jvriesem

We still don't know a whole lot about what the interiors are like, but with more missions like Cassini and Juno, we'll be getting a much better picture.

Let's travel from the outside inward. The planets have the same layers as on Earth, though their natures vary.

The outermost layer of the atmosphere is the thermosphere (well, technically the exosphere, but I consider that to be effectively space). It's called the **thermo**sphere because temperature increases a lot in this region. Most people would call this a total vacuum, but as scientists, we know that it's close to a total vacuum, but there's still plenty going on. We have very little idea what the winds are like here. This is where we see aurora. There are significantly less types of chemicals to keep track of, which is nice for us. ;-)

Near the bottom of the thermosphere is the ionosphere, which is where a lot of interesting chemistry happens. There, radiation from the Sun breaks apart molecules and frees electrons from atoms — a process known as "photoionization". The result is a partial plasma layer. On Earth, this is partially responsible for reflecting certain radio communications back down towards Earth (more info). This is an important layer because it helps mediate the atmosphere below and the magnetosphere above. Powerful electrical currents come back and forth from the magnetosphere, and they interact with the atmosphere here.

The mesosphere and stratospheres are the next two layers. (Though, evidently Jupiter lacks a mesosphere!) There, temperatures typically decrease with height. Things called gravity waves and planetary waves often play a significant role.

The troposphere is where the cloud deck is. Lots of stuff happening there! Finally, the atmosphere is thick enough that we can it! We can infer wind speeds by tracking clouds!

Now we're getting past the traditional atmospheric layers. The pressure is getting too much for us. Lots of clouds everywhere. I wonder what it would look like! The Galileo atmospheric probe only got to 132 km below the 1 bar level, so we've never *directly* probed below this. What we know is based on indirect measurements.

We expect that dynamics will play a big role. Other measurements suggest that the wind systems (jet streams) we see from the clouds extend much deeper into the atmosphere — likely thousands of kilometers (source)! There's also a lot of different chemical species and tons of different kinds of chemical reactions going on here. It's a really complex place.

You've probably heard of exotic things down below, like weird chemicals "raining" down. Recent evidence suggests helium rain might fall in Jupiter (more info). The pressure here is mind-boggling: many millions of times higher than Earth's surface pressure.

**Is there a" surface"?** It's a complicated question.

At deeper levels and higher pressures, the pressure pushes gas molecules so close together that it becomes a "supercritical fluid", with no distinction between the liquid and gaseous phases. Different chemicals experience these sorts of phase changes at different levels, so it'd be a really unusual mix of some things being solid, some things being liquid, and some things being gaseous — possibly like the worst smog you can imagine in the middle of a blizzard (where the solid stuff is the snowflakes; except a really hot blizzard, and not snow).

The "mantles" are deeper than we know, but our models can help give some good guesses, based on other indirect observations we've made. The exact composition depends on the planet. Jupiter and Saturn have large layers of metallic hydrogen surrounded by a thinner liquid hydrogen layer. We believe Uranus and Neptune have more water, methane and ammonia.

We do expect that the cores contain some mixture of "rock" and metal. This makes sense: rock and metal are pretty dense, and would sink to the bottom. At this point, perhaps there'd be a solid core? AFAIK, this is still poorly understood.

More great info is available here: https://lasp.colorado.edu/outerplanets/giantplanets_interiors.php.

Saturn's magnetic field is — as far as we have been able to see — *perfectly* aligned with its rotation axis. All of the other planets have their magnetic field at an angle (or even offset from the center!). Uranus and Neptune are especially wonky (image). Dynamo theory — which represents our understanding of how rotating planets generate planetary magnetic fields through the rotation and convection of their fluid interiors — says that this shouldn't happen for Saturn. Why it is so closely aligned is a big mystery!

Another thing! We see a lot of matter in galaxies, but the galaxies behave as if they have a lot more material than we see. This is borne out from several different types of experiments. We infer that there must be a lot more material there than we can see, but since we can't see it, we call it "dark matter". We still have no idea what it is!

That's a neat question! I don't know if I'd call it a failed binary star...it's a bit too small for that. But that's an insightful way of putting it!

In planetary science, we actually use Jupiter as a standard of mass. The mass of Jupiter is...**drumroll**..."*one Jupiter mass*" 😂. The minimum mass an object needs to be to begin fusing hydrogen in its core — and therefore be considered a star — is about 80–90 Jupiter masses.

However, something with 13 Jupiter masses can be a brown dwarf!

More info here.

I did! I expected maybe a dozen or so comments, but not 136!

EDIT: over 200....

I would LOVE to read more! Unfortunately, I'm a pretty slow reader, so I typically read only one or two books per year. I'm also an avid gamer, so that limits things, too. ;-)

Right now, I'm reading Lord Kalvan of Otherwhen, by H. Beam Piper. It was given to me by a dear friend.

Favorite? Hmm..back when I read a lot more as a teen, I was all over Star Wars books. The Han Solo trilogy was one of my favorites. I'm still bear a grudge against Disney for summarily dismissing the established canon. I think one of my other favorites was The Martian Chronicles, by Ray Bradbury. Again, I wish I had time to read so much more!!

Favorite fiction book overall, however, is definitely The Lord of the Rings. :-)