First - our own Moon is drifting outwards, yes. That happens if a moon orbits its planet slower than the planet's spin rate. While if the moon orbits at a rate faster than the spin rate, it spirals inwards and eventually hits the planet. Charon is the only moon that orbits at exactly Pluto's spin rate, because both are tidally locked, so there are no long term tidal effects on Charon from Pluto.
Though the Moon spirals outwards, the Earth's spin rate is also slowing down due to the same tidal effects. Eventually the two will be tidally locked like Charon and Pluto. After that, the tidal effects of the sun are dominant, and the question then is - what happens after that? Some think the Moon would start to spiral inwards, both Moon and Earth speed up, remaining tidally locked together until over an immense period of time the Moon will hit the Earth. That is if the Moon and Earth survive the red giant phase of the sun which I think nobody knows quite at present...
So anyway to answer your question, yes there are a few moons that are so close to their planet that their orbital period is faster than the planet's spin rate.
Phobos's orbital period is 8 hours, and Mars's rotation period is 24 hours, 37 minutes and 22 seconds. So Phobos is spiraling inwards. Deimos with its orbital period of 30 hours is spiraling outwards.
Jupiter's rotation period is 9 hours, 55 minutes and its innermost two moons Metis and Adrastea orbit in less than a Jupiter day so are spiraling inwards.
Wikipedia has a comprehensive list: Tidal acceleration
Then Neptune's Triton is in a retrograde orbit, so any tidal effects cause it to spiral inwards. That's true of any moon in a retrograde orbit - orbiting in opposite direction to the spin of its parent planet - but most are so distant from their planet that the effect is negligible.
Several moons of Saturn are within the Roche limit for fluid bodies, though not for solid bodies including Pan (moon) has an orbital period of a bit over 13 hours. But they are too far away to spiral inwards because Saturn's rotation period is about eleven and a half hours Length of Saturn's Day Measured Like Never Before
So it's a separate question if a moon is within the Roche limit for fluid or solid bodies, and whether the moon is so close it spirals inwards rather than outwards. If the planet is massive, and rotating rapidly enough it can have moons within the Roche limit that are spiraling outwards due to tidal effects. For list of examples of moons within the roche limit, see Roche limit
Though the Moon spirals outwards, the Earth's spin rate is also slowing down due to the same tidal effects. Eventually the two will be tidally locked like Charon and Pluto. After that, the tidal effects of the sun are dominant, and the question then is - what happens after that? Some think the Moon would start to spiral inwards, both Moon and Earth speed up, remaining tidally locked together until over an immense period of time the Moon will hit the Earth. That is if the Moon and Earth survive the red giant phase of the sun which I think nobody knows quite at present...
So anyway to answer your question, yes there are a few moons that are so close to their planet that their orbital period is faster than the planet's spin rate.
Phobos's orbital period is 8 hours, and Mars's rotation period is 24 hours, 37 minutes and 22 seconds. So Phobos is spiraling inwards. Deimos with its orbital period of 30 hours is spiraling outwards.
Jupiter's rotation period is 9 hours, 55 minutes and its innermost two moons Metis and Adrastea orbit in less than a Jupiter day so are spiraling inwards.
Wikipedia has a comprehensive list: Tidal acceleration
- Around Mars: Phobos
- Around Jupiter: Metis and Adrastea
- Around Saturn: none, except for the ring particles (like Jupiter, Saturn is a very rapid rotator but has no satellites close enough)
- Around Uranus: Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Cupid, Belinda, and Perdita
- Around Neptune: Naiad, Thalassa, Despina, Galatea and Larissa
Then Neptune's Triton is in a retrograde orbit, so any tidal effects cause it to spiral inwards. That's true of any moon in a retrograde orbit - orbiting in opposite direction to the spin of its parent planet - but most are so distant from their planet that the effect is negligible.
Several moons of Saturn are within the Roche limit for fluid bodies, though not for solid bodies including Pan (moon) has an orbital period of a bit over 13 hours. But they are too far away to spiral inwards because Saturn's rotation period is about eleven and a half hours Length of Saturn's Day Measured Like Never Before
So it's a separate question if a moon is within the Roche limit for fluid or solid bodies, and whether the moon is so close it spirals inwards rather than outwards. If the planet is massive, and rotating rapidly enough it can have moons within the Roche limit that are spiraling outwards due to tidal effects. For list of examples of moons within the roche limit, see Roche limit
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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