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Post Info TOPIC: Saturn's inner moons


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Saturn's inner moons
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Title: Accretion of Saturn's mid-sized moons during the viscous spreading of young massive rings: solving the paradox of silicate-poor rings versus silicate-rich moons
Authors: Sébastien Charnoz, Aurélien Crida, Julie C. Castillo-Rogez, Valery Lainey, Luke Dones, Özgür Karatekin, Gabriel Tobie, Stephane Mathis, Christophe Le Poncin-Lafitte, Julien Salmon

The origin of Saturn's inner mid-sized moons (Mimas, Enceladus, Tethys, Dione and Rhea) and Saturn's rings is debated. Charnoz et al. (2010) introduced the idea that the smallest inner moons could form from the spreading of the rings' edge while Salmon et al. (2010) showed that the rings could have been initially massive, and so was the ring's progenitor itself. One may wonder if the mid-sized moons may have formed also from the debris of a massive ring progenitor, as also suggested in Canup (2010). However, the process driving mid-sized moons accretion from the icy debris disks has not been investigated in details. In particular, this process does not seem able to explain the varying silicate contents of the mid-sized moons (from 6% to 57% in mass). Here, we explore the formation of large objects from a massive ice-rich ring (a few times Rhea's mass) and describe the fundamental properties and implications of this new process. Using a hybrid computer model, we show that accretion within massive icy rings can form all mid-sized moons from Mimas to Rhea. However in order to explain their current locations, intense dissipation within Saturn (with Qp<2000) would be required. Our results are consistent with a satellite origin tied to the rings formation at least 2.5 Gy ago, both compatible with either a formation concurrent to Saturn or during the Late Heavy Bombardment. Tidal heating related to high-eccentricity post-accretional episodes may induce early geological activity. If some massive irregular chunks of silicates were initially present within the rings, they would be present today inside the satellites' cores which would have accreted icy shells while being tidally expelled from the rings (via a heterogeneous accretion process)while those still present in the rings are interpreted as today Saturn's rings' propellers and ring-moons (like Pan or Daphnis).

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Date:
Saturn's inner moons
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According to team of Cassini scientists, Saturn's inner moons, such as Calypso, Telesto, Pan, Atlas, Janus, Epimetheus, Pandora and Prometheus, may not be solid chunks of ice, but rather made up of "rubble piles" of material built up around small central cores.

"...We didn't have good pictures of them. We didn't have measurements of their shape. It could have been that they were collisional shards, monolithic pieces of ice" - Carolyn Porco, Cassini Imaging Science Team leader from the Space Science Institute in Boulder, Colorado, US.

The scientists examined the moons' shapes, masses and their densities and found that they, the "rounded football" shapes are characteristic of accreted bodies – where material has built up around a core.
The very low densities of between 0.4 - 0.6 grams per cubic centimetre, calculated for Telesto and Calypso, support the "rubble pile" theory.

The team will present their results at the Lunar and Planetary Science Conference in Houston, Texas, US, in March 2006.

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