Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Refractor on 15/10/2010 10:56:17
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What causes the seemingly enormous variety in the moons around the gas giants? Titan has an atomoshere, Europa is covered in ice, Io is volcanic, etc. There's remarkable differences, not just in size but in composition, geography, etc. What caused this?
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The moons are very different because they have different quantities of stress associated with the different orbits that they follow round their primary planets. The large effect of the internal heating to t the moon caused by gravitational stresses in the moon was not anticipated until the moons were observed at close quarters.
Heat flows very slowly in most solids and any object big enough to pull itself gravitationally into a sphere if subjected to a varying gravitational field (tides) caused by an elliptical orbit or the presence of other moons will be subject to internal heating and this heat cannot escape easilty and warms the interior considerably.
Radioactive elements can have a big effect too it is estimated that most of the earth's internal heat is now due to the radioactive decay of uranium etc.
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OK, thanks for that. But how do gravitational stresses account for the big differences in composition?
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If a moon like Io gets very hot because of gravitational stresses it boils off its lighter elements notably water the other moons probably have similar compositions with plenty of water Ice. However the gravitational stress heating has driven differentiation since accretion in most of the moons but Europa,and Enceladus appear to have liquid water layers in them while Ganymede has a molten iron centre (magnetic field) and an active but probably solid surface. The outer moon Callisto appears to be undifferentiated and dead like our moon. Some of the moons of uranus and neptune display structure which indicates stresses and Triton may also be quite active.
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The planets formed from a rotating disc of material (http://en.wikipedia.org/wiki/Protoplanetary_disk). Gravity and centrifugal forces have sorted this material (http://en.wikipedia.org/wiki/Terrestrial_planet#Density_trends) resulting in high-density rock/metal inner planets and low density gas (giants) in the outer solar system.
A similar mechanism could be responsible for the differing properties of Jupiter’s moons.