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Jarmo
2009-Jan-24, 07:30 PM
One simple thing has been bugging me for a while. I didn't find an answer in the forum, but if there's a thread discussing this, kindly point me to the right direction.

Let's take a regular ~5 mm diameter round droplet of water at room temperature and normal pressure. If we expose it to vacuum of space, what happens to it and how fast? It starts to radiate its heat away, by boiling and evaporation. Does the "core" of the droplet freeze due to rapid depressurization or does the whole thing just evaporate away? What happens to the evaporates, do they get immediately split into OH and H? How long do each of these processes take, are we talking about milliseconds, seconds, minutes..?

I'm assuming all this happens at 1 AU from the Sun.. How would the situation differ if the droplet was a) in sunlight or b) in shadow? Or c) in deep space?

A followup question: If the droplet was suspended in a liter of pressurized air and the whole thing gets exposed in the vacuum, will the depressurization of the air cause the droplet to freeze? The way I see this, is that the air expands, and thus cools down. And while it is cooling, there is still some gas around the slightly hotter droplet. Thus convection and conduction would take care of freezing at least the surface of the droplet... Am I totally off track here?

Jarmo

Tarkus
2009-Jan-24, 08:40 PM
I once sucked some water up to 140,000 feet in a vacuum chamber and it boiled away. On return to zero the inside of the chamber was all wet with condensation. I wasn't recording gas levels so can't comment on cracking back to elements but it seemed to stayas water just went from liquid to gas state and returned..

If that helps..

grant hutchison
2009-Jan-24, 08:57 PM
The evaporation takes energy, which comes from the remaining water, reducing its temperature until it freezes. The proportion of the original droplet which ends up frozen depends on its original temperature.
The evaporated water will just hang around as water molecules, unless it is hit by a sufficiently energetic particle to break it into ions.
The frozen remnant will continue to lose molecules to vacuum. At Earth's distance from the sun, in sunlight, that'll happen reasonably quickly, so the droplet will dwindle away to nothing. Beyond Jupiter's orbit, equilibrium temperatures are cool enough for water ice to persist for geologically significant time scales. So a droplet protected by shadow, or far enough from the sun, might leave a persistent remnant of ice.

Grant Hutchison

grant hutchison
2009-Jan-24, 10:16 PM
I knew I'd done some sums on this topic previously, and I've tracked them down on the "burial in space" thread. Here (http://www.bautforum.com/1024770-post11.html) is the post in which I figured the evaporative history of a kilogram of water, neglecting radiation. It took the evaporation of about a quarter-kilogram of water to convert the rest to "snow line" temperature ice.
So a water droplet at room temperature ejected into shaded vacuum is going to produce a significant blob of ice. Radiative cooling will only increase the fraction of the droplet that freezes.

Grant Hutchison

AonSao
2009-Jan-24, 11:19 PM
Would it be possible that there are some vast reservoirs of water just floating freely in space? Water interacts with light in interesting ways. Maybe this could explain some phenomena.

Just brain storming :) No real logic to back up my thoughts here.

SkepticJ
2009-Jan-24, 11:26 PM
Would it be possible that there are some vast reservoirs of water just floating freely in space?

Comets. They're really dirty, though. Be sure to filter it.

AonSao
2009-Jan-24, 11:37 PM
Edit: Just deleting my question, it distracts from the point of this post.

Jarmo
2009-Jan-25, 04:05 AM
I knew I'd done some sums on this topic previously, and I've tracked them down on the "burial in space" thread. Here (http://www.bautforum.com/1024770-post11.html) is the post in which I figured the evaporative history of a kilogram of water, neglecting radiation. It took the evaporation of about a quarter-kilogram of water to convert the rest to "snow line" temperature ice.
So a water droplet at room temperature ejected into shaded vacuum is going to produce a significant blob of ice. Radiative cooling will only increase the fraction of the droplet that freezes.
Grant Hutchison

Thanks Grant for inspiring answers! :)

So, let me get this straight just to see if I got something wrong: Basically, the blob boils and evaporates, and due to the cooling it experiences, the remainder eventually freezes. Now, depending on whether it is in sunlight or shadow (at 1 AU) the ice further evaporates or stays intact for eons, respectively (not counting all the cosmic or reflected radiation etc that will evaporate unprotected ice anyway, eventually).

