QUESTION:
Would you elaborate on the theories concerning the "raining out of
helium and neon" during the formation of Jupiter?

ANSWER from Glenn Orton on February 16, 1996:
The Helium rain-out theory was first proposed by David Stevenson, a graduate
student at Cornell, now at Caltech, as a means to explain the low He
abundance in Saturn's atmosphere.   In this model, the formation of the
"metallic" hydrogen region in the interior of Saturn would also imply that
a 'noble' gas, such as helium, would be misicble (able to be mixed in
solution) with the hydrogen.  The helium is heavier than the hydrogen,
however, and would slowly sink toward the interior.  The energy that this
movement would release (since being closer to the interior means that the
helium had lost gravitational potential energy) would be felt as heating, and
this would be consistent with Saturn's high internal heat.

Our first exploration of Jupiter's atmosphere through the Pioneer and Voyager
spacecraft showed a helium mixing ratio on the order of 10%, too high to
imply any movement of helium to the center.  However, the discovery of less
helium (closer to 7.4%) (and less neon, another noble gas which is miscible
with metallic hydrogen) from the Galileo Probe implies that this process is
probably taking place in Jupiter's interior, as well as Saturn's.

ANSWER from Claudia Alexander on February 19, 1996:
I'm not sure how much elaborating I can do.  The basic idea is that 
the lowest portion of the atmosphere/upper interior seems to be 
depleted of these materials compared to what is expected from solar 
abundances, yet the deeper interior of the planet seems to have a 
super-abundance of these materials. This state suggests that what 
seems to have happened is the following, there is a region on the 
planet where conditions transition from the atmosphere to the interior, 
i.e. the pressure increases, and the temperature increases from freezing 
temperatures to moderately warm temperatures.  As the planet was forming, 
in this transition region, helium and neon "rained" from the 
lower-atmosphere/upper-interior to the deeper interior.  In other 
words, conditions were just right for helium and neon to condense 
into raindrops and fall to a region deeper inside the planet.  The 
effect of this "rain" would be to remove the affected elements 
from that region of the atmosphere/interior.