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Thoughts on the Composition of Gas Giant Planets: The Ooze Zone

by Roger Bourke White Jr., copyright Jan 2007


Little is known about the composition of the interior of gas giant planets. What follows is my speculation about those interiors, based on basic physics and the few things we do know with fair certainty. This analysis is the basis for my story “Pressure Point” in the book The Honeycomb Comet.

The Ooze Zone, where Gas Giant Gas meets Gas Giant Solid

The gas giant planets of the solar system -- Jupiter, Saturn, Uranus, and Neptune -- are the biggest planets, and they all share having a thick layer of atmosphere surrounding a rocky core that is roughly the size of Mars. The differences between them are differences of how much gas surrounds the Mars-size core and distance from the sun. Compared to the distant twin planets of Neptune and Uranus, Jupiter is big and close to the sun, and Saturn is in-between in size and distance. What makes the gas giants big, compared to the inner planets, is their thick atmospheres, and this thickness has other interesting effects, as well.
One of those other interesting effects is that the planet's "surface" -- the place where it turns from gas into solid -- is not located at a chemical composition boundary. On the rocky planets, the surface occurs where the chemical composition of the planetary “layer” changes from primarily nitrogen (in the case of Earth) or carbon dioxide (in the case of Venus and Mars) or vacuum (in the case of Mercury) into a silicon and aluminum oxide layer (better known as rock). To put it another way: On the inner planets, "above the surface" is air, and "below the surface" is rock.
On the gas giants, this is not true. On gas giants, above the surface is atmospheric gas acting as a gas does, and below the surface is atmospheric gas squeezed so hard it is acting as a solid does. If this is true, then there is likely to be a layer between the purely air layer and the purely solid layer that is a mixture of both. I am calling that layer the Ooze Zone.

Characteristics of the Ooze Zone

The Ooze Zone is likely to be a thick layer. The chemical composition in the layer will be pretty much the same from top to bottom, but the pressure change is enough to change the mix from acting like air to acting like solid. Above this Ooze Zone transition layer, the "air" of all the gas giants is a mix of hydrogen, helium, methane, water, and ammonia, and below the transition layer, the "rock" is roughly the same mix -- just at a high enough pressure to make it act like a solid.
Between the fluid "air" layer and the "solid" bedrock layer is the "ooze" layer. This ooze layer exists because this mix of hydrogen, helium, methane, water, and ammonia is likely to be somewhat thixotropic in its solid state -- it acts more solid when it stays at rest, and becomes more fluid the more it is sheared, forced to move around a bit. Earthly examples of thixotropic materials are sticky mud on a path and modeling clay in a person's hands -- the more you push them around, the softer they get. Turbulence, convection currents, and coriolis effects move this semi-solid atmosphere around in the gas giant. The result is a layer of atmosphere which is a mix of oozy mud and solid boulders. (Coriolis effect is what causes hurricanes to swirl in circles. Look it up if you’d like more details.)
The rock solid air and the muddy air are both the same material, so the air changes between these states constantly –- one moment it is acting gas-like, and another it is acting rock-like, then back. In the upper part of this layer, the air is mostly mud-like. In the lower part it is mostly boulder-like.
The layer below the Ooze Zone is too pressurized to move, and in this essay I will call it the Bedrock Zone. This is not a sharp transition -- the upper layers of bedrock can move, and vigorous cracks from the Ooze Zone routinely bring some motion to the upper layer of bedrock. The big difference between bedrock and the ooze layer above is that the cracks don't happen often, and they "heal" back to solid state quickly.
Likewise, above the Ooze Zone, the atmosphere is always in motion. There are only occasional places where the air is still for long enough to harden into lumps, and these quiet spots are small in size. The Ooze Zone is somewhat arbitrarily defined as the layer at which the lumps are routinely in contact with other lumps, so the motion of the mix becomes more like that of mud than that of air with snow in it. In this upper part of the ooze layer, wind slows down considerably compared to the layers above the ooze because of the friction that comes from blowing around solid chunks of ooze.
The top of the ooze zone acts like a perpetual fall day, where there are constantly leaves floating around, and they are endlessly shuffling in the breeze. Above the Ooze Zone, the wind blows vigorously, but the friction of moving "leaves" around in the Ooze Zone slows the wind down a lot. And the leaves are transitory, if you were to take any particular leaf in your hand and squeeze it, the squeezing motion would transform it back into air, and it would disappear. Conversely, if you were to cup your hands for a few seconds, the calm air inside would congeal into a leaf.
At the bottom of the Ooze Zone, the layer is mostly rock-solid, and the scale of motion changes from leaf size to plate tectonic size. The motion at the bottom is that of continental-size plates grinding against each other as coriolis force spins these huge blocks round and round horizontally. Between the huge grinding plates are huge boulders, acting like crude ball bearings, and cracks of liquid, acting like crude lubricating oil. The oil and boulders are warmed by the friction, so they try to rise. The huge blocks are a bit cooler, so they try to settle. But these blocks of the lower Ooze Zone are not strong enough to avoid being moved, so even as they settle, they continue to grind. (The blocks below the Ooze Zone, those in the Bedrock Zone, are glued together strongly enough to resist the coriolis shear effect, and that ability to hold still defines the top of the Bedrock Zone.)

The veining of the Ooze Zone

This grinding and churning of the Ooze Zone has been going on since the gas giant planet was formed, so it has been going on for billions of years now. This churning is primarily a mixing process, but it is also a melting and refreezing process, which means it is also a distilling process -- the melting and refreezing will try to separate out the ooze's constituent minerals into distinct groups. This separating tendency is counteracted by the general mixing of the grinding, turbulence, and convection, but veins of different materials will be created, and those veins created in the lower part of the zone will survive for long periods when they become part of a quiet area.
Because of this veining -- this concentrating of trace materials -- the Ooze Zone is likely to contain a gas giant's most interesting materials –- interesting, in both the scientific and commercial sense. When mankind figures out how to explore the atmospheres of the gas giants, the Ooze Zone will be one of the most exciting places to explore.


Is there an Ooze Zone in the gas giant planets?
I don't know; this is speculation.
But since gas giants are believed to have cores of solid hydrogen, and they are known to have normal gas-like gases composed mostly of hydrogen in their outer atmospheres, there has to be some kind of transition between these two states. I think an Ooze Zone of the nature I have described is very likely. And if it does exist, it will be the most interesting part of the planet for science, commerce and science fiction stories.

-- The End --

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