What about the time scale, how long does it take for the blob to boil to freezing point and further on to oblivion? Have any formulas for that?

According to my calculations, cooling water down from 25 deg centigrade and freezing it (to 0 deg ice) will cause 20% or original mass to be evaporated... why did you freeze it down to -100 degrees in your earlier example? Is zero deg ice somehow not stable in shadow in vacuum?

mugaliens
2009-Jan-25, 08:11 AM
Let's take a regular ~5 mm diameter round droplet of water at room temperature and normal pressure. If we expose it to vacuum of space, what happens to it and how fast? It starts to radiate its heat away, by boiling and evaporation. Does the "core" of the droplet freeze due to rapid depressurization or does the whole thing just evaporate away?

It's temperature wouldn't change due to the change in pressure, at least not directly, as it's a liquid, not a gas. Rather, as the pressure drops below the point where it boils at room temperature, it begins boiling. It's the latent heat of evaporation (http://en.wikipedia.org/wiki/Enthalpy_of_vaporization)which cools the remaining water. This reduces the rate of boiling, as it requires 2,260 kJ/kg just to vaporize the water. This graph of zinc (http://en.wikipedia.org/wiki/File:Heat_Content_of_Zn(c,l,g).PNG)is similar to that of water, and shows how much heat we're talking about.

The water does not have enough inherent energy at 70 degrees to finish the boiling process. However, it doesn't quite freeze, either. Rather, the boiling process reduces as the temperature goes down. Then, something has to give. Either the droplet freezes, then later sublimates (http://en.wikipedia.org/wiki/Heat_of_sublimation), or it receives enough energy (sunlight?) to keep from freezing and slowly boils off.

What happens to the evaporates, do they get immediately split into OH and H?

Those are ions. Water vapor exists as distinct molecules.

How long do each of these processes take...

Minutes for the initial part, hours for the second.

I'm assuming all this happens at 1 AU from the Sun.. How would the situation differ if the droplet was a) in sunlight or b) in shadow? Or c) in deep space?

a vs b wouldn't matter much, though it would boil/sublimate faster in sunlight. In deep space, what's left after it cooled would freeze solid, then sublimate over a very long time as whatever energy is out there hits it.

A followup question: If the droplet was suspended in a liter of pressurized air and the whole thing gets exposed in the vacuum, will the depressurization of the air cause the droplet to freeze?

Immediately upon exposure to the vacuum, the air rapidly escapes and the physics return to what's been described above.

grant hutchison
2009-Jan-25, 02:03 PM
According to my calculations, cooling water down from 25 deg centigrade and freezing it (to 0 deg ice) will cause 20% or original mass to be evaporated... why did you freeze it down to -100 degrees in your earlier example? Is zero deg ice somehow not stable in shadow in vacuum?At zero degrees, ice doesn't last very long if there's no ambient vapour phase. You can see that on the surface of the Earth, when a cold dry wind causes patches of ice on the ground to sublime away.
So I took it down to -100C, which is a nice round number for the equilibrium temperature at the "snow line" in the solar system, beyond which we see icy bodies persist for geological time periods. At that point we can be pretty sure that evaporative cooling is no longer the dominant effect driving the temperature lower, and our blob of ice can settle into radiative equilibrium.

Grant Hutchison

dhd40
2009-Jan-25, 08:05 PM
...
Beyond Jupiter's orbit, equilibrium temperatures are cool enough for water ice to persist for geologically significant time scales. So a droplet protected by shadow, or far enough from the sun, might leave a persistent remnant of ice.

Grant Hutchison

Isnīt that akin to the processes on Enceladus which supply material to Saturnīs ring(s)?

Jarmo
2009-Jan-26, 12:13 AM
The water does not have enough inherent energy at 70 degrees to finish the boiling process. However, it doesn't quite freeze, either. Rather, the boiling process reduces as the temperature goes down. Then, something has to give. Either the droplet freezes, then later sublimates (http://en.wikipedia.org/wiki/heat_of_sublimation), or it receives enough energy (sunlight?) to keep from freezing and slowly boils off.

Where did you come up with 70 degrees?

Those [OH & H] are ions. Water vapor exists as distinct molecules.

