Physics
Today: In a paper
published
in
Physical Review Letters
, University of California
Berkeley scientists Hugh F. Wilson and
Burkhard
Militzer have come up with an explanation for why the top
layers of Jupiter's atmosphere are severely depleted in helium
and neon compared to other
protosolar
values.
The conclusions are from the
Galileo space probe that plunged into Jupiter's
atmosphere in 1995 and computer simulations of the data sent
back before the probe was crushed.
A failed starThe
Galileo data confirmed that Jupiter could be
considered a "failed star," —it didn't have enough size
to ignite like the Sun but contains similar elements from
around the time the solar system formed 4.56 billion years ago.
The one crucial and unexpected difference was for the amount of
helium and neon. Neon stood out because it was one-tenth as
abundant as it is in the Sun.
Caltech's
David Stevenson had predicted neon depletion on Jupiter
prior to the
Galileo measurements, but he never published a reason
for his guess.
Simulations by Wilson and Militzer, however, suggest that
helium condenses and drops into the deep interior like rain,
because at that level in Jupiter's atmosphere, hydrogen has
turned into a metal around 5000 °C and 1 million-2 million
times Earth's atmospheric pressure. Helium is not yet a metal
under those conditions and does not mix with metallic hydrogen,
so it forms drops, like those of oil in water.According to the
simulations, neon easily dissolves in the helium drops as they
fall, pulling both elements deeper into the planet. Eventually
the two elements separate out where helium and neon again mix
with metallic hydrogen (see image at right; credit: Burkhard
Militzer). That would explain why the upper layers of the
atmosphere are depleted."Helium condenses initially as a mist
in the upper layer, like a cloud, and as the droplets get
larger, they fall toward the deeper interior," said Wilson.
"Neon dissolves in the helium and falls with it. So our study
links the observed missing neon in the atmosphere to another
proposed process, helium rain."Militzer notes that "rain" is an
imperfect analogy to what happens in Jupiter's atmosphere. The
helium droplets form about 10 000 to 13 000 kilometers below
the tops of Jupiter's hydrogen clouds, under pressures and
temperatures so high that "you can't tell if hydrogen and
helium are a gas or a liquid," he said. A better way to
visualize it, he said, is to consider the rain as droplets of
fluid helium mixed with neon falling through a fluid of
metallic hydrogen."Our research will help refine models of
Jupiter's interior and the interiors of other planets," said
Wilson. Modeling planetary interiors has become a hot research
area since the discovery of hundreds of extrasolar planets
living in extreme environments around other stars. The study
will also be relevant for
NASA's Juno mission to
Jupiter, which is scheduled to be launched next year.Paul
GuinnessyRelated link
Sequestration
of noble gases in giant planet interiors
Simulations by Wilson and Militzer, however, suggest that
helium condenses and drops into the deep interior like rain,
because at that level in Jupiter's atmosphere, hydrogen has
turned into a metal around 5000 °C and 1 million-2 million
times Earth's atmospheric pressure. Helium is not yet a metal
under those conditions and does not mix with metallic hydrogen,
so it forms drops, like those of oil in water.According to the
simulations, neon easily dissolves in the helium drops as they
fall, pulling both elements deeper into the planet. Eventually
the two elements separate out where helium and neon again mix
with metallic hydrogen (see image at right; credit: Burkhard
Militzer). That would explain why the upper layers of the
atmosphere are depleted."Helium condenses initially as a mist
in the upper layer, like a cloud, and as the droplets get
larger, they fall toward the deeper interior," said Wilson.
"Neon dissolves in the helium and falls with it. So our study
links the observed missing neon in the atmosphere to another
proposed process, helium rain."Militzer notes that "rain" is an
imperfect analogy to what happens in Jupiter's atmosphere. The
helium droplets form about 10 000 to 13 000 kilometers below
the tops of Jupiter's hydrogen clouds, under pressures and
temperatures so high that "you can't tell if hydrogen and
helium are a gas or a liquid," he said. A better way to
visualize it, he said, is to consider the rain as droplets of
fluid helium mixed with neon falling through a fluid of
metallic hydrogen."Our research will help refine models of
Jupiter's interior and the interiors of other planets," said
Wilson. Modeling planetary interiors has become a hot research
area since the discovery of hundreds of extrasolar planets
living in extreme environments around other stars. The study
will also be relevant for
NASA's Juno mission to
Jupiter, which is scheduled to be launched next year.Paul
GuinnessyRelated link
Sequestration
of noble gases in giant planet interiors
