Earth: The Ring World 28
An anonymous reader writes "Sandia Labs is reporting on the 100,000 years or so when the Earth might have had a debris ring like Saturn. They need the rings to help explain climatic shifts and after all, what happened to all those ejected rocks when the larger meteors hit the Earth?"
That would explain one for earth... (Score:2)
Re:That would explain one for earth... (Score:4, Informative)
The Roche limit. It has to do with tides, and tides have to do with orbits. The best way to understand how orbits work is to understand how they don't. Picture a solid rotating disc, like a record on a record player. Every point on the disc has the same angular velocity; in other words, it takes the same amount of time for a point on the inside of the disc to make a revolution as it takes for a point on the outside of the disc. This is obvious, because the disc is solid.
Orbits aren't like that. Two bodies orbiting at different distances from the parent body will have different angular velocities, because the force of gravity attracting them to the parent body varies with distance.
Now, think about what happens when an orbiting body speeds up or slows down. Think spaceman with a jetpack here. If the spaceman, who is orbiting the Earth in a circular path, slows down, he'll start to fall in toward the Earth. If he speeds up, he'll rise away from the Earth. If your velocity at a given distance from the parent body is lower than orbital velocity for that distance, you'll fall toward the body. If it's faster than orbital velocity, you'll tend to rise away from the body.
Now, consider the moon. The moon is a thick body; the distance from the center of the Earth to the near face of the moon is less than the distance to the far face of the moon. The moon orbits the Earth at a velocity appropriate for a body orbiting at the distance of the moon's center of mass. Because the moon is rigid, both the near half and the far half of the moon must also orbit at that same velocity, even though the near side of the moon tends to want to fall in toward the Earth (because its orbital velocity is slightly too low to maintain a circular path) and the far side tends to want to rise way from the Earth (because it's moving too fast).
The result, once you sum everything up, is that there's a net force on the moon pulling it apart along the line between the centers of the moon and the Earth. Because the total tensile strength of the moon-- its own gravity, plus the friction holding it together, plus the strength of the rock that forms it-- is greater than this net force, the moon is in a stable orbit.
But the tidal force has had an effect: long ago, the moon stopped revolving with respect to the Earth. The near face of the moon is held toward the Earth because it is "heavier" (sort of) than the far face of the moon. Tides did that.
Now, if you move the moon closer to the Earth, things will change. Because gravity varies with the square of the distance, the difference between the pull of gravity on the near side and the pull of gravity on the far side will increase as the moon gets closer to the Earth. As the moon moves closer, there's a point where the difference in gravity between near and far sides exceeds the aggregate tensile strength of the moon. At that point, the moon will fracture, and part of it will fall in toward the Earth while the rest rises into a higher orbit.
That distance is called the Roche limit.
Whew.
If you want a really good understanding of orbital motion and tidal forces, pick up a couple of science fiction books by Larry Niven. His Neutron Star includes a short story that uses the tidal force as a key plot point, as does The Integral Trees. No math, just lots of descriptions of how human-scale objects behave in close orbit around stellar-scale objects. You might have to read the stories a few times before you get the idea, but it's worth it.
Re:That would explain one for earth... (Score:3, Informative)
The Roche limit is technically the distance at which a body of zero tensile strength will tend to fall apart in orbit. Real objects, like moons, can be stable well inside the Roche limit of their parent body because their tensile strength is higher than zero. It's even possible, if the parent body is big enough and the satellite is small enough, that the practical Roche limit for a pair of bodies might be inside the parent body. In that case, the satellite would just spiral down to the parent body and crash before breaking up due to tides.
I think you probably got the idea anyway, but I just wanted to be a little more clear.
Re:That would explain one for earth... (Score:2)
Of course, it's a silly exercise. Ample evidence exists for the accretion theory of lunar formation, which would suggest that the moon as a solid body was formed outside the Roche limit of the Earth. So extrapolating back in time based on recent measurements of lunar motion is kind of like extrapolating your age relative to your little sister. "I'm ten but she's only five; I'm twice her age. So when she's 50 I'll be 100 years old!"
Re:That would explain one for earth... (Score:2)
Re:That would explain one for earth... (Score:2)
Slashdot should be like a nuclear missile silo. In order to post a comment, two ranking officers should have to insert their keys into boxes on opposite sides of the room and turn them at the same time.
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Re:That would explain one for earth... (Score:1)
http://www.talkorigins.org/faqs/moonrec.html
The current theory for the origin of the moon puts it as a collision between the proto-earth and a mars-sized planet about 4.5 Billion years ago.
Abiogenesis does not, as far as I know, require tides. Some early models did, but that was a while ago.
Re:That would explain one for earth... (Score:2)
Re:That would explain one for earth... (Score:2)
Re:That would explain one for earth... (Score:1)
Lets you placed a bomb at the core and exploded it. It would eject energy (hopefully evenly) that could sperator the parts. Gravity would tend to "pull" pieces back together.
IF the release was large enought and non-even - then a few parts could get enough energy to "escape", but may not escape all surrounding mattter (earth) and stay in an orbit around earth, hence a new moon(s).
IF the release was real big... maybe a new orbit around the sun.
IF so big that it is sent to random space... most like the earth will be gone too!
Now bury nuclear waste on the dark side and that goes cri... Damn - "Space 1999" already done.
Re:That would explain one for earth... (Score:2)
But how would you explain the rings around saturn or jupiter? If I am not mistaken there is no solid surface for a big rock to hit to be able to eject debris
Not saying this is the answer, only one possible answer: if a 'roid hits hard enough, it would likely eject some liquid and gaseous "debris", which would tend to collect in lanes, freeze, and coagulate. Voila! Rings.
So basically... (Score:3, Insightful)
I guess that's one way to get published.
Missing matter? (Score:1)
Re:Missing matter? (Score:1)
Re:Missing matter? (Score:2)
I'm no astrophysicist, though, so don't quote me on that.
Re:Missing matter? (Score:2)
Talking of rings... (Score:2)
that's not a Ringworld (Score:2, Insightful)