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Origin of Saturn Moons

June 10, 2010

Saturn with it’s rings and numerous moons have always raised curiosity among astronomers and general public alike. Saturn is infact full of moons- 52 officially named while there might be more. I posted some of the pictures of Saturn moons in one of my earlier post. Basically these moons can be divided into two sets- one which are large in size and mass and farther away from the planet while the others which are smaller in size and closer to the rings. Scientists have always been trying to understand if these two sets of moons originated in the similar fashion, from the planet itself. In general, satellites are formed from the same pro-planetary disk from which the planets are formed and therefore similar in composition. With time, these satellites migrate away from the planet, just like our moon is moving away from earth. These two sets of moons not only differ in size and distance but also in composition. The smaller moons closer to the ring are less dense and devoid of silicates, the compounds which are present both in other larger moons and the planet itself.  The following plot from the paper shows comparison of two sets of moon and how the slopes differ in mass vs distance plot. (Plot is linked to Nature article and so if its not visible, you might need subscription to the journal.)

Recent paper published today in Nature, suggests that these smaller moons or moonlets have formed from the ring itself by accretion of material coming out of the ring. Also, these moonlets are very young and also suggests that many more moons might be forming in future from these rings. This explains, their proximity to the ring, similarity to their icy composition and density with the ring, and fairly smooth surfaces . Explaining in layman’s terms, Saturn’s icy ring is expanding and moving outward. As the icy material moves farther than 140,000 kms from the center of Saturn (Roche Limit), the icy particles on the fringes become unstable due to lack of gravitational force balance and they start oozing out. With time, the material accretion takes place and forms smaller moonlets like Janus of Saturn. Now these moonlets start exerting gravitational push on the ring and push it back to below 140,000 kms, beyond the Roche limit, where the ring is again in gravitational balance and thus process of newer moon formation stops. With time, the newly formed moon moves away from the ring and the ring also starts moving out and same process repeats again. That means number of Saturn moons will keep on increasing by this process and also goes on to show that how our universe is so dynamic and ever evolving. Here is the abstract of the paper:

The regular satellites of the giant planets are believed to have finished their accretion concurrent with the planets, about 4.5 Gyr ago1, 2, 3, 4. A population of Saturn’s small moons orbiting just outside the main rings are dynamically young5, 6 (less than 107 yr old), which is inconsistent with the formation timescale for the regular satellites. They are also underdense7 (~600 kg m−3) and show spectral characteristics similar to those of the main rings8, 9. It has been suggested that they accreted at the rings’ edge7, 10, 11, but hitherto it has been impossible to model the formation process fully owing to a lack of computational power. Here we report a hybrid simulation in which the viscous spreading of Saturn’s rings beyond the Roche limit (the distance beyond which the rings are gravitationally unstable) gives rise to the small moons. The moonlets’ mass distribution and orbital architecture are reproduced. The current confinement of the main rings and the existence of the dusty F ring are shown to be direct consequences of the coupling of viscous evolution and satellite formation. Saturn’s rings, like a mini protoplanetary disk, may be the last place where accretion was recently active in the Solar System, some 106–107 yr ago.

Nature Volume: 465, Pages: 752–754  Date published: (10 June 2010) DOI: doi:10.1038/nature09096

Top Image Credit: NASA/JPL/Space Science Institute

Plot credit: Nature article The recent formation of Saturn’s moonlets from viscous spreading of the main rings. Sébastien Charnoz,Julien Salmon& Aurélien Crida

Filed under: Research,Science

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