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RedHerringxx t1_ixq40xr wrote

All planets are slowly ripping their associated moons apart.

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Reniconix t1_ixqf7rv wrote

Ours isn't. And Mars is only ripping one of 2 apart.

Moons only get ripped apart if they orbit faster than the rotation of their host planet. This is because tidal forces are working to rob them of energy, further decreasing their already short distances from their host planet.

This list is Phobos (Mars), Metis and Andrastea (Jupiter), Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Cupid, Belinda, Perdita, Puck, and Mab (Uranus), and Naiad, Thalassa, Despina, Galatea, Larissa, and Hippocamp (Neptune). All of these moons are listed from nearest to furthest to their host.

All others are safe, as they are robbing energy from their host planet, speeding up and drifting away until they eventually slow their host rotation to tidally lock with them.

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Exploding_Antelope t1_ixqlniv wrote

Isn’t our moon already tidally locked?

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Reniconix t1_ixqlqd9 wrote

It is, but I'm talking about the opposite: Earth becoming tidally locked to the Moon, etc.

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Exploding_Antelope t1_ixqm9qw wrote

Got it. That got me wondering, do we know how long that’ll take for Earth? And what I read is that by the time it happens, the sun will mostly likely have expanded anyway, which would obviously have much bigger effects on the whole system.

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Reniconix t1_ixqoftb wrote

The sun will engulf the earth before this happens.

The sun expanding will actually not change anything past Earth's orbit. The center of gravity will be unchanged, thus expansion alone cannot affect orbits.

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peterabbit456 OP t1_ixriwbj wrote

No, there is something called the Roche limit.

Moons inside that limiting distance will be ripped apart. I think only Saturn and Mars has moons inside the limit. You can see what happened at Saturn.

(Edit: There is a much better response about this, higher up. If I read it right, Uranus and Jupiter are also ripping moons apart.)

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