3 ms·

if planets are effectively amorphous liquids with no surface tensions at planetary scales, then why would you see areas on the surface that experience different

if planets are effectively amorphous liquids with no surface tensions at planetary scales, then why would you see areas on the surface that experience different amounts of gravitation. The rock of the planet experiences that gravitation all the same as the surface of the rock, mass experiencing low gravitation must equilibrate with mass experiencing high gravitation. So what stops mass experiencing 0.65 units of gravity from displacing mass experiencing 0.3 units? You would think this would result in somewhat consistent gravity around the planet. This result is unexpected to me.

AlanSE 13y ago
I'm not sure if I understand entirely what you're asking, but I think it comes down to "planetary differentiation". It's the same process by which water floats on top of oil because of density. The heavy stuff floats to the bottom and the light stuff floats to the top. That process isn't 100% complete obviously, but that's because near the surface material strength matters a great deal. We still have mountains on Earth, which are clearly not behaving like a liquid. If it was behaving like a liquid, then you're right that the heavier parts would upturn the lighter parts and sink. But Earth still has density variations because the process isn't complete, and tectonic movement keeps rearranging it all.
- dredmorbius 13y ago
  Friction and timescales also matter. The Earth is highly plastic ... on geological timescales. The continents move, the Himalaya are rising at the rate of about 5 mm per year, and one theory of the post-Messinian flooding of the Mediterranean basin (https://en.wikipedia.org/wiki/Post-Messinian_flood_of_the_Mediterranean https://en.wikipedia.org/wiki/Post-Messinian_flood_of_the_Me...) is that a fragment of one of the colliding plates in the region broke off and sank into the lithosphere. The whole system is dynamic, and driven by both latent gravitational heat of formation and by radioactive decay (including postulated naturally occurring fission reactors within the outer core / inner mantel: http://www.nature.com/news/2008/080515/full/news.2008.822.html http://www.nature.com/news/2008/080515/full/news.2008.822.ht... -- a fission reactor near the surface was found in Gabon, within Africa, active about 1.7 billion years ago: https://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor https://en.wikipedia.org/wiki/Natural_nuclear_fission_reacto...). But it's an interesting rock we crawl about.
gliese1337 13y ago
Because "magnitude of the gravitational force" =/= "gravitational potential" and potential is what matters- the shape is stable if the potential is equal across the entire surface. This corresponds to the gravitational force vector always being perpendicular to the surface (i.e., gravity always points 'down'). As long as the gravitational force vector is perpendicular to the surface, nothing will get pulled sideways, therefore nothing will move, therefore the shape is statically stable, and the magnitude of the perpendicular force can vary as much as you like. Now, you may be thinking "but shouldn't the bits of planet-fluid under stronger gravity sink and push on the fluid around them and thus indirectly cause stuff to move around to the sides?" This is where equal potential really becomes important. For one bit of the planet to sink (losing potential energy), another bit of the planet would have to rise (gaining potential energy). If the shape of the planet currently conforms to an equipotential surface, any marginal redistribution of which bits are up and which bits are down will end up requiring at least as much total energy as the original configuration. Thus, the shape is a local optimum in the energy configuration space, and nothing will move.
- GhotiFish 13y ago
  > Now, you may be thinking "but shouldn't the bits of planet-fluid under stronger gravity sink and push on the fluid around them and thus indirectly cause stuff to move around to the sides?" that was what I was thinking. So the reason we see gradients is because areas that are experiencing low force have lots of mass, and areas experiencing high force have little mass. Looking at the diagrams, I guess I can see that is the case. Thanks for clearing that up!

AlanSE 13y ago

I'm not sure if I understand entirely what you're asking, but I think it comes down to "planetary differentiation". It's the same process by which water floats on top of oil because of density. The heavy stuff floats to the bottom and the light stuff floats to the top. That process isn't 100% complete obviously, but that's because near the surface material strength matters a great deal. We still have mountains on Earth, which are clearly not behaving like a liquid. If it was behaving like a liquid, then you're right that the heavier parts would upturn the lighter parts and sink. But Earth still has density variations because the process isn't complete, and tectonic movement keeps rearranging it all.

dredmorbius 13y ago
Friction and timescales also matter. The Earth is highly plastic ... on geological timescales. The continents move, the Himalaya are rising at the rate of about 5 mm per year, and one theory of the post-Messinian flooding of the Mediterranean basin (https://en.wikipedia.org/wiki/Post-Messinian_flood_of_the_Mediterranean https://en.wikipedia.org/wiki/Post-Messinian_flood_of_the_Me...) is that a fragment of one of the colliding plates in the region broke off and sank into the lithosphere. The whole system is dynamic, and driven by both latent gravitational heat of formation and by radioactive decay (including postulated naturally occurring fission reactors within the outer core / inner mantel: http://www.nature.com/news/2008/080515/full/news.2008.822.html http://www.nature.com/news/2008/080515/full/news.2008.822.ht... -- a fission reactor near the surface was found in Gabon, within Africa, active about 1.7 billion years ago: https://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor https://en.wikipedia.org/wiki/Natural_nuclear_fission_reacto...). But it's an interesting rock we crawl about.

gliese1337 13y ago

Because "magnitude of the gravitational force" =/= "gravitational potential" and potential is what matters- the shape is stable if the potential is equal across the entire surface. This corresponds to the gravitational force vector always being perpendicular to the surface (i.e., gravity always points 'down'). As long as the gravitational force vector is perpendicular to the surface, nothing will get pulled sideways, therefore nothing will move, therefore the shape is statically stable, and the magnitude of the perpendicular force can vary as much as you like. Now, you may be thinking "but shouldn't the bits of planet-fluid under stronger gravity sink and push on the fluid around them and thus indirectly cause stuff to move around to the sides?" This is where equal potential really becomes important. For one bit of the planet to sink (losing potential energy), another bit of the planet would have to rise (gaining potential energy). If the shape of the planet currently conforms to an equipotential surface, any marginal redistribution of which bits are up and which bits are down will end up requiring at least as much total energy as the original configuration. Thus, the shape is a local optimum in the energy configuration space, and nothing will move.

GhotiFish 13y ago
> Now, you may be thinking "but shouldn't the bits of planet-fluid under stronger gravity sink and push on the fluid around them and thus indirectly cause stuff to move around to the sides?" that was what I was thinking. So the reason we see gradients is because areas that are experiencing low force have lots of mass, and areas experiencing high force have little mass. Looking at the diagrams, I guess I can see that is the case. Thanks for clearing that up!