question about mars

[Skeptic Ginger]

The atmospheric pressure of mars is much lower than that of earth.
Is it so much lower that it would instantly kill a human outside without a space suit, such as the famous scene from the movie 'Total Recall'?

Would it be possible to stand on mars with nothing but an oxygen tank and mask? To dig in the dirt with my bare hands?

Too cold. Even in the Sun the surface temperature can be warm but only about an inch off the ground. A foot or more from the warmed surface and you start to see temperatures hundreds of degrees below freezing.


I haven't read the other replies yet so forgive me from being redundant.

 

[Third Eye Open]

Oddly enough, yes. Volunteers had spent as much as 30 minutes with both hands in a vacuum chamber, with no worse effects than some swelling. So if your space suit had airtight cuffs, then yes you could stick your hand into dirt on Mars. Or on the Moon.

Somehow this makes me unreasonably happy.

Maybe, but that dirt is unlikely to be a comfortable temperature.

Too cold. Even in the Sun the surface temperature can be warm but only about an inch off the ground. A foot or more from the warmed surface and you start to see temperatures hundreds of degrees below freezing.

Wikipedia says that martian summers can have highs of 68F, is this incorrect, or am I misunderstanding it?

Thanks for the replies all!

 

[R.Mackey]

Yep. The thing that matters for respiration is oxygen partial pressure. On the Earth's surface, your body gets about 2.9 psi of oxygen mixed with nitrogen and whatnot...

I bet that a very low-grade pressure suit----a handful of rubber straps around the chest, at the level that'd make it difficult to inhale on Earth---would take a lot of the load off of breathing. Maybe you could get up to 1.0 or 1.2 psi.

Good post. A couple of additional details, though:

The effective suit pressure is a little bit higher because of two practical concerns. First is that there will also be a vapor pressure -- your own aspiration will displace some of the oxygen as you lose water into the super-dry tank air. Second is that there is some additional minimum operating pressure in your lungs.

So the actual minimum pressure for a 100% oxygen mix in a spacesuit works out to be a shade over 4 PSI, if we're delivering the same amount of oxygen as we're used to. If we're running at the ragged edge and don't mind blacking out, 3 PSI might be survivable.

Pure oxygen is rather dangerous and uncomfortable, however. Most spacesuits contain some inert gas in the mix, albeit not much since you have to carry it all with you. The A7L suits used in Apollo ran at about 5-6 PSI, running about 80% oxygen and 20% nitrogen. Yes, nitrogen -- unlike deep sea diving, there's no reason not to keep nitrogen in your system instead of flushing with helium. If you decompress, the bends will be the least of your worries...

Running at low pressure in a spacesuit helps in a few ways, one being it makes the suit less prone to bursting, another that it lightens the tanks. But some space applications are much closer to a "shirt-sleeve" environment. The International Space Station runs at full sea level pressure. This cuts down on the risk of fire, makes working more efficient, and provides some added protection from radiation.

It isn't clear what mix we'd use on Mars. I suspect it will be closer to the high pressure mix used on Station, but it's a complicated decision.

 

[I Ratant]

I recall Spandex was promoted a long time back for a non-pressurized space suit capable of protection from internal problems for a short but useful period.
That fancy rip-stop suit seems to be a generation or two newer and probably more capable yet.
If anyone ever gets into real space, not LEOs, something like that may be the uniform, with the full-pressure suits for extended times outside in vacuum.
(Keep the oxygen mask close by. )

 

[Skeptic Ginger]

Wikipedia says that martian summers can have highs of 68F, is this incorrect, or am I misunderstanding it?

Thanks for the replies all!

It's misleading. That is the surface temperature, but like I said, one only needs to get a few inches off the surface to see an incredible drop in temp.


Atmospheric temperatures are the featured Pathfinder meteorological observations

The temperatures on the two Viking landers, measured at 1.5 meters above the surface, range from + 1° F, ( -17.2° C) to -178° F (-107° C). However, the temperature of the surface at the winter polar caps drop to -225° F, (-143° C) while the warmest soil occasionally reaches +81° F (27° C) as estimated from Viking Orbiter Infrared Thermal Mapper.

(emphasis mine)


But 1°F at 1.5 meters up is much warmer than I had recalled. You could tolerate that temperature with a good coat. It would however, be during a very limited time window. And if you think about it, the warmest soil is 80°F while the warmest air at 1.5 meters off the surface is 1°F. That is an 80° temperature gradient from your toes to your waist. So what would the temperature be at 2 meters up then? It might be 40 below at your head even if it was 1°F at your waist.

 

[Soapy Sam]

Oddly enough, yes. Volunteers had spent as much as 30 minutes with both hands in a vacuum chamber, with no worse effects than some swelling. So if your space suit had airtight cuffs, then yes you could stick your hand into dirt on Mars. Or on the Moon.

I envisage a tiny frostbite problem...

 

[shadron]

It's misleading. That is the surface temperature, but like I said, one only needs to get a few inches off the surface to see an incredible drop in temp.


Atmospheric temperatures are the featured Pathfinder meteorological observations(emphasis mine)


But 1°F at 1.5 meters up is much warmer than I had recalled. You could tolerate that temperature with a good coat. It would however, be during a very limited time window. And if you think about it, the warmest soil is 80°F while the warmest air at 1.5 meters off the surface is 1°F. That is an 80° temperature gradient from your toes to your waist. So what would the temperature be at 2 meters up then? It might be 40 below at your head even if it was 1°F at your waist.

