Gravity and atmospheric pressure

I have another response to an "Ask Tsana" question today.

Brookelin asked:

I was wondering... with planets like Europa and possibly Ganymede, who possible have oceans, if humans made future settlements under said oceans, would the pressure from the water above counteract the effects of reduced gravity on the human body?

Interesting question. A preliminary point: it's Jupiter's moons Europa and Callisto that probably have sub-surface oceans (especially Europa), not Ganymede which is a solid rocky moon.

Europa, one of Jupiter's moons, has a vast ocean beneath
its surface. Credit: Galileo Project, JPL, NASA;
reprocessed by Ted Stryk

So, how do pressure and gravity work? In this context, gravity is the force that holds a planet/moon/star together and which attracts other objects to it. So we're all being pressed into the surface of Earth due to Earth's gravity. Pressure is the force a surrounding fluid (air, water, etc) exerts on something. So the atmospheric pressure we feel on Earth is pushing at us from all sides (well, OK, not out from the ground) and is due to all the air in Earth's atmosphere.

When you go swimming, the further you dive down, the higher the water pressure around you gets. This is because the deeper you are, the more water is above you to press down on you and the more water is above the bits of water on either side of you, also pressing into you. If you've ever been snorkelling (or scuba diving, I suppose but I can't vouch for that due to lack of experience) you might have noticed that it gets harder to breath the deeper you go (assuming a long enough snorkel). This is due to the water pressing down on your chest. Air does the same thing, but we're used to it, so we don't notice. The other thing that happens under water is that the water underneath you pushes up on you: this is called the buoyancy force and it's why things (people, tennis balls, icebergs, etc) float.

The higher up you go from sea level on Earth, the thinner the atmosphere gets (basically, the less atmosphere left above you). To halve the atmospheric pressure you experience, you need to go 5 km above sea level. (On the other hand, to double the pressure, you only need to be about 10 metres under water.) At that height, gravity is still pretty much the same as at sea level (the difference is about an eighth of a percent) and your main problems are getting enough oxygen (not a huge problem if your lung capacity is OK) and possibly altitude sickness (potentially a problem).

We need some amount of air pressure around us to survive which is part of the reason astronauts wear space suits. However, there is a range at which we can still function and that range increases if we have extra oxygen (and don't get altitude sickness). People have climbed Mt Everest (8.8 km above sea level) which has an atmospheric pressure of about a third that at sea level at it's peak without oxygen, but even doing it with oxygen requires training and acclimatisation and isn't something anyone can just decide to do one morning (well, unless they also decide to put in all the training).

On the surface of Europa or Callisto, there is no atmosphere and hence no atmospheric pressure. The ground is frozen water (probably not pure water, if only due to meteorite bombardment, but that's beside the point), but let's suppose we somehow got under the surface and set up a habitat. Since we're human and breathe air (a particular mix of mostly nitrogen, with some oxygen, carbon dioxide and misc) we'd have to have some sort of bubble habitat under the sea. But it's not just the air part that we need, we also need it to be around one (Earth) atmosphere of pressure. So we build a habitat with solid walls and fill it with the right amount of air... and then we're inside an air bubble and the water outside the bubble is having no effect on our bodies directly. The only way it would is if we went out into the water without pressure suits. Which probably wouldn't be the best idea in the world for a variety of health and safety reasons that don't necessarily have to do with the water pressure.

Now let's talk about gravity. The main way we detect small changes in pressure is though our ears, for example when they pop on taking off and landing in aeroplanes. The main way we detect changes in apparent gravity (which is the same as changes in acceleration) is when we feel lighter or heavier. If you're standing, this might manifest as extra strain on your legs, if the apparent gravity has increased, or a feeling like your stomach is moving upwards (possibly accompanied by nausea), if the apparent gravity has decreased. You don't experience the same feeling underwater or up a tall mountain because the gravity doesn't change in those places although the pressure does.

So what I'm ultimately trying to say is that the effects of gravity and atmospheric pressure are different. You can't compensate for a decrease in gravity by increasing pressure. Pressure is a force applied from all directions simultaneously, while gravity acts in just one direction. We know about the effects of Earth gravity, high gravity (from fighter pilots for example) and zero/microgravity (like on the space station) on people but much less about the effects of gravitational fields less than Earth's and more than zero. Europa's and Callisto's accelerations due gravity at the surface are about 13% Earth's and for comparison, the moon's is about 17% Earth's) so while we have had some experience with the moon landings during the Apollo missions, we don't really know how serious the health problems associated with spending prolonged periods at such low accelerations would be. There almost certainly would be some, but they probably wouldn't be as severe as zero gees. So while we can't use water pressure to compensate for gravity, it's not impossible for people to live on one of the moon's of Jupiter. We just don't know enough about what long term problems might arise.