physics question #349

Sara, a female from the Internet asks on February 24, 1998,


What are the surfaces of Uranus and Neptune like? Is there wind or precipitation on either planet's surface? Is there water on them? What's the gravity of these two planets compared to Earth's?

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the answer

Donald J. Barry answered on February 24, 1998, A:

Understanding and visualizing the structure of the Gas Giant planets requires that we "rewire" our everyday conceptions of physics a bit. To let you know what one would experience as you descend into their atmospheres, we need to define exactly what we mean by the standard states of matter, "solid", "liquid", and "gas". Because as you'll see shortly, these words are inadequate to completely describe the environments you'll encounter as you go deep down inside them.

Normally we say a planet has a surface if it has a solid layer, covered either by liquid or gas. But our concept of "solid" is pretty much defined by the image of something you can push against and which won't change until you push hard enough to break it. Similarly, a "liquid" in our everyday conception is something that we can push against and go through, but which is itself incompressible (we can't squeeze a water balloon to make it smaller, as we can a balloon filled with air). A "gas", of course, is also something you can both push against and through, but which is also "squeezable" (compressible).

On a trip into the atmosphere of any of the giant planets, one would first encounter gas, starting very thin, and eventually reaching a pressure and temperature similar to that we breathe, except that it wouldn't be breathable because it's mostly hydrogen, helium, methane, and ammonia. On further descent, pressures and temperatures would increase; eventually we would find the atmosphere began to exhibit a substantial "thickness" (viscosity). Still lower, the compressibility of the atmosphere would begin to decrease and the thickness rise: if one could imagine the circumstances there, it would resemble an exceedingly hot, exceedingly dense taffy. Still further below, the atmosphere, primarily hydrogen, would somewhat suddenly become conductive, but still be thick, relatively incompressible (at this point), and viscous. Probably our picture of a "liquid" describes this state best, but it is arrived at quite gradually as one descended.

At the very central regions of any of these worlds, one would find the composition suddenly becoming enriched in heavy elements, but the pressure, thickness, and viscosity, already very high in all categories by this point, would not suddenly change, though the density would. Indeed, the only real "breaks" in any measures of the behavior of the structure surrounding you would be the density and conductivity of hydrogen at one point, and the composition and density very close to the center. Neither would represent a change between "solid", "liquid", and "gas", because at the extreme conditions deep inside these worlds, these words hardly apply.

High in the atmospheres of these worlds, where pressures and temperatures are similar to those at the surface of the Earth, water could exist as a liquid, if ever enough were brought together in one place and prevented from evaporating. Extrapolating from the measurements made two years ago by a probe which entered Jupiter's atmosphere, however, we can relatively safely say that the water that is present in the atmosphere of these worlds is far too spread out to exist in liquid form. There just isn't enough of it for worlds so large. High in their atmospheres, where temperatures are quite cold, clouds of ammonia ice do exist, somewhat like cirrus clouds here on Earth. The Galileo atmosphere probe continued falling into Jupiter's atmosphere until it reached temperatures sufficient to vaporize it a few hours after entry. The same would happen to any person unfortunate enough to try the same thing (though even if one could stop and levitate in a layer of the atmosphere, say with a blimp of heated hydrogen, a gravity of 2.6 times that of the Earth's surface is incompatible with human life, unless you were supported in a water bath and aided in respiration).

The gravities on any of these worlds are quite high compared to the Earth. On Jupiter, objects weigh about 2.6 times what they would on Earth. On Saturn, Uranus, and Neptune, the ratios are 1.1, 0.9, and 1.2 respectively.

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