A somewhat silly but interesting question came up during lunch at work the other day.
What would happen if you put (and held in place) an “unobtainium” tube between the atmospheres of two planets with ends at different atmospheric pressure levels, and pressurized the tube to the higher of the two pressures. Would there be flow between the atmospheres until pressures equalized?
I’m sorry if this has been asked before, for some reason search is not working.
Nothing would happen. The presence of a tube makes no difference here; atmospheres have pressure because the bodies they surround have gravity. Teh same gravity which keeps the atmosphere from flowing away, even though space is at roughly zero pressure.
My gut answer is that the pressure of the atmosphere at the high-pressure end would not overcome that planet’s gravity; the air in each end of the tube would be pulled toward the nearby planet and the pressure in the tube would drop.
Yes. Stretching a tube between to planetary surfaces would be precisely equivalent to enclosing an area of each surface with a fence and then extending both fences upward until one met the other, forming the tube. The air at each planetary surface would simply sit on the ground as it had before the fences were built, leaving vacuum between.
Now, for real action, you want a tube with zero internal length between the mouths, possibly as a result of a hyperspace bypass. Place one mouth at each planetary surface and then things will start to flow.
However, I specified that the tube is pressurized first (i.e. before the ends are opened up). What happens to the gas inside?
The air in the tube is the same pressure as the higher-pressure end, right? Air will definitely flow out of the tube at the low pressure end, due to both the pressure difference and that planet’s gravity. This will lead to an area of lower pressure propagating up the tube. But if that pressure gradient reaches the other end of the tube, it will essentially match the normal conditions there, so the flow will stop. And it might not even get all the way there, because as it nears the other planet, gravity will start acting against it.
Assuming you mean the gas throughout the tube is initially pressurized to the same pressure everywhere in the tube, the gas would flow out both ends of the tube, until there was a vacuum between the planets, in the region where there’s already a vacuum. It will flow out of boh ends, because the gas near the planet with the higher pressure will fall down the pipe towards the planet, increasing the pressure at the end of the tube.
In other words, it would reach thermodynamic equilibrium, i.e. the pressure inside the tube will become the same as the ambient pressure outside the tube at a given altitude. The only way this wouldn’t happen is if the amount of gas in the tube was substantially more than the gas on the planets…in which case it would leak out, fill the atmospheres of the planets, and start leaking out into space.
You can, of course, make all sorts of qualifications about this, but since it is a totally hypothetical thought experiment with no practical implementation there isn’t much point, really.
Stranger
The empty space between the planets is already just such a tube. That is, you didn’t put an upper limit on the tube diameter, and the space does the same thing a tube would.
Though you did fill it first. So, that would come rushing out the bottoms of the tube, blowing out onto the two planets. The more massive planet should take more of the air, but both would get nearly the same amount, because the parts of the tube close to the planets would empty as the air fell out, and the middle part of the tube that is far from either planet would empty more slowly as the air gradually expanded and blew toward the ends.
That makes perfect sense. Thanks. No chance for a trans-planetary hydrogen pipeline from a gas giant without some sort of a power source then.
Exactly. You can’t suction-pump Jupiter’s hydrogen away (with a vacuum above), but you could push it up (with high pressure below). The mathematics is left as an exercise for the student. 
>Exactly. You can’t suction-pump Jupiter’s hydrogen away (with a vacuum above), but you could push it up (with high pressure below).
I’m not exactly sure you could.
As you start pushing hydrogen in on Jupiter, the pressure would go up, though this tube would radiate heat away in space. So, surely you would be liquifying the hydrogen before long. But you keep pushing, and perhaps you would solidify it. I don’t know much about solid hydrogen, but it is a metal,and for all I know it could be very hard to force solid hydrogen into a tube. If you keep pushing and pushing, and there is not only the weight but the friction and deformation force to worry about, could the pressure get high enough to approach degeneracy?
You wouldn’t want to be responsible for that, would you??
As fun as Napier’s speculation is, I think achieving metallic hydrogen is a bit of a, um, stretch. Conceivably, by adding more pressure to Jupiter’s atmosphere, you’re creating more of the metallic hydrogen that hypothesized to exist at the planet’s core, but my off-the-cuff impression is that you’re orders of magnitude short of achieving it in the tube.
>Conceivably, by adding more pressure to Jupiter’s atmosphere, you’re creating more of the metallic hydrogen that hypothesized to exist at the planet’s core, but my off-the-cuff impression is that you’re orders of magnitude short of achieving it in the tube.
Well, if only one Jovian radius is already enough to hypothetically create it at the somewhat elevated temperatures in Jupiter’s core, wouldn’t a great many Jovian radii have to create it in the long tube we propose, which would also certainly get much colder? Realize that we would have to integrate outward from one radius to very many of them, multiplying the density of the tube contents at that condition by the declining gravitational field strength. Or, what would be simpler, would be to start with a little more than no pressure at a great distance, and move towards Jupiter, having higher pressures such that all the tube we have gone past is still full and supported, and the hydrogen at each point we pass is getting denser and the field is getting stronger, so that we have to add much higher pressure hydrogen for each incremental length. I think it’s the same problem as making Jupiter large enough so that, at low temperature, its atmosphere would extend all the way to wherever we’re trying to pump the hydrogen. To Earth, I guess. Then you ask whether the hydrogen turns metallic before you get to where the atmosphere of Jupiter was before we started messing with it.