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View Diary: Getting to Know Your Solar System (33): Dione (18 comments)

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  •  i am a new reader here and (2+ / 0-)
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    The Marti, Troubadour

    to things astronomical, so please allow for that when i ask a question or two (plus you have so much material to digest - which ought to be awarded great notice if it hasn't already been - that i am sure the answers are there but...)

    About the origin of Dione:  can we know if it is a piece of the planet Saturn?   You say in Saturn (Vol.2) that the planet has a metal / rock core.  So is it possible that a chunk was ejected?  Or what is the probability of a different origin?  Can an analysis (like spectral) be done to determine and compare the composition of both?

    Another question: what makes Dione spherical (or the Earth's moon or any moon for that matter)?

    Thank you for this post and all the others.

    •  Most major moons of big planets (2+ / 0-)
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      Khun David, ume

      are thought to have formed from the same cloud of material that formed the planet, but the gravity well of a planet like Saturn is much too deep for any likely collision to blast something off from it.

      But you raise something important: Because they both formed from the same cloud of material, Dione's core and the rocky part of Saturn's core probably have some compositional similarities.  That isn't known for sure though, because Saturn's core can't be directly observed - so far its properties have to be inferred from gravitational behavior.  

      To really get a handle on internal structures you need two probes to fly in formation in low orbits over a body and sort of triangulate gravitational effects down to a precise level.  They recently did this for the Moon with the GRAIL mission.  

      The thing that makes any body round is gravity, and there's a threshold of gravity where it's intense enough to cause a surface to gradually sag and erode inward until it's roughly spheroidal.  Below this limit, you get weirdly-shaped objects like asteroids because gravity there isn't enough to even out their surfaces.  Above it, you get increasingly rounded shapes.  Dione has enough gravity to make itself spheroidal.  

      Rotation or strong tidal effects can warp a body into an ovoid (Mimas is egg-shaped due to tides, and Saturn itself rotates so rapidly that the poles are flattened from angular momentum), but Dione isn't substantially affected in those ways.  

      The term, BTW, for being roughly spheroidal is hydrostatic equilibrium.  Because even solids behave like liquids over very long periods of time, under high enough gravity and with long enough time periods, they "flow" into smoothness like a blob of water in space.  You can actually see this happening in images of crater walls collapsing on Mars, the Moon, and other places.  But in a place like Dione (and the Moon) there are still impacts happening that create sharp new features even as old ones erode like this.  

      Process defines product.

      by Troubadour on Sun May 19, 2013 at 10:27:35 AM PDT

      [ Parent ]

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