NEOs and Seismic Activity

[Meikle]

Is it possible that the gravitational pull of an Near-Earth Object (such as Toutatis) could result in the recent spate of earthquakes and eruptions on Earth? Thoughts?

[granthutchison]

Is it possible that the gravitational pull of an Near-Earth Object (such as Toutatis) could result in the recent spate of earthquakes and eruptions on Earth? Thoughts?

You get much bigger variations in tidal forces on the Earth as the Moon moves around it each month. The effects of a NEO are trivial by comparison. So I don't think it's very likely.

Grant

[Evil Dr Ganymede]

Is it possible that the gravitational pull of an Near-Earth Object (such as Toutatis) could result in the recent spate of earthquakes and eruptions on Earth? Thoughts?

I'd go further than grant and say that it's not remotely possible. Toutatis' mass is tiny compared to the Earth's, and as grant pointed out the effects of the Sun and the Moon are far larger.

[ArneB]

I've read that Triton will eventually "fall" down on Neptune due to tidal effects. Will something similar happen to the moon? Will the Earth's rotation slow down with time? What decides whether tidal effects deprive a system of it's kinetic energy or not? Following my own physical intuition, I'd say that the composition of the planet/moon would matter much. A planet full of gas or liquid would have much more mass "moved" due to tidal forces, and it would also be easier for the energy to "radiate away" due to friction work in the fluid. A rocky planet will be elastic and will produce less friction heat. Does anyone know if this reasoning is right, or merely BS?

[selden]

Tidal effects are changing their orbits over long periods.

For prograde orbits, where the planet rotates in the same direction as the satellite orbits around it, if the primary body rotates faster than the satellite orbits around it, then the satellite gradually moves away from the planet and the planet's rotation gradually slows down. If the primary body rotates more slowly than the satellite orbits around it, then the satellite gradually approaches the planet and the planet's rotation gradually speeds up.

This is because the satellite distorts the shape of the body it's orbiting around, producing a "lump" on the the near and far sides. The nearside lump pulls on the satellite, too. If the planet rotates rapidly, then that lump tends to be in front of the satellite, accelerating it into a more distant orbit, while the satellite drags back on the planet, slowing its rotation.

Triton is in a retrograde orbit: it orbits in the opposite direction from the direction that Neptune rotates. As a result, the near-side lump in Neptune's shape is always behind Triton, slowing it down, dragging it into a lower orbit. At the same time Triton is pulling back against Neptune's rotation, slowing it down, too.

The Moon is in a prograde orbit, and its 28day orbit is slower than the Earth's 1 day rotational period, so tides on the Earth are always ahead of the Moon, pulling the Moon away from us. The pull of the moon on that lump is slowing Earth's rotation. Eventually the Moon and the Earth will be tidally locked, with the Earth's rotational period the same as the Moon's orbital period. That'll take a very long time, though, maybe 5 billion years or so.

The tidal distortions do cause some energy to be released as heat, but I think that's a smaller effect.

Does this clarify things at all?

[ArneB]

Thanks! That almost made perfect sense to me except for one thing.
The first to cases (with prograde orbits) were not so difficult to understand, as the energy is conserved. (Rotation slows down, potential energy increases, rotation speeds up, potential energy decreases) But in the retrograde case (rotation slows down, potential energy decreases) where does the energy go? How about entropy production as I suggested? I think I am wrong here, but as long as I don't understand, I've got to argue.

Eventually the Moon and the Earth will be tidally locked, with the Earth's rotational period the same as the Moon's orbital period

I guess this is what has happened to Pluto/Charon

[selden]

Arne,

You're right: the lost energy has to go somewhere. I suspect it goes into heat. Hopefully someone more familiar with planetary dynamics can comment.

[Evil Dr Ganymede]

Arne,

You're right: the lost energy has to go somewhere. I suspect it goes into heat. Hopefully someone more familiar with planetary dynamics can comment.

You're right, it does go into heat - tidal dissipation is what's heating up Io. Though the situation there is more complicated, because Io is being heated by tidal flexing as it travels around Jupiter on its eccentric orbit. Ordinarily the orbit would circularise very rapidly since it's so close to Jupiter, but its eccentricity is being "pumped" by the orbital resonances with Europa and Ganymede.

[cpotting]

Arne,

You're right: the lost energy has to go somewhere. I suspect it goes into heat. Hopefully someone more familiar with planetary dynamics can comment.

You're right, it does go into heat - tidal dissipation is what's heating up Io.

I would have thought that you didn't need heat to explain the conservation of energy. In the second example Triton is slowing down in its retrograde orbit - or in other words, it is being accelerated in the prograde direction. The energy to do this should equal the energy being expended in slowing Neptune's rotation - conservation of energy is maintained.

Granted, in all such interactions there will be energy converted into other forms, such as heat, but I would have thought it would be no more than "usual" (i.e. than for the prograde version of the same interaction).

Or am I way off base and showing my community college education again?

[Evil Dr Ganymede]

Well, I meant that tidal heating is one way to lose energy within each body. What happens is that there is conservation of angular momentum in the system (which I think is what people mean by "energy" here?) - as the satellite is pushed out, the orbital angular momentum increases, so the rotational angular momentum of the planet has to decrease for angular momentum to be conserved. And vice versa.

When you consider the sun in as part of the system (so you get a three body system), then that complicates things because it's trying to get the planet tidelocked to it. That sucks angular momentum out of the planet-moon system, which complicates the evolution somewhat.

And don't ask what happens when you have multiple satellites - then you have orbital resonances and other such fun things to REALLY complicate matters .

[tony873004]

But in the retrograde case (rotation slows down, potential energy decreases) where does the energy go?

Rotation speeds up. If the Moon orbited Earth retrograde, it would speed up the Earth's rotation while being pulled into a smaller, faster orbit.

If the primary body rotates more slowly than the satellite orbits around it, then the satellite gradually approaches the planet and the planet's rotation gradually speeds up.

This is why Phobos will ultimately crash into Mars, even though its orbit is prograde.