Europa’s surface water ice crystallinity: Discrepancy between observations and thermophysical and particle flux modeling

Abstract

Physical processing of Europan surface water ice by thermal relaxation, charged particle bombardment, and possible cryovolcanic activity can alter the percentage of the crystalline form of water ice compared to that of the amorphous form of water ice (the "crystallinity") on Europa's surface. The timescales over which amorphous water ice is thermally transformed to crystalline water ice at Europan surface temperatures (80-130 K) suggests that the water ice there should be primarily in the crystalline form, however, surface bombardment by charged particles induced by Jupiter's magnetic field, and vapor deposition of water ice from Europan plumes, can produce amorphous water ice surface deposits on short timescales.The purpose of this investigation is to determine whether the Europan surface water ice crystallinity derived from ground-based spectroscopic measurements is in agreement with the crystallinity expected based upon temperature and radiation modeling. Using a 1D thermophysical model of Europa's surface, we calculate an integrated full-disk crystallinity of Europa's leading hemisphere by incorporating the thermal relaxation of amorphous to crystalline water ice and the degradation of crystalline to amorphous water ice by irradiation. Concurrently, we derive the full-disk crystallinity of Europa's leading hemisphere using a comparison of near-infrared ground-based spectral observations from Grundy et al. (1999), Busarev et al. (2018), and the Apache Point Observatory in Sunspot, NM, with laboratory spectra from Mastrapa et al. (2008) and the Ice Spectroscopy Lab at the Jet PropulsionLaboratory. We calculate a modeled crystallinity significantly higher than crystallinities derived from ground-based observations and laboratory spectra. This discrepancy may be a result of geophysical processes, such as by vapor-deposited plume material, or it may arise 1 arXiv:2002.04132v1 [astro-ph.EP] 10 Feb 2020 from assumptions and uncertainties in the crystallinity calculations.

Europa’s Surface Water-ice Crystallinity and Correlations between Lineae and Hydrate Composition

“…We estimated a crystallinity of ∼15% based on the crystalline and amorphous water-ice abundances in the 17e009 NIMS cube (Figure 17). This was slightly lower than, but still consistent with, spectroscopically derived crystallinities of ∼30% for the full-disk leading hemisphere, which is dominated by warmer (and therefore more crystalline), lower-latitude locations (Berdis et al 2020). As discussed previously, Adonis Linea crosses the NIMS observation at ∼124°-125°W.…”

“…A lower crystallinity near the pole is expected owing to (1) colder temperatures, which indicates a longer amount of time for the amorphous water-ice to crystallize (Kouchi et al 1994;Jenniskens et al 1998;Mastrapa et al 2013), and (2) migration of amorphous plume material (Kouchi et al 1994;Hansen & McCord 2004). These crystallinities are in agreement with those derived using full-disk ground-based spectroscopic observations of Europa's leading hemisphere and laboratory-produced crystalline and amorphous water-ice (Berdis et al 2020). They found that the spectroscopically derived crystallinity of Europa's full-disk leading hemisphere is ∼27%-36%, whereas the crystallinity derived from thermophysical modeling and particle flux is ∼80%-95%.…”

“…The crystallinity of Europa's leading hemisphere cannot be reproduced by thermophysical and charged particle bombardment modeling alone (Berdis et al 2020). This discrepancy may indicate that additional processes have existed in the past (or are currently active) that may be influencing Europa's surface and altering its crystallinity.…”

Strength Evolution of Ice Plume Deposit Analogs of Enceladus and Europa

“…(1) The surface of Enceladus is dominated by the same hexagonal ice I h as used in our experiments (Filacchione et al, 2007, 2010). (2) The surface of Europa may contain a small fraction of amorphous ice due to irradiation effects, but it appears nevertheless dominated by ice I h (Berdis et al, 2020; Hansen & McCord, 2000). (3) Low H 2 O vapor pressure and airless conditions at the surface of Europa and Enceladus would ease release and escape of H 2 O, which suggests that ice could redistribute less efficiently within plume deposits than in our experiments.…”

UV-Induced Formation of Ice XI Observed Using an Ultra-High Vacuum Cryogenic Transmission Electron Microscope and its Implications for Planetary Science

“…As a result, layered structures, from outermost ice Ih to inner high-pressure ices, were formed. Although the temperature of the icy satellite’s surface differs among each satellite and between equatorial and polar regions (e.g., Berdis et al, 2020 ), we will discuss using average temperatures for simplicity. We assume that the surface temperatures of icy satellites