1 00:00:03,279 --> 00:00:05,640 Speaker 1: Welcome to Stuff to Blow Your Mind, a production of 2 00:00:05,640 --> 00:00:10,880 Speaker 1: I Heart Radio. Hi. My name is Joe McCormick, and 3 00:00:10,960 --> 00:00:14,480 Speaker 1: this is the Artifact, a short form series from Stuff 4 00:00:14,520 --> 00:00:18,560 Speaker 1: to Blow Your Mind, focusing on particular objects, ideas, and 5 00:00:18,680 --> 00:00:23,759 Speaker 1: moments in time. The Sun orbits the center of the 6 00:00:23,800 --> 00:00:27,080 Speaker 1: Milky Way galaxy roughly once every two hundred and thirty 7 00:00:27,120 --> 00:00:32,080 Speaker 1: million years. Given this measure, it was roughly one galactic 8 00:00:32,240 --> 00:00:35,239 Speaker 1: year ago that a reptile clade we now know as 9 00:00:35,320 --> 00:00:39,440 Speaker 1: the dinosaurs first appeared in the fossil record. And if 10 00:00:39,440 --> 00:00:45,000 Speaker 1: you go one galactic year before that, the euryptorid predator penticopterus, 11 00:00:45,280 --> 00:00:48,800 Speaker 1: a type of giant sea scorpion of the Ordovician waters, 12 00:00:49,200 --> 00:00:52,520 Speaker 1: was one of the most fearsome animals on Earth. Within 13 00:00:52,680 --> 00:00:56,520 Speaker 1: that galactic orbit, you can zoom down into smaller frames 14 00:00:56,560 --> 00:01:00,080 Speaker 1: of satellite time. Of course, our planet orbits the on 15 00:01:00,280 --> 00:01:03,920 Speaker 1: every three hundred and sixty five and one quarter earth days, 16 00:01:04,440 --> 00:01:06,959 Speaker 1: and then of course the Moon orbits the Earth in 17 00:01:06,959 --> 00:01:10,640 Speaker 1: a little over twenty seven days. But then you hit 18 00:01:10,680 --> 00:01:13,880 Speaker 1: a wall where you have to ask a question, is 19 00:01:13,920 --> 00:01:18,600 Speaker 1: that as far down as natural orbital frames go, can 20 00:01:18,680 --> 00:01:21,640 Speaker 1: the moon of a planet have a smaller moon of 21 00:01:21,680 --> 00:01:26,440 Speaker 1: its own. In a paper published in twenty nineteen and 22 00:01:26,600 --> 00:01:31,440 Speaker 1: monthly Notices of the Royal Astronomical Society, Astronomer's Juna A. 23 00:01:31,680 --> 00:01:36,679 Speaker 1: Colemeyer and Sean M. Raymond address exactly this question. Can 24 00:01:36,760 --> 00:01:41,040 Speaker 1: moons have their own moons? And if so, where are they? 25 00:01:42,360 --> 00:01:44,720 Speaker 1: The answer to this first question depends on how you 26 00:01:44,760 --> 00:01:47,680 Speaker 1: frame it. To start off with, we have to clarify 27 00:01:47,800 --> 00:01:50,800 Speaker 1: what we mean by moon. To count as a moon, 28 00:01:51,120 --> 00:01:53,880 Speaker 1: an object in orbit around a planet or moon needs 29 00:01:53,880 --> 00:01:58,040 Speaker 1: to be a natural satellite. As of this recording, NASA's 30 00:01:58,120 --> 00:02:02,200 Speaker 1: Lunar Reconnaissance Orbiter is an orbit around Earth's moon right now, 31 00:02:02,280 --> 00:02:06,120 Speaker 1: but nobody would call this vehicle a moon. In addition 32 00:02:06,160 --> 00:02:08,880 Speaker 1: to being natural, a moon should really be at least 33 00:02:09,000 --> 00:02:13,240 Speaker 1: semi permanent. At any given time, Earth could potentially be 34 00:02:13,400 --> 00:02:17,480 Speaker 1: orbited by a number of tiny natural satellites in addition 35 00:02:17,560 --> 00:02:21,080 Speaker 1: to the Moon. One recent example is an object called 36 00:02:22,040 --> 00:02:26,320 Speaker 1: c D three, a dim near Earth asteroid probably a 37 00:02:26,360 --> 00:02:30,120 Speaker 1: little smaller than a car, which was discovered orbiting Earth 38 00:02:30,160 --> 00:02:33,760 Speaker 1: in February, and which has since been flung out of 39 00:02:33,880 --> 00:02:37,000 Speaker 1: orbit and sent on its way. This is not the 40 00:02:37,040 --> 00:02:40,720 Speaker 1: first temporary captured orbiter in Earth's history and it won't 41 00:02:40,720 --> 00:02:43,120 Speaker 1: be the last. But it's hard to call it a moon. 42 00:02:44,160 --> 00:02:47,040 Speaker 1: So if we can find our question to large natural 