WEBVTT - The Artifact: Submoon Fever

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<v Speaker 1>Welcome to Stuff to Blow Your Mind, a production of

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<v Speaker 1>I Heart Radio. Hi. My name is Joe McCormick, and

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<v Speaker 1>this is the Artifact, a short form series from Stuff

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<v Speaker 1>to Blow Your Mind, focusing on particular objects, ideas, and

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<v Speaker 1>moments in time. The Sun orbits the center of the

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<v Speaker 1>Milky Way galaxy roughly once every two hundred and thirty

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<v Speaker 1>million years. Given this measure, it was roughly one galactic

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<v Speaker 1>year ago that a reptile clade we now know as

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<v Speaker 1>the dinosaurs first appeared in the fossil record. And if

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<v Speaker 1>you go one galactic year before that, the euryptorid predator penticopterus,

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<v Speaker 1>a type of giant sea scorpion of the Ordovician waters,

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<v Speaker 1>was one of the most fearsome animals on Earth. Within

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<v Speaker 1>that galactic orbit, you can zoom down into smaller frames

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<v Speaker 1>of satellite time. Of course, our planet orbits the on

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<v Speaker 1>every three hundred and sixty five and one quarter earth days,

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<v Speaker 1>and then of course the Moon orbits the Earth in

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<v Speaker 1>a little over twenty seven days. But then you hit

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<v Speaker 1>a wall where you have to ask a question, is

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<v Speaker 1>that as far down as natural orbital frames go, can

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<v Speaker 1>the moon of a planet have a smaller moon of

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<v Speaker 1>its own. In a paper published in twenty nineteen and

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<v Speaker 1>monthly Notices of the Royal Astronomical Society, Astronomer's Juna A.

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<v Speaker 1>Colemeyer and Sean M. Raymond address exactly this question. Can

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<v Speaker 1>moons have their own moons? And if so, where are they?

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<v Speaker 1>The answer to this first question depends on how you

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<v Speaker 1>frame it. To start off with, we have to clarify

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<v Speaker 1>what we mean by moon. To count as a moon,

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<v Speaker 1>an object in orbit around a planet or moon needs

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<v Speaker 1>to be a natural satellite. As of this recording, NASA's

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<v Speaker 1>Lunar Reconnaissance Orbiter is an orbit around Earth's moon right now,

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<v Speaker 1>but nobody would call this vehicle a moon. In addition

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<v Speaker 1>to being natural, a moon should really be at least

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<v Speaker 1>semi permanent. At any given time, Earth could potentially be

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<v Speaker 1>orbited by a number of tiny natural satellites in addition

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<v Speaker 1>to the Moon. One recent example is an object called

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<v Speaker 1>c D three, a dim near Earth asteroid probably a

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<v Speaker 1>little smaller than a car, which was discovered orbiting Earth

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<v Speaker 1>in February, and which has since been flung out of

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<v Speaker 1>orbit and sent on its way. This is not the

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<v Speaker 1>first temporary captured orbiter in Earth's history and it won't

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<v Speaker 1>be the last. But it's hard to call it a moon.

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<v Speaker 1>So if we can find our question to large natural

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<v Speaker 1>satellites that stick around for millions or billions of years,

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<v Speaker 1>like Earth's moon, can moons have their own According to

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<v Speaker 1>the modeling done by Coalmeyer and Raymond, the answer is yes,

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<v Speaker 1>but ut. In order to have its own submon, a

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<v Speaker 1>moon has two major hurdles to clear. It needs to

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<v Speaker 1>be large, and it needs to be pretty far out

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<v Speaker 1>from its host planet. The main reason for these requirements

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<v Speaker 1>is the influence of tidal forces from the host planet.

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<v Speaker 1>Tidal forces refer to a process where the host planet

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<v Speaker 1>causes stretching and deformation of the moon or sub moon

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<v Speaker 1>through the influence of its gravity. Tidal forces from Jupiter,

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<v Speaker 1>for example, are the cause of the internal stretching and

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<v Speaker 1>friction that it heats up the core of Io, the

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<v Speaker 1>innermost of the Galilean moons, and drives volcanic eruptions on

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<v Speaker 1>Io's surface. In the case of sub moon retention. The

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<v Speaker 1>authors of the twenty nineteen paper showed that if a

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<v Speaker 1>moon is too close to its host planet, any potential

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<v Speaker 1>sub moons in orbit around it will experience too much

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<v Speaker 1>interference from tidal forces, which will eventually destabilize the orbit

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<v Speaker 1>of the submon, cause sing it to fly off into

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<v Speaker 1>space or crash down into its host. Now, an obvious

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<v Speaker 1>question is do any moons in our Solar system meet

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<v Speaker 1>the criteria to host a submoon? Actually, yes, Jupiter's moon Callisto,

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<v Speaker 1>Saturn's moon Iapodus, and our moon, the moon of Earth.

