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March 2023 ESRFnews

ID15B tastes Jovian salty ices

Jupiter’s moons Europa and

Ganymede, as well as Saturn’s moons

Enceladus and Titan, are believed to

contain oceans of water under a crust

of ice. Quite how the water remains

liquid at such low temperatures

has been a mystery, however, as

spectroscopic signatures of the most

obvious antifreeze – common salt,

or sodium chloride – are absent from

the surface. Now a collaboration led

by the University of Washington in

Seattle, US, and the ESRF thinks it

has the answer, in newly discovered

salt hydrates that exist at extreme

conditions

Baptiste Journaux at the University

of Washington and colleagues

exposed salt water at low temperatures

and high pressures of 25 GPa to

Xrays at the ID15B beamline Here

Xray diffraction revealed two

previously unknown salt hydrates

that is salt crystals that trap water

within their crystalline structures

The first SC13 has one sodium

chloride molecule for every 13 water

molecules while the second SC85

which has two sodium chlorides for

every 17 water molecules, is predicted

by the researchers to be stable in the

conditions of the icy moons (PNAS

120 e2217125120). Until now,

scientists had known of only one

hydrate of sodium chloride, and it

contains much less water.

“[SC8.5] has the structure that

planetary scientists have been waiting

for to explain the mysterious surface

spectra of icy moons,” says Journaux.

“This will permit us to identify where

the best places on their surface are to

explore and eventually land and dig

to look for signs of life

Over the next two years two

missions are being launched to

study icy moons the European

Space Agencys JUpiter ICy moon

Explorer JUICE and NASAs

Europa clipper The team led by the

University of Washington and the

ESRF is planning to study other salt

species and to measure their spectral

properties in the hope that they will

be detected when these missions enter

Jupiters orbit early next decade

Thermoelectrics probed

Thermoelectrics have long been used

in specialist applications to convert

waste heat into usable electricity, but

the exact mechanism of their operation

has been unclear. Now researchers

at Université Bourgogne-Franche-

Comté in France, Oak Ridge National

Laboratory (ORNL) in Tennessee, US,

Columbia University and Brookhaven

National Laboratory in New York,

US, and the ESRF have uncovered

a detailed mechanism for one

thermoelectric, germanium telluride

(GeTe).

The team used the brilliant, high-

energy X-rays of the ESRF’s ID15A

beamline to investigate GeTe at

common operating temperatures

of around 550 °C, using the X-ray

technique known as the pair

distribution function. They

observed local structural deviations

that at first glance appeared to

represent static disorder, but turned

out, after complementary neutron

studies at ORNL, to be dynamic.

Simulations on ORNL supercomputers

confirmed the possibility of

picosecond atomic vibrations, which

disrupt heat flow and help to provide

the thermoelectric effect.

The X-ray data also revealed that

the crystal structure of GeTe is much

more rigid in some directions than

others. Theorists in the team believe

this is because the local structural

deviations reinforce chemical bonding

in those directions, favouring electrical

conductivity. They also believe that

their model is general enough to be

applied to many other thermoelectric

materials (Nat. Mater. 22 311).

“The high-energy X-rays available

at ID15 were crucial for this study, as

they allowed us to chart the real-space

atomic displacements relevant to

thermoelectricity in exquisite detail,”

says Simon Kimber, who was an ESRF

postdoc at the time of the experiments.

S K I M B E R

Above a fragment of the disordered

GeTe structure required to fit the data

The Ge sites purple must be displaced

and split from their positions

This salt

hydrate has

the structure

that planetary

scientists have

been waiting for

to explain the

mysterious

surface spectra

of icy moons

N A S A/J P L-C A L T E C H/S E T I I N S T I T U T E

Despite its very

cold temperatures,

Jupiter’s moon

Europa harbours

liquid water under

a crust of ice.