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Could a planet have a moon that itself had a smaller moon?
A former subsatellite would help explain some of the mysteries of Iapetus, one of Saturn's moons. For starters, Iapetus is not a sphere—it's a bit squished. And its flattened shape implies that Iapetus once spun very quickly, completing a rotation in 16 hours. It now takes 79 days. So what put on the brakes?
Maybe it was a onetime moonlet of Iapetus, explained Kevin Walsh of the Southwest Research Institute in Boulder, Colorado, at a recent planetary science meeting in France. The subsatellite would have spiraled slowly away from Iapetus until Saturn grabbed it.
But not before its outward drift sapped rotational energy from Iapetus and slowed it down.
The moonlet could account for another feature of Iapetus, too. The moon has a tall ridge running along its equator, like a walnut's seam.
If the short-lived moonlet emerged from a debris disk, as Earth's moon did, the moonlet could have forced leftover debris onto Iapetus to form the walnut ridge.
The moonlet idea is still preliminary. But solving two mysteries with one hypothesis means that it's not so nutty.
—John Matson
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5 Comments
Add CommentSo the moonlet orbits Iapetus, which orbits Saturn, which orbits the Sun, which orbits the Galatic core, which orbits around the local cluster. Nice.
Reply | Report Abuse | Link to thisKinda makes your head spin, doesn't it? Good informative, but too short article.
Reply | Report Abuse | Link to thisJeesch! First we got dozens of wild animals loose in Ohio, now we've lost a moon.
Reply | Report Abuse | Link to thisWhat's next? A war in Burundi?
Oh, never mind.
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The hypothesis that Iapetus once had a smaller satellite sounds reasonable to me, considering its shape plus the rotational momentum conservation law as the supposed satellite withdraws. However, in the case of the Earth-Moon system the tidal drag of the Moon's gravity on the oceans as Earth rotates beneath, added to tidal friction near continental margins is the cause of Earth's decreasing rotational period. This decrease in turn results in loss of kinetic energy by Earth. This energy must be "taken up" by the Moon to maintain a gravitationally-balanced state. As energy is transferred to the Moon, the Moon accelerates in its orbit and in consequence recedes from Earth. SO: Since Iapetus is too small to have oceans, just what caused the recession and ultimate loss of its hypothetical satellite? Also, are there other small satellites in the Solar System that suggest a similar past event?
Reply | Report Abuse | Link to thisMoonlet orbited Iapetus, Iapetus orbits Saturn, Saturn orbits Sun, Sun orbits Milky way's center which is a massive Black Hole,Milky Way(Galaxy) in turn orbits the center of the Local Group(Cluster),Local Group orbits the center of mass of Milky Way Galaxy and Andromeda Galaxy, Local Group Super Cluster comprising of Virgo Cluster and Local Group Cluster orbits around Virgo Cluster and thus it goes ad infinitum.This is the fractal view or the Primary-centric view of the Universe. Universe looks like a cosmic web.
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