Modeling how cores end up off-center

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willpittenger

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As an object's rotation becomes gravitationally locked, the object's core, if still surrounded by a liquid or semi-liquid core, it will slowly become closer to the object at the center of the orbit.<br /><br />If the core ends up off-center before the surrounding mantle and liquid core freeze, it will slow the object's rotation down. Both friction and gravitational tides would cause this.<br /><br />One or both can be seen in the Moon and possibly Mercury. <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>
 
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eosophobiac

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Is <i>'gravitationally locked</i> the same as 'tidally locked'?<br /><br /> <div class="Discussion_UserSignature"> <p> </p><p> </p> </div>
 
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nexium

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The planet Mercury was thought to be tide locked a century ago, but it rotates and revolves at 88 days and 66 days, or there abouts, so it is not tide locked. Earth's moon is the (only?) tide locked body known in our solar system. Neil
 
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willpittenger

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I agree on Mercury, but its rotation is directly tied to its revolution time by the Sun's tidal forces. As for other objects locked by gravitational tides, I have to include all four Galilean moons and both Pluto and Charon. An observer on Pluto would never see Charon move or rotate. <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>
 
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CalliArcale

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There are actually many tidally locked bodies in the solar system. The technical term is "synchronous rotation", which means that there is a 1:1 ratio to the length of its orbital period and the length of its rotational period. Pluto and Charon are said to be "mutally synchronous" -- this means that they are both locked to one another, such as that one Pluto day is one Charon day which is also one Charon "month". That's what willpittenger is describing above. That's the only known example of mutally synchronous bodies, although there could well be others out in the Kuiper Belt and among the other minor planets.<br /><br />Most of Saturn's moons also rotate synchronously. This is especially interesting for bodies like Iapetus. Iapetus is astonishingly dark on its leading hemisphere, but brilliantly white on its trailing hemisphere. Dione and Rhea are the other way around; they are bright on their leading hemispheres, and somewhat darker on their trailing hemispheres (although the difference is not anywhere near as pronounced as it is with Iapetus). The synchronous rotation and bicolor pattern of Iapetus was first theorized centuries ago by its discoverer, Giovanni Cassini. He noticed that he could only see Iapetus on one side of Saturn. Therefore, it had to rotate synchronously, just like our own Moon, and its leading hemisphere had to be almost pitch black. When the Voyagers arrived at Saturn in the late 20th Century, they proved him right. <img src="/images/icons/wink.gif" /> Just an interesting random factoid. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>
 
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