VLT/Sinfoni Observations of Europa: New Insights Into the Surface Composition

Abstract

We present new insights into Europa’s surface composition on the global scale from linear spectral modeling of a high spectral resolution data set acquired during a ground-based observation campaign using SINFONI4, an adaptive optics near-infrared instrument on the Very Large Telescope (ESO). The spectral modeling confirms the typical “bullseye” distribution of sulfuric acid hydrate on the trailing hemisphere, which is consistent with Iogenic sulfur ion implantation. However, the traditional hypothesis of the presence of sulfate salts on the surface of the satellite is challenged as Mg-bearing chlorinated species (chloride, chlorate, and perchlorate) are found to provide improved spectral fits. The derived global distribution of Mg-chlorinated salts (and particularly chloride) is correlated with large-scale geomorphologic units such as chaos and darker areas, thus suggesting an endogenous origin. Based on the 1.65 μm water-ice absorption band shape and position, the surface temperature is estimated to be in the range 110–130 K, and water ice is found to be predominantly in its crystalline state rather than amorphous. While amorphous water ice exhibits a strong correlation with the expected intensity of the Ionian plasma torus bombardment, crystalline water ice is instead more associated with distinct geomorphological units. Endogenous processes such as jets and ice heating due to active geology may explain this relationship. Otherwise, no evidence of a correlation between grain size for the water ice and the sputtering rate has been detected so far.

Magnetospheric considerations for solar system ice state

“…This interpretation of only crystalline water ice (at depth) is also supported by Earth based telescopic observations ( Ligier et al, 2016 ). While Hansen and McCord (2004) pointed out that energetic ion bombardment is only one mechanism for transforming crystalline ice to amorphous ice, the increasing crystallinity of surface ice correlating with the satellites' Jovian distance makes this mechanism a very likely explanation.…”

“…For instance, in Fig. 11a of Ligier et al (2016) , one can see this effect even though high latitude sampling is limited. The very shallow depth of the amorphous ice that is detected is consistent with protons and heavy ions being a critical agent, since they have very short ranges in materials.…”

“…are equal, the polar regions would retain the most amorphous ice. Finally, we would like to note that the interpretation of amorphous ice by Ligier et al (2016) is based on an assumption about the ice temperature. They have carefully considered temperature range and the small variations that can come about at other temperatures in that paper.…”

“…A careful study of Fig. 11 of Ligier et al (2016) , with summing of the amounts in the two principal ice states, reveals the relative amounts of non-ice.…”

“…The heavy energetic ions, which have their own precipitation patterns, remove ice in the very top layer (see below) and also preferentially eliminate smaller grains (e.g., Cassidy et al, 2013 ). In addition, the possible presence of endogenic activity, such as a plume on Europa ( Roth et al, 2014;Sparks et al, 2016 ), may create local discrepancies in the spatial distribution of grain sizes and therefore correlations with geomorphological units ( Ligier et al, 2016 ).…”