MadSci Network: Chemistry

Re: can an icecube melt in outer space floating around?

Date: Tue Apr 27 15:17:55 1999
Posted By: John Haberman, Space Scientist, NASA Goddard Space Center, Greenbelt MD
Area of science: Chemistry
ID: 924652629.Ch

Dear Kathleen,

    We know that ice can exist in space from our studies of comets.  These 
studies have shown that comets are made from ice and dirt: comets are 
often referred to as 'dirty snowballs'.  

    Ice in outer space will not melt in the way that we think of melting 
here on the Earth.  The process of melting is an equilibrium process.  On 
the Earth, at one  atmosphere of pressure and zero degrees Celsius (32 
degrees Fahrenheit, water exists at the same time in equilibrium in the 
solid, liquid, and gas phases.  This equilibrium has been studied for many 
years and has resulted in what are referred to as phase diagrams.  When 
you change the temperature and the pressure, the behavior of water can be 
predicted by referring to its 'phase diagram'.  You might expect that in 
space, where we know that it is very cold, that water would exist only as 
a solid: ice.  But the pressure in space is very low.  Knowing this we 
expect that ice will not melt but will evaporate.  This is what happens.  
An ice cube will slowly evaporate: it will change directly from the solid 
to the gas phase.  This process, on Earth, is very useful and is called 
freeze drying.

John Haberman

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