Speaker 1: Welcome to Stuff to Blow Your Mind, a production of
Speaker 1: I Heart Radio. Hi. My name is Joe McCormick, and
Speaker 1: this is the Artifact, a short form series from Stuff
Speaker 1: to Blow Your Mind, focusing on particular objects, ideas, and
Speaker 1: moments in time. The Sun orbits the center of the
Speaker 1: Milky Way galaxy roughly once every two hundred and thirty
Speaker 1: million years. Given this measure, it was roughly one galactic
Speaker 1: year ago that a reptile clade we now know as
Speaker 1: the dinosaurs first appeared in the fossil record. And if
Speaker 1: you go one galactic year before that, the euryptorid predator penticopterus,
Speaker 1: a type of giant sea scorpion of the Ordovician waters,
Speaker 1: was one of the most fearsome animals on Earth. Within
Speaker 1: that galactic orbit, you can zoom down into smaller frames
Speaker 1: of satellite time. Of course, our planet orbits the on
Speaker 1: every three hundred and sixty five and one quarter earth days,
Speaker 1: and then of course the Moon orbits the Earth in
Speaker 1: a little over twenty seven days. But then you hit
Speaker 1: a wall where you have to ask a question, is
Speaker 1: that as far down as natural orbital frames go, can
Speaker 1: the moon of a planet have a smaller moon of
Speaker 1: its own. In a paper published in twenty nineteen and
Speaker 1: monthly Notices of the Royal Astronomical Society, Astronomer's Juna A.
Speaker 1: Colemeyer and Sean M. Raymond address exactly this question. Can
Speaker 1: moons have their own moons? And if so, where are they?
Speaker 1: The answer to this first question depends on how you
Speaker 1: frame it. To start off with, we have to clarify
Speaker 1: what we mean by moon. To count as a moon,
Speaker 1: an object in orbit around a planet or moon needs
Speaker 1: to be a natural satellite. As of this recording, NASA's
Speaker 1: Lunar Reconnaissance Orbiter is an orbit around Earth's moon right now,
Speaker 1: but nobody would call this vehicle a moon. In addition
Speaker 1: to being natural, a moon should really be at least
Speaker 1: semi permanent. At any given time, Earth could potentially be
Speaker 1: orbited by a number of tiny natural satellites in addition
Speaker 1: to the Moon. One recent example is an object called
Speaker 1: c D three, a dim near Earth asteroid probably a
Speaker 1: little smaller than a car, which was discovered orbiting Earth
Speaker 1: in February, and which has since been flung out of
Speaker 1: orbit and sent on its way. This is not the
Speaker 1: first temporary captured orbiter in Earth's history and it won't
Speaker 1: be the last. But it's hard to call it a moon.
Speaker 1: So if we can find our question to large natural
Speaker 1: satellites that stick around for millions or billions of years,
Speaker 1: like Earth's moon, can moons have their own According to
Speaker 1: the modeling done by Coalmeyer and Raymond, the answer is yes,
Speaker 1: but ut. In order to have its own submon, a
Speaker 1: moon has two major hurdles to clear. It needs to
Speaker 1: be large, and it needs to be pretty far out
Speaker 1: from its host planet. The main reason for these requirements
Speaker 1: is the influence of tidal forces from the host planet.
Speaker 1: Tidal forces refer to a process where the host planet
Speaker 1: causes stretching and deformation of the moon or sub moon
Speaker 1: through the influence of its gravity. Tidal forces from Jupiter,
Speaker 1: for example, are the cause of the internal stretching and
Speaker 1: friction that it heats up the core of Io, the
Speaker 1: innermost of the Galilean moons, and drives volcanic eruptions on
Speaker 1: Io's surface. In the case of sub moon retention. The
Speaker 1: authors of the twenty nineteen paper showed that if a
Speaker 1: moon is too close to its host planet, any potential
Speaker 1: sub moons in orbit around it will experience too much
Speaker 1: interference from tidal forces, which will eventually destabilize the orbit
Speaker 1: of the submon, cause sing it to fly off into
Speaker 1: space or crash down into its host. Now, an obvious
Speaker 1: question is do any moons in our Solar system meet
Speaker 1: the criteria to host a submoon? Actually, yes, Jupiter's moon Callisto,
Speaker 1: Saturn's moon Iapodus, and our moon, the moon of Earth.
