The idea of
compressing water is foreign to our daily experience.
Nevertheless, an
accurate estimate of water’s shrinking volume under the huge gravitational
pressures of large planets is essential to astrophysicists trying to model the
evolution of the universe. They need to assume how much space is taken up by
water trapped under high density and pressure, deep inside a planet, to
calculate how much is needed of other elements to flesh out the planet’s
astronomical image.
In a challenge to
current astrophysical models, researchers at Sandia National Laboratories and
the University of Rostock in Germany have found that current
calibrations of planetary interiors overstate water’s compressibility by as
much as 30%. The work was reported in the paper “Probing the Interior of the
Ice Giants” in Physical Review Letters.
“Our results question
science’s understanding of the internal structure of these planets,” said
Sandia lead author Marcus Knudson, “and should require revisiting essentially all
the modeling of ice giants within and outside our solar system.”
To come up with the
composition of the so-called ice-giants Neptune and Uranus, as well as any of
the ice-giant exoplanets being discovered in distant star systems,
astrophysicists begin with the orbit, age, radius, and mass of each planet.
Then, using equations that describe the behavior of elements as the forming
planet cooled, they calculate what light and heavy elements might have
contributed to its evolution to end up with the current celestial object.
But if estimates of
water volume are off-target, then so is everything else.
The measurements—10
times more accurate than any previously reported—at Sandia’s Z accelerator
agree with results from a modern simulation effort that uses the quantum
mechanics of Schrödinger’s wave equation—the fundamental equation of wave
mechanics—to predict the behavior of water under extreme pressure and density.
The model, developed
through a University
of Rostock and Sandia
collaboration, is called “First Principles Modeling” because it contains no
tuning parameters.
“You’re solving
Schrödinger’s equation from a quantum mechanical perspective with hydrogen and
oxygen as input; there aren’t any knobs for finagling the result you want or
expect,” Knudson said.
The model’s results
are quite different from earlier chemical pictures of water’s behavior under
pressure, but agree quite well with the Z machine’s test results, said Knudson.
These results were achieved by using Z’s magnetic fields to shoot tiny plates
40 times faster than a rifle bullet into a water-sample target a few
millimeters away. The impact of each plate into the target created a huge shock
wave that compressed the water to roughly one-fourth its original volume,
momentarily creating conditions similar to those in the interior of the ice
giants.
Sub-nanosecond
observations captured the behavior of water under pressures and densities that
occur somewhere between the surface and core of ice giants.
“We took advantage of
recent, more precise methods to measure the speed of the shock wave moving
through the water sample by measuring the Doppler shift of laser light
reflected from the moving shock front, to 0.1% accuracy,” said Knudson.
The re-shocked state
of water was also determined by observing its behavior as the shock wave
reflected back into the water from a quartz rear window (its characteristics
also determined) in the target. These results provided a direct test of the
First Principles model along a thermodynamic path that mimics the path one
would follow if one could bore deep into a planet’s interior.
Multiple experiments
were performed, providing a series of results at increasing pressures to create
an accurate equation of state. Such equations link changes in pressures with
changes in temperatures and volumes.
Z can create more
pressure—up to 20 megabars—than at Earth’s core (roughly 3.5 megabars), and
millions of times Earth’s atmospheric pressure. The Z projectiles, called flyer
plates, achieve velocities from 12 to 27 km a second, or up to 60,000 mph. The
pressure at the center of Neptune is roughly 8
megabars.
Water at Z’s
ice-giant pressures also was found to have reflectivity like that of a weak
metal, raising the possibility that water’s charged molecular fragments might
be capable of generating a magnetic field. This could help explain certain
puzzling aspects of the magnetic fields around Neptune and Uranus.
“Reducing uncertainty
on the composition of planetary systems by precisely measuring the equation of
state of water at extreme conditions can only help us understand how these
systems formed,” Knudson said.
These experimental
techniques also are used at Z to study materials of critical importance to the
nuclear weapons program. In addition to producing the largest amount of X-rays
on Earth when firing, the huge pressures generated by Z make it useful to
astrophysicists seeking data similar to that produced by black holes and
neutron stars.
