Re: Does the energy for Jupiter's weather come primarily from the sun?

You ask a very insightful question. First of all, you're exactly right that the intensity of the Sun's light should drop as one over the distance to the Sun squared. So Jupiter, at about 5.2 AU (where 1 AU is the Earth-Sun distance) gets about 1/27th the energy per unit area we get here on Earth. On Earth, we intercept about 1353 Watts of Sunlight per square meter. This energy is intercepted by the Earth's cross-section, pi r², but distributed over Earth's surface area, 4 pi r², so that the average solar energy that each square meter of Earth gets is about 338 Watts. (Of course, some areas, like the Arctic, receive less than the average energy. Others, like the tropics, receive more.) This compares with about 0.055 Watts per square meter leaking out from Earth's interior due to radioactive heating. So well more than 97% of the energy used to drive weather on Earth comes from the Sun. (The internal energy, however, is what causes plate tectonics and volcanic eruptions.)

So Jupiter should have some pretty weak storms. Saturn (at 10 AU) should have weaker still, and Uranus and Neptune almost none at all. This isn't what we see, however! What's wrong? The answer is that three of those four planets (Uranus is the odd-ball) radiate a lot more energy than they take in from the Sun. This was pretty surprising when it was first discovered by planetary scientists, but we think we can explain it. Interestingly, we have a different explanation for each of three planets.

• We think Jupiter is still contracting from it's initial formation. As material settles into a more compact state, gravitational energy gets turned into heat.
• Saturn might have a helium rain in it's atmosphere. As the rain falls, its gravitational energy turns into heat.
• Neptune's heat might be leftover from its formation.

So the jovian planets have enough heat to drive convection (vertical motion driven by hot gas rising abd cool gas descending). But how do the winds compare to Earth? The answer is that the winds are much faster, hundreds of kilometers per hour in some places. The reason that the winds are so fast is that the planets all rotate very quickly. East/West winds are created by the Coriolis force, which is stronger in faster rotation. The jovian planets all rotate in about 10-18 hours (Jupiter and Saturn have about 10-hour days, while Neptune and Uranus take nearer 18 hours to rotate.) This drives winds to much higher speeds, leading to stronger storms.

So the jovian planets can have much stronger weather than Earth can, despite the feeble sunlight in their part of the solar system. You can look up more details about each of the planets at The Nine Planets Website. If you want some more discussion, I recommend The New Solar System by J. Kelly Beatty (Editor), Carolyn Collins Petersen (Editor), Andrew Chaikin (Editor), Andrew L. Chaikin. I looked up my numbers in William Hartmann's book, Moons and Planets.