the order of millimeters, although their abundance is lower than is expected from the extrapolation of the power-law distribution for the larger size range. Cohesive force between particles has been proposed to explain the paucity of sub-centimeter particles, but its strength depends on impact velocity. In order to better understand recent Cassini observations about small ring particles, we examine impact velocity (vimp) between ring particles using N-body simulation including size distribution. We find that most collisions take place at low velocities with vimp≲0.1 cm s−1; even in dense rings where particles' velocity dispersion is enhanced by gravitational wakes, they collide at such low velocities because particles tend to move coherently in the wakes. This velocity is too low to detach sub-centimeter particles that are attached to the surface of large ones due to the cohesive force, explaining their paucity in the main rings. On the other hand, impacts at higher velocities occasionally occur, either as collision between particles in the high-velocity tail in dilute rings, or as collisions between adjacent wakes in the case of dense rings. Such infrequent high-velocity impacts release millimeter-sized particles strongly attached to the surface of large ones, explaining the observed free millimeter-sized particles with relatively low abundance. Our results suggest that studies of small particles in Saturn's rings would provide constraints on the cohesive force between icy particles, which is important not only for the origin and evolution of planetary rings but also for other astrophysical problems including dust evolution and planetesimal formation in protoplanetary disks.

Systematic Search for Rings around Kepler Planet Candidates: Constraints on Ring Size and Occurrence Rate

Abstract We perform a systematic search for rings around 168 Kepler planet candidates with sufficient signal-to-noise ratios that are selected from all of the short-cadence data. We fit ringed and ringless models to their light curves and compare the fitting results to search for the signatures of planetary rings. First, we identify 29 tentative systems, for which the ringed models exhibit statistically significant improvement over the ringless models. The light curves of those systems are individually examined, but we are not able to identify any candidate that indicates evidence for rings. In turn, we find several mechanisms of false positives that would produce ringlike signals, and the null detection enables us to place upper limits on the size of the rings. Furthermore, assuming the tidal alignment between axes of the planetary rings and orbits, we conclude that the occurrence rate of rings larger than twice the planetary radius is less than 15%. Even though the majority of our targets are short-period planets, our null detection provides statistical and quantitative constraints on largely uncertain theoretical models of the origin, formation, and evolution of planetary rings.

Ring in the new

Planets are no longer the only Solar System bodies sporting ring systems. Two dense rings have been detected encircling a Centaur object — a relatively small, icy interloper from the distant reaches of the Solar System. See Letter p.72 Observations of a stellar occultation by (10199) Chariklo, a Centaur-class outer-system asteroid orbiting between Saturn and Uranus, reveal that it has a ring system, a feature previously observed only for the four giant planets. Chariklo, with a diameter of about 250 km, has two narrow and dense rings separated by a small gap, probably due to the presence of a (yet-to-be-found) kilometre-sized satellite. The discovery of these rings raises questions about the formation and dynamical evolution of planetary rings. For one thing, it seems likely that planetary rings are much more common than previously thought.

Wave angular momentum and the evolution of planetary rings

A theory is proposed that provides angular momentum conservation through the acoustic drift mechanism for gap and ringlet formation in planetary rings. In this context, a new variant of drift mechanism is found, which is dominant if the collision frequency is of the order of the orbital frequency or less. An angular pseudo-momentum equation is derived for gravitational acoustic waves with dissipation. An explicit expression for angular pseudo-momentum, taking into account vertical wave structure, is obtained. The relation between Eulerian- and Lagrangian-averaged mass fluxes is also derived, and the advantages of the Lagrangian one are emphasized.

A belt of moonlets in Saturn’s A ring

The origin and evolution of planetary rings is one of the prominent unsolved problems of planetary sciences, with direct implications for planet-forming processes in pre-planetary disks. The recent detection of four propeller-shaped features in Saturn's A ring proved the presence of large boulder-sized moonlets in the rings. Their existence favours ring creation in a catastrophic disruption of an icy satellite rather than a co-genetic origin with Saturn, because bodies of this size are unlikely to have accreted inside the rings. Here we report the detection of eight new propeller features in an image sequence that covers the complete A ring, indicating embedded moonlets with radii between 30 m and 70 m. We show that the moonlets found are concentrated in a narrow 3,000-km-wide annulus 130,000 km from Saturn. Compared to the main population of ring particles (radius s < 10 m), such embedded moonlets have a short lifetime with respect to meteoroid impacts. Therefore, they are probably the remnants of a shattered ring-moon of Pan size or larger, locally contributing new material to the older ring. This supports the theory of catastrophic ring creation in a collisional cascade.

The new rings: Contributions of recent ground-based and space observations to our knowledge of planetary rings

Recent observations of planetary rings have dramatically improved our knowledge of these objects. Still mysterious ten years ago, the rings now appear rich representatives of a more general class of objects in astrophysics, namely the disks, like galaxies, accretion disks or proto solar systems. It is striking to note in this progress the close interaction between ground-based and space observations. Out of the four ring systems so far observed, three were discovered form the Earth, allowing a careful planning of the space missions and thus, a harvest of high-quality data. As examples, we can think of Uranus rings, whose orbits were known with an internal accuracy of a few hundreds of meters just before the Voyager 2 encounter with Uranus in January 1986. Also, Neptune's arcs, first discovered from the ground, will be hopefully observed during the Voyager 2 encounter in August 1989. Finally, we stress the importance of these observations for triggering new dynamical studies, sometimes of unexpected far-reaching consequences. In particular, rings could give us some major hints as to the mechanisms at work during the accretion processes in the proto Solar System. For instance, many highly efficient confining mechanisms are now observed in the rings. First assumed as essentially featureless disks, the rings exhibit very complicated and fine structures, where the matter is led to be confined in very narrow and precise regions. New techniques and new theoretical approaches are now needed for fully understanding the origin and the evolution of planetary rings.