The rotations of comets and asteroids are affected by outgassing volatilities. Io is very volcanically active. Does this give Io a varying orbit and a slow rotation? Tidal forces should take longer than the effect of a large eruption. Tidal locking means that any volcano alway pushes Io in the same direction (radially, tangential or inclined) accumulating the same kind of orbital change during all of its its active lifetime.

  • $\begingroup$ Not quite what you asked for, but polar wander of Io's axis has been proposed (this is based on modelling of volcano positions, I haven't been able to find any measurements proving such an effect): onlinelibrary.wiley.com/doi/10.1029/2001GL014539/pdf $\endgroup$
    – Andy
    Apr 8, 2016 at 15:06
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    $\begingroup$ On comets and asteroids the gases are easily accelerated to the relatively tiny escape velocities. (Angular) Momentum can therefore be carried away from the body by the escaping gases. Larger bodies usually can't manage this: the material fails to achieve escape velocity due to the higher gravity, and so expelled material falls back to the body. In Io's case, though, charged particles can be pulled off by Jupiter's magnetic field (and there's a lot of interesting magnetism connected to Io as a result). All of which is to say: interesting question, because maybe? $\endgroup$ Apr 9, 2016 at 2:56
  • $\begingroup$ @zibadawa timmy First the eruption causes a reaction force on the moon. Then, the ejecta impacting around the volcano should give another push in the same direction. While flying, the ejecta has a gravitational pull on the moon in the opposite direction, but the impacts might cause heat and outgassing. Since it has no atmoshpere, I would think that eruptions push Io even though the ejecta falls back. $\endgroup$
    – LocalFluff
    Apr 9, 2016 at 8:09
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    $\begingroup$ @LocalFluff If it stays bound, then everything balances out. You have to transfer angular momentum away from the moon to alter orbit or rotation. As long as the material stays bound to Io, then no momentum is actually being removed from the moon as a whole. If material gets unbound from Io, such as charged particles being pulled off by Jupiter's magnetic field, then that's when some angular momentum can conceivably be lost from Io. And Io loses about 1 tonne/sec in exactly this way. How much momentum goes along with it, I have no idea. $\endgroup$ Apr 9, 2016 at 9:42
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    $\begingroup$ @LocalFluff Some of it is returned, but most of it is radiated away. Though don't forget where that heat primarily came from: the tidal flexing from Jupiter. Which remains ongoing. The eruptions are basically just part of Io maintaining (thermal, hydrostatic) equilibrium: it needs to dissipate the excess energy the flexing is adding. The tidal flexing itself is a factor in orbit/rotation, as that's how it ended up tidally locked in the first place. But the volcanoes are more symptom than cause here. $\endgroup$ Apr 9, 2016 at 19:08

2 Answers 2


Is Io's orbit or rotation affected by its volcanism?

The rotations of comets and asteroids are affected by outgassing volatilities. Io is very volcanically active. Does this give Io a varying orbit and a slow rotation?

Jupiter's moon Io has a lot of volcanoes.

A similar answer to your question is provided at Quora's question: "Could the eruptions of all the volcanoes on Earth simultaneously knock it off its rotation or change its orbit?".

There are two differences: Eruptions on Io can reach escape velocity (2.558 km/s), and matter is subject to gravity as it is propelled away from the moon allowing some to escape and some to return; so it is a source of wobbling and rotational speed changes.

"Jupiter's rocky moon Io is the most volcanically active world in the solar system, with hundreds of volcanoes, some erupting lava fountains dozens of miles (or kilometers) high. Io’s remarkable activity is the result of a tug-of-war between Jupiter's powerful gravity and smaller but precisely timed pulls from two neighboring moons that orbit farther from Jupiter – Europa and Ganymede.


Orbit and Rotation

Although Io always points the same side toward Jupiter in its orbit around the giant planet, the large moons Europa and Ganymede perturb Io's orbit into an irregularly elliptical one. Thus, in its widely varying distances from Jupiter, Io is subjected to tremendous tidal forces.

These forces cause Io's surface to bulge up and down (or in and out) by as much as 330 feet (100 meters). Compare these tides on Io's solid surface to the tides on Earth's oceans. On Earth, in the place where tides are highest, the difference between low and high tides is only 60 feet (18 meters), and this is for water, not solid ground.

Io's orbit, keeping it at more or less a cozy 262,000 miles (422,000 kilometers) from Jupiter, cuts across the planet's powerful magnetic lines of force, thus turning Io into a electric generator. Io can develop 400,000 volts across itself and create an electric current of 3 million amperes. This current takes the path of least resistance along Jupiter's magnetic field lines to the planet's surface, creating lightning in Jupiter's upper atmosphere.


The tidal forces generate a tremendous amount of heat within Io, keeping much of its subsurface crust in liquid form seeking any available escape route to the surface to relieve the pressure. Thus, the surface of Io is constantly renewing itself, filling in any impact craters with molten lava lakes and spreading smooth new floodplains of liquid rock. ...".

The two-body problem explains how multiple interacting objects affect the orbit of each other. While the orbital eccentricity of Io is quite low (0.0041) the orbits of three of the Galilean moons (Ganymede, Europa and Io, excluding Callisto) have a 1:2:4 resonance. The "pushing" will act in periodic repetition to have a cumulative effect on the motion.

Orbits of the Galilean moons

Orbital resonances greatly enhance the mutual gravitational influence of the bodies, i.e. their ability to alter or constrain each other's orbits. In most cases, this results in an unstable interaction, in which the bodies exchange momentum and shift orbits until the resonance no longer exists. Under some circumstances, a resonant system can be stable and self-correcting, so that the bodies remain in resonance.

The small effect of the volcanoes is corrected by the much greater effect of resonance. The website space.com estimates Io's age at about 4.5 billion years old, the same age as Jupiter. If the orbit was unstable and changing it may well have been destroyed long ago.

  • $\begingroup$ " Thus, in its widely varying distances from Jupiter" and then "the orbital eccentricity of Io is quite low (0.0041)". This is what most people don't understand about tidal forces. They work also with zero eccentricity.. $\endgroup$ Jun 1, 2018 at 9:52

If you read the Astronomy magazine, you should have seen the star charts, and the article about Jupiter and the moons. So, the answer is no, the activity does not affect it's orbit.


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