S C

IE N

T IF

IC H

IG H

LI G

H T

S E

N V

IR O

N M

E N

T, E

A R

T H

A N

D P

L A

N E

T A

R Y

S C

IE N

C E

S

1 3 5 I H I G H L I G H T S 2 0 2 3

PRINCIPAL PUBLICATION AND AUTHORS

On the identification of hyperhydrated sodium chloride hydrates stable at icy moon conditions, B. Journaux (a), A. Pakhomova (b,c), I.E. Collings (c,d), S. Petitgirard (e), T.B. Ballaran (f), J.M. Brown (a), S.D. Vance (g), S. Chariton (h), V.B Prakapenka (h), D. Huang (e), J. Ott (a), K. Glazyrin (b), G. Garbarino (c), D. Comboni (c), M. Hanfland (c), Proc. Natl. Acad. Sci. U.S.A. 120, 9, e2217125120 (2023); https:/doi.org/10.1073/pnas.2217125120 (a) Department of Earth and Space Sciences, University of Washington, Seattle (USA) (b) Deutsches Elektronen-Synchrotron, Hamburg (Germany) (c) ESRF (d) Center for X-ray Analytics, Empa - Swiss Federal Laboratories for Materials Science and Technology, Dübendorf (Switzerland) (e) Institute of Geochemistry and Petrology, ETH Zürich, Zürich (Switzerland) (f) Bayerisches Geoinstitut, University of Bayreuth (Germany) (g) Jet Propulsion Laboratory, California Institute of Technology, Pasadena (USA) (h) Center for Advanced Radiations Sources, University of Chicago (USA)

REFERENCES

[1] B. Journaux et al., Space Sci. Rev. 216, 7 (2020).

facilitates the dissociation of Na+ and Cl- ions, and possibly other alkali halindes species, to form hyperhydrated phases.

The thermodynamic stability of these phases was studied through measuring melting curves, performing isothermal transect and quenching phases down to ambient pressure from 150 K to 300 K. From these data, it appears that SC8.5 and SC13 are stable phases, while it remains unclear for the third phase. SC13 is stable above 275 K and 1.5 GPa, up to 2.3 GPa. SC8.5 is stable below 275 K at high pressures, but the results also suggest that it is stable at ambient pressure below 235 K for less than 27.6 wt% NaCl. This implies an update to the canonical H2O NaCl phase diagram, the first in 150 years, to include the stability range of SC8.5 (Figure 109).

Because icy moon surface temperatures are generally below 150 K, the results of this work suggest that SC8.5 should be the most common NaCl hydrate at the surface of icy worlds, rather than hydrohalite (SC2). This discovery provides a simple explanation for the lack of hydrohalite detection at the surface of Europa, while clear NaCl signatures have been observed as well as spectral features suggesting high water of hydration chlorine-rich hydrates. The results of this study suggest that a large diversity of hyper-hydrated water-salt phases, stable at low temperature, remains to be discovered. Such work is crucial to support the next generation of robotic space missions, such as ESA JUICE, NASA Europa Clipper and NASA DragonFly, that will study the surface mineralogy of icy worlds and explore the potential habitability of their ocean.

Fig 109: Updated H2O-NaCl phase diagram at 1 atmosphere with the addition of SC8.5 stability field below 235 K eutectic reported.