Hey, you, who's reading this in the future: you can find out who I am today at https://rarf.zone/about/. πŸ’™


E-mail me at:
info@rarf.zone
Mastodon/Fediverse
yiff.life/@katja

ireneista
@ireneista

we're not even going to list off every astrophysical problem with this


polymath
@polymath

because I did a whole bit on this once so I already have notes. much of it is fairly obvious but I ran some numbers so i'm quite confident in most of the specifics.


the version I heard was that Saturn would float in a glass of water β€” which is just silly because a glass of water is small and Saturn is far away. You can't make Saturn small because it's only held together by its own gravity so it would just evaporate before it got near the water. On the other hand, if you made the glass of water big, well, you might as well be talking about a "big sea" because I'm pretty sure the glass would explode.

Like, the glass wants to be a cylinder and the water wants to be a sphere and that's going to create a lot of pressure around what I suppose is the equator of the glass of water. There's an equation for the burst pressure of a pipe, and while it might be based on these "reasonable assumptions" that engineers have an annoying habit of making (such as "pipes are smaller than gas giants" or "you do not have ready access to 2 septillion tonnes of water") it says that the burst pressure is proportional to the ratio of the wall thickness to the pipe diameter, which means making a glass huge doesn't actually make it any stronger. I don't know how much pressure a regular tumbler can withstand but surely less than a saturn's worth of water trying to break out of it.

So the glass of water or whatever is really a sphere of water with some bits of broken glass in it. The surface is boiling in the low pressure, but also freezing in the low temperature, so we've got a proper planet with land masses and an atmosphere. It's got a solid core of glass, as well as exotic forms of ice formed under high pressure, which unlike regular ice sink in liquid water, and if the glass of water started out on Earth (which most glasses of water do) then it is harbouring life (although significantly less life and smaller life that Earth had been until it was destroyed by the sudden appearance of this enormous glass of water and its overwhelming gravitational field). But assuming there's enough open water near the surface, let's bring in Saturn.

Carefully.

If you just drop Saturn in then concepts like "floating" and "sinking" won't really apply β€” these are two huge objects with massive gravitational pulls and the energy of a freefall collision from really any relevant distance would at best result in a huge explosion and then the water planet maybe having a cute little moon.

On the other hand if you bring in Saturn too slowly then you're quite near the Roche Limit, so you're in danger of tidal forces tearing it apart until your water planet has a lovely set of rings, which is more like "orbiting" than "floating" per se.

But if you can control the descent just so, the water planet's gravity will smush Saturn out like any other cloud of gas and it will combine with the atmosphere of the planet. If you want to call that "floating" then sure, I guess. Maybe some of Saturn will be moving fast enough to punch into the water and then bob up, like a football forced underwater but exponentially more violent. I mean that is floating, strictly, but I feel like the breezy, chill word "floating" doesn't really capture the absolute maelstrom of destruction we're talking about here.

Also: my understanding is that Saturn probably has a rocky core. We're not 100% sure (and this experiment would be a terrible way to find out) but probably it does, and that rocky core would sink. This is exactly like if you dropped the Earth into a massive ocean β€” the rocky part would sink and the gas part would not. We know this, because the Earth is already mostly submerged in a massive ocean. Literally the only difference with Saturn is that more of it is atmosphere (and we are more confident that the rocky surface of Earth definitely exists).

And yet Nasa are happy to say "saturn would float in a giant bathtub" with no qualifications at all and I just think that's a waste of a lot of fun opportunities to imagine cataclysmic explosions.


You must log in to comment.

in reply to @ireneista's post:

I mean, it really is a way to illustrate the remarkably low overall density of the planet in an easily-imaginable fashion, even if it absolutely fails to capture any detail of why this wouldn't work.

And that's a good thing! This is a neat way of communicating "Saturn is less dense to water" to an audience of laypeople, who are a different audience from those who think about pressures and gravity. It's not meant to be accurate, since you're not the target audience.

That said, unsimplifying is still very fun!

Which is, largely, most people! Given my previous employment, I can tell you that people tend to vastly underestimate everything about the world if it exists past the horizon. Going off-planet, that underestimation and misunderstanding explodes. (I still short out a bit that the photo we took of another galaxy's supermassive black hole was with light the accretion disc emitted when our ancestors were still tiny tree shrews.)

in reply to @polymath's post:

This would also allow you to do the "make Saturn small" version of the experiment, although not as a piece of science communication because basically the demo is just an inflated condom bobbing about in the sink