Hypervelocity impact studies on Enceladus analogue ices

Enceladus is an icy moon of Saturn, with a surface composed predominantly of water ice [1]. It has large water vapour plumes, which eject icy grains and volatiles from its subsurface ocean into space [1]. The presence of a subsurface liquid water ocean and evidence of hydrothermal activity in the moon’s interior [2] has made Enceladus a key target in the search for life beyond Earth. Determining the ocean composition is an important consideration in assessing the moon’s habitability, but direct analysis is impossible because it is covered by a thick icy crust [3]. Enceladus’ surface, however, can offer some insights into the processes operating within and on the satellite, but is influenced by several processes that modify its composition. For example, plume material either falls back to the surface of the satellite or is ejected at high enough velocities that it orbits Saturn, forming Saturn's E-ring [4]. Since Enceladus orbits within this structure, E-ring material is redeposited on the surface of the moon. While the distribution and abundance of volatiles deposited by the plumes may be indicative of their presence in the subsurface ocean, E-ring grain deposits have been processed by Saturn’s magnetosphere, experiencing irradiation and erosion in the space environment. Being able to distinguish between volatile profiles of ices from fresh plume material and those subjected to impact by E-ring grains is important for understanding which surface material best represents the ocean composition.

To achieve this requires a simulation of impact processes onto Enceladus ices. Ices representative of feasible volatile compositions have been grown in a bespoke cryogenic vacuum system. Ices are grown at temperatures between -95°C and -120°C, at pressures of 10^-4 mbar to 10^-5 mbar. These conditions have been chosen to best replicate the environment around the Enceladus plumes, without rapidly sublimating the ices at ultra-high vacuum pressures. A Quadrupole Mass Spectrometer (QMS) is used to analyse the composition of the pristine ices prior to impact simulation using the All-Axis Light Gas Gun (LGG) at The Open University. The LGG can achieve hypervelocity impacts at velocities in the range of those predicted for E-ring impact (0.5 km/s to 5 km/s [5, 6, 7]). Salt grains, extracted from simulated E-ring ice particles via sublimation using the cryogenic vacuum system, will be used as projectiles. The volatile profile of impacted Enceladus ice analogues is then analysed using a QMS and compared to the original ices.

However, it is critical to understand sources of contamination within the impact simulation, since these may mask modifications that are key to understanding the space weathering processes and invalidate icy impact experiments at low pressures. Any volatile contamination in the headspace of the gun is likely to cryotrap onto the pristine ice analogues and it is expected that, as projectiles are fired, materials used in the shot, such as nylon from the piston and sabot material, will volatilise. Although the chamber is cleaned after each use and nitrogen is used to purge the chamber, the level of contamination from volatilising substances is expected to be high. 

A QMS has been used to investigate the volatiles within the LGG for the duration of the impact experiments. After the projectiles and shell casing have been loaded and the range of the gun evacuated down to vacuum pressures, the QMS is exposed to the headspace of the small experimental chamber, where targets are placed. It is set up to detect masses between 0 – 200 amu and takes readings throughout the firing process. This includes the initial set up, loading hydrogen in the pump tube, firing the gun, evacuating the range, nitrogen purging, and air purging.

The preliminary results indicate that hypervelocity impact studies using Enceladus ice analogues are feasible despite the contamination in the gun and the QMS has proven a valuable method of analysing the headspace of the gun during the firing process. While loading hydrogen in the pump tube in preparation for firing, the results from QMS analysis have shown slight leaks between the pump tube and range (Fig. 1A). This is to be expected as the pump tube is not hermetically sealed from the range, but may be improved upon. It has also shown the presence of higher mass (>100 amu) molecules that are only present for <1 min during the firing process (Fig. 1B). This is potentially the main source of recurring debris and soot observed on the walls of the blast tank post-impact, and may be a source of contamination if cryotrapped onto ices. This is currently undergoing further investigation. However, results show that this is the only stage of the firing process with a volatile profile not indicative of air leaks. The full results of the contamination analysis will be presented, alongside future work involving Enceladus ice analogues.

Figure 1. Initial results of the QMS analysis, showing the molecules present at various stages of the firing process. A) shows mass fragments (0-45 amu) in the range of the gun before any shots have been fired, indicative of air. B) shows heavier mass fragments (100-200 amu) detected during the firing process. These peaks are detected at the limits of the QMS sensitivity, which is expected to be why they are detected at the same pressures.

 

References

[1] Brown, R. et al. (2006). Composition and physical properties of Enceladus' surface. Science, 311(5766):1425-1428.

[2] Hsu, H. et al. (2015) Ongoing hydrothermal activities within Enceladus. Nature, 519(7542):207–210

[3] Cadek, O. et al. (2016) Enceladus’s internal ocean and ice shell constrained from Cassini gravity, shape, and libration data. Geophysical Research Letters, 43(11):5653–5660

[4] Kempf, S. et al. (2010). How the Enceladus dust plume feeds Saturn’s E ring. Icarus, 206(2):446–457

[5] Juhász, A. et al. (2007). Signatures of Enceladus in Saturn’s E ring. Geophysical Research Letters, 34(9)

[6] Howett, C. et al. (2018). Ring and Magnetosphere Interactions with Satellite Surfaces. In Enceladus and the Icy Moons of Saturn, pages 343–360. University of Arizona Press

[7] Spahn, F., et al. (2006). E- ring dust sources: Implications from Cassini’s dust measurements. Planetary and Space Science, 54(9-10):1024–1032.