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5.27 Effects of Tidal Forces

The tidal force is a universal consequence of Newton's law of gravity, and we can see its effects throughout the universe. The force that causes the Earth's oceans to rise and fall also operates elsewhere in the Solar System and beyond. Large objects in close proximity exert the strongest tidal forces, but any time two large objects orbit each other closely, tides are important. This is true of planets, stars, and even entire galaxies!

Tidal locking results in the Moon rotating about its axis in about the same time it takes to orbit the Earth. Except for libration effects, this results in it keeping the same face turned towards the Earth, as seen in the figure on the left. (The Moon is shown in polar view, and is not drawn to scale.) If the Moon didn't spin at all, then it would alternately show its near and far sides to the Earth while moving around our planet in orbit, as shown in the figure on the right. Click here for original source URL.

The same tidal force that stretches a satellite also tends to slow its rotation until the longest axis of the satellite lines up with the planet. Just as the Earth's rotation is slowing due to the Moon's tidal force on it, the Moon's rotation has slowed until it is locked into this position. This is why most satellites, like the Moon, face toward their planet - they are tidally locked"" in that orientation. We always see the same face of the Moon from the Earth because the Moon's rotation period is the same as the time it takes to complete one orbit around the Earth. Another way of saying this is that the Moon is in a 1:1 spin orbit resonance — the ratio of its rotational (spin) period to its orbital period is 1 to 1. Examples of this are common in our Solar System. Pluto and Charon are tidally locked to each other. Mercury's eccentric orbit prevents it from being in a 1:1 spin orbit resonance. Instead, it's in a 3:2 resonance — in other words, Mercury's day is two-thirds as long as its year.

Closer to the Roche limit the body (an exoplanet) is deformed by tidal forces. Click here for original source URL.

If a satellite (or a passing body) comes very close to a planet, the tidal forces can be destructive. The closer two objects are in space, the stronger the gravity between them, and the stronger the tidal force. So the closer an object comes to a planet, the more it's stretched. Within a certain distance called the Roche limit, stretching forces can break it apart. That's why we don't see satellites orbiting too close to planets — instead, we see ring systems within a certain distance of the planet. These rings are the remnants of bodies that were broken up by tidal forces.

Even when there's no water to respond to tidal forces, the solid mass of a planet feels the stress caused by these forces. In fact, tidal forces can heat the interior of a satellite in an elliptical orbit. In that case, the satellite comes closer to the planet during one part of its orbit and there it's subjected to strong stretching forces. As it moves away from the planet the stress is partly released and the body relaxes back toward a more spherical shape. This continual flexing of the satellite creates heat through internal friction, in the same way that if you flex a tennis ball enough times it becomes warm. This effect is called tidal heating. The more elliptical the orbit, the stronger the tidal heating. Jupiter's large Galilean satellites experience this kind of heating — enough to produce extensive volcanism on Io, and possibly create liquid-water oceans beneath the surface of Europa.