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| Titel |
A Driving Mechanism for Moonlets Formation in Saturn's A Ring |
| VerfasserIn |
Evgeny Griv, Michael Gedalin |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250037079
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| Zusammenfassung |
| Tiscareno et al. (2006, 2008) and SremÄević et al. (2007) have detected in recent
Cassini images of Saturn’s A ring localized features – “propellers”– which may be
interpreted as signatures of small moonlets of some 100 m in size embedded within
the ring. The features, believed to be disturbances generated by unseen embedded
small moonlets (tens to hundreds of meters in diameter), are concentrated in three
bands in the mid-A ring. The propellers are most abundant in a 3000 km-wide belt,
about 130000 km from Saturn’s center. It is estimated that the A ring contains
thousands of such objects. Some very large propellers (from > 100 m objects)
are found in the outermost A ring farther from Saturn than the population in the
propeller-rich belt. It was especially noted that the lack of significant brightening
at high phase angle indicates that these bodies are likely composed primarily of
macroscopic particles, rather than dust. Herein, the linear stability of the Saturnian ring
disk of mutually gravitating and physically colliding macroscoping particles is
examined. Jeans’ instabilities of small-amplitude gravity perturbations (e.g., those
produced by a spontaneous disturbance) are analyzed analytically through the use of
dynamical equations of a compressible fluid. The approach taken in this article differs
from traditional dynamical views by taking into account the three-dimensional
effects. The simple model of the system is considered: the ring disk is considered
to be a thin slab with plane-parallel symmetry and its structure is considered in
a horizontally local short-wave approximation. The used is a viscous isothermal
fluid, viscosity is driven by physical collisions between particles, and the analysis is
linear. It is shown that Jeans’ gravitational instability (discussed first by Lin and
Shu in context of the formation of spiral arms of normal galaxies) of both radial
and spiral perturbations can lead to formation of porous moonlets with diameters
- 100 - 200 m of preferred mass - 107 g each embedded in broad rings, and this
process is believed to occur at distances r -³ 130000 km from Saturn, near the
center and in the outer regions of the A ring. Although this has yet to be directly
measured.
This work was sponsored by the Israel Science Foundation and the Binational U.S.–Israel
Science Foundation. |
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