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Properties of Matter in Vacuum (Space)

  1. Jun 25, 2014 #1

    This is something I've been wondering about. If one looks at a phase chart like this:


    you notice something. First off, as the pressure is reduced, the liquid range shrinks and then disappears at the "triple point". So you cannot have any liquid water in a vacuum. However, wouldn't this also apply to any liquid as well? If so, then how do you get big flows of lava on vacuum-exposed bodies like Jupiter's moon Io, for example, or even our own Moon? Wouldn't lava ejected from a volcano, for example, boil rapidly due to the lack of pressure, thereby sucking its heat out and leaving what remains frozen, just like with water? And so then at the bottom of your volcano you don't get a nice lava flow but a bunch of "lava pellets" that froze while transiting the vacuum?

    Second off, the other thing that one notices is that the gap between absolute zero and the solid-gas line seems to continue to shrink with a reduction in pressure. Linear-scale, or more like it, anyways, diagrams show this line even converging right on absolute zero at zero pressure! So does that mean that under zero pressure (total vacuum), no solid phase even is possible? Does this also apply with substances other than water? But then how does one reconcile that with the existence of solid bodies like comets (in large part water) and asteroids which have been in space for billions of years and haven't just evaporated away?
  2. jcsd
  3. Jun 25, 2014 #2


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    It depends upon the "vapor pressure" of the liquid. Vibration free vacuum chambers - such as electron microscopes - often used oil diffusion pumps to reduce the vacuum. The oil used has a very high vapor pressure - and costs about $500 per ounce. Yes, it evaporates, but very slowly.

    I would expect the same or similar cause for the presence of an apparent fluid flow on the moon.

    See https://en.wikipedia.org/wiki/Vapor_pressure
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