Why are the natural satelites (moons) of all planets - including the moons of the gas giants - solid or rocky, and not gaseous?

  • $\begingroup$ What would you consider Saturn's rings? A moon that never formed, a billion tiny moons? A solid? A billion tiny solids? Composed mainly of water ice, are they solid or frozen gas? $\endgroup$ – Marc Oct 2 '14 at 4:07

To answer this, we have to consider the definition of an atmosphere. A popular way of looking at it is to think of an atmosphere as a layer of gases surrounding a body. by that definition, we can say that gas giants are really just planets that are massive enough to accrete substantially large atmospheres (because deep down, they have a rocky core - although not a core similar to the "rocky" inner planets in our solar system). So now we can reduce the question to "Why do most natural satellites not have extremely large atmospheres?"

This comes down to mass. The more massive an object is, the more gravitational pull it has on objects around it. This means that a very massive object traveling through a region of gas would attract the gas more than a less massive object would. In the early Solar System, the more massive bodies far enough out in the protoplanetary disk managed to gain a substantial amount of matter via core accretion, as gas and dust were collected by a rocky core. Moons are unable to do this because they are very low-mass; therefore, they cannot become gas giants (in fact, many moons around giant planets may be the remains of the material the planets never accreted).

Gas dwarfs, low-mass gas planets with small envelopes, are certainly possible. However, they would have to have radii at least 1.5-2 times the radius of the Earth, and masses scaled appropriately. Moons are nowhere near that size.

  • $\begingroup$ So does cores of Gas Giants in Solar System have more Mass than the mass mass of Earth? I mean to hold all the gases in atmosphere will require stonger gravitational pull and hence more mass, as the atmosperic pressure of gas giants is greater than that of Earth. $\endgroup$ – CrownedEagle Sep 21 '14 at 17:57
  • $\begingroup$ We don't know much about the cores of the gas giants (estimates for Jupiter's range from 12 to 45 Earth masses), but the masses of the core and mantle for all four gas giants are each at least about 9 times the mass of Earth, with some being higher. $\endgroup$ – HDE 226868 Sep 21 '14 at 18:02
  • $\begingroup$ Does the Sun also have a rocky core? $\endgroup$ – Scottie Sep 21 '14 at 20:56
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    $\begingroup$ A molten rocky core is still a rocky core, right? The Earths core is molten, and we consider it rocky, don't we? $\endgroup$ – Scottie Sep 21 '14 at 21:00
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    $\begingroup$ @Pere That has to do with formation. The reason giant planets are able to form is that there are enough volatiles ("ices") where they form (beyond the frost line). Nothing inside that is cold enough for volatiles to form; you couldn't have any significantly large core. Besides, stars form differently than planets; they come directly from the collapse of a protostellar cloud, rather than a protoplanetary disk. $\endgroup$ – HDE 226868 Nov 27 '16 at 23:48

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