This is somewhat of a follow-up question to my ammonia-based world question. I've found out that the melting point of ammonia is at 25°C at a pressure of about 1 MPa (about 10 times the atmospheric pressure on earth). I think that should be definitely warm enough for life. However, could higher land-based life actually exist on a planet with such high pressure, especially considering the higher gravitation that would be needed to create such a high pressure?
$\begingroup$ Just a sidenote: keep in mind that higher pressure means higher density, which means better heat transfer, which means faster cooling (in case your life forms need to maintain higher body temperature). At a certain pressure level, it makes more sense to think of everything as a liquid... $\endgroup$– PlasmaHHNov 3, 2014 at 11:59
Actually you can create high pressures without needing extraordinarily high gravity.
Venus for example has 92 times our atmospheric pressure (9 times what you need!) but slightly less than earth's gravity.
We know that life can exist in high pressure environments (they just keep the same internal as external pressures), so the simple answer is yes.
You could have a 1G planet with high pressure and reasonable temperatures and life could in theory survive those conditions.
This would have some interesting effects as well, for example flight would be much easier in high pressure but standard gravity environments.
$\begingroup$ Can you have those higher pressures (like Venus) without the higher temperatures (like Venus)? $\endgroup$– corsiKaNov 3, 2014 at 9:39
4$\begingroup$ @corsiKa Yes. Thicker atmospheres tend to increase temperature but you can compensate by placing the planet further from the Sun. $\endgroup$– Tim BNov 3, 2014 at 10:09
1$\begingroup$ Thank you in particular for the Venus example. It shows indeed quite impressively that the strong relation between gravity and pressure which I had been assuming doesn't actually exist. $\endgroup$– celtschkNov 3, 2014 at 21:11
$\begingroup$ As for flying, see What-If XKCD: Interplanetary Cessna. $\endgroup$– userNov 5, 2014 at 14:27
To answer your implied question about the possibility of this high pressure: Jupiter's surface has a pressure of 100 times the Earth's. Cosmically, pressures this high are trivial: think of "small" cosmic bodies like the Sun, which has a pressure around 50 times that of Jupiter. Scaled up, the pressures become unimaginably high.
Human bone has a compressive strength of around 170 MPa, meaning it would still be able to hold the human frame up at 1 MPa pressure. However, human body temperature is 37 degrees Celsius. High or low ambient temperatures can cause hyperthermia or hypothermia respectively, the former more lethal faster (hyperthermia can kill at 42C, compared to hypothermia at 27C). Pressure increases temperature, so a planet with a higher pressure would initially at least have a higher temperature and a higher chance of causing hyperthermia. The planet could cool down when the pressure stabilises though, making it once again suitable for life, and of course planets further away from their stars are cooler.
It depends. If the ambient conditions are right, life could survive such a planet. But, it would need the prerequisites for life, such as water, and the current perception is that any habitable planet must be in the Goldilocks zone of its star.
3$\begingroup$ "pressure increases temperature" yes, but generally only the moment when you compress something. Compressed gas can just cool down, otherwise the poor guys using pneumatic nail guns (30MPa) would need quite some heat protection gear... Just put the planet far enough away from its star should work fine. $\endgroup$– PlasmaHHNov 3, 2014 at 12:06
Pressure increases temperature, so a planet with a higher pressure would have a higher temperature. Higher than what? Adding pressure increases temperature. (more pressure would possibly imply more greenhouse gas, though). $\endgroup$– njzk2Nov 3, 2014 at 15:03
$\begingroup$ @PlasmaHH: Many nail guns use springs... but I take the point. Edits in progress. $\endgroup$ Nov 3, 2014 at 17:52
$\begingroup$ @ArtOfCode Wish I could ping you somewhere else, but just wanted to say that I was wrong in rejecting the tag wiki edit. I misread, and thought it was for the wrong section. I should have approved it. I'm really sorry about this. $\endgroup$– HDE 226868 ♦Dec 7, 2014 at 22:20
$\begingroup$ @HDE226868: Ah, don't worry. It's not that much to have lost. It might still be hanging around the revision or review history somewhere, so feel free to find it and reapply it if you feel too bad about it :) $\endgroup$ Dec 7, 2014 at 22:30
Life could definitely exist at 10 atmospheres of pressure because humans can live at that pressure. A SCUBA diver feels 10 atmospheres of pressure at a depth of ~100 meters, and they do just fine at that depth. The only problems they experience have to do with the changing pressure when they come back up. So if typical humans can survive 10 atmospheres of pressure, I think it's a safe bet that life could evolve and flourish on your planet.
$\begingroup$ Half correct. At 100 meters they are experiencing some rapture of the deep. Just because humans aren't adapted doesn't mean it's impossible for animals, though--some whales dive much deeper than that. $\endgroup$ Nov 3, 2014 at 0:24
$\begingroup$ It might be worth mentioning that 10 atm ~= 1MPa (1.013MPa), so as to answer the question in the units it us using. $\endgroup$ Nov 3, 2014 at 0:25
$\begingroup$ @LorenPechtel hopefully you won't have rapture of the deep if sticking to your dive plan. Basic level SCUBA is good for 100 feet depth. Beyond that you need "technical diving" and breath different mixtures. The concentration of oxygen in plain compressed air becomes toxic at 140 feet (if memory serves) so you'd be dead before you got high on nitrogen. $\endgroup$– JDługoszJul 1, 2015 at 2:53
Pressure has little difference in effect of land vs. sea creatures. The pressure pushes all around, so its not like the pressure is crushing them to the ground. Remember, until science told you there was air around you, you didn't even notice it!
In the Mariana's trench, they have found sea life at 108MPa, so there's proof it can work.
The bigger issue would be that higher pressures would increase the rate of chemical reactions in the atmosphere. However, that is not a part of land life, just life in general.
As a scuba diver I know that pressure does not squeeze a diver. Only body parts that are not luquid or solid are affected by the pressure, i.e. parts containing gas (mainly the lung). But if this gas has the same pressure as the outside, effectively no force is applied to the body as well.
However, when diving at 90 meters (i.e. at a pressure of 1 MPa), a special breathing gas is needed. If air was used (21 % O₂), the partial pressure of the oxygen would be 2.1 bar, which is toxic for a human body. This can be circumvented by replacing some of the air with Helium.
As already stated in Mike's answer, mainly a reduction of the pressure is dangerous for the diver, which would not occur in a constant pressure environment on your planet.
Conclusion: I see no problems for life at 1 MPa!