43 00:02:47,120 --> 00:02:51,000 Speaker 1: satellites that stick around for millions or billions of years, 44 00:02:51,040 --> 00:02:55,720 Speaker 1: like Earth's moon, can moons have their own According to 45 00:02:55,760 --> 00:02:59,040 Speaker 1: the modeling done by Coalmeyer and Raymond, the answer is yes, 46 00:02:59,800 --> 00:03:02,320 Speaker 1: but ut. In order to have its own submon, a 47 00:03:02,360 --> 00:03:05,640 Speaker 1: moon has two major hurdles to clear. It needs to 48 00:03:05,639 --> 00:03:08,440 Speaker 1: be large, and it needs to be pretty far out 49 00:03:08,520 --> 00:03:12,520 Speaker 1: from its host planet. The main reason for these requirements 50 00:03:12,560 --> 00:03:15,919 Speaker 1: is the influence of tidal forces from the host planet. 51 00:03:16,480 --> 00:03:19,720 Speaker 1: Tidal forces refer to a process where the host planet 52 00:03:19,760 --> 00:03:23,320 Speaker 1: causes stretching and deformation of the moon or sub moon 53 00:03:23,760 --> 00:03:27,760 Speaker 1: through the influence of its gravity. Tidal forces from Jupiter, 54 00:03:27,880 --> 00:03:31,320 Speaker 1: for example, are the cause of the internal stretching and 55 00:03:31,400 --> 00:03:35,000 Speaker 1: friction that it heats up the core of Io, the 56 00:03:35,040 --> 00:03:39,360 Speaker 1: innermost of the Galilean moons, and drives volcanic eruptions on 57 00:03:39,440 --> 00:03:44,040 Speaker 1: Io's surface. In the case of sub moon retention. The 58 00:03:44,080 --> 00:03:46,880 Speaker 1: authors of the twenty nineteen paper showed that if a 59 00:03:46,920 --> 00:03:50,520 Speaker 1: moon is too close to its host planet, any potential 60 00:03:50,680 --> 00:03:54,200 Speaker 1: sub moons in orbit around it will experience too much 61 00:03:54,280 --> 00:03:58,720 Speaker 1: interference from tidal forces, which will eventually destabilize the orbit 62 00:03:58,720 --> 00:04:01,120 Speaker 1: of the submon, cause sing it to fly off into 63 00:04:01,120 --> 00:04:04,600 Speaker 1: space or crash down into its host. Now, an obvious 64 00:04:04,680 --> 00:04:08,680 Speaker 1: question is do any moons in our Solar system meet 65 00:04:08,720 --> 00:04:14,920 Speaker 1: the criteria to host a submoon? Actually, yes, Jupiter's moon Callisto, 66 00:04:15,240 --> 00:04:19,599 Speaker 1: Saturn's moon Iapodus, and our moon, the moon of Earth. 67 00:04:20,520 --> 00:04:24,120 Speaker 1: So if these moons are technically capable of supporting submons, 68 00:04:24,760 --> 00:04:31,200 Speaker 1: where are they? We can't know for sure, but the 69 00:04:31,240 --> 00:04:35,400 Speaker 1: authors speculate about likely reasons that if these moons ever 70 00:04:35,520 --> 00:04:39,359 Speaker 1: had submoons, they would no longer exist today. In the 71 00:04:39,400 --> 00:04:42,440 Speaker 1: case of Callisto, it's worth remembering that Jupiter has a 72 00:04:42,480 --> 00:04:45,719 Speaker 1: lot of relatively large moons, and it seems likely that 73 00:04:45,760 --> 00:04:50,000 Speaker 1: the gravitational interference posed by the other Galilean moons would 74 00:04:50,040 --> 00:04:54,280 Speaker 1: reduce Callisto's stable submon sphere to nothing. There's just too 75 00:04:54,320 --> 00:04:57,440 Speaker 1: much going on in the neighborhood. In the case of Earth, 76 00:04:57,760 --> 00:05:00,840 Speaker 1: the best theory of our moon's origin is that it 77 00:05:00,880 --> 00:05:04,400 Speaker 1: was created four point five billion years ago when an 78 00:05:04,400 --> 00:05:08,000 Speaker 1: object about the size of Mars literally collided with the 79 00:05:08,080 --> 00:05:12,239 Speaker 1: young planet Earth. This colossal impact through a large chunk 80 00:05:12,279 --> 00:05:15,520 Speaker 1: of Earth's mass off into orbit around our planet, and 81 00:05:15,560 --> 00:05:20,240 Speaker 1: this mass eventually coalesced and became the moon. But the 82 00:05:20,360 --> 00:05:24,640 Speaker 1: young Moon was really close, only a few Earth radii, 83 00:05:24,880 --> 00:05:30,760 Speaker 1: or something like twenty kilometers away. It's been retreating ever since. Today. 