Tidal Locking

[FrostySnow]

 

Tidal locking is said to be the reason that the moon always has the same face towards earth. Jupiter and Saturn have 62 - 63 moons most of which are also in synchronous orbit. In the absence of a liquid ocean and the crustal structure of the earth how do they still pull this off?

[swansont]

Tides deform solids and gases, too.

[Dekan]

Tidal locking is said to be the reason that the moon always has the same face towards earth. Jupiter and Saturn have 62 - 63 moons most of which are also in synchronous orbit. In the absence of a liquid ocean and the crustal structure of the earth how do they still pull this off?

 

"Tidal locking" has always seemed to me vaguely suspect.

 

Mainly because the Earth isn't tidally locked to the Sun. Surely it ought to be? Or at least, the Earth's axial rotation should be quite slow - dragged by Solar tidal effects - resulting in a slowly-rotating Earth.

 

Yet this isn't so - the Earth seems to display a complete disregard for tidal effects. It spins fast on its axis - 365 times in just 1 orbit round the Sun!

 

Where's the tidal locking?

[D H]

Surely it ought to be?

Surely not.

 

Tidal locking doesn't happen instantaneously. It takes time. How much time? That depends on a number of factors. One key factor is the distance between the two bodies. All other things being equal, the time to tidal locking is proportional to the distance raised to the sixth power. The distance between the Earth and Sun is very large compared to the distance between the Earth and the Moon. That power of six means that while the Moon became tidally locked to the Earth in a relatively short period of time, it will take tens to hundreds of billions of years for the Earth to become tidally locked to the Sun.

[Dekan]

Surely not.

 

Tidal locking doesn't happen instantaneously. It takes time. How much time? That depends on a number of factors. One key factor is the distance between the two bodies. All other things being equal, the time to tidal locking is proportional to the distance raised to the sixth power. The distance between the Earth and Sun is very large compared to the distance between the Earth and the Moon. That power of six means that while the Moon became tidally locked to the Earth in a relatively short period of time, it will take tens to hundreds of billions of years for the Earth to become tidally locked to the Sun.

Thanks D H, appreciate your interesting post. You say that a key factor is the distance between the two bodies.

 

The distance between the Earth and the Sun is about 93 million miles. And you suggest that at such a distance, tidal locking can't have an effect - at least, not for hundreds of billions of years.

 

At what distance would tidal locking take effect? For example, suppose the Earth were closer to the Sun - say at a distance of about 63,000,000 miles- like the planet Venus.

 

Is Venus tidally locked?

[D H]

Is Venus tidally locked?

At face value, obviously not, since Venus' rotation is retrograde. However ...

 

Let's start with Mercury. Is it tidally locked? Since Mercury completes three rotations for every two orbits, Mercury is not tidally locked to the Sun by the strictest definition of "tidally locked". It is however tidally locked in the sense that this 3:2 ratio is stable, thanks largely to Mercury's rather large eccentricity. If Mercury's rotation is slightly perturbed by some other planet, it will quickly regain that 3:2 rotation rate to orbital rate ratio.

 

This suggests a broader meaning of "tidally locked", which is that the average (average in a virial sense) total angular momentum is constant. In this sense, Venus may well be tidally locked. Venus has an incredibly thick atmosphere. Like Earth's atmosphere, Venus' upper atmosphere has a fast prograde rotation. In fact, Venus' upper atmosphere rotates almost as fast as does the Earth's -- in spite of the fact that Venus proper has a slow retrograde rotation. The slow retrograde planetary rotation and the fast prograde atmospheric rotation is consistent with the hypothesis that Venus as a whole is tidally locked. However, planetary scientists don't yet know enough about Venus as a whole to say whether Venus is tidally locked in the sense that it's total angular momentum is more or less constant.

[Dekan]

Thanks DH - I cited Venus (as you probably guessed!) - because its retrograde rotation seems to blow a hole in the tidal-lock idea. Venus is rotating in the wrong direction. Going backwards, so to speak.

 

This anomalous behaviour seems hard to explain by tidal lock. Of course there are Velikovskian explanations - about Venus being a kind of rogue comet shot out from the Sun. But this seems chemically and physically implausible. And is confuted by the almost perfect circularity of the Venusian orbit - not an orbit likely to be taken up by a rogue body.

 

So we have to get back to normal explanations. And in this connection, I'm grateful to you for drawing attention to the thick Venusian atmosphere. Before reading your post, I hadn't given any consideration to planetary atmospheres, and their possible effect on rotation.

 

It seems, intuitively, that the atmospheric effect must be very small. But that may be a facile impression.

