QUESTION:
Knowing that Jupiter radiates twice as much heat energy as it gets from the
sun, did the probe send you any information about the source of that energy?

ANSWER from Bob Gounley on February 21, 1996:  
The Galileo Probe did shed some light on the subject, although we already
had a pretty good idea of the heat source before then.

Jupiter is so large that it has kept much of the heat left over from the
formation.  The center of any object holds heat best since from there it
takes the longest to reach the outside.  That's one reason why Jupiter's
temperatures are relatively cool just below the cloud layers and get
progressively warmer the deeper you go.  It's a lot like a freshly baked
loaf of bread. 

The cooling takes a long time because Jupiter's gravity puts a lot of
pressure on everything in the middle.  Whenever you squeeze something and
try to make it smaller, it gives off heat.  That is the heat source --
Jupiter's gravity trying to make it into a smaller ball. 

So why isn't Jupiter a small ball?  Well, when you heat heat something, it
tries to grow.  Now you have gravity and heat fighting each other, one
trying to make Jupiter smaller and the other trying to make it bigger. 
Jupiter is the size it is now because the two forces balance each other. 

The Sun gives Jupiter some heat, though it mostly goes to warm the
cloudtops and the air just below it.  We can't send a probe all the way to
the center of Jupiter -- it would be crushed.  However, the Galileo Probe
had sensors on it to tell how much heat came from above (the Sun) and how
much from below (gravitational heating). With this information, scientist
will get a better idea of how warm things are inside. 

Now let's play a "what if" game.  What if Jupiter started out a whole lot 
bigger?  The pressures and temperatures in the middle would be really high.  
Warm and squeeze hydrogen atoms enough and they helium atoms and heat.  A lot 
more heat.  You would have a star, just like our sun.

Think about this the next time you see the sky at night.  If Jupiter had
been bigger, there would be a second sun in the sky.  Much of the time,
its light would hide the stars.  The rest of the year, with Jupiter and
the Sun in the same part of the sky, you would cast two shadows.  Can you
think of other things that would be different?