16/11/2014
Berkeley - October 1,
1999 - If experiments at
the University of
California, Berkeley, are
any indication, future
explorers of our solar system may
well find diamonds hailing down
through the atmospheres of Neptune
and Uranus.
These planets contain a high
proportion of methane, which UC
Berkeley researchers have now
shown can turn into diamond at the
high temperatures and pressures
found inside these planets.
"Once these diamonds form, they fall
like raindrops or hailstones toward
the center of the planet," said Laura
Robin Benedetti, a graduate student in
physics at UC Berkeley.
The team, led by Benedetti and
Raymond Jeanloz, professor of
geology and geophysics, produced
these conditions inside a diamond
anvil cell, squeezing liquid methane to
several hundred thousand times
atmospheric pressure. When they
focused a laser beam on the
pressurized liquid, heating it to some
5,000 degrees Fahrenheit, diamond
dust appeared.
They report their experimental
findings in a paper in the Oct. 1 issue
of Science.
The demonstration that methane can
convert to diamond as well as other
complex hydrocarbons in the interiors
of giant planets like Neptune hint at a
complex chemistry inside gaseous
planets and even brown dwarf stars.
Brown dwarfs are small, dim stars
barely larger than the largest gas
giant planets.
"This is opening the door to study of
the interesting types of chemical
reactions taking place inside planets
and brown dwarfs," Jeanloz said.
"Now that technology is able to
reproduce the high pressures and
temperatures found there, we are
getting much better quality
information on the chemical reactions
taking place under these conditions."
"It is not amazing that chemistry like
this happens inside planets, it's just
that most people haven't dealt with
the chemical reactions that can
occur," Benedetti said. "The interior
of these planets may be much more
complicated that our current picture."
A simple calculation, for example,
shows that the energy released by
diamonds settling to the planet's core
could account for the excess heat
radiated by Neptune, that is, the heat
given off by Neptune in excess of
what it receives from the sun.
"What's exciting to us is the
application of this high-pressure
chemistry to understanding the outer
planets," Jeanloz said.
"As more planets are found in
unexpected orbits around other stars,
the effects of internal chemical
processes will need to be further
clarified in order to obtain a general
understanding of planet formation and
evolution," the authors concluded in
the Science paper.
Our solar system's other gas giant
planets -- Jupiter and Saturn -- may
also contain diamonds produced under
such conditions, though they contain
proportionately less methane than
Neptune and Uranus. Based on
theoretical calculations, Neptune and
Uranus are estimated to contain about
10 to 15 percent methane under an
outer atmosphere of hydrogen and
helium. (See graphic for presumed
internal structure of Neptune,
Several groups of researchers have
suggested that the methane in these
planets could conceivably turn into
diamond at fairly shallow depths,
about one tenth of the way to the
center. Nearly two decades ago, a
group at Lawrence Livermore National
laboratory shocked some methane and
reported the formation of diamond
before the stuff evaporated. That
group was led by retired scientist
Marvin Ross and researchers William
Nellis and Francis Ree.
Recently some theorists in Italy also
concluded that diamonds were likely.
Benedetti and Jeanloz decided to try
the obvious experiment -- squeeze
liquid methane and see if they could
make diamond dust.
The liquid methane, cooled with liquid
nitrogen, was placed in a diamond
anvil cell and squeezed to between 10
and 50 billion pascals (gigapascals),
or about 100,000 - 500,000 times
atmospheric pressure. The
researchers then heated the
compressed methane with an infrared
laser to about 2,000 to 3,000 Kelvin
(3600-5400 degrees Fahrenheit).
"It's really cool to watch," said
Benedetti. "When you turn on the
laser the methane turns black
because of all the diamonds created.
The black diamond specks float in a
clear hydrocarbon liquid melted by
the laser."
Raman spectroscopy confirmed the
identity of the suspended specks, as
did subsequent analysis with X-ray
crystallography. The flecks were
diamonds interspersed with
hydrocarbons.
Jeanloz said that the high
temperature breaks up methane (CH4)
into carbon and hydrogen, while high
pressure condenses the carbon to
diamond. Other types of hydrocarbons
-- doubly and triply bonded carbon --
also were produced, apparently in the
cooler areas outside that illuminated
by the laser.
Jeanloz and his team plan next to see
what happens to other constituents of
these planets -- ammonia and water
-- at high temperatures and
pressures.
Coauthors of the paper with Benedetti
and Jeanloz are post-doctoral
researcher Jeffrey H. Nguyen, now a
scientist at Lawrence Livermore
National Laboratory; geology graduate
student Wendell A. Caldwell, Chinese
visiting scholar Hongjian Liu and
Michael Kruger, a former graduate
student now in the physics
department at the University of
Missouri, Kansas City.