84 00:05:30,760 --> 00:05:32,720 Speaker 1: The Moon is still edging away from the Earth at 85 00:05:32,720 --> 00:05:35,840 Speaker 1: a rate of about four centimeters every year. But at 86 00:05:35,880 --> 00:05:38,080 Speaker 1: the time when the young Moon could have captured a 87 00:05:38,160 --> 00:05:40,560 Speaker 1: sub moon of its own, it wouldn't have had the 88 00:05:40,600 --> 00:05:44,840 Speaker 1: distance it needed. Saturn's moon Iapodus might be the most 89 00:05:44,880 --> 00:05:48,279 Speaker 1: interesting case. The most notable feature on the surface of 90 00:05:48,320 --> 00:05:53,280 Speaker 1: Iapodus is its creepy equatorial ridge, a ring of mountains 91 00:05:53,279 --> 00:05:55,920 Speaker 1: wrapped like a belt around the mid section of the 92 00:05:56,120 --> 00:05:59,280 Speaker 1: icy moon. When I see these peaks, I imagine the 93 00:05:59,360 --> 00:06:03,560 Speaker 1: spine of some ice incrusted gigaresque monster poking up through 94 00:06:03,600 --> 00:06:07,520 Speaker 1: the skin of its back. One possible explanation among several 95 00:06:07,640 --> 00:06:11,240 Speaker 1: for the equatorial ridge of Iapetus today is that it 96 00:06:11,400 --> 00:06:15,400 Speaker 1: is the scattered flesh of an obliterated ancient sub moon. 97 00:06:16,200 --> 00:06:19,520 Speaker 1: In fact, this would be consistent with the hypothesis argued 98 00:06:19,560 --> 00:06:23,920 Speaker 1: by Andrew J. Dombard, Andrew F. Chang, William B. McKinnon, 99 00:06:23,960 --> 00:06:27,400 Speaker 1: and Jonathan P. K in a paper published in the 100 00:06:27,440 --> 00:06:31,640 Speaker 1: Journal of Geophysical Research Planets. The authors of this paper 101 00:06:31,720 --> 00:06:35,000 Speaker 1: right quote. We expand upon our previous proposal that the 102 00:06:35,080 --> 00:06:39,360 Speaker 1: ridge ultimately formed from an ancient giant impact that produced 103 00:06:39,360 --> 00:06:43,320 Speaker 1: a sub satellite around Diapotus. The orbit of this sub 104 00:06:43,400 --> 00:06:47,400 Speaker 1: satellite would then decay once Diapotus itself had de spun 105 00:06:47,520 --> 00:06:51,440 Speaker 1: due to tides raised by Saturn, until tidal forces from 106 00:06:51,440 --> 00:06:56,520 Speaker 1: Iapotus tore the sub satellite apart. The resultant debris formed 107 00:06:56,520 --> 00:07:00,880 Speaker 1: a transient ring around Diapotus, the material of which rained 108 00:07:00,960 --> 00:07:04,280 Speaker 1: down on the surface to build the ridge. And I 109 00:07:04,320 --> 00:07:06,599 Speaker 1: love that image of the ring falling down to the 110 00:07:06,600 --> 00:07:10,000 Speaker 1: surface of the Moon. But finally I wanted to ask 111 00:07:10,240 --> 00:07:13,360 Speaker 1: to continue the logic we started with, can a sub 112 00:07:13,400 --> 00:07:19,040 Speaker 1: moon have its own sub submoon? Actually yes, but as 113 00:07:19,040 --> 00:07:23,120 Speaker 1: you might guess, each orbital level you descend, the maximum 114 00:07:23,200 --> 00:07:27,160 Speaker 1: size of the object goes down. And given the size 115 00:07:27,160 --> 00:07:30,160 Speaker 1: of the planets in our Solar system, and thus the 116 00:07:30,200 --> 00:07:33,200 Speaker 1: maximum size of moons that can orbit them, and thus 117 00:07:33,240 --> 00:07:36,440 Speaker 1: the maximum size of submoons that can orbit the moons, 118 00:07:36,800 --> 00:07:39,040 Speaker 1: it's not likely that our solar system could have a 119 00:07:39,080 --> 00:07:43,480 Speaker 1: subsubmon bigger than about a kilometer wide, but I take it. 120 00:07:48,720 --> 00:07:52,080 Speaker 1: Tune into new editions of the Artifact every Wednesday, hosted 121 00:07:52,120 --> 00:07:55,160 Speaker 1: either by Robert or by myself. As always, you can 122 00:07:55,240 --> 00:07:58,679 Speaker 1: email us at contact at stuff to Blow your Mind 123 00:07:58,920 --> 00:08:09,120 Speaker 1: dot com. Stuff to Blow Your Mind is a production 124 00:08:09,200 --> 00:08:11,920 Speaker 1: of I Heart Radio. 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