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TEAM WORKSHOP

Physics of Dusty Rings

20 to 24 June 2005

ISSI, Bern, Switzerland


Abstract

Dusty rings around all four giant planets have been discovered by Voyager. Some dusty phenomena were related to the dense rings, others to satellites, and still others were found in isolation. In July of 2004 Cassini started its in-situ investigation of the Saturn ring system – a few years after Galileo performed its detailed study of the Jovian ring system. Models (sometimes conflicting) have been developed to explain some properties of a specific ring. With the in depth investigation of two dusty ring systems and their environments it is time to reflect commonalities and diversities in order to obtain a deeper understanding of this exciting widespread astrophysical phenomenon. The workshop is intended to bring together leading observers and modelers in order to provide a stimulating environment for new ideas and amalgamation of diverse views.

Scientific Rationale

Dense rings around Saturn and Uranus were first observed from the ground while faint dust rings around all giant planets have been discovered in recent years by spacecraft. The first detections of dust clustered near the equatorial planes of Jupiter and Saturn were accomplished a quarter-century ago by pressurized ”beer-can” experiments aboard the Pioneer spacecraft. But it was the flybys of the giant planets by the Voyager spacecraft that showed the diversity and complexities of the ring systems. Some dusty phenomena were related to the dense rings, others to satellites, and still others were found in isolation. Radio occultation experiments with the Voyager spacecraft showed that the dense rings consist of cm to m sized particles, while dust rings are mostly composed of micron sized particles. While the discoveries and early observations of dusty rings were made by chance a much more systematic survey of Jupiter’s and Saturn’s environs was and is planned for the Galileo and Cassini missions.

Observational techniques used to analyze circumplanetary dust rings are images, spectra, occultation profiles, charged particle absorption signatures, and in-situ sampling of ring dust. Since 1996 we have obtained detailed studies of the Jovian ring system by the Galileo spacecraft including recent passages through the ring system enabling us to analyze in-situ grains orbiting the planet. In July 2004 Cassini will start its in-situ investigation of the Saturn ring system and first observations will be made within a years time. In recent years also ground- and space-based astronomical observations over a wide of wavelengths of dust rings have been possible providing additional information on these rings.

Several motivations provoke interest in dusty rings. First, ring dynamicists are challenged by the wide range of forces to which these grains are subject and by the counter-intuitive behavior of some of this material. Second, circumplanetary grains intimately interact with the surrounding magnetospheric plasma and with neighboring satellites, in some cases being derived from the latter and in others modifying those surfaces. Thirdly, dust probes conditions in the surrounding magnetospheric plasma and, through its response, calibrates the nature of those fields.

The applicable physics and dynamics acting on dusty rings are distinct from those pertinent to interplanetary dust, because the dust orbits through a magnetosphere and about a central mass other than the radiation source. But circumplanetary dust particles are not classical ring particles either. In the latter case, collisions dominate and the resultant structures can be studied with the tools of fluid dynamics and kinetic theory. For faint rings, collisions among ring particles are rare, and each particle behaves as a miniature independent satellite circling its primary; thus the methods of single-particle dynamics may be applied. However, as constituent particles are generally tiny, non-gravitational forces (electromagnetism, radiation and drag) must be included.

Dynamics of circumplanetary dust particles is extremely rich: Physical and dynamical processes acting on circumplanetary dust are electrical charging, forces including planetary gravity, radiation forces, electromagnetic forces, drag forces from interactions with radiation (Poynting-Robertson effect) and neutral and charged particles, destruction and generation of grains, interactions with nearby satellites, and resonances - even collective effects are suggested. The ranking of these dynamic effects is hotly discussed.

Sometimes the source of a ring is obvious, in other cases no source has been identified. Models (sometimes conflicting) have been developed in the past to explain characteristics of a specific ring. Because of the wealth of new data on all dusty rings that is becoming available in very recent years it is time to start a reflection on commonalities and diversities in order to obtain a deeper understanding of this exciting widespread astrophysical phenomenon.

It is time to synthesize all these new observations and to try to develop a unified view of all dusty rings, especially those in the Jupiter and Saturn system. The workshop is intended to bring together a team of leading observers and modelers in order to provide a stimulating environment for new ideas and amalgamation of diverse views.

Questions to be addressed by the workshop are:

<!--[if !supportLists]-->-         <!--[endif]-->What are the physical properties of distinct regions in the dusty rings?

<!--[if !supportLists]-->-         <!--[endif]-->What are the most important dynamic mechanisms determining the shape of the rings?

<!--[if !supportLists]-->-         <!--[endif]-->What are the chemical compositions of the different rings?

<!--[if !supportLists]-->-         <!--[endif]-->What is the mass flux into the various ring systems?

<!--[if !supportLists]-->-         <!--[endif]-->What are the influences of the magnetosphere and plasma environment of the rings?

<!--[if !supportLists]-->-         <!--[endif]-->What are the sources of dust grains and what is their production rate?

<!--[if !supportLists]-->-         <!--[endif]-->What is the lifetime of dust grains?

<!--[if !supportLists]-->-         <!--[endif]-->How old are planetary rings?

<!--[if !supportLists]-->-         <!--[endif]-->How do dust rings evolve?

<!--[if !supportLists]-->-         <!--[endif]-->What causes the tenuous ring systems to differ so much?

<!--[if !supportLists]-->-         <!--[endif]-->Why are some faint rings confined while others are vertically or radially extended?

<!--[if !supportLists]-->-         <!--[endif]-->Which of the phenomena displayed by dusty rings are relevant to the collisionally dominated classical ring systems?

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TEAM MEMBERS (preliminary)


Michel Blanc F
Shawn Brooks USA
Joe Burns USA
Hiroshi Daisaka Japan
Stan Dermott USA
Valeri Dikarev D, RU
Amara Graps I
Eberhard Grün (Team Lead) D
Doug Hamilton USA
Ove Havnes N
Mihaly Horanyi USA
Antal Juhasz HU
Sascha Kempf D
Alexander Krivov D
Harald Krüger D
Tadashi Mukai Japan
Imke de Pater USA
Sho Sasaki Japan
Mark Showalter USA
Frank Spahn D
Ralf Srama D
Henry Throop USA