Another point to remember is that while the air temperature may be very cold (or hot), it doesn't transfer much heat away from/to the body. Vacuum is just about the perfect insulator (for conducted and convected interchange), and atmospheric pressures below 1 atm are probably as good as Thermos gives you. It's even better than that in space. When the Skylab overheated, all that was needed was a thin mylar sheet to shadow the hull and the temperature inside dropped.

 

[Skeptic Ginger]

Another point to remember is that while the air temperature may be very cold (or hot), it doesn't transfer much heat away from/to the body. Vacuum is just about the perfect insulator (for conducted and convected interchange), and atmospheric pressures below 1 atm are probably as good as Thermos gives you. It's even better than that in space. When the Skylab overheated, all that was needed was a thin mylar sheet to shadow the hull and the temperature inside dropped.

OK, now that's an interesting concept. So would a human outside a space suit lose heat to that frozen vacuum? They may have to re-write a few sci fi scripts.

 

[Ziggurat]

OK, now that's an interesting concept. So would a human outside a space suit lose heat to that frozen vacuum? They may have to re-write a few sci fi scripts.

In a vacuum, you can still lose heat through radiation. The power radiated for a blackbody is P = a s T⁴, where a is the area, s is the Stefan-Boltzmann constant, and T is absolute temperature. So the surface area of an adult man is about 1.9 m^{2. Arbitrarily picking a surface temperature of about 17 C (290 K), this gives us an upper bound to radiated power of roughly 300 Watts. Typical body power output is around 100 Watts. So yes, you can freeze to death in deep space, but it won't be terribly quick (just imagine how long it would take three 100-Watt bulbs to thaw a turkey).}

 

[Skeptic Ginger]

In a vacuum, you can still lose heat through radiation. The power radiated for a blackbody is P = a s T⁴, where a is the area, s is the Stefan-Boltzmann constant, and T is absolute temperature. So the surface area of an adult man is about 1.9 m^{2. Arbitrarily picking a surface temperature of about 17 C (290 K), this gives us an upper bound to radiated power of roughly 300 Watts. Typical body power output is around 100 Watts. So yes, you can freeze to death in deep space, but it won't be terribly quick (just imagine how long it would take three 100-Watt bulbs to thaw a turkey).}

^{But your body is also making heat. So does the calculation mean one loses more heat than one's metabolism creates? I love learning new stuff.}

 

[dasmiller]

In a vacuum, you can still lose heat through radiation. The power radiated for a blackbody is P = a s T⁴, where a is the area, s is the Stefan-Boltzmann constant, and T is absolute temperature. So the surface area of an adult man is about 1.9 m^{2. Arbitrarily picking a surface temperature of about 17 C (290 K), this gives us an upper bound to radiated power of roughly 300 Watts. Typical body power output is around 100 Watts. So yes, you can freeze to death in deep space, but it won't be terribly quick (just imagine how long it would take three 100-Watt bulbs to thaw a turkey).}

^{If the person in question isn't actually naked, then there's insulation from the clothing, hair, etc. I'm not energetic enough to do the math (and my thermal analysis courses were among my least favorite) but I suspect that even relatively light clothing would be quite sufficient. You specifically said "deep space," and I'm interpreting that to mean very far from any stars. But if someone was floating around in space in our neighborhood (1 AU from the sun), exposed to sunlight, then hyperthermia is a real possibility. Of course, there are still the suffocation and lethal-radiation issues to deal with.}

 

[BenBurch]

... Oddly enough, yes. Volunteers had spent as much as 30 minutes with both hands in a vacuum chamber, with no worse effects than some swelling. So if your space suit had airtight cuffs, then yes you could stick your hand into dirt on Mars. Or on the Moon.

Might get painful after a while;

http://www.centennialofflight.gov/essay/Explorers_Record_Setters_and_Daredevils/Kittinger/EX31.htm

 

[ben m]

Actually, much of the discussion of vacuum/heat conduction is incorrect. Vacuum is indeed a good insulator, but 0.1 psi is too high a pressure to "count" as vacuum. In fact, the thermal conductivity of air at 0.1 psi is almost identical to that at 1 atmosphere. (In a nutshell: as you lower the pressure, there are fewer molecules around to carry energy, but each molecule moves farther between collisions. The two effects cancel out.)

That goes both for conduction from one surface to another, as well as conduction to the air itself (I think). So yeah, Mars will freeze you by conduction---and do it quickly.

Convection is a bit harder to think about---I think most of the the relevant dimensionless numbers (Prandlt number, Raleigh number) have the same density/diffusion cancellation as the conductivity does, but they also have a gravity term---Mar's lower surface gravity might give you a bit of "insulation" by lowering the convection rate. (As you warm up the air around you, buoyancy won't carry it away as quickly in low gravity.)

 

[R.Mackey]

The Rayleigh number is a little bit lower due to the lower gravity, but otherwise it's relatively unaffected by lower pressure. So convection on Mars is still quite significant. Even without this, however, there would still be convection -- actually advection -- because there's quite a lot of wind. The thin atmosphere also sets up very large scale convection patterns rapidly in daylight, large enough to be seen in terrestrial telescopes.

It's not as cold as the Lunar night, but you will be very unhappy without thermal control.