The Artifact: Submoon Fever

the dinosaurs first appeared in the fossil record. And if you go one galactic year before that, the euryptorid predator penticopterus, a type of giant sea scorpion of the Ordovician waters, was one of the most fearsome animals on Earth. Within that galactic orbit, you can zoom down into smaller frames of satellite time. Of course, our planet orbits the on every three hundred and sixty five and one quarter earth days, and then of course the Moon orbits the Earth in

a little over twenty seven days. But then you hit a wall where you have to ask a question, is that as far down as natural orbital frames go, can the moon of a planet have a smaller moon of its own. In a paper published in twenty nineteen and monthly Notices of the Royal Astronomical Society, Astronomer's Juna A. Colemeyer and Sean M. Raymond address exactly this question. Can moons have their own moons? And if so, where are they? The answer to this first question depends on how you

frame it. To start off with, we have to clarify what we mean by moon. To count as a moon, an object in orbit around a planet or moon needs to be a natural satellite. As of this recording, NASA's Lunar Reconnaissance Orbiter is an orbit around Earth's moon right now, but nobody would call this vehicle a moon. In addition to being natural, a moon should really be at least semi permanent. At any given time, Earth could potentially be orbited by a number of tiny natural satellites in addition

to the Moon. One recent example is an object called c D three, a dim near Earth asteroid probably a little smaller than a car, which was discovered orbiting Earth in February, and which has since been flung out of orbit and sent on its way. This is not the first temporary captured orbiter in Earth's history and it won't be the last. But it's hard to call it a moon.

So if we can find our question to large natural satellites that stick around for millions or billions of years, like Earth's moon, can moons have their own According to the modeling done by Coalmeyer and Raymond, the answer is yes, but ut. In order to have its own submon, a moon has two major hurdles to clear. It needs to be large, and it needs to be pretty far out from its host planet. The main reason for these requirements is the influence of tidal forces from the host planet.

Tidal forces refer to a process where the host planet causes stretching and deformation of the moon or sub moon through the influence of its gravity. Tidal forces from Jupiter, for example, are the cause of the internal stretching and friction that it heats up the core of Io, the innermost of the Galilean moons, and drives volcanic eruptions on

Io's surface. In the case of sub moon retention. The authors of the twenty nineteen paper showed that if a moon is too close to its host planet, any potential sub moons in orbit around it will experience too much interference from tidal forces, which will eventually destabilize the orbit of the submon, cause sing it to fly off into space or crash down into its host. Now, an obvious question is do any moons in our Solar system meet

the criteria to host a submoon? Actually, yes, Jupiter's moon Callisto, Saturn's moon Iapodus, and our moon, the moon of Earth. So if these moons are technically capable of supporting submons, where are they? We can't know for sure, but the authors speculate about likely reasons that if these moons ever

had submoons, they would no longer exist today. In the case of Callisto, it's worth remembering that Jupiter has a lot of relatively large moons, and it seems likely that the gravitational interference posed by the other Galilean moons would reduce Callisto's stable submon sphere to nothing. There's just too

much going on in the neighborhood. In the case of Earth, the best theory of our moon's origin is that it was created four point five billion years ago when an object about the size of Mars literally collided with the young planet Earth. This colossal impact through a large chunk of Earth's mass off into orbit around our planet, and this mass eventually coalesced and became the moon. But the young Moon was really close, only a few Earth radii,

or something like twenty kilometers away. It's been retreating ever since. Today. The Moon is still edging away from the Earth at a rate of about four centimeters every year. But at the time when the young Moon could have captured a sub moon of its own, it wouldn't have had the distance it needed. Saturn's moon Iapodus might be the most

interesting case. The most notable feature on the surface of Iapodus is its creepy equatorial ridge, a ring of mountains wrapped like a belt around the mid section of the icy moon. When I see these peaks, I imagine the spine of some ice incrusted gigaresque monster poking up through the skin of its back. One possible explanation among several for the equatorial ridge of Iapetus today is that it is the scattered flesh of an obliterated ancient sub moon.

In fact, this would be consistent with the hypothesis argued by Andrew J. Dombard, Andrew F. Chang, William B. McKinnon, and Jonathan P. K in a paper published in the Journal of Geophysical Research Planets. The authors of this paper right quote. We expand upon our previous proposal that the ridge ultimately formed from an ancient giant impact that produced

a sub satellite around Diapotus. The orbit of this sub satellite would then decay once Diapotus itself had de spun due to tides raised by Saturn, until tidal forces from Iapotus tore the sub satellite apart. The resultant debris formed a transient ring around Diapotus, the material of which rained down on the surface to build the ridge. And I love that image of the ring falling down to the surface of the Moon. But finally I wanted to ask to continue the logic we started with, can a sub

moon have its own sub submoon? Actually yes, but as you might guess, each orbital level you descend, the maximum size of the object goes down. And given the size of the planets in our Solar system, and thus the maximum size of moons that can orbit them, and thus the maximum size of submoons that can orbit the moons, it's not likely that our solar system could have a subsubmon bigger than about a kilometer wide, but I take it. Tune into new editions of the Artifact every Wednesday, hosted

either by Robert or by myself. As always, you can email us at contact at stuff to Blow your Mind dot com. Stuff to Blow Your Mind is a production of I Heart Radio. For more podcasts for my heart Radio, visit the I Heart Radio app, Apple Podcasts, or wherever you listen to your favorite shows.