Hypervelocity impacts into water ice: measuring shockwave propagation to validate hydrocodes

Impact processes are generally regarded as destructive events in the history of our Solar System. However, previous experiments have shown that hypervelocity impacts into simple ice mixtures (e.g., H2O, CO2, NH4) can produce sufficient energy to synthesise more complex matter, such as amino acids [e.g., 1], as well as alter existing organic material, such as forming longer-chain organics when impacting with a polymer projectile [e.g., 2]. The energy produced on impact varies depending on the impactor and target compositions, as well as the impact velocity, which influence the peak temperatures and pressures achieved on impact. Currently, we rely on computational (‘hydrocode’) modelling to inform us on the conditions achieved during impact, which subsequently inform us on the conditions required for synthesis. However, water ice has a complex solid phase diagram, and consequently is a difficult material to model, therefore experimental data (specifically temperature and pressure) are sorely needed to test and validate such models. 

Previous work has demonstrated the ability to experimentally determine temperatures achieved on impact through studying impact flashes (‘self-luminous plumes’) produced during high velocity phenomena [3,4], with a preliminary concept for experimentally measuring impact pressures [5]. This work, developed by Spathis et al., used prototype strain gauges embedded in epoxy, which were subsequently implanted into ice targets. However, it was difficult to calibrate these sensors and, as they were bulky, that may have interfered with the shock propagation through the ice. In conjunction with Odin Space Ltd, we have now refined our pressure sensor system to ones which are calibrated to the 10s of GPa range. These more compact sensors are then implanted into the ice as before and connected to front-end electronics to read the sensors’ output. 

Initial impact experiments were performed using the All-Axis Light-Gas Gun at the Open University, where water ice targets (Fig. 1) were impacted using a 2 mm stainless steel projectile at 1.40 kms-1 and 5.14 kms-1. Results (e.g., Fig. 2) show the change in the speed of the shockwave for the two shot regimes as well as the decrease in the amplitude, although the wave seems to decelerate very quickly moving between the sensors. We have data to calibrate the pressure seen by the sensors to validate the readings for comparison with computational data, and hydrocode modelling is ongoing using the strength model described in Ma et al., 2023 [6]. Furthermore, studies into the structure of the ice will be performed using cryo-Raman spectroscopy, in collaboration with colleagues at the Open University, to investigate how the compression experienced during impact events influences the ice lattice. Finally, we hope to resurrect our homemade high-speed spectrometer constructed from an electric toothbrush, presented at EPSC 2025 [7], to investigate the spectra of the impact flashes. 

With ice being prevalent across bodies in our Solar System and studies into material and organic synthesis increasing, it is important to validate hydrocode modelling so we can confidently model impact events impossible to recreate in the lab (due to sizes/impact speeds). Additionally, with the search for life in our Solar System turning towards icy worlds, and missions such as NASA’s Europa Clipper and the European Space Agency’s first Large-class mission of Voyage 2050, ‘L4’, to Enceladus and the Saturn System aiming to search for habitability and signs of life on the icy moons, it is important to understand what detections (e.g., of organic molecules) have a biological origin, and which may be the result of environmental processes. Developing this system will allow us to experimentally constrain the conditions required for synthesis, or those where we expect none, which can provide valuable insight into sampling depth considerations, as well as useful references for contextualising mission data. 

Acknowledgements: The authors would like to thank Dr Z. Emerland and Dr M. Sylvest from the Open University Hypervelocity Impact Laboratory for operating the light-gas gun. 

References: [1] Z. Martins et al. (2013). Nat. Geoscience, 6, 1045.; [2] V. Spathis et al., 2021, LPSC Abstract # 1623; [3] V. Spathis et al., 2021, LPSC Abstract # 1625; [4] Spathis et al., 2021, IAC Abstract # 64949; [5] M. C. Price et al., 2021, LPSC, Abstract # 1328; [6] Ma et al., 2023, IJIE, 172, 104375; [7] M.C. Price & V. Spathis, 2025, EPSC Abstract # 200. 

Fig.1: Water ice target with embedded sensor system prior to impact 

 

Fig.2: Output from the pressure sensors embedded in an ice target impacted at 5.14 km/s with a 2-mm diameter stainless-steel projectile (blue: top sensor, red: middle sensor, black: bottom sensor). The decrease in the speed and amplitude of the shock wave can be clearly seen.