Would there be atmospheric moisture on Laythe?

[Patrick Kerbivan]

Based on established KSP lore, would it be possible for there to be appreciable moisture in Laythe's atmosphere? According to Laythe's page on the KSP wiki the presence of liquid water despite the planet's sub-freezing temperatures is most likely due to the presence of salt or another ion in the water. Would these conditions counter-indicate the presence of H2O vapor?

To provide a little context, I'm starting preliminary work on some modules intended for long-term colonization of Laythe (based on TAC life support's resource system), but I'm trying to stay as "lore friendly" as possible. One module I'm considering is a moisture extractor to create clean water from intake air, but I don't know if that would actually be possible in terms of realism.

[Boomerang]

Similar conditions to what you'd find around our poles, I imagine. When the air temperatures are below freezing, sea water can remain liquid due to the salt content. The air will be very dry, but it won't be entirely devoid of moisture either.

As an instinctual guess, I'd say that trying to get appreciable amounts of water from the atmosphere in those conditions would be difficult to do with any practicality. I might be wrong though and of course you can hand-wave all sorts of climate explanations for Laythe to make it work.

[Brotoro]

Cold air can't hold much moisture. But I would expect almost continuous volcanic eruptions on Laythe that would put water vapor into the atmosphere, which I would expect to rain out soon.

[Jesrad]

Don't forget that atmospheric pressure is lower on Laythe, too, and its composition differs from Earth and Kerbin. We know it contains dioxygen because jet engines work there, and that excludes most flammable gases from being in it too for obvious reasons, but that's about it. Could be dinitrogen, could be something else entirely.

Laythe temps range +9°C to -40°C (AFAIK), pressure at sea level is typically around 0.8 bar, so you'd get anywhere between 0.1 to 10 mBar of water vapor partial pressure in the air.

Edited August 26, 2014 by Jesrad

[Tommygun]

I wonder if you could get an "ice fog" near the surface of the water as the liquid water evaporates into the air and then freezes.

[Patrick Kerbivan]

Laythe temps range +9°C to -40°C (AFAIK), pressure at sea level is typically around 0.8 bar, so you'd get anywhere between 0.1 to 10 mBar of water vapor partial pressure in the air.

Thanks for the thorough explanation!

Taking it a bit further... With a sea level atmospheric pressure of .8 bar and an approximation of 5 mBar water vapor partial pressure, would it then be reasonable to calculate the portion of water vapor in the atmosphere as 5 mbar / 800 mbar = 0.0625%, ie 62,500 cubic centimeters of water vapor in 1 cubic meter of atmosphere? If so, how much further is it from there to determining what volume of liquid water at a reasonable ambient temperature that would equate to?

The moisture extraction is primarily intended to offset entropy in the water cycle I'm setting up, i.e. losses due to conversion of consumed H2O into other chemicals by both kerbals (who I'm assuming are metabolically identical to small humans) and the food-bearing plants in the hydroponics lab I'm creating (still have to decide what sorts of plant/s to model that on). I still have to work out exactly how much H2O that would be, but if it doesn't stray too far from what could be reasonably extracted then I think I have the makings of a scientifically-plausible self-sustaining outpost on Laythe!

[Jesrad]

Well you need to convert the pressure at given temp into molar volume, then this molar amount into mass. I'm not quite sure we can do the first part because of absent density, temperature and composition data for Laythe's atmosphere. Using ideal gas values I get Vm = RT/P = 0.11355 L/mol at 0°C and 5 mBar, or 8.8 mol / L. 62500 cm^3 is 0.625 L so that would amount to 5.5 mol of water for one cubic meter. Molar mass of water is 18, so that would be just shy of 100g of water per cubic meter of Laythe atmosphere at 0°C - given all the going assumptions and ideal gas properties.

Sounds like quite a bit of water to me, I expected a lot less. At 0.1 mBar of partial pressure that'd be just 2 grams of water per cubic meter of atmo, and vastly more challenging for your extractor

Condensing water from air requires cooling the air (or decreasing its pressure) in order to lower the dew point enough that the vapor turns liquid. It's a LOT harder to do when the temperature is already low (see on the second graph I posted, the steep part of the curve is on the high temp range, that's where it is easy to pull water from the air, whereas in the range where the curve is almost flat you're just out of options). You cannot extract more than a fraction of the water vapor in the air in the first place, and that fraction gets "eaten" quickly as temperature drops. Thermodynamic cycles and all that jazz.

IMO it would just make more sense to pump water from the liquid oceans (the easy way) or even mine solid ice at the poles directly.

Edited August 27, 2014 by Jesrad

[Patrick Kerbivan]

IMO it would just make more sense to pump water from the liquid oceans (the easy way) or even mine solid ice at the poles directly.

Thanks again for the explanation! I agree that either of those options would be more practical in reality, but then I run into the complication of building a mod sensitive to its proximity to liquid water or polar ice, which is beyond my skill at this point; for now I'm just making use of the same "requires O2 astmosphere" check used by the existing air-breathing modules to limit operation to Kerbin and Laythe. I'll do a bit more research on realistic extraction rates for atmospheric water generators and test out the values to see if they're actually useful in play without having to take too many sci-fi liberties with the device's efficiency. For future endeavors a polar ice collector mod would actually be a pretty cool idea though! (bah-dum-psh)

[magico13]

but then I run into the complication of building a mod sensitive to its proximity to liquid water or polar ice, which is beyond my skill at this point;

The simple (though maybe not the best) solution to this would be to check for "Splashed Down" for liquid water (so the vessel would have to be physically in the water) and latitude for polar ice (assuming it starts at roughly the same latitude for every longitude).

[lajoswinkler]

I hope it helps.