[Note: Robin Bendetti is a scientist who recreates the thermodynamic conditions of planetary interioris using a diamond anvil cell. A diamond anvil cell is a machine used to create extremely high pressures. When heated with a laser, specimens inside the diamond anvil cell reach the temperatures and pressures which approach those of planetary cores {10 gigapascals, 2000 Kelvin).

Neptune and Uranus are composed largely of methane gas.

Under extremely high pressures and temperatures, methane gas transforms to diamond, hydrogen and other hydrocarbons.

This means the cores of Neptune and Uranus are most likely made of diamond.]

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Q: So what was it you actually saw in the lab?

Bendetti: What we saw was a transformation from methane to diamond, hydrogen, and some other hydrocarbons (with double and triple carbon bonds) at pressures above 10 GPa and temperatures around 2000 Kelvin.

Q: Can you compare 10-50 gigapascals to a pressure we can understand?

Bendetti: Let’s see. 10/50 gigapascals is 100-500 kilobar, which is 100,000-500,000 atmospheres of pressure. That’s not super meaningful to me, but it is a start — 1 atmosphere is what you feel all the time from the air around you.

Do you know how when you dive deep in a swimming pool you feel pressure in your ears? That’s about 0.0004 gigapascals.

Here’s another way. I converted to psi (pounds per square inch). 10 gigapascals is about 29 million psi. The pressure in a car tire is ~30 psi (I think) and in a skinny bicycle tire, it might be as high as 100 psi.

I’m not sure that any of these comparisons really give a sense of what a gigapascal feels like. I don’t think I know. The way that I have any intuition at all is really from watching material in a diamond anvil cell through a microscope… seeing how the pressure changes materials. Water, for example, freezes to ice at room temperature when you give it a pressure between .5 and 1 gigapascals. Coal turns to diamond at 7 gigapascals (I think), but you have to also make it hot. Basically, these pressures are so high, they don’t just make a gas move around, they don’t just deform shapes, they make the atoms in many solids completely rearrange themselves at a microscopic level.

Q: What do you think the planets look like? How do you think they’re structured?

Bendetti: Under the conditions inside Neptune, under the hydrogen-and-helium atmosphere, I have a mental picture that is much like an ocean, but a murky one. In the experiment I did, I saw that methane at high pressures is a clear solid, like ice; then when I heated it, it formed solid carbon (diamond), but what didn’t make solid diamond (or some other form of carbon) formed liquid. I’ve seen, too, with ammonia and water (though I haven’t published this yet), that when I laser heat them I am looking at liquids.

So really an ocean is the right metaphor… picture an ocean of methane, ammonia, and water, which are all clear fluids, much like water on the earth’s surface. They are similar in density, so they should be pretty well mixed, but the lighter one, methane, might be a little more represented at the top of the ocean, while there’s more water at the bottom.

Q: Is there debris floating in the liquid? Or diamond particles?

Bendetti: There are certainly other things mixed into the ocean. These experiments that I’ve done are about hydrocarbons, but I actually expect a whole host of other things. Some will be liquids, and mostly transparent, but some (like carbon) may be solids. There will almost be some amount of rocky and metal material there too, but most of the rocky/metal material in Neptune and Uranus may have settled to the bottom to create a core which is differentiated from the ocean. We cannot tell from observations of the planet whether this has happened. If it has, there would be some solid rocky material in the ocean. If it hasn’t, there would be lots of rocky material in the ocean. I imagine there is at least some core.

So the picture that I am seeing is really a lot like what you might see if you went to the beach and opened your eyes underwater. Mostly but not completely transparent liquid flowing around… with a lot of suspended particles (like sand)… which over short times flow with the water, but over long times settle to the bottom of the ocean.

The other difference is that the densities of everything are higher than the density of water on earth…. more than twice as dense… but still not really as dense as rock… but because it’s so hot the dynamics may not be that different.

Q: Would it be possible to build some sort of craft that could land on liquid ammonia?

Bendetti: I’m going to try to start with facts for this one and work up to an intelligent answer.

Firstly, I’m not sure there will actually be a surface to the ammonia-water- (and maybe some methane) ocean. The pressures and temperatures of the hydrogen atmosphere are so high, that the ammonia, water, and methane, and I think even the bottom of the hydrogen are all above the critical point. What that means is that while there is still gas and liquid, the difference is not going to be obvious, not like the earth’s ocean. So it is possible that as you go down into the planet, it might not have a real distinct surface, where it goes from being in the atmosphere to being in the liquid. I don’t really know just how to describe what it will be like, it will just get denser and denser, but I don’t have a very good physical picture of that process myself.

Q: Perhaps some sort of foam base would solve our problem…

Bendetti: Maybe the right understanding to take away from my experiments is not “Neptune is a huge hunk of diamond” but something closer to “giant planets likely have diamond in them, possibly a lot of it, but they are also extraordinarily complex inside, with the actual materials subject to a lot of physics and chemistry.”

Q: You know who we should ask? NASA.