Salt hydrates at high pressure and low temperature: Implications for icy moon explorations

A picture of Europe, with its white surface scarred with pinkish lines. — Europa ©ESA

Summary

The icy moons of Jupiter and Saturn such as Europa, Ganymede, Titan and Enceladus, composed of a silicate rock mantle covered by water and ice envelopes (hydrosphere), are promising candidates for hosting habitable extra-terrestrial environments [1]. Within these ice-rich moons, hydrated forms of salts are expected to form in the icy crust and at the bottom of the ocean where dense high-pressure ice polymorphs are present [Figure 1]. The cycle of these salts in solution and in solid form as hydrates are key for shaping icy ocean world internal structures as well as compositions of their surfaces, and may be crucial in promoting habitable conditions.

Little is known about the structures that water and salt adopt at the high pressure and low temperature conditions of icy moon and exoplanet hydrospheres [Figure 1]. This project goal is to extend the database of known salt hydrates by characterising a range of water-salt mixtures as a function of both pressure and temperature using in-situ single-crystal diffraction. The expanded database of salt hydrates will allow the identification of important fundamental chemical-structural relationships and directly support the study of icy moons and their potential habitable oceans.

Figure 1. Schematic of icy moon and ocean exoplanet interiors, highlighting the role of ice and salt convection in the icy mantle. Figure reproduced from Ref. [2].

Project Description

Possible chemical compositions of icy moon hydrospheres include salt-water mixtures of [NaCl, MgSO4, MgCl2, KCl, CaCO3]-n[H2O] [1]. Most of the currently known salt hydrate phases have only been studied at ambient conditions, yet low temperatures and high pressures are important thermodynamic variables to stabilise new forms of salt hydrates [3, 4, 5]. Performing in-situ experiments are crucial for identification of new salt hydrates in extreme environments found in icy moons. This is highlighted by the recent discovery of new sodium chloride hydrates, with three new structures at high pressure and low temperature showing unique structural configuration unseen for that type of salts [4]. In this study, it is shown that the application of pressure and cooling favours the formation of new hyperhydrated salts structures (dissociated ions) [Figure 2], but this remains to be studied for many other salt systems to better constrains the controlling factors of salt hydrate structural diversity.

Figure 2. Schematic phase diagram of NaCl-water mixtures as a function of pressure and temperature. Hyperhydrated structures of sodium chloride hydrates are formed when both pressure and cooling are applied.

Project goals

The main aims of this project are to:

i) survey novel salt systems for which high pressure and low temperature exploration has not yet been achieved.

ii) study the structural diversity of salt hydrates and identify controlling factors on the hydration level to identify general chemical-structural relationships.

iii) measure the new salt hydrates using spectroscopic methods (Raman and Infra-red) and provide characterisation data for the instruments onboard space missions exploring outer solar system icy ocean worlds.

To achieve this, we will use in-situ low-temperature high-pressure single-crystal diffraction to systematically characterise the hydration formation and structure of salt hydrates. The majority of the work will be performed at NHM on the synergy single-crystal diffractometer with the use of a cryo-cooled diamond anvil cell. This in-house research will be supplemented by neutron and synchrotron experiments at facilities such as the ISIS neutron source, Diamond Light Source and the European Synchrotron Radiation Facility. Spectroscopic characterisation (Raman and Infra-red) will be performed in collaboration with Prof. Baptiste Journaux at the University of Washington and at UCL. This work is directly relevant to ongoing exploration efforts of icy moon composition with the recently launched ESA Jupiter Icy Moons Explorer (JUICE) and upcoming NASA Europa Clipper space missions [6, 7].

The student will learn high-pressure experimental techniques, such as diamond-anvil cell (DAC) preparation, DAC loadings, and pressure measurements using ruby fluorescence. The student will become proficient in conducting and analysing single-crystal and powder diffraction data performed at ambient conditions and high pressure. A high-pressure and low-temperature setup will be developed and put in place for this project using a 3D-printed compact cryostat based on a model developed at the European Synchrotron Radiation Facility. The student will become familiar with performing experiments at central facilities such as the European Synchrotron Radiation Facility and ISIS neutron source.

Suggested Skills and Background

We seek an enthusiastic and inquisitive person with a strong background in crystal chemistry and an interest to carry out complex in-situ experiments at NHM and at different facilities worldwide. The student will become integrated into the Planetary Materials Group at the Natural History Museum and will be affiliated to University College London. The student will benefit from STFC-led training opportunities throughout the studentship.

For informal enquiries or further information, please contact Paul Schofield (p.schofield@nhm.ac.uk).

Application Process

Deadline: Sunday 4th February 2024.

Please upload the following documents:

Curriculum vitae
Covering letter outlining your interest in the PhD project, relevant skills training, experience and qualifications, and a statement of how this PhD project fits with your career development plans.
Transcripts of undergraduate and Masters’ degree results.
Two academic references including (if applicable) Masters’ project supervisor.

We strongly recommend contacting potential supervisors in advance, so that you can ask any questions, and also meet the supervisory team before you apply. This project is eligible for funding from the Science and Technology Facilities Council, who provide information for students: https://www.ukri.org/what-we-do/developing-people-and-skills/stfc/training/studentship-information-for-students/

This is a competitive application process. All applications will be reviewed by the project supervisory team and an academic panel. Shortlisted applicants will be invited for an interview, which usually lasts 30-60 minutes.

As part of the process, shortlisted applicants will also be offered the opportunity to visit the NHM, to meet the wider research group, and tour relevant facilities. Shortlisted applicants will usually find out the outcome a few days after all interviews have been held.