I'm from the Denver area (5,340, in my case) but I was at 9,600 for the last two days with no noticeable differences. Most airliners are pressurized to about 8,000 feet and almost everyone from sea-level is fine with that for hours. I've spent days camping and even waterskiing in Leadville, Colorado (10,000+ feet) with no ill effects. The town of Leadville is above 10,500 feet and lots of people visit and stay there from many other places. Altitude sickness is a very, very serious thing but it's easy to avoid entirely if you follow a few very simple rules (don't exercise until you acclimate, stay hydrated, don't drink alcohol while you acclimate, acclimate in steps like sea-level to Denver to high altitude if possible).My point is, for healthy people, even those who grew up at sea level, living at an equivalent pressure above 10,000 feet should be easy within a couple of days. Higher oxygen concentrations and lower pressures come with lousy flammability limits.
The martian atmosphere is 1.9% nitrogen, and about 1% of Earth sea level pressure so all one has to do is extract all the nitrogen from 3500 times the volume of your air supply and you have your nitrogen. 3000 if you keep the pressure equivalent to Lee Jay's Denver.
I think we ought to optimize for the plants rather than the humans and plants do not like lower pressure, it interferes with plant water flows leading to water stress. Plants also do not like if the atmosphere contains too little or much CO2.
I HIGHLY doubt we will copy Earth's biosphere for the interior of the habitat. It'd have to be so enormous (and so expensive) that even a large colony could only support a tiny human population.And we'll probably develop plants that can survive straight on Mars's surface, if not right away then immediately after terraforming starts to slightly thicken the atmosphere.
I think we ought to optimize for the plants rather than the humans and plants do not like lower pressure, it interferes with plant water flows leading to water stress.
However, in addition to the problems mentioned above is flammability, Lee Jay's last point. If you keep the oxygen content constant and reduce the nitrogen things burn easier, and fire in spacecraft is something NASA works hard to avoid. The Apollo 1 fire, now just over a half century ago, is the extreme example, but it doesn't have to be 100% oxygen or 1 Atm to dramatically increase the risk.
However, lichen isn't a particularly hearty food, and for that reason, reindeer will eat 4 to 11 pounds (1.8 to 4.9 kilograms) of reindeer moss each day [source: Dieterich and Morton]. That's why reindeer pack on the pounds in the warmer months when there's more to choose from. In fact, these animals gradually lose weight starting in the fall and continuing to March [source: University of Alaska Fairbanks].
Quote from: AegeanBlue on 02/02/2017 11:49 pmI think we ought to optimize for the plants rather than the humans and plants do not like lower pressure, it interferes with plant water flows leading to water stress. Quite to the contrary, plants generally like low nitrogen partial pressures, so long as the O2 and CO2 partial pressures remain the same. Not always, but most often. And they can survive much lower pressures than humans.Here's some random studies from the last time I checked (need to translate my notes from Icelandic...). Since you guys normally seem to work in PSI, I'll note that 1 atmosphere is 101,325 pascals.Mansell et al, 1968: No negative effects in Brassica rapa (bok choy / turbip) at 50kPa, just more water loss.Rule and Staby, 1981: Tomatoes @17 kPa constricted; @33kPa were stronger that tomatoes grown at @100kPa, but not bigger.Daunicht and Brinkjans, 1992: tomatoes @40kPa and @70kPa constrictedAndre & Richaux, 1986: barley grows better @3kPa in a nitrogenless atmosphere than a conventional O2/N2 one.Gale, 1972: CO2 is easier to take up when the pressure is lower.Smith & Donahue, 1991: At 50kPa+, CO2 uptake is inversely proportional to pressure.Andre & Massimino, 1992: Wheat can sprout @10 kPa and grows better at @20kPa than @100kPa if N2 is low (O2@14 kPa, N2@ 3,4kPa, CO2@ 3,4 kPa). Appears that lowering N2 in general helps.Musgrave et al, 1988: Mung beans grow independent of O2 partial pressure, and general pressure reduction is negative (tested @21kPa)Goto et al, 2002: Rice can grow @25kPa and @50kPa; having O2 partial pressure at least.10 kPa prevents damage.Spanarkel & Drew, 2002: Lettuce @ 70kPa grows similar or better than @101kPaHe et al, 2003: Plants in general grow similar or better at @ 30kPa vs. @1atm because of better removal of ethylene.Wheeler et al, 2001: Corey et al, 2002: Plants grow similar or better at 30kPa Ferl et al, 2002: In addition to plants already doing well at low pressures, there's significant potential for genetic improvement to increase it (aka avoiding the drought response).
As you'll note, most were "same or better" - without any selective breeding / genetic engineering. Plants uptake CO2 and remove waste products better when the nitrogen partial pressure is reduced. Given that reducing pressure also reduces system mass, there's no reason not to do it.
Quote from: Rei on 02/04/2017 12:32 amAs you'll note, most were "same or better" - without any selective breeding / genetic engineering. Plants uptake CO2 and remove waste products better when the nitrogen partial pressure is reduced. Given that reducing pressure also reduces system mass, there's no reason not to do it.This has been stated several times in this thread but I can find little evidence for it. Do you have a reference?
Actually, yeah. You shouldn't expect people to spend an entire day picking through a long list of references for a single fact you mentioned.
I am not claiming to redo a natural terran biosphere, I am trying to show how we need to put our emphasis on viability elsewhere than just people. The average American eats a little under 1 metric ton of food every year. We have mentioned the ISS astronaut in the forum, I do not remember if it is 500 kgs or 800 kgs per year but even if we use the 500 kgs food/yr value producing it requires another 1000 kgs of plant biomass, above and below ground. But let us assume that for simplicity's sake this food is harvested for times a year and that we fully recycle the biomass of one crop to the next (impossible). So for every 75 kg astronaut there are 1500/4 = 375 kgs of plant biomass providing food. As Napoleon said, armies march on their stomach. So do astronauts.
TBH I was thinking more in terms of terraforming than food, where a fairly light cover would provide a few pounds over pressure...
I mean a literal planetwide floating row cover, transparent polymer withstanding 1ATM, loads transferred to fibre reinforcement to catenary curtains to cables, with a net loading on them of ~20 tonnes per square meter (something in that ballpark).
Quote from: john smith 19 on 02/07/2017 10:43 pmTBH I was thinking more in terms of terraforming than food, where a fairly light cover would provide a few pounds over pressure...This statement got me off and thinking (I was going to doubling the radius of a dome doubles tensile requirements in the envelope, so such Mars domes would either have to be kept proportionally small, or have an elaborate reinforcement system). But the side thought:Ignoring all the issues of difficulty in producing such a thing:has anyone ever proposed a system of terraforming a planet involving weighing down the whole atmosphere? I mean a literal planetwide floating row cover, transparent polymer withstanding 1ATM, loads transferred to fibre reinforcement to catenary curtains to cables, with a net loading on them of ~20 tonnes per square meter (something in that ballpark).I'm not saying I find it a realistic option. But when we're talking crazy megaengineering plans.... I mean, you could do that sort of thing on any body, even barren moons, and simultaneously reduce gas escape. The only difference is you need a lot more mass to weigh it down when gravity is lower (although the stresses on the materials remain the same)
As I mentioned in another thread, I spent a week in Cusco, and few people seemed to have any trouble at 9.4 psi after the first day. And the city is all walking up and down steep hills to get around.