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<v Speaker 1>So if these moons are technically capable of supporting submons,

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<v Speaker 1>where are they? We can't know for sure, but the

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<v Speaker 1>authors speculate about likely reasons that if these moons ever

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<v Speaker 1>had submoons, they would no longer exist today. In the

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<v Speaker 1>case of Callisto, it's worth remembering that Jupiter has a

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<v Speaker 1>lot of relatively large moons, and it seems likely that

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<v Speaker 1>the gravitational interference posed by the other Galilean moons would

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<v Speaker 1>reduce Callisto's stable submon sphere to nothing. There's just too

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<v Speaker 1>much going on in the neighborhood. In the case of Earth,

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<v Speaker 1>the best theory of our moon's origin is that it

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<v Speaker 1>was created four point five billion years ago when an

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<v Speaker 1>object about the size of Mars literally collided with the

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<v Speaker 1>young planet Earth. This colossal impact through a large chunk

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<v Speaker 1>of Earth's mass off into orbit around our planet, and

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<v Speaker 1>this mass eventually coalesced and became the moon. But the

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<v Speaker 1>young Moon was really close, only a few Earth radii,

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<v Speaker 1>or something like twenty kilometers away. It's been retreating ever since. Today.

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<v Speaker 1>The Moon is still edging away from the Earth at

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<v Speaker 1>a rate of about four centimeters every year. But at

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<v Speaker 1>the time when the young Moon could have captured a

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<v Speaker 1>sub moon of its own, it wouldn't have had the

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<v Speaker 1>distance it needed. Saturn's moon Iapodus might be the most

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<v Speaker 1>interesting case. The most notable feature on the surface of

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<v Speaker 1>Iapodus is its creepy equatorial ridge, a ring of mountains

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<v Speaker 1>wrapped like a belt around the mid section of the

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<v Speaker 1>icy moon. When I see these peaks, I imagine the

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<v Speaker 1>spine of some ice incrusted gigaresque monster poking up through

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<v Speaker 1>the skin of its back. One possible explanation among several

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<v Speaker 1>for the equatorial ridge of Iapetus today is that it

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<v Speaker 1>is the scattered flesh of an obliterated ancient sub moon.

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<v Speaker 1>In fact, this would be consistent with the hypothesis argued

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<v Speaker 1>by Andrew J. Dombard, Andrew F. Chang, William B. McKinnon,

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<v Speaker 1>and Jonathan P. K in a paper published in the

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<v Speaker 1>Journal of Geophysical Research Planets. The authors of this paper

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<v Speaker 1>right quote. We expand upon our previous proposal that the

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<v Speaker 1>ridge ultimately formed from an ancient giant impact that produced

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<v Speaker 1>a sub satellite around Diapotus. The orbit of this sub

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<v Speaker 1>satellite would then decay once Diapotus itself had de spun

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<v Speaker 1>due to tides raised by Saturn, until tidal forces from

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<v Speaker 1>Iapotus tore the sub satellite apart. The resultant debris formed

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<v Speaker 1>a transient ring around Diapotus, the material of which rained

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<v Speaker 1>down on the surface to build the ridge. And I

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<v Speaker 1>love that image of the ring falling down to the

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<v Speaker 1>surface of the Moon. But finally I wanted to ask

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<v Speaker 1>to continue the logic we started with, can a sub

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<v Speaker 1>moon have its own sub submoon? Actually yes, but as

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<v Speaker 1>you might guess, each orbital level you descend, the maximum

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<v Speaker 1>size of the object goes down. And given the size

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<v Speaker 1>of the planets in our Solar system, and thus the

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<v Speaker 1>maximum size of moons that can orbit them, and thus

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<v Speaker 1>the maximum size of submoons that can orbit the moons,

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<v Speaker 1>it's not likely that our solar system could have a

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<v Speaker 1>subsubmon bigger than about a kilometer wide, but I take it.

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<v Speaker 1>Tune into new editions of the Artifact every Wednesday, hosted

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<v Speaker 1>either by Robert or by myself. As always, you can

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<v Speaker 1>email us at contact at stuff to Blow your Mind

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<v Speaker 1>dot com. Stuff to Blow Your Mind is a production

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<v Speaker 1>of I Heart Radio. For more podcasts for my heart Radio,

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<v Speaker 1>visit the I Heart Radio app, Apple Podcasts, or wherever

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<v Speaker 1>you listen to your favorite shows.