Evidence of superionic ice provides new insights into unusual magnetic
fields of Uranus and Neptune
How a conductive form of ice is formed at several thousand degrees and
millions of times atmospheric pressure.
Date:
October 14, 2021
Source:
GFZ GeoForschungsZentrum Potsdam, Helmholtz Centre
Summary:
Not all ice is the same. The solid form of water comes in more than
a dozen different - sometimes more, sometimes less crystalline -
structures, depending on the conditions of pressure and temperature
in the environment. Superionic ice is a special crystalline form,
half solid, half liquid - and electrically conductive. Its existence
has been predicted on the basis of various models and has already
been observed on several occasions under - very extreme - laboratory
conditions. New results provide another piece of the puzzle in the
spectrum of the manifestations of water. And they may also help
to explain the unusual magnetic fields of the planets Uranus and
Neptune, which contain a lot of water.
FULL STORY
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Not all ice is the same. The solid form of water comes in more than
a dozen different -- sometimes more, sometimes less crystalline --
structures, depending on the conditions of pressure and temperature in
the environment.
Superionic ice is a special crystalline form, half solid, half liquid --
and electrically conductive. Its existence has been predicted on the basis
of various models and has already been observed on several occasions under
-- very extreme -- laboratory conditions. However, the exact conditions
at which superionic ices are stable remain controversial. A team of
scientists led by Vitali Prakapenka from the University of Chicago,
which also includes Sergey Lobanov from the German Research Center for
Geosciences GFZ Potsdam, has now measured the structure and properties
of two superionic ice phases (ice XVIII and ice XX). They brought water
to extremely high pressures and temperatures in a laser-heated diamond
anvil cell. At the same time, the samples were examined with regard to
structure and electrical conductivity. The results were published today
in the journal Nature Physics. They provide another piece of the puzzle
in the spectrum of the manifestations of water. And they may also help
to explain the unusual magnetic fields of the planets Uranus and Neptune,
which contain a lot of water.
==========================================================================
Hot ice? Ice is cold. At least type I ice from our freezer, snow or
from a frozen lake.
In planets or in laboratory high-pressure devices, there are different
species of ice, type VII or VIII, for example, which exist at several
hundred or thousand degrees Celsius. However, this is only because of
very high pressures of several ten Gigapascal.
Pressure and temperature span the space for the so-called phase diagram of
a substance: Depending on these two parameters, the various manifestations
of water and the transitions between solid, gaseous, liquid and hybrid
states are recorded here -- as they are predicted theoretically or have
already been proven in experiments.
Linking fundamental physics with geological questions The higher the
pressure and temperature, the more difficult such experiments are. And
so the phase diagram of water -- with ice as its solid phase -- still
has quite a few inaccuracies and inconsistencies in the extreme ranges.
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"Water is actually a relatively simple chemical compound consisting of
one oxygen and two hydrogen atoms. Nevertheless, with its often unusual
behaviour, it is still not fully understood. In the case of water, the
fundamental physical and geoscientific interests come together because
water plays an important role inside many planets. Not only in terms of
the formation of life and landscapes, but -- in the case of the gaseous
planets Uranus and Neptune - - also for the formation of their unusual
planetary magnetic fields," says Sergey Lobanov, geophysicist at GFZ
Potsdam.
Unique conditions in the lab Sergey Lobanov is part of the team led
by first author Vitali Prakapenka and Nicholas Holtgrewe, both from
the University of Chicago, and Alexander Goncharov from the Carnegie
Institution of Washington. They have now further characterized the phase
diagram of water at its extremes. Using laser-heated diamond anvil cells
-- the size of a computer mouse -- they have generated high pressures
of up to 150 Gigapascal (about 1.5 million times atmospheric pressure)
and temperatures of up to 6,500 Kelvin (about 6,227 degrees Celsius).
In the sample chamber, which is only a few cubic millimetres in size,
conditions then prevail that occur at the depth of several thousand
kilometres inside Uranus or Neptune.
The scientists used X-ray diffraction to observe how the crystal structure
changes under these conditions. They carried out these experiments using
the extremely bright synchrotron X-rays at the Advanced Photon Source
(APS) of the Argonne National Laboratory at the University of Chicago. A
second series of experiments at the Earth and Planets Laboratory of the
Carnegie Institution of Washington used optical spectroscopy to determine
the electronic conductivity.
Structural changes in ice as it passes through phase space: formation of
superionic ice The researchers first produced ice VII or X from water at
room temperature by increasing the pressure to several tens of Gigapascal
(see the phase diagram).
And then, at constant pressure, they increased the temperature by heating
it with laser light. In the process, they observed how the crystalline
ice structure changed: First, the oxygen and hydrogen atoms moved a
little around their fixed positions. Then only the oxygen remained
fixed and formed its own cubic crystal lattice. As the temperature
rose, the hydrogen ionised, i.e. gave up its only electron to the
oxygen lattice. Its atomic nucleus -- a positively charged proton --
then whizzed through this solid, making it electrically conductive. In
this way, a hybrid of solid and liquid is created: superionic ice.
==========================================================================
Its existence was predicted on the basis of various models and has already
been observed on several occasions under laboratory conditions. The
scientists have now been able to synthesize and identify two superionic
ice phases -- ice XVIII and ice XX -, and to delineate the pressure and
temperature conditions of their stability. "Due to their distinct density
and increased optical conductivity, we assign the observed structures
to the theoretically predicted superionic ice phases," explains Lobanov.
Consequences for the explanation of the magnetic field of Uranus and
Neptune In particular, the phase transition to a conducting liquid
has interesting consequences for the open questions surrounding the
magnetic field of Uranus and Neptune, which presumably consist of more
than sixty percent water. Their magnetic field is unusual in that it
does not run quasi parallel and symmetrically to the axis of rotation
-- as it does on Earth -- but is skewed and off-centre. Models of its
formation therefore assume that it is not generated -- as on Earth --
by the motion of molten iron in the core, but by a conductive water-rich
liquid in the outer third of Uranus or Neptune.
"In the phase diagram, we can draw the pressure and temperature in the
interiors of Uranus and Neptune. Here, the pressure can roughly be taken
as a measure of the depth inside. Based on the refined phase boundaries
we have measured, we see that about the upper third of both planets
is liquid, but deeper interiors contain solid superionic ices. This
confirms the predictions about the origin of the observed magnetic field,"
Lobanov sums up.
Outlook The geophysicist emphasises that further investigations to
better clarify the inner structure and the magnetic field of the two
gas planets will be carried out at the GFZ. Here, in addition to the
diamond anvil cells already in use, there is both the corresponding
high-pressure laboratory and the highly sensitive spectroscopic measuring
equipment. Lobanov set up the latter as part of his funding as head
of the Helmholtz Young Investigators Group CLEAR to investigate the
phenomena of the deep Earth with unconventional ultra-fast time-resolved
spectroscopy techniques.
Funding: The work of Sergey Lobanov was supported within the Helmholtz
Young Investigators Program CLEAR (VH-NG-1325).
==========================================================================
Story Source: Materials provided by
GFZ_GeoForschungsZentrum_Potsdam,_Helmholtz_Centre.
Original written by Uta Deffke. Note: Content may be edited for style
and length.
==========================================================================
Journal Reference:
1. Vitali B. Prakapenka, Nicholas Holtgrewe, Sergey S. Lobanov,
Alexander F.
Goncharov. Structure and properties of two superionic ice
phases. Nature Physics, 2021; DOI: 10.1038/s41567-021-01351-8
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Link to news story:
https://www.sciencedaily.com/releases/2021/10/211014131203.htm
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