Can Moons Have Moons?

#Science - because science should blow your mind 🤯

Can Moons Have Moons?

Have a question about #Science? Drop me a line!

This week on the blog, a loyal reader asks, “Can moons have their own moons?” I think this is my favorite question yet, and the answer might surprise you.

It turns out that, a little bit like wormholes, astronomers don’t really know the answer. Humans have never seen a moon orbiting a moon, but they can use math and science to theorize, and they think that it’s possible. Most often they call these moons of moons “submoons” (which my computer insists on correcting to summons every single time—maybe it’s trying to summon a submoon). Much like the theoretical physicists who use math to prove that wormholes are theoretically possible, two researchers published a paper just last year demonstrating that submoons could exist if the conditions are just right. (1)

What would it take for a moon to have a moon?

First, the primary moon would need to be large enough that its mass creates a strong enough gravitational pull to support a submoon so it won’t be affected by their planet’s gravitational force. Titan, one of the planets orbiting Saturn, is a perfect example of this type of moon. (2)

Secondly, the primary moon needs to have a wide enough orbit, for the same reason—so that the submoon won’t be affected by their planet’s stronger gravitational force. Iapetus, another of Saturn’s moons, and Callisto, one of Jupiter’s moons, both fall into this category. (2)

Together, moon mass versus planet mass and orbital distance combine to create a zone called a Hill Sphere, which is the area of space around the moon in which a submoon could exist without breaking apart, an area where the moon’s gravitational pull is stronger than the planet’s. Not all moons will have a viable Hill Sphere, but Earth’s Moon, our Moon, does. It is both far enough away from the Earth and has a large enough mass that that scientists think it would be capable of supporting its own submoon. Isn’t that amazing? (2)

So, it’s mathematically possible for a moon to have a moon, but why haven’t we ever seen one?

There could be lots of reasons that we haven’t found one yet. Even though our Moon satisfies the Hill Sphere requirement, the Moon’s mass is not consistent. (1) It is not a perfectly weighted sphere like a tennis ball. There are pockets called Mascons on the moon where the crust is more dense than in other areas, sort of like if you didn't mix a pie dough well enough or roll it out perfectly flat, you might have a lumpy crust, and that means that the Moon’s gravitational pull can actually change depending on where a satellite is in relation to those Mascons. The center of the moon’s mass isn’t even in the center of the moon! It is actually weighted more towards the Earth. We know from the Apollo missions that these pockets of density can cause orbit problems for spacecraft if they're not prepared for them (4), so it's logical to assume that they would cause similar difficulties to an orbiting submoon.

Another reason that we might not find submoons, other than the simple fact that they would be small and hard to see, is that the formation of a planet and its moons isn't really conducive to forming submoons. Scientists believe that when our Moon first formed it was much closer to the Earth than it is today. It is only the strength of the tides that have caused the Moon’s orbit to shift further and further away. So when it first formed, the Moon's Hill Sphere would not have been able to support a submoon. (2)

There’s also quite a lot happening in space, between planets and moons, and other objects—lots of things that could cause a small moon’s sensitive orbit to destabilize even within the Hill Sphere (1). One theory suggests that Saturn’s moon, Iapetus, might have had it’s own submoon once upon a time. There's a unique rocky mountain range that circles Iaepetus’s equator, and no one knows for sure how it formed. It’s possible that Iapetus once had a submoon, but the pressures of gravity from Saturn pulling it one way and fellow moons pulling it another, over and over as they all passed each other along their different orbits finally made the submoon slip too close to Iapetus, destabilize, and break apart. Then the pieces might have crashed down along the equator creating that unusual ridge. (2)

Even though we haven’t found one yet, those scientists who published a paper on the topic last year, Juna Kollmeier and Sean Raymond, are searching beyond our solar system, hoping to find an example of a moon with a moon. Right now they are focused on a planet the size of Jupiter with the very boring name Kepler-1625b. They believe it has a moon as big as Neptune, and they think that moon is a great candidate to support a submoon. (1) They won’t give up looking. Sean even wrote a poem about it, that you can read on his blog, here.

☄️Fun fact: even though we haven’t found moons with moons, did you know that scientists have found over 300 asteroids that have their own moons? Some are as small as a football field, and some are as long as a mile (seventeen football fields). Usually occurring when two asteroids collide with each other, the very first one was discovered in 1993 when a 35-mile long asteroid called Ida was photographed. Since then, scientists have seen asteroids with as many as three moons (4), and I guess what I’m saying is that you could have a whole celestial football tournament on a moon orbiting an asteroid orbiting the sun!

★ Want to learn more? Check out these awesome resources that helped me write this little #Science article:

Jean M. Malone - August 2020