Boiling point of water in space
My name is Mike and I reside in the United Kingdom. This is a very short question:
at what temperature would water boil at in Outer Space?
It all started with a conversation between a friend of mine and myself. We understood that at higher altitudes water boils at less than 100 degrees C, and I was interested to know that mountain climbers used to use this theory to determine how high they were.
I have searched the Internet for an answer, all currently to no avail, and I was hoping that you could shed some light for us.
Reply
Formally, ANY temperature can serve as "boiling point in space," but to understand how and why, you need know more about "vapor pressure" (vapour pressure in the UK). For instance, you may look up
http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/vappre.html
Vapor pressure of a liquid (or even a solid, though its value may be vanishingly small) is a measure of how much its molecules "want" to convert to gas in the surrounding space. For instance, suppose you pump out all the air in a big bulb. The bulb is connected to a reservoir of water, and you let a cupful of water at room temperature, 27 deg C, be sucked into it. What will happen?
The water will boil, and as it does, part will be converted to water vapor. The process will stop when the pressure of the vapor reaches the value of the vapor pressure of liquid water at 27 degrees. At this point, an equilibrium exists--the rate of evaporating molecules is exactly balanced by the rate at which others consense out of the vapor.
Let's say that pressure equals P atmospheres (you can find tables of P against temperature in handbooks). If instead the bulb already contains air at pressure P, the water will be just at its boiling point. Heat it to 40 deg and it will boil, until the steam released adds enough pressure to P for the sum to equal the vapor pressure of water at 40 deg. At 40 degrees the vapor pressure of water is larger than P, because vapor pressure of a liquid increases with temperature.
The rule is, then--water will boil at the temperature where the external pressure equals its vapor pressure. The vapor pressure of water at 100 deg. C is one atmosphere, so at sea level, it boils at 100 C. On a mountaintop where the external pressure is only 0.8 atmosphere, it boils at a lower temperature, the one where the vapor pressure of liquid water only equals 0.8 atmosphere.
And in space, where the external pressure is zero... water boils at any temperature at which it has some vapor pressure. Which in principle means, any temperature above absolute zero.
Suppose an astronaut vents water to the outside. It will always evaporate, but not immediately, because there is also energy to consider. Turning water from liquid to gas requires about 236 calories per gram, more than twice the heat it takes (on the ground) to warm it from near freezing to boiling. That's why sweating on a hot day cools your body: as the water evaporates, it needs extra energy, and by supplying it, your skin gets cooled.
If water is vented to space, part of it immediately boils away, but the rest is cooled to where it freezes, into a sort of instant snow. Ultimately the frozen part also evaporates, since no equilibrium can exist between water (in any form) and a vacuum, but that happens more slowly.
Comets are apparently mostly ice, so they are expected ultimately to evaporate. However, they are so far from the Sun, and therefore so cold, that I would not be surprised if (as long as they stay distant) "ultimately" turns out to be much longer than the age of the universe.
Short question, long answer. If you know any better way, please tell me; but at least, I hope you and your friends have now learned a little more physics, and also have a bit more appreciation for the complexity of nature..