328 research outputs found
Free Unstable Modes and Massive Bodies in Saturn's Outer B Ring
Voyager images and Cassini occultation data have previously shown that the
behavior of the outer edge of Saturn's massive B ring is determined only in
part by a static response to the 2:1 inner Lindblad resonance with Mimas. In
Cassini images of this region, we find, in addition to the expected
wavenumber-2 forced distortion, evidence for unforced self-excited
wavenumber-3, wavenumber-2, and wavenumber-1 normal modes. These are the first
observations to suggest substantial wave amplification in Saturn's broad rings.
Moreover, the presence of these free modes strongly implicates viscous
overstability as their underlying cause and, by inference, the cause for most
if not all of the unforced structures throughout the high-mass-density B ring
and in other high-mass-density regions in Saturn's rings. Analysis of each of
the inferred waves reveals a consistent lower bound on the average surface mass
density of ~ 44 g/cm^2 for the outer 250 km of the ring, though the true
surface density could be as high as 100 g/cm^2 or higher. Interference between
the forced and free wavenumber-2 modes yields a total wavenumber-2 pattern that
varies in amplitude and orientation with a characteristic period of ~ 5.5
years. We also find localized disturbances, including 3.5-km-tall vertical
structures, that provide circumstantial evidence for embedded massive bodies in
the Mimas resonance zone. The presence of such bodies is supported by the
presence of a shadow-casting moonlet ~ 0.3 km wide near the ring's edge.Comment: 38 pages, 6 figure
Tidal Control of Jet Eruptions on Enceladus as Observed by Cassini ISS between 2005 and 2007
Observations of Enceladus have revealed active jets of material erupting from cracks on its south polar surface. It has previously been proposed that diurnal tidal stress, driven by Enceladus' orbital eccentricity, may actively produce surface movement along these cracks daily and thus may regulate when eruptions occur. Our analysis of the stress on jet source regions identified in Cassini ISS images reveals tidal stress as a plausible controlling mechanism of jet activity. However, the evidence available in the published and preliminary observations of jet activity between 2005 and 2007 may not be able to solidify the link between tidal stress and eruptions from fissures. Ongoing, far more comprehensive analyses based on recent, much higher resolution jetting observations have the potential to prove otherwise
Tidal Control of Jet Eruptions Observed by Cassini ISS
Observations by Cassini's Imaging Science Subsystem (ISS) of Enceladus' south polar region at high phase angles has revealed jets of material venting into space. Observations by Cassini's Composite Infrared Spectrometer (CIRS) have also shown that the south polar region is anomalously warm with hotspots associated with geological features called the Tiger Stripes. The Tiger Stripes are large rifts near the south pole of Enceladus, which are typically about 130 km in length, 2 km wide, with a trough 500 m deep, and are l1anked on each side by 100m tall ridges. Preliminary triangulation of jets as viewed at different times and with different viewing geometries in Cassini ISS images taken between 2005 and 2007 have constrained the locations of eight major eruptions of material and found all of them associated with the south polar fractures unofficially the 'Tiger Stripes', and found four of them coincident with the hotspots reported in 2006 by CIRS. While published ISS observations of jet activity suggest that individual eruption sites stay active on the timescale of years, any shorter temporal variability (on timescales of an orbital period, or 1.3 Earth days, for example) is more difficult to establish because of the spotty temporal coverage and the difficulty of visually isolating one jet from the forest of many seen in a typical image. Consequently, it is not known whether individual jets are continuously active, randomly active, or if they erupt on a predictable, periodic schedule. One mechanism that may control the timing of eruptions is diurnal tidal stress, which oscillates between compression/tension as well as right and left lateral shear at any given location throughout Enceladus' orbit and may allow the cracks to open and close regularly. We examine the stresses on the Tiger Stripe regions to see how well diurnal tidal stress caused by Enceladus' orbital eccentricity may possibly correlate with and thus control the observed eruptions. We then identify possible mechanisms by which tidal stress can provide access to the surface for volatile material and implications for observed jet activity
Physical characteristics and non-keplerian orbital motion of "propeller" moons embedded in Saturn's rings
We report the discovery of several large "propeller" moons in the outer part
of Saturn's A ring, objects large enough to be followed over the 5-year
duration of the Cassini mission. These are the first objects ever discovered
that can be tracked as individual moons, but do not orbit in empty space. We
infer sizes up to 1--2 km for the unseen moonlets at the center of the
propeller-shaped structures, though many structural and photometric properties
of propeller structures remain unclear. Finally, we demonstrate that some
propellers undergo sustained non-keplerian orbit motion. (Note: This arXiv
version of the paper contains supplementary tables that were left out of the
ApJL version due to lack of space).Comment: 9 pages, 4 figures; Published in ApJ
The discovery and dynamical evolution of an object at the outer edge of Saturn's A ring
This work was supported by the Science and Technology Facilities Council (Grant No. ST/F007566/1) and we are grateful to them for financial assistance. C.D.M. is also grateful to the Leverhulme Trust for the award of a Research Fellowshippublisher PDF not permitted, withdraw
The instant sequencing task: Toward constraint-checking a complex spacecraft command sequence interactively
Robotic spacecraft are controlled by sets of commands called 'sequences.' These sequences must be checked against mission constraints. Making our existing constraint checking program faster would enable new capabilities in our uplink process. Therefore, we are rewriting this program to run on a parallel computer. To do so, we had to determine how to run constraint-checking algorithms in parallel and create a new method of specifying spacecraft models and constraints. This new specification gives us a means of representing flight systems and their predicted response to commands which could be used in a variety of applications throughout the command process, particularly during anomaly or high-activity operations. This commonality could reduce operations cost and risk for future complex missions. Lessons learned in applying some parts of this system to the TOPEX/Poseidon mission will be described
Saturn in hot water: viscous evolution of the Enceladus torus
The detection of outgassing water vapor from Enceladus is one of the great
breakthroughs of the Cassini mission. The fate of this water once ionized has
been widely studied; here we investigate the effects of purely neutral-neutral
interactions within the Enceladus torus. We find that, thanks in part to the
polar nature of the water molecule, a cold (~180 K) neutral torus would undergo
rapid viscous heating and spread to the extent of the observed hydroxyl cloud,
before plasma effects become important. We investigate the physics behind the
spreading of the torus, paying particular attention to the competition between
heating and rotational line cooling. A steady-state torus model is constructed,
and it is demonstrated that the torus will be observable in the millimeter band
with the upcoming Herschel satellite. The relative strength of rotational lines
could be used to distinguish between physical models for the neutral cloud.Comment: submitted to Icarus updated: references fixe
Rings in the Solar System: a short review
Rings are ubiquitous around giant planets in our Solar System. They evolve
jointly with the nearby satellite system. They could form either during the
giant planet formation process or much later, as a result of large scale
dynamical instabilities either in the local satellite system, or at the
planetary scale. We review here the main characteristics of rings in our solar
system, and discuss their main evolution processes and possible origin. We also
discuss the recent discovery of rings around small bodies.Comment: Accepted for the Handbook of Exoplanet
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