Physics
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..780l&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Physics
Scientific paper
The concept of moonlets embedded in rings that are too small to open gaps has a rich history of numerical exploration [1,2,3]. Their simulations, both using fluid descriptions and N-body codes, show the formation of propeller shaped structures with low density regions near the moon and higher density bands beyond that. The dimensions of these structures, especially in the radial direction, are well determined by the size of the moonlet and vary linearly with the Hill radius of that body. The arrival of Cassini at Saturn has allowed for observations of the disturbances caused by these moonlets [4,5,6]. Several were found in the initial high resolution insertion images. The list of observed features has now expanded to include well over 100 objects. They are seen through narrow bands in the mid-A ring in both lit and dark side images. The features that are seen have radial extents that would imply moonlets with diameters between 40 and 500m. This talk presents the results of numerical studies in which particle size distributions and self-gravity were included in N-body simulations. Early simulations were performed with two different moon sizes varying the properties of the background particles. Fig. 1 shows the structures that form in a few of these simulations. These simulations all used an optical depth of 0.1 and include a moonlet that is 26 m in diameter. The top two frames are from simulations including ~760,000 particles, all 1 m in diameter with the lower of those two including the effects of selfgravity. The lower two frames have a power law size distribution with a differential slope of q=-2.8 and the particles diameters between 0.5-4 m. They include ~490,000 particles. What stands out most in these simulations is that adding particle self-gravity and size distributions reduce the formation of propellers. Borrowing the terminology used by Tiscareno et al. [6], the moonlet wakes disappear in many of the simulations and the propeller shaped gaps disappear in a few of them. What determines whether these features are present in the simulation is the magnitude of the free eccentricity of the background particles relative to the forced eccentricity imparted on the particles in a close encounter with the moonlet. These have to be very different for the moonlet wakes to form and as they get closer the propeller shaped gaps disappear as well. The most realistic case is when both size distributions and particle self-gravity are considered. In this type of system, the small particles are kept fairly dynamically hot because of their interactions with the larger particles. As a result, the moonlet wakes are missing in all of the simulations. What is more, even the propeller shaped gaps disappear unless the moonlet is significantly larger than the largest bodies in our power-law size distribution. The lack of moonlet wakes in all simulations involving both size distributions and particle selfgravity poses an interesting challenge for the interpretation of observations. The features are observed as bright spots in images of both lit and unlit sides of the ring in all of the images they have been detected in. Therefore, one must assume that they contain more material that can reflect light, but not enough to block the light from passing through. The insensitivity to phase angle also leads to the conclusion that what is being seen is not dust [6]. There are two main proposals for explaining what is seen. Sremčević et al. [5] have proposed that the moonlet liberates ring particle regolith which is then free for a while until it is swept back up by the ring particles. An alternate model is that the moonlet disrupts gravity wakes and effectively exposes of many particles that are normally held in ephemeral self-gravitating clumps to increase the reflective surface area [6]. Both of these models would also lead to an increase in the brightness observed on the unlit side of the rings. The last section of this talk will focus on more recent simulations aimed at determining the relative importance of these two models. The simulations include a power-law size distribution for the background particles and include the effects of selfgravity. The moonlet size is selected to be representative of observations and the low end of the size distribution is in the range of a few cm in diameter to closely resemble the lower end of the expected size distribution. The maximum particle size in the distribution is varied to find a configuration where the moonlet produces observable features.
Lewis Matthew
Stewart Gay
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