Yes, and distinct water molecules are broken down into OH and H ions by energetic particles and radiation, are they not? And those things exist in abundance at 1 AU from the Sun. How fast is this breakdown process?

minutes for the initial part, hours for the second.

Time scale based on what? And the initial part is boiling, and second part is the freezing, or what?

a vs b wouldn't matter much, though it would boil/sublimate faster in sunlight. In deep space, what's left after it cooled would freeze solid, then sublimate over a very long time as whatever energy is out there hits it.

I beg to differ... as far as I know, shadowed places (b) anywhere in interplanetary space are rather similar to "deep space" (c). The only difference would be the existence of the solar wind, which is a quite sparse medium. Less dense than in terrestrial vacuum. What would cause ice sublimation in e.g. permanently shadowed craters, or on the dark side of the Moon? Cosmic rays etc, maybe, but is the time scale really different form that of the situation in deep space?

immediately upon exposure to the vacuum, the air rapidly escapes and the physics return to what's been described above.

So the expansion of the air causes no further cooling of the droplet? How come?

grant hutchison
2009-Jan-26, 02:01 AM
So the expansion of the air causes no further cooling of the droplet? How come?Essentially no further cooling of the droplet. The air will cool as it expands, but a very low mass of this cooled air will have the chance to interact with the surface of the water droplet before the air dissipates. I'd imagine that the air would therefore make a minimal contribution to the cooling of the droplet.

Grant Hutchison

Jarmo
2009-Jan-26, 02:04 AM
A followup question:

As radiative heat loss is so inefficient, what happens when the droplet gets in contact with a conductive medium? E.g. if you drop the water onto the Lunar surface, what will happen to it then? Again, starting from regular pressure and room temperature, and exposing the droplet otherwise to vacuum.

Assuming this occurs in shadow and during Lunar night (surface temperature around -200 deg C) my feeling is that the droplet would freeze immediately...?

Further factors I can think of but can't figure out how they change the experiment result.. the first three are rather trivial, but what about the last two?
- Changing temperature of the surface (varies from -230 to +120 deg C)
- Doing this in sunlight or shadow (radiative heat from the Sun)
- Dispersing of the droplet into smaller pieces at impact (increasing the area exposed to the environment)
- Surface tension of Moon dust (how much of the water will actually touch the surface?)
- Material composition of the surface (No water in minerals, sp do chemical reactions take place immediately? Hard to think of this like pouring acid on limestone..)

Jeff Root
2009-Jan-26, 05:15 AM
You'll probably find something by Googling "Space Shuttle" and "urine dump".

-- Jeff, in Minneapolis

mugaliens
2009-Jan-26, 06:16 PM
Where did you come up with 70 degrees?

Room temp. If you're going to find a single droplet of water in space, it stands a very high probability of having been ejected from a spacecraft occupied by humans.

I won't hazard any guesses as to the quality of that water...

Yes, and distinct water molecules are broken down into OH and H ions by energetic particles and radiation, are they not? And those things exist in abundance at 1 AU from the Sun. How fast is this breakdown process?

Very, very slow. There are some 50 billion trillion molecules of water in that droplet. The amount of energy required to break down even one-millionth of those molecules would flash the droplet into highly energetic plasma. In other words, it's an interesting theory, but no, that's not how it works. You will not have OH and H ions. You will have H2O molecules, some of which don't boil off and will freeze.

Time scale based on what? And the initial part is boiling, and second part is the freezing, or what?

If I were to hazard a guess, I'd say a few minutes.

I beg to differ... as far as I know, shadowed places (b) anywhere in interplanetary space are rather similar to "deep space" (c). The only difference would be the existence of the solar wind, which is a quite sparse medium. Less dense than in terrestrial vacuum. What would cause ice sublimation in e.g. permanently shadowed craters, or on the dark side of the Moon? Cosmic rays etc, maybe, but is the time scale really different form that of the situation in deep space?

In permanent shadow, there would be little sublimation. But it would still occur, as cosmic rays stream throughout our galaxy - they're not just from the Sun, and they do exist in deep space.

So the expansion of the air causes no further cooling of the droplet? How come?

Because it expands very rapidly, then is gone. No more air = no more cooling from that air.