 

Therefore I'd be glad of your guidance on these two questions, please:

 

1. What is the mass of the Venusian atmosphere - as a percentage of the mass of the planet's rocky body?

 

2. What contribution does the atmosphere make - in percentage terms - to the planet's angular momentum?

[md65536]

All other things being equal, the time to tidal locking is proportional to the distance raised to the sixth power. The distance between the Earth and Sun is very large compared to the distance between the Earth and the Moon. That power of six means that while the Moon became tidally locked to the Earth in a relatively short period of time, it will take tens to hundreds of billions of years for the Earth to become tidally locked to the Sun.

The gravitational pull of the sun is much greater than the pull of the moon, but because the moon is much closer the gravitational gradient is higher, so the moon causes greater tidal effects.

 

Wouldn't that imply that the Earth would become tidally locked to the moon? I don't suppose it's possible to be tidally locked to both, unless the moon was at a Lagrange point?

 

 

I suppose that because of the directions the Earth is spinning and the moon is orbiting, simply slowing the Earth's spin would cause the moon to become geosynchronous before the sun would be. At that point the moon would cause no torque on the Earth but the sun would still slow the Earth's spin ever so slightly. Then the moon would appear to "move backwards" slowly through the sky and try to slow the Earth's spin in the opposite direction. Would it eventually stabilize into "mostly locked to the moon, but with a very very slight rotation caused by solar tidal effects?

 

Or would permanent tides cause permanent deformation of the Earth that would keep it locked to the moon forever?

 

 

[Janus]

Thanks D H, appreciate your interesting post. You say that a key factor is the distance between the two bodies.

 

The distance between the Earth and the Sun is about 93 million miles. And you suggest that at such a distance, tidal locking can't have an effect - at least, not for hundreds of billions of years.

 

At what distance would tidal locking take effect? For example, suppose the Earth were closer to the Sun - say at a distance of about 63,000,000 miles- like the planet Venus.

 

Is Venus tidally locked?

 

How long it takes for a satellite to become tidally locked to its primary depends on the following:

 

The initial rotation rate of the satellite.

The distance between the primary and satellite (to the power of 6)

The mass of the satellite.

The dissipation function of the satellite

 

The mass of the primary (squared)

The radius of the satellite (cubed)

The Love number of the satellite

 

The first 4 increase the time to tidal lock and the last three decrease it.

 

The dissipation factor and Love number are only well known for the Earth-Moon pair and otherwise can only be roughly estimated.

 

If we assume these last two and the initial rotation rate are the same, we can estimate that it would take ~60,000 times longer for the Earth to tidally lock to the Sun than the Moon would take to lock to the Earth, and the Earth at Venus' distance from the Sun would take ~ 1/16 as long as that.

 

There is no "distance at which tidal locking takes effect", It's just a matter of how much time it takes for tidal locking to occur for different distances.

 

The gravitational pull of the sun is much greater than the pull of the moon, but because the moon is much closer the gravitational gradient is higher, so the moon causes greater tidal effects.

 

Wouldn't that imply that the Earth would become tidally locked to the moon? I don't suppose it's possible to be tidally locked to both, unless the moon was at a Lagrange point?

 

 

I suppose that because of the directions the Earth is spinning and the moon is orbiting, simply slowing the Earth's spin would cause the moon to become geosynchronous before the sun would be. At that point the moon would cause no torque on the Earth but the sun would still slow the Earth's spin ever so slightly. Then the moon would appear to "move backwards" slowly through the sky and try to slow the Earth's spin in the opposite direction. Would it eventually stabilize into "mostly locked to the moon, but with a very very slight rotation caused by solar tidal effects?

 

Or would permanent tides cause permanent deformation of the Earth that would keep it locked to the moon forever?

 

If the Earth-Moon system were to last long enough (and this would mean surviving the Red giant stage of the Sun.), the following would happen:

 

Once the Earth and Moon became tidally locked to each other, tidal braking by the Sun would indeed work against the Earth's rotation. However, if the Earth rotates slower than the Moon orbits, the tidal effects between Earth and Moon work to speed up the Earth's rotation while drawing in the Moon. So what ends up happening is that the Earth's rotation will get faster and faster again while the Moon falls in closer. Eventually the moon would pass inside the Roche limit, break apart and form a ring.

[xyz]

On 10/21/2012 at 3:21 PM, FrostySnow said:

 

Tidal locking is said to be the reason that the moon always has the same face towards earth. Jupiter and Saturn have 62 - 63 moons most of which are also in synchronous orbit. In the absence of a liquid ocean and the crustal structure of the earth how do they still pull this off?

info I have read somewhere

*removed attachment*

 

[CharonY]

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