Going with the Tide

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I have a very good friend who loves the ocean, a sentiment that I personally cannot get behind (give me the vast, existential reaches of space over the ocean any day. Things in the ocean have teeth and stingers and so far nothing in space does). Of course, loving the ocean is a sensible thing to do when you live on a planet that is famously 73% covered in oceans. And one of the most characteristic things about those oceans I avoid with such alacrity are their tides.

But here’s the thing: tides aren’t an ocean thing, they’re a space thing! The ocean tides just happen to be a very visible way this space thing manifests on the surface of our planet. So in the interest of reclaiming this oceanic term back to the space realm from which it hails, let’s talk about what tides actually are and why we see them everywhere from our local beach to the far ends of intergalactic space.

Push and Pull

Tides as we see them in the ocean are the rise and fall of local sea levels on a regular, predictable cadence. A more general, less Earth-centric definition of tides can be found as the fourth entry down in the Merriam-Webster Dictionary’s entry on “tide”: “a periodic distortion on one celestial body caused by the gravitational attraction of another.”

That is what tides actually are. For all that tides are identified so strongly with the oceans, it’s really a function of the way the Sun and (mostly) the Moon are pulling on Earth with their gravity. You just see the effect more easily in the oceans than you do anywhere else! Well…anywhere else on Earth anyway.

Ebb and Flow

The regular, predictable cadence with which our oceans rise and fall is determined by where the Moon is in the sky. The Sun absolutely does contribute to ocean tides, but at a much lower level. I’ll get to that in a moment, but for now let’s focus on the Moon.

Earth, obviously, tugs on the Moon gravitationally. That’s why the Moon sticks around in orbit instead of going winging off into space. But the Moon also exerts a smaller, yet measurable pull on Earth. And it’s close enough, at only about 240,000 miles away, that it pulls a little more strongly on the side of Earth that is closest to it than it does on Earth’s center or the side farthest away.

It helps to not think of the Earth as a single solid object, in this case. Think of it as three balls in a line (this is how my college astronomy textbook visualized it, and I found it helpful): a green ball and a blue ball with a red ball between them. The green and blue balls represent the oceans on the two opposite sides of the Earth and the red ball represents Earth’s center. Now plunk the Moon down somewhere closer the green ball than the other two. 

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The Moon will pull on all three balls, but it will pull hardest on the green ball because it’s closest. It will, therefore, draw the green ball away from the other two. It will pull next hardest on the red ball. As a result the red ball will be pulled towards it, just not as hard as the green one. So the red ball won’t move as far towards the Moon as the green one, but it will pull away from the blue one. The blue one, pulled the least, will move a little.

Now imagine you’re standing on the red ball (in this analogy this corresponds to the Earth-centric view). From your perspective, it looks like both the green ball and blue ball have moved away from you—the green ball because it has moved away, and the blue ball because you have moved away from it.

Now replace those green and blue balls with oceans and the red ball with the planet itself. The effect is that there is an ocean bulge on either side of the Earth, one just about under where the Moon is and one on the exact opposite side of the planet. These are the high tides of Earth’s oceans. They appear to move around the Earth not because the Moon is moving that fast but because Earth is actively rotating beneath these tidal bulges, meaning different parts of the world are running into them as it does. Low tides are the parts of the ocean at right angles to the Moon.

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So what is the Sun’s role in all of this? The same, just less. The gradient of the Sun’s gravitational pull on the different parts of Earth is not nearly as big as the Moon’s because the Sun is farther away (the main driver in this case isn’t the total gravitational pull, it’s how much difference there is in how different parts of the Earth feel it). If we had no Moon, we would still have Sun-caused tides, they would just be smaller than the ones we have (which the Sun does affect some).

Indoor Pools

So that’s tides as we see them on Earth because we have something as flowy as water all over our surface. But what if you don’t have a nice liquid ocean to provide a convenient visual of tidal forces? That doesn’t mean you can’t get tides. In fact, tidal forces are one way you can get liquid oceans…just not above ground.

Several moons of the outer solar system, from Europa to Enceladus to Triton, are believed to have liquid water oceans under their surfaces despite being far enough from the Sun that water with any sense would definitely be frozen. Where does the heat to keep these oceans liquid come from? Tidal forces!

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The gravity of the planets that these moons orbit tugs on them the same way the Moon tugs on Earth. It has the same effect, though far less obviously visible—the moons flex as the parts of them exactly facing and exactly facing away from the planets bulge a little. When it’s rock that’s flexing instead of water, that generates a whole heckuva ton of friction, and if you’ve never rubbed your hands together really hard on a cold day to warm them up you know friction makes heat.

This heat caused by the moons’ tidal flexing radiates outward from their interiors. If they’ve got a lot of ice in their makeup this heat can be enough to melt a thick layer of that ice, making it possible for moons really far from the Sun to support underground oceans, some of which we think may be capable of supporting life.

Taking It to the Extreme

Then there’s the Jovian moon Io where tidal forces have really gone berserk. The same sort of flexing is happening on Io thanks to Jupiter’s ridiculous gravity, but Io is also getting pulled hard in the opposite direction by some of Jupiter’s big moons, Europa and Ganymede (with whom Io is in resonance on top of everything else).

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The overall effect is that Io experiences tidal heating on a level unlike anything else in the solar system. Io experiences tides just like the Earth does, only way bigger and these tides are in the rock surface of the moon. According to NASA, these tidal bulges can reach a height of 330 feet (100 m), making them far larger than even the biggest tidal bulges in Earth’s oceans!

That’s a lot of rock rubbing together really hard, so a lot of friction, so a lot of heat! This is what is driving Io’s insane volcanism, making this tiny moon the most volcanically active place in the entire solar system, constantly spewing fresh lava onto its roiling surface.

Going Galactic

So far we’ve only talked about how tidal forces affect fairly solid objects like planets and moons. But one of the places we see them most at work in the universe is when galaxies get together, whether that’s a fly-by or whether they’re preparing to merge.

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Imagine two galaxies drawing close to each other. For the sake of argument we’ll say they’re roughly the same size, and we can even say they’re both nice, neat spirals like the Milky Way. With such large objects, once they’re close enough the pull they each exert on the other’s leading edge will be significantly greater than that exerted on the center, which will be much greater than that exerted on the trailing edge. Those spiral shapes will begin to stretch and distort. As the process continues, the galaxies can even form tails, known as tidal tails, as their leading edges and centers get pulled farther and farther form their trailing edges which will linger behind like, well, like a tail.

Eventually the spiral shape gets completely pulled apart and you wind up with a big ol’ mashup of gas and dust and solar systems. Which, as it turns out, is good conditions for star formation, so you’re probably going to see a whole bunch of baby solar systems get born!

Spaghetti!

I would be remiss not to include something about everyone’s favorite, most extreme form of tidal force, spaghettification. It remains a glorious thing to me that this is a real term that is used by actual scientists, and it refers very specifically to what happens around black holes.

The gradient of gravitational forces close to a black hole is insane. If you are flying feet first into a black hole, the gravitational force on your feet will be way more powerful than that on your head. The most extreme tidal forces in the universe will go to work on your body—pulling your feet away from your pelvis and your pelvis away from your head (RIP), yes, but also pulling the atoms on the bottom of your feet away from the atoms on the top of your feet.

Pretty soon you are no longer a solid form but a spaghetti stream of atoms, which is why they call it spaghettification. This happens whether you’re you or you’re a star or you’re a gas cloud or you’re a planet. Get too close to a black hole and spaghetti is your destiny.

And I cannot think of a more magnificent thing to end on than spaghettification, so think on that the next time you’re at the beach watching the tide roll in!