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You might be pretty confident that when a moon is there it’s there to stay, but that’s not always the case. Moons may have a history of disappearing.

Hosted by: Savannah Geary (they/them)

05:19 Well, this is half right. The moons don't actually orbit the same distance from Mars and orbit on opposite sides of the synchronous radius. Sorry about the mistake!

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It’s just one of those days, you know?

You’re on  your way out the door, you pat your pocket, and   you don’t hear the familiar jingle of your keys.  So it’s time to turn the house upside down! If   a planet had feelings, maybe it would experience  the same sense of confusion and frustration when   a moon that it swears was there the last time  it looked has now vanished.

A disappearing moon   may sound like Science Fiction, but there are  a lot of clues out there that suggest this has   happened in our own solar system more than once.  Let’s start out with the planet that has the most   moons to keep track of, 83 so far, and the most  obvious evidence that a moon has gone missing:   Saturn. It’s famous for its gorgeous rings, vast  collections of pebbles, ice, and dust that circle   the planet. By reflecting the Sun’s rays, they  shine almost as brightly as the planet itself.   But that light, which gives clues about the  composition of the rings, hides a secret:   The rings are a lot younger than the planet.

Back  in 2017, the Cassini spacecraft was approaching   the end of its 20-year mission studying Saturn  and its many moons. So astronomers felt it was   time to take a risky series of dives between the  rings. And those dives revealed the total mass of   all that ring matter was smaller than it ought  to be, at least assuming they had been around   for Saturn’s entire four and a half billion year  lifespan.

Because they’re largely made of ice,   the rings would start off bright and shiny, and  would dull with time as they accumulated more and   more dust. The heavier the rings, the older they  are. But Cassini’s data suggest they’re only tens   of millions of years old.

Why were they so late  to the planet-making party? Well, according to   a paper published in 2022, some astronomers think  that Saturn ripped apart one of its own moons that   strayed a little too close. You see, as a small  body like a moon gets closer to a really large one   like a planet, the tidal forces get stronger and  stronger.

Tidal forces are a result of the fact   that the strength of gravity decreases quickly  with distance. So as a planet tugs on a moon,   the gravity is stronger on the side of the  moon that’s facing the planet, and weaker on   the side that isn’t. The closer that moon is to  the planet, the stronger the gravitational force,   and the more important the difference between  close- and far-side becomes.

If a moon gets   too close, those tidal forces are so large that  they overpower the moon’s own gravity holding all   of its bits and pieces together. So there’s a big  no-go zone around the planet. A point of no return   called the Roche Limit.

Any moon that slips inside  will be torn asunder and smeared out into a ring.   Bad news for the moon, good news for anyone who  likes looking at pretty planetary structures. This   transformation from moon to ring led the team to  propose the name “Chrysalis” for this hypothetical   destroyed moon. So Saturn didn’t exactly lose  Chrysalis.

It’s more like it forgot it was in   its back pocket and sat down, smushing it. But  Saturn’s rings aren’t the only clue that a moon   went missing. If Chrysalis really did exist, its  disappearance would also help explain why Saturn   has the specific tilt it does.

To be clear, there  are other hypotheses that explain Saturn’s awkward   tilt, but none can also explain why Saturn’s rings  are so much younger than the rest of it. Moving   one planet closer to the Sun, Jupiter also seems  to have lost some moons. Today it has at least 80,   but it probably managed to misplace a bunch more,  billions of years ago.

Unlike Saturn, which may   have proverbially sat on one of its moons, Jupiter  may have eaten them. See, based on simulations of   the early Solar System, astronomers have estimates  for how much matter should have been collapsing   to form Jupiter, with leftover chunks becoming  moons. Four of those leftover chunks are Jupiter’s   main moons.

Ganymede, Callisto, Io, and Europa.  They’re collectively named the Galilean moons,   after the guy who discovered them, and make up  over 99% of all the mass orbiting Jupiter. But   according to these simulations, there was enough  stuff to make at least four more sets of Galilean   moons, which aren’t there. The simulations may be  inaccurate.

They are trying to explain events that   happened four and a half billion years ago, after  all. But some researchers have proposed that these   moons did exist way back when. Unfortunately for  those moons, because there was so much building   material around them, they got too massive for  their own good.

After reaching some critical mass,   these early moons just weren’t moving fast  enough to escape their planet’s grasp. They   started spiraling downward, and eventually got  gobbled up. Which, for you mythology nerds,   is really more of a Saturn thing.

During the  millions of years it took Jupiter to form,   this could have happened several times over.  According to this hypothesis, the Galilean   moons were part of the last generation of moons,  and survived because there wasn’t enough material   for them to reach that critical size. Compared  to these gas giants, the terrestrial worlds have   way fewer moons. But that doesn’t mean they’re  any more careful with them!

Mars has two tiny   natural satellites, named after the Greek gods of  fear and despair: Phobos and Deimos. And there’s a   bit of an argument going on in the astronomical  community about where these moons came from.   Did they form alongside Mars? Did a lonely Mars  capture two wayward asteroids that strayed too   far from the asteroid belt?

Well, in 2021 one team  of astronomers suggested that Mars captured just   one asteroid about 30 kilometers across, the size  of Phobos and Deimos combined. Somewhere between   1 and 2.7 billion years ago, they suggest it got  smashed apart. Phobos and Deimos are just what’s   left.

The team noticed that the moons orbit on  opposite sides but the same distance from Mars.   That’s a weird coincidence if they were just  two independently captured asteroids. However,   they got called out by another team in 2022, or  at least the math they used did. The second team   took issue with the fact that the first group  didn’t fully simulate what would happen after   the breakup.

According to their newer study,  if Phobos and Deimos formed from some large   asteroid breaking apart, they would have hit  each other pretty quickly and broken up into   even more pieces! That’s not what we see when  we look up in the night sky, so they say the   first group is wrong. So Saturn may have smushed  a moon, Jupiter might have eaten lots of moons,   and Mars, well, until we get a better idea of  where Phobos and Deimos came from, Mars is missing   moons in more ways than one!

Understanding how a  planet can manage to lose a moon is fascinating   in its own right, but it also gives us tools to  better understand Earth’s relationship with our   own natural satellite, whether it eventually  leaves us or not. If you’re wondering if the   Earth has ever lost a moon, the answer is yes!  Maybe. It depends on your definition of “moon.”   We’ve already released a video on that, and you  can check it out here!

And as for your keys,   the science is still out on that. Maybe try inside  the fridge. And thanks for watching SciShow Space.