Why does the Earth not have rings like Saturn or Uranus?
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2 Answers
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A moon is held together by its own gravity, and pulled apart by the tidal action of a planet. If a moon comes too close to a planet it will be ripped apart by the planet's gravity and become a ring. The closest a moon can come to a planet is known as the Roche limit, and it is dependent on the mass and density of the planet and moon.
A large planet, such as as Saturn, has a large Roche limit, and has collected many moons over its history, some of which have been ripped apart to make rings.
The Roche limit for the Earth-moon system is at a radius of about 10000 km, which is very close to the surface of the earth (about 6400km). And unlike the gas giants, terrestrial planets don't have many moons. If any rings did form at one time (such as from the matter resulting from the "giant impact" that formed the moon), they would be short-lived features. They would be cleared from orbit in a few thousand years by perturbations and drag from the outer parts of the atmosphere.
So with few moons, and small Roche limits, the opportunities for ring formation are rare. That said, Phobos, one of the satellites of Mars, is now so close to Mars, that it is held together as much by its rigidity as by gravity, and is slowly getting closer to Mars. Within 50 million years it will be ripped apart and form a (small) ring around Mars
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5$\begingroup$ Just thought I'd add that when you say "very close to the surface of the earth" we're still talking about a distance that is beyond Low Earth Orbit (LEO)--it's just very close to Earth's surface compared to the distance of the Moon. $\endgroup$– called2voyage ♦Commented Sep 18, 2015 at 21:09
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$\begingroup$ 1) Another interesting question here (that I should be able to find out with a little research) is why Jupiter doesn't have more massive rings than Saturn (large magnetic field?). 2) I propose that if mankind makes it through the current global warming/ over population/ etc. crisis that there may be rings around the earth in another millennia. $\endgroup$ Commented Dec 1, 2015 at 1:00
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$\begingroup$ @JackR.Woods Rings form under specific circumstances. Saturn's outer E ring seems to be fed by enormous jets and volcanism from Enceladus but that doesn't explain the inner rings. A disintegrated moon is perhaps likely what formed the more densely concentrated inner rings. If Io drifts inside Jupiter's Roche limit, it could break apart and give Jupiter glorious rings, even bigger than Saturns, but I have no time-table on when or if that might happen. The 4 moon system in orbital resonance might be long-term stable. $\endgroup$– userLTKCommented Mar 1, 2016 at 0:17
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$\begingroup$ @James K . Prometheus, moon of saturn have orbit inside F-ring of saturn, so why isnt it ripped off and converted to ring? $\endgroup$ Commented Sep 20, 2016 at 9:04
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Earth does have rings! Not icy ones like Saturn, but ones made up mostly of solid masses a few meters across. A relatively sharp ring is centred on geostationary orbit. A rather more diffuse one (although probably with higher total mass) reaches up to altitudes of a few hundred km from the surface. The second one is constantly losing mass back to the planet due to friction from the atmosphere, but is replenished by a complex biological process.
answered Feb 24, 2018 at 17:48
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$\begingroup$ I see what you did there.:) And I guess Jack Woods may have also been alluding to artificial rings. $\endgroup$– PM 2RingCommented Feb 25, 2018 at 13:20
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6$\begingroup$ -1 for talking in vague terms. A 'beginner' in astronomy wouldn't understand this. $\endgroup$– user1569Commented Feb 25, 2018 at 15:36