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I have read questions such as these two about aliens adapted to environments with high atmospheric pressure experiencing negative effects when exposed to Earth's comparatively low atmospheric pressure. This question is different because I want to know how to prevent negative effects from occuring with the opposite situation.

Mars has ~1% of the atmospheric pressure and 38% of the surface gravity of Earth. Some Martians, who are comparable in mass and body plan to Humans, want to visit Earth for cultural exchange. But exposure to a hundred times the air pressure and nearly three times the gravity under which they've evolved will very probably kill them. Humans offer to provide them with pressure suits, but a Human spacesuit is designed to maintain high internal pressure compared to vacuum.

Would a suit allowing a Martian to withstand Earth's conditions, by maintaining low internal pressure, have to be significantly different in structure and design from the spacesuits that Humans use? If she were supplied with the CO2 and H2 she breathes instead of deadly oxygen, could a Martian gear up in a standard Apollo-era NASA spacesuit with no design alterations and survive on Earth? Otherwise, what changes would be needed for the suit to maintain low internal pressure without collapsing? Also, Earth's gravity isn't really an issue addressed by spacesuits made on Earth, but even with proper (de)pressurization, would it need to provide her with additional support to stand and walk under Earthly gravity while wearing it?

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    $\begingroup$ I suggest meeting them on the ISS. While the problems could be overcome it seems that it wouldn't be a pleasant for them and communication wouldn't be possible in a suitable way either. E.g. they probably would need to lie down all the time or be in a fluid tank (resulting in problems with the pressure) to survive gravity. While certainly not easy a room of the ISS could be reduced to the neccessary pressure and one could communicate through a window. $\endgroup$ – Christoph Sep 21 '17 at 6:44
  • $\begingroup$ With advances in plastics the way they are, a subtle frame could be created in the suit to help a person deal with higher gravity and use some sort of power assist hinge at the limbs. The suit wouldn't have to be bulky if the tech was advanced enough. $\endgroup$ – JFA Sep 21 '17 at 14:29
  • $\begingroup$ @Christoph A common meeting point in space is a good idea - although the particulars of this story make the ISS an unfeasible choice. $\endgroup$ – undine_centimeter Sep 22 '17 at 21:13
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Spacesuits are designed to keep an individual pressurized in a low pressure environment.

What you are looking for is something along the lines of an atmospheric diving suit. They are designed to keep a individual at a much lower pressure than the surrounding environment. It's likely that the suit could be a lot less bulky than a diving hardsuit because the pressure differential is much lower between Earth and Mars than between sea level and 700m below sea level.

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    $\begingroup$ Pressure changes due to depth are not equivalent to a change of weight resulting from higher G forces. A person visiting Jupiter would weight 2.4 times as much. As their blood now weights this much more the heart is put under considerable strain. People can certainly adapt to doubling their weight but this is generally a slow process, not something that would happen in hours (or possibly even minutes). Free divers can very rapidly decent to depths of 214m (No-limits apnea record) but the weight of their blood is still constant, which is not the case under higher Gs. $\endgroup$ – Quaternion Sep 21 '17 at 20:40
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The aliens pressure concerns would likely be more similar to deep ocean divers than astronauts since there is a higher pressure outside than you want to maintain inside. This would require something similar to an atmospheric diving suit, rigid with bulky joints to keep the lower pressure interior from collapsing inwards. It would not need to be as strong since it is only holding out a 1 atm difference instead of 10s of atm.

But your aliens might not need a pressure suit at all, trained divers have reached 534 metres deep without atmospheric diving suits. Thats 50 times atmospheric pressure. So it may not be unreasonable for your martians to survive 100 times there own atmospheric pressure. You would need to take into account some things for this to work though. Deep diving requires special air mixtures to avoid oxygen toxicity and you might assume there would be similar issues with H2 based respiration. And compression and decompression must be done carefully over a period of time to avoid having gasses that liquified in the body due to pressure suddenly turn back into gas.

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Space suits are designed to endure low pressures.

Dive suits are designed to endure high pressures.

Simply use stuff similar to deep dive equipment.

Examples: Deep pressure doesn't express an explosive risk. Matter is matter, for the most part. Diving into high pressure environment's risk comes mainly from compression of gasses or even worse, their phase transition into liquids, not from the pressure itself. So what needs to be addressed is any materials that would change from gas to liquid at such pressures. As fully-contained gasses aren't safe to evolve -anyway- (walk up a hill, vital organs go pop), most are generally adjustable as needed. The harder risk is phase transitions, at which point alternatives may be needed (such as a liquid that functions like their breathable air to their biology) The other main concern would primarily be mobility, as their locomotive method is more adapted to a less resistive substance.

So, breathing tanks filled with -something-, and a segway, and training on how to use them appropriately.

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All the below assumes a somewhat humanoid anatomy.

As sphennings said, a relatively light suit with plastic hard panels would work. It is possible that they would only need the panels around the head, neck and torso. It really depends on how fragile the flesh on their limbs are and if they will be able to circulate blood to the limbs against the pressure.

If they are from a low G world as well as low pressure (such as Mars) they may leave the pressure protection off of their lower legs anyway to help pump the blood back up to their heart.

The suit would also have to be sealed. I would imagine that the O2 in the air would be damaging to their skin even if they don't breathe it.

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Since you say "Some Martians, who are comparable in mass and body plan to Humans, want to visit Earth for cultural exchange", only thing you need is SCUBA gear, and tanks with gasses not poisonous to the Martians at 1 atm of pressure. If Earth atmosphere is not poisonous in this pressure, then even the SCUBA gear is not necessary, but this seems unlikely, considering how evil our atmospheric Oxygen is.

This is because human-like body plan implies they are liquid (such as water) based creatures. And liquids are essentially uncompressible, so outside pressure will not crush a liquid based creature. This is demonstrated by humans being able to dive deep underwater, to tens of atmospheres of pressure, with only a few issues to be taken care of. These issues mostly involve gasses becoming poisonous under high pressure, and gasses getting dissolved in blood in high pressure, and then causing problems when pressure returns to normal.

Therefore another precaution needed is, Martians need to return to their normal pressure slowly enough, similar to how SCUBA divers need to return to surface slowly enough, to avoid decompression sickness.

The important thing to understand here is, gas-filled body cavities (lungs and ear canals for humans) need to have same internal pressure, as the pressure outside. When normally on surface, breathing air, this happens automatically, the air we breathe is at the 1 atm, which matches the air pressure of 1 atm (duh). What makes SCUBA gear special is, it is able to provide air (from high-pressure tank) automatically at same pressure as the water surrounding a diver, so the air in lungs will push back against the water pressure just right, and diver chest will not get crushed (too low air pressure) or exploded (too high air pressure from the high-pressure tank).

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