Jupiter vs. Saturn: Puzzling differences between giants
                 
Scientists aren't sure what the interiors of Jupiter and Saturn
look like or how the planets formed. But a new study of their
insides suggests they took different paths to giant status.

The elemental makeup of Jupiter and Saturn is similar, so why
would only Saturn end up with a heavy core?

NASA/JPL
Researchers modeled 50,000 what-ifs of internal structure
using real data from the two planets and lab experiments
that show how material behaves under extreme pressure.
They found Saturn has a huge core and Jupiter may have
none.

"Heavy elements are concentrated in Saturn's massive core,
while those same elements are mixed throughout Jupiter,
with very little or no central core at all," said Didier
Saumon of the Los Alamos National Laboratory.

Why so different?
Both planets are about 70% hydrogen. Most of the rest of
their makeup is helium. The remaining "heavy elements"
include iron, silicon, carbon, nitrogen and oxygen. 

But why would only Saturn end up with a heavy core? The
question is a troubling one for planet-formation theorists. 

The leading model for giant planet formation involves first
growing a core of rock and ice several times the mass of
Earth. Once big enough, this core easily attracts and holds
onto gas, sweeping through the nascent solar system and
glomming on to hydrogen and other elements left behind in
the Sun's formation.

But this so-called core-accretion model has a problem. It
takes millions of years to build a gas giant, and
observations of other star systems suggests the gas swirling
around a newborn star doesn't hang around that long.

"We don't really believe the two planets would have formed by
two processes so different," he said. 

By Robert Roy Britt, SPACE.com

Posted 7/20/2004 12:31 PM
http://www.usatoday.com/tech/news/2004-07-20-jupiter-vs-saturn_x.htm

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