You might have heard that the #tides that cause #Earth's #oceans to rise and fall twice each day are caused by our Moon, but did you know that the same #Physics has the power to flex & churn other worlds to the point of melting?

Here is #Jupiter's moon #Io captured in the early 2000s by the Galileo spacecraft. It's surface looks really odd, so many different yellows, reds, and oranges. In the image a strange blue plume gives us a hint to what is going on...

#Volcanoes! 🌋and lots of them!

Io's close orbit to Jupiter leads to intense tidal forces which cause the moon to flex and squeeze like a rubber ball. That flexing grinds rocks inside the moon against one another creating _a lot_ of friction & #heat. There is so much heat that the entire moon's surface is covered by 🌋 and #lava flows!

The best part? Io isn't alone in experiencing this phenomenon! Here is #Saturn's moon #Enceladus taken in 2009 by the #Cassini spacecraft. There are strange whisps flowing from the surface.

By flying closer (and even through!) those clouds we learned that they are actually plumes of #water, #ice, and salts ejected from large gashes in #enceladus' south pole that we call the #tiger #stripes (📷: Close view of plumes taken by Cassini).

But Enceladus is tiny! Only about 15% the size of our Moon. Something so small should not have the energy to melt this much water let alone eject it into #space; the moon should be a frozen ball of ice and rock!

The reason? You guessed it, #tides!

But how do #tides work? Why is this happening to these moons and not other ones? What does this mean for our Moon and the Earth? What about #exoplanets??

Well my coffee has cooled down almost to Enceladus' temperature so we will have to pick this back up later today!

I ran out of coffee so its time to return to #tides!

We left off with some cool worlds that are affected by tidal forces, but what causes these forces?

To answer that we have to talk about #gravity! You know the force that keeps us down all the time? That stuff. Gravity is also what keeps our #Moon orbiting #Earth. Did you know the Moon is actually falling just like us? But it is also moving really fast to the side, so it keeps missing the Earth, we call this an #orbit.

Get ready for some #science (I will leave the #math for another day 😉)!

The force of #gravity is stronger the closer you are to a more massive object, like a #planet or #star, and weaker the further away you are.

In this image there is large circle which represents a planet. The arrows represent the pull of gravity. Their length represents the strength of that pull being exerted by a massive object (shown as a black dot). The further away you are from the dot, the weaker the pull of gravity.

You might wonder, if the force of #gravity is stronger the closer you are to an object, does that mean that if you are standing upright that your feet, being closer to the center of the Earth, feel a stronger pull of gravity than your head? You'd be right! A 1.7 meter tall person's head at sea level experiences about 0.00005% stronger pull than their feet!

This is wayyy too small for us to notice or to impact our #biology.

#Earth, our #Moon, and even you and I are mostly #rigid such that when one part of us moves the rest (hopefully!) follows pretty quickly. So the fact that our feet are being pulled slightly more than our heads doesn't really matter when it comes to keeping us on the ground. The _average_ force exerted across our body is more important.

This is also true for planets & moons, but in some circumstances all those little differences can start to matter.

These tiny differences are what we call #tidal #forces. Going back to the image from before, if we subtract the average length of all the arrows from each individual arrow we are left with this new image on the right: arrows closer to the dot are still pointing towards it while those on the opposite side are now pointing away from it!

This is what tidal forces look like on #Earth! Those peaks or "tidal bulges" are where our #ocean's high #tide occur (the dot in that case would be our Moon).

These tidal bulges happen on all #planets and #moons but are only important and noticeable on those who orbit very close to very large objects.

@dpthorngren actually just talked yesterday about how tidal forces can be so strong on #exoplanets the size of #jupiter orbiting *very* close to their host star that the planet becomes very misshaped and can start to look like an American football 🏈! This is an extreme example of tidal distortion.

Now let's go back to #Io and #Enceladus. I hope you can see now that these small #moons that #orbit close to their host #planets can experience #tidal #forces, but that doesn't really explain why they are being heated...

It turns out that as these moons orbit, those tidal bulges begin to dance across the surface and interior of these worlds. Each time the bulge moves it drives material to move in different directions, leading to #frictional #heating as the rock & ice grind against each other!🌋

Now #heat is a kind of #energy and energy has to come from somewhere! It turns out that tides have the unique ability to extract energy trapped in a planet/moon's #orbit and/or #rotation and transform it into internal heat!

Over millions of years this effect is powerful enough to alter a planet/moon's orbital distance, #eccentricity (how elliptical an orbit is), rotation rate (how long a planet's day is), and more. This has actually happened to Earth and our Moon! But we will get back to that.

But how much energy is transformed into heat? Why is #Saturn's other moon #Mimas not as #geologically active as its sister #Enceladus? What does this mean for #Earth?

These are all big questions and leads us to our final chapter of this thread. But first, I need to take a break and wash our dishes!

Alright, it is time to wrap up this thread and talk about the final #tidal puzzle piece 🧩 ...

... #rocks ...

Specifically this type of rock called #olivine which makes up the majority of #Earth's upper #mantle. We suspect, based off density measurements, that it is also common in the mantles of other #planets and #moons.

Remember when we talked about how #tidal forces can cause the interior of a planet or moon to flex and churn? This is the stuff that is doing that flexing!

This & other types of #rocks are made up millions of #microscopic grains of various #minerals. When you apply a force to them, like a #tidal force, they will begin to deform and slide against one another. This is the mechanism that transforms #orbital motion into frictional #heat, these tiny little bits of rock & #ice!

Variations in composition, the temperature the rocks are held at, and how much moisture is present all can have a dramatic impact on how quickly this energy transfer occurs.

So in order to fully understand #tides we actually need to understand #rocks, #ices, and #magma.

That is what I love about tidal dynamics: It is the study of how microscopic grains of rock can end up melting an entire planet, or making another one potentially #habitable by providing energy for #life!

This field is the melding of #geophysics, #geology, #orbital #mechanics, #planetary #science, and more.

💎 + 🪐 = 🌋 + 🐟

I briefly mentioned that tides can change the orbits and rotation rates of planets and moons, and that this has happened to Earth and our Moon.

Specifically, tides have caused Earth's rotation rate to slow down, meaning our day has gotten longer. We think, billions of years ago, that a "day" used to only be ~6--10 hours long!

Those same forces have caused our Moon to recede away from us, which it is still doing today at a rate of around 1.5 inches per year.

(📷 : Our Moon by Gregory Revera)