QUESTION:
How can Jupiter have sulphuric gas clouds when sulphur is a solid?

ANSWER from Bob West on January 12, 1996:
Pure sulfur, as we commonly know it at the surface of the Earth, 
is a solid and does not form a cloud.  However pollution from cars 
and industry and volcanos put sulfur into the atmosphere as sulfur 
dioxide (SO2), sulfuric acid (H2SO4), and hydrogen sulfide (H2S), 
which combine with water to form cloud droplets of sulfuric acid.  

The sulfur in Jupiter is well mixed in the atmosphere since the 
atmosphere extends to very deep levels where the temperature is 
several thousands of degrees and sulfur cannot exist as a solid.  
There is so much hydrogen in Jupiter's upper atmosphere that the 
sulfur reacts with hydrogen to form hydrogen sulfide (H2S) gas.  
Nitrogen also reacts with hydrogen to form ammonia (NH3) gas.  
Observers have seen the spectral lines of ammonia, so we know 
ammonia is in Jupiter's upper atmosphere.  We have not seen the 
spectral signature of H2S, but we believe significant amounts of 
H2S are in the deeper part of the atmosphere where we cannot see 
with ground-based instruments.  Our belief is based on the observation 
that most of the elements (hydrogen, helium, carbon, and nitrogen) 
that we do see are roughly in the same relative abundance as in the sun.  

If sulfur is also as abundant as it is in the sun, hydrogen sulfide 
and ammonia would react in Jupiter's upper atmosphere to form 
ammonium hydrosulfide (NH4SH).  Ammonium hydrosulfide would exist as 
a gas at a pressure in Jupiter's atmosphere higher than about 1.6 bars 
(roughly, 1.6 times the pressure at sea level on the earth).  
At lower pressure (higher altitudes) Jupiter's atmosphere is cooler 
and the gas molecules condense to form solid particles of ammonium 
hydrosulfide.  These particles form clouds, just as water molecules 
in the earth's atmosphere condense to form water ice clouds 
(the cirrus clouds) at high altitudes on Earth.