Salt grains in hypervelocity impacts in the laboratory: Methods to sample plumes from the ice worlds Enceladus and Europa

C R Fisher,M C Price,M J Burchell

doi:10.1111/maps.13729

Abstract

AbstractThe plumes naturally erupting from the icy satellite Enceladus were sampled by the Cassini spacecraft in high‐speed fly‐bys, which gave evidence of salt. This raises the question of how salt behaves under high‐speed impact, and how it can best be sampled in future missions to such plumes. We present the results of 35 impacts onto aluminum targets by a variety of salts (NaCl, NaHCO3, MgSO4, and MgSO4·7H2O) at speeds from 0.26 to 7.3 km s−1. Using SEM‐EDX, identifiable projectile residue was found in craters at all speeds. It was possible to distinguish NaCl and NaHCO3 from each other, and from the magnesium sulfates, but not to separate the hydrous from anhydrous magnesium sulfates. Raman spectroscopy on the magnesium sulfates and NaHCO3 residues failed to find a signal at low impact speeds (<0.5 km s−1) where there was insufficient projectile material deposited at the impact sites. At intermediate speeds (0.5 to 2–3 km s−1), identifiable Raman spectra were found in the impact craters, but not at higher impact speeds, indicating a loss of structure during the high speed impacts. Thus, intact capture of identifiable salt residues on solid metal surfaces requires impact speeds between 0.75 and 2 km s−1.

Highlights

Subsurface oceans are believed to exist on several icy satellites and bodies in the outer solar system
The gun was originally optimized to fire at speeds above 1 km s−1, but has recently been adjusted to permit lower speeds down to around 250 m s−1 (Hibbert et al 2017)
An evolution in crater shape can be seen as impact speed increases (Table 2)

Summary

Introduction

Subsurface oceans are believed to exist on several icy satellites and bodies in the outer solar system. That the oceans are brine rich (e.g., see Sohl et al [2010], or Grasset et al [2017], for a discussion of the nature of the oceans) implies that the plumes may contain salts. The Cassini space mission detected dust grains when passing close to Enceladus in 2005 (Spahn et al 2006), and later analysis identified salt (NaCl via signals for sodium and potassium)-rich grains (Postberg et al 2009, 2011). It is suggested that the Europan ocean is rich in NaCl (Hand and Carlson 2015). Hydrated magnesium sulfate salts are present, and epsomite (MgSO4Á7H2O) has been suggested to be present on the surface of Europa, for example, as a radiation product (Brown and Hand 2013)

Methods

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Discussion

Conclusion