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Where do ocean tides get their energy from?
Category: Earth Science
Published: June 11, 2024
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
Ocean tides get most of their energy from the rotation of the earth. As tidal energy is created, the rotational kinetic energy of the earth literally decreases, which gradually slows down the rate at which the earth is spinning. The agent that makes this happen is the gravitational tidal force exerted on the earth from the moon and the sun, with the moon's influence dominating. As the earth gradually decreases its rotational rate, the length of a day on earth is literally gradually increasing. But don't worry—this change in the length of a day is so gradual that you'll never notice it in your lifetime. Specifically, a day on earth is getting longer by 2.3 milliseconds per century. This is miniscule on human timescales, but it's significant enough to be measurable over geological timescales. The key point here is that energy that is given to the ocean tides is coming from the earth's rotational kinetic energy, for the most part, rather than being created out of nothing. That should make sense because energy can never be locally created out of nothing.
Ocean tides carry energy. Tides involve large masses of ocean water moving across the globe, which is ultimately caused by the earth rotating beneath the ocean's tidal bulges. Along the coasts, this leads the ocean water to climb up the shore during high tide and then to recede down again during low tide. The cycle of high tide and low tide repeats twice a day (in most places). This is because there are two tidal bulges. As the large masses of ocean water move in the form of tides, they rub and slam back and forth against the land and against each other. This converts their kinetic energy to thermal energy. This means that some of the rotational kinetic energy of the earth is converted to tidal energy, which then ultimately ends up as thermal energy. You can think of this as a type of friction. Just as the friction between a sliding box and the floor converts the box's kinetic energy to thermal energy, the friction-like effect between the ocean masses and the land, and between the ocean masses and each other, converts the kinetic energy of the earth's rotation to thermal energy. As a result, this friction-like tidal effect slows down earth's rotation and heats up the earth. For this reason, this process is called "tidal heating". To be clear, gravity is not creating tidal energy. Gravity is just the agent that facilitates this conversion of rotational energy to tidal energy.
Tidal heating is not unique to the earth. Any planet-and-moon pair that is not already completely tidally locked is experiencing tidal heating. If neither the moon nor the planet is tidally locked to the other, both of them experience tidal heating. If the moon is tidally locked to the planet but the planet is not tidally locked to the moon, only the planet experiences tidal heating from their interaction. Being "tidally locked" to another object means that the planet or moon always shows the same side to the other object. If both the moon is tidally locked to the planet and the planet is tidally locked to the moon, then neither one experiences tidal heating.
What you need to understand is that the rotation of the planet or moon relative to its tidal bulges is what creates this friction-like effect that converts rotational energy to thermal energy. Once a planet or moon is tidally locked, its rotation is stationary relative to its tidal bulges. With no relative motion between the moon or planet and its tidal bulges, there's no rubbing or slamming, no friction-like effect, no dynamic stresses, no tidal energy, no change of the tides, and therefore no tidal heating. Once tidally locked, a planet or moon stops decreasing its spinning rate because the mechanism that was slowing it down is gone. Note that what I've been calling here a "friction-like effect" also includes time-varying stresses and strains inside the planet or moon, which also contribute to tidal heating.
Our moon is already tidally locked with the earth, and therefore the moon is not experiencing tidal heating from the earth's gravitational field. The moon being tidally locked to the earth means that it always shows the same side to the earth. Interestingly, the moon was not always like this. Long ago, the moon was not tidally locked to the earth and did indeed experience tidal heating. However, tidal heating gradually slowed down the moon's rotation rate until it became tidally locked to the earth. Once tidally locked, it stopped decreasing its rotation rate. In other words, the moon got stuck in a state of always rotating just right so that it's near side always faces the earth. This ultimate fate of getting stuck is why we call it tidal locking.
"Wait a minute!" you may say, "If a moon does not have any oceans, how can it have tides?" The answer is that gravitational tidal forces are applied to the entire moon or planet, and not just to the water on its surface. The entire moon or planet experiences tidal bulging. However, because solid rock is not as deformable as ocean water, the tidal bulging of the rock inside a moon or planet is not as large as that of earth's oceans. The varying stresses and strains occurring in the rocks inside a moon or planet because of tidal bulging also act as a friction-like effect and convert rotational energy to thermal energy. For a moon or planet that is not tidally locked, tidal heating happens to all of it, and not just to its oceans. When it comes to the tidal bulging of the solid rock inside a moon or planet, it's not like ocean water flowing across the globe. Rather, the moon or planet becomes stretched and deformed outward along the moon-planet axis while being squeezed and deformed inward in the plane perpendicular to this axis.
All moons and planets that are experiencing stable tidal heating are gradually moving toward this fate of being tidally locked. Even the earth is doing this. Just as the moon is already tidally locked with the earth, the earth will eventually become tidally locked with the moon (assuming that the earth survives that long). This will mean that the rotation rate of the earth will be just right so that it will always show the same side to the moon. When that happens, people living on the near side of the earth will always see the moon in the sky—day and night—while people living on the far side of the earth will never see the moon in the sky. Furthermore, the near-side people will always see the moon sitting in the same location in the sky, day and night. This assumes that the earth and humans survive long enough to experience earth becoming tidally locked. It's possible that the sun's red giant stage will involve the sun engulfing the earth, which would happen long before the earth becomes tidally locked to the moon. However, recent research suggests that in its red giant stage, the sun may not grow large enough to engulf the earth.
Note that tidal effects are not caused by the centrifugal force that arises from the moon or planet traveling along a circular orbital path. The centrifugal force that arises from the motion of a moon or planet along a circular orbital path is experienced approximately the same throughout that moon or planet and therefore cannot cause bulging.
In addition to the earth experiencing tidal effects from the moon, it also experiences tidal effects from the sun. In other words, the tidal effects that the earth experiences arise from the combination of the moon's and the sun's gravitational tidal field. However, the tidal effects of the moon exerted on the earth dominate over the tidal effects of the sun on the earth. As a result, there are just two tidal bulges on earth at any given moment, instead of four. The sun's tidal effects on the earth just influence the ocean tides that were already caused by the moon, making the tides particularly weak at certain times of the month and particularly strong at other times of the month, which are respectively called neap tides and spring tides.
Also note that stable tidal heating only occurs for a certain range of moon-and-planet sizes and distances. If the planet is far larger than the moon and they are close enough, the gravitational tidal force on the moon will be so strong that it rips the moon to pieces rather than just deform it. More accurately, these ripping forces will prevent a moon from even forming in the first place for this type of situation.
In summary, tidal heating, which is the process whereby thermal energy is generated in a moon or planet because of tidal effects, gets its energy from the rotational kinetic energy of the moon or planet. There are other, weaker effects that can come into play because of the tidal interaction between a moon and a planet. However, the dominant mechanism is tidal bulging converting the moon's or planet's rotational kinetic energy to tidal energy and then to thermal energy.