Ignoring how one would get it there, what would happen to water spilled on the moon?
Start with a half-liter. Then a tanker truck full. Then a Super Tanker’s worth.
Finally,(OOPS! Clumsy me!), I dropped Lake Superior again.
Any thoughts?
Thanks Ted
Wow. Great question. Would it freeze by the low lemperature, or evaporate for the low atmospheric pressure, or immediately soak into the non-compacted surface?
Not a physicist, but:
Mostly it would boil immediately to vapor. Depending on whether or not it was poured into sunlight or shadow either it would all vaporize, or at least some of it would freeze to the rock until sun fell on it. At which point it would sublimate - go straight from ice to vapor.
If you dumped it out during the “day”, it would quickly boil away. If you dumped it at “night”, it would freeze, until “day”, then it would boil away. {Source}
Even if you dumped it at night, some of it would still boil away before it froze. By day, it’d all boil.
Small volumes of water (where the free molecular flow of water molecules to the vacuum will escape before the latent heat of water is lost) will evaporate (“boil”) rapidly even in absence of sunlight or other thermal sources. With larger (“tanker truck full”) volumes the water will partially evaporate and then the surface will crystalize as evaporation carries away enough heat energy to cause the shell to freeze. The interior will remain liquid for some time as heat slowly flows from the interior through the frozen exterior which will continue to slowly sublimate, until the entire structure is frozen through. (Due to the expansion of freezing water, the outer shell will expand and likely crack, which may allow water to flow out, but this will also freeze and create a fairly stable outer layer which inhibits heat flow.) Once frozen, the water will slowly sublimate in shadow or evaporate in sunlight.
Stranger
Is there a technical reason you used two different words? I read that sentence as saying that the frozen water in sunlight will go directly to vapor, which is what I thought sublimate meant. Or are you implying that it will necessarily go though a liquid phase first?
The two words sublimate and evaporate have precise scientific definitions.
Sublimate is solid -> gas
Evaporate is liquid -> gas.
Melting is solid -> liquid.
When he said the ice will evaporate, you can assume the liquid phase would occur first. The ice melts and then the liquid evaporates.
Of course, there may be a mix of sublimation, and evaporation after melting at “day”, but evaporation should be much faster, for the same square meters of surface area exposed.
The process by which a solid turns to a gas is sublimation. The verb used to describe the process is sublime, as in “the ice sublimed.” You will find this all through the article linked to by Der Tris.
Sublimate is better reserved for its psychological definition.
So, if it all evaporated, would the Moon have a moist atmosphere?
Currently, the Moon has NO atmosphere. I think it would take more than several supertankers’ worth of water dumped on the surface to result in something slightly resembling an atmosphere.
I was going to make a wiseass guess like 1 part in a kabillion, but after I thought of it, I think this is a question worthy of posing to Cecil or Randall over at What If? / xkcd.
That’s why I was asking. It all turns to vapor at some point, but how long is the middle phase? Would you see puddles of water at the surface or would it be essentially instantaneous?
Okay, I just posed the question to What If?
Well, the OP said Lake Superior.
Actually, Earth’s Moon has a very tenuous atmosphere which is mostly composed of ionized particles and some very tiny traces of helium and oxygen which are liberated by thermal differentials and will eventually escape the Moon’s sphere of influence. This isn’t enough to be of any use for flying or breathing, of course, but it is enough to be concerned about optical effects and buildup of static charge on mechanisms and structures.
In sublimation the average surface temperature never raises to the level of liquid water so you would see no liquid phase of any kind and the individual water molecules will escape from the surface and enter into free molecular flow, i.e. there isn’t enough of an atmosphere to get any significant interactions and gas law effects do not apply. However, if a block of ice is heated by sunlight or some other source, the surface will very briefly flash to liquid, and then will boil off, possibly explosively depending on the original temperature of the ice and the heating rate. Again, because there is no atmosphere this wouldn’t rise like steam but would radiate directly outward as a fine mist until it also enters free molecular flow or is absorbed by the surrounding regolith.
Stranger
You could dump water in one of the regions of permanent shadow near the poles and a portion of it would remain as ice, mostly after seeping into the regolith.
In fact some of the water that seeps into the regolith might remain, even if the shading is not complete; so long as the temperature of the regolith doesn’t exceed melting/ boiling point. This could happen in deeply shadowed craters near the poles that only get a short period of sunlight.
Well, maybe; it depends on the volume to surface area ratio of the water you dump and the initial temperature of the water. At essentially no surface pressure, the surface of your body of water will not freeze until it reaches about 200 K, as can be seen from this phase diagram. that means if you dump out water which is initially at standard temperature and pressure (STP) the surface immediately vaporizes. However, the fluid that is boiling off will create a temporary “atmosphere” that increases the pressure locally. Once the surface reaches the triple point (Ih, the first entry in the table) at 273.16 K and ~0.61 kPa (around 0.6% of the air pressure at Earth’s surface) the water will stop going into vapor phase and start solidifying. The surface will then slide down the solid/vapor line until it cools to 200 K. However, as the surface freezes into a crystalline state, it also expands creating tensile stresses that will tend to rupture the surface, allowing liquid below to flow out and vaporize. This, as well as conductive transfer from liquid water inside to the frozen shell will create some highly dynamic conditions until the mass forms a sufficiently thick layer to prevent stresses from breaching the shell and moderating the rate of heat flux from the interior to the exterior. With a very small amount of water the vaporization will happen too fast to come to equilibrium; with a larger mass, it will happen eventually but there will still be significant loss of water during the freezing period.
If you add any significant heat flux to the system (e.g. sunlight or some other thermal source) then there is no static equilibrium and the surface will continue to evaporate (visible flash to vapor). If the mass is in permanent shadow then you’ll just see slow sublimation (no visible effects). If you add some fine material–regolith, calcium carbonate, carbon soot, or even cellulose–this will tend to stabilize the thermal condition and give the water something to nucleate around, making a more stable solid mass when it freezes, and also giving better material properties analogous to lime or fly ash to make concrete.
Stranger
There is about 7000 km2 of permanently shadowed crater at the Moon’s north pole, and 6000 km2 at the south pole. This seems like a lot, but it is very small compared to Lake Superior (mentioned in the OP).
If you poured Lake Superior into these shadowed craters much of it would vapourise; but there would probably be a significant amount left as ice when the system stabilised, assuming there is any room left.
Chandrayaan-1 has apparently detected millions of tonnes of water ice on the Moon, so maybe all the available reservoirs are already full.
Vaguely related: the cooling system of the Apollo spacesuits used sublimation. Basically the cooling water oozed out through a porous plate, onto which it would freeze, and then slowly sublimate off into space, carrying away the heat with it.
The length of time the astronauts could spend on the surface was, I believe, limited by the supply of cooling water they could carry, as it was a consumable.
To provide sufficient cooling required about 600ml of water, or 1.25 US pints, per hour.
Edit to add link: Apollo Lunar Surface Journal : Apollo PLSS Images
Note that this isn’t pure water ice, but water-bearing strata containing approximately 4-6% water ice by mass.
Stranger