Speaker 1: So if these moons are technically capable of supporting submons,
Speaker 1: where are they? We can't know for sure, but the
Speaker 1: authors speculate about likely reasons that if these moons ever
Speaker 1: had submoons, they would no longer exist today. In the
Speaker 1: case of Callisto, it's worth remembering that Jupiter has a
Speaker 1: lot of relatively large moons, and it seems likely that
Speaker 1: the gravitational interference posed by the other Galilean moons would
Speaker 1: reduce Callisto's stable submon sphere to nothing. There's just too
Speaker 1: much going on in the neighborhood. In the case of Earth,
Speaker 1: the best theory of our moon's origin is that it
Speaker 1: was created four point five billion years ago when an
Speaker 1: object about the size of Mars literally collided with the
Speaker 1: young planet Earth. This colossal impact through a large chunk
Speaker 1: of Earth's mass off into orbit around our planet, and
Speaker 1: this mass eventually coalesced and became the moon. But the
Speaker 1: young Moon was really close, only a few Earth radii,
Speaker 1: or something like twenty kilometers away. It's been retreating ever since. Today.
Speaker 1: The Moon is still edging away from the Earth at
Speaker 1: a rate of about four centimeters every year. But at
Speaker 1: the time when the young Moon could have captured a
Speaker 1: sub moon of its own, it wouldn't have had the
Speaker 1: distance it needed. Saturn's moon Iapodus might be the most
Speaker 1: interesting case. The most notable feature on the surface of
Speaker 1: Iapodus is its creepy equatorial ridge, a ring of mountains
Speaker 1: wrapped like a belt around the mid section of the
Speaker 1: icy moon. When I see these peaks, I imagine the
Speaker 1: spine of some ice incrusted gigaresque monster poking up through
Speaker 1: the skin of its back. One possible explanation among several
Speaker 1: for the equatorial ridge of Iapetus today is that it
Speaker 1: is the scattered flesh of an obliterated ancient sub moon.
Speaker 1: In fact, this would be consistent with the hypothesis argued
Speaker 1: by Andrew J. Dombard, Andrew F. Chang, William B. McKinnon,
Speaker 1: and Jonathan P. K in a paper published in the
Speaker 1: Journal of Geophysical Research Planets. The authors of this paper
Speaker 1: right quote. We expand upon our previous proposal that the
Speaker 1: ridge ultimately formed from an ancient giant impact that produced
Speaker 1: a sub satellite around Diapotus. The orbit of this sub
Speaker 1: satellite would then decay once Diapotus itself had de spun
Speaker 1: due to tides raised by Saturn, until tidal forces from
Speaker 1: Iapotus tore the sub satellite apart. The resultant debris formed
Speaker 1: a transient ring around Diapotus, the material of which rained
Speaker 1: down on the surface to build the ridge. And I
Speaker 1: love that image of the ring falling down to the
Speaker 1: surface of the Moon. But finally I wanted to ask
Speaker 1: to continue the logic we started with, can a sub
Speaker 1: moon have its own sub submoon? Actually yes, but as
Speaker 1: you might guess, each orbital level you descend, the maximum
Speaker 1: size of the object goes down. And given the size
Speaker 1: of the planets in our Solar system, and thus the
Speaker 1: maximum size of moons that can orbit them, and thus
Speaker 1: the maximum size of submoons that can orbit the moons,
Speaker 1: it's not likely that our solar system could have a
Speaker 1: subsubmon bigger than about a kilometer wide, but I take it.
Speaker 1: Tune into new editions of the Artifact every Wednesday, hosted
Speaker 1: either by Robert or by myself. As always, you can
Speaker 1: email us at contact at stuff to Blow your Mind
Speaker 1: dot com. Stuff to Blow Your Mind is a production
Speaker 1: of I Heart Radio. For more podcasts for my heart Radio,
Speaker 1: visit the I Heart Radio app, Apple Podcasts, or wherever
Speaker 1: you listen to your favorite shows.