Neutron Scattering as a Powerful Tool for Studying Multi-Scale Structure for Planetary Sciences

Neutrons are a powerful probe and vital tool for a wide range of scientific and industrial fields. Neutron scattering is used to obtain a direct and detailed insight into the structure and dynamics of condensed matter – matching dimensions in space from single atoms to macromolecules and in time from atomic vibrations to diffusion of large molecular units – all depending on the instrument and facility used. Compared to other scattering probes, neutrons are non-interacting and scatter off of the nucleus - allowing for much deeper penetration. This unique mechanism gives it the ability to ‘see’ hydrogen with high sensitivity, which is present in most molecules relevant to planetary and interstellar environments, and allows it to differentiate between isotopes. By exploiting the isotopic differentiation, such as using selective H/D substitution, we can also highlight interatomic correlations of interest. Another important advantage is that complex sample environments can be used, such as vacuum chambers and pressure cells, to recreate the conditions across various regions. This range of techniques has much to offer the Planetary Sciences and Astrochemistry communities and there is much to be gained from its increased use/collaboration. In this presentation, I will present what is on offer to these communities, starting with an introduction to the main techniques (diffraction, small-angle, dynamics, spectroscopy and imaging) and then I will go through examples of neutron work we have previously and are currently undertaking with relevance to both the Solar System and the Interstellar Medium (ISM).

The presentation will focus on some recent examples in detail. We have uniquely employed an in-situ ice vapour-deposition experiment (high vacuum) on the instruments NIMROD and Sans2d, at the ISIS Neutron and Muon Source in the UK, to study the structure of Amorphous Solid Water (ASW), as a function of growth and thermal evolution.^1–3 Neutrons gave us the ability to directly probe (non-destructively) the general bulk structure of ASW, along with its porosity (volume fraction, pore sizes, shapes etc.), surface area and crystallinity, and how this all evolves with time and temperature. Altogether, the results converge to form a new picture of the ASW structure and nanoporosity during growth and annealing, involving microporous islands with voids between them (see Fig.1). This work has drastically changed our picture of ice astrophysics and the role it plays in planet- and star-formation processes.

Figure 1. Cartoon representation of the structure of ASW theorised in this work – islands/grains with voids between them (not to scale).

Another avenue of ice studies conducted using NIMROD and Polaris at the ISIS Facility led to the discovery of a new metastable dihydrate of sodium chloride at ambient pressure.^2,4,5 Neutrons were key here with their particular sensitivity to the position of hydrogen, which makes up a large part of the NaCl water ice mixture. This dihydrate should be stable at the surfaces of icy worlds, such as Europa and Enceladus, and if it were detected then that would indicate regions of recent activity where subsurface brines have frozen rapidly, which are priorities for upcoming planetary missions, such as ESA’s JUICE and NASA’s Europa Clipper.

NIMROD and SANDALS at the ISIS Facility specialise in studying disordered materials such as liquids and glasses.^2,7 They allow for the full structural characterisation of such a material and the interactions between all its components. One example of long-term work using both instruments is the study of the role of magnesium perchlorate (Mg(ClO₄)²) in the subsurface water on Mars. Initially, it was found that the presence of Mg(ClO₄)² has a significant effect on the structure of water, making it appear as though it is under massive pressure, even when no external pressure is applied.⁶ Although this is the case, it was still found in a follow up study that amino acids, such as glycine in this case, can still self-assemble under these conditions, even though the Mg(ClO₄)² disrupts its hydration and hydrogen bonding ability.⁸ This happened more readily at low temperatures and so it seems possible to have biological molecules forming in the Martian environment. Finally, it was recently found that the presence of the osmolyte trimethylamine N-oxide (TMAO) could undo the pressuring effect of Mg(ClO₄)² on water, which would help prevent damage to biological systems.⁹

References
[1] Z. Amato, T. F. Headen, S. G ¨artner, P. Ghesqui `ere, T. G. A. Youngs, D. T. Bowron, L. Cavalcanti, S. E. Rogers, N. Pascual, O. Auriacombe, E. Daly, R. E. Hamp, C. R. Hill, R. K. TP and H. J. Fraser, Phys. Chem. Chem. Phys., 2025, 27, 6616–6627.
[2] ISIS, NIMROD, 2025, https://www.isis.stfc.ac.uk/Pages/nimrod.aspx, Accessed: 02-04-2025.
[3] ISIS, Sans2d, 2025, https://www.isis.stfc.ac.uk/Pages/Sans2d.aspx, Accessed: 02-04-2025.
[4] R. E. Hamp, C. G. Salzmann, Z. Amato, M. L. Beaumont, H. E. Chinnery, P. Fawdon, T. F. Headen,
P. F. Henry, L. Perera, S. P. Thompson and M. G. Fox-Powell, The Journal of Physical Chemistry Letters, 2024, 15, 12301–12308.
[5] ISIS, Polaris, 2025, https://www.isis.stfc.ac.uk/Pages/Polaris.aspx, Accessed: 02-04-2025.
[6] S. Lenton, N. H. Rhy, J. J. Towey, A. K. Soper and L. Dougan, Nat Commun, 2017, 8, 919.
[7] ISIS, SANDALS, 2025, https://www.isis.stfc.ac.uk/Pages/sandals.aspx, Accessed: 02-04-2025.
[8] H. Laurent, A. K. Soper and L. Dougan, Molecular Physics, 2019, 117, 3398–3407.
[9] H. Laurent, T. G. A. Youngs, T. F. Headen, A. K. Soper and L. Dougan, Communications Chemistry, 2022, 5, 116.