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From: (Henry Spencer)
Subject: Io (was Re:
Date: Fri, 26 Jan 2001 03:40:33 GMT

In article <uiHb6.88145$>,
JL <> wrote:
>This is a multi-part message in MIME format.

Post in plain text, please.

>I thought IO's orbit was VERY circular.

Yes and no.  Its *average* orbital eccentricity, which is what you will
find in the data tables, is negligible.  But Io's orbit is constantly
changing slightly, due to regular perturbations from Europa, and at any
given instant it typically has a significant eccentricity, up to 0.004.
(That doesn't sound like much, but so close to Jupiter it makes a big

Think of Io as wandering around Jupiter in a wide groove:  the groove's
*overall* shape is circular, but Io wanders from side to side within the
groove all the time, in a complicated way which averages out to zero.

Recognition of this odd situation is what led Peale et al to predict
strong tidal heating and a possibility of active volcanism... in a paper
published one week before the discovery of Io's volcanos.

>I thought the tidal heating was a result of it basically
>pushing/interacting with Europa and Ganymede in the 4:2:1 orbit

The tidal heating comes from Jupiter, but the orbital resonance with
Europa (and indirectly Ganymede) keeps pumping Io's orbit up as the tidal
effects try to circularize it.
When failure is not an option, success  |  Henry Spencer
can get expensive.   -- Peter Stibrany  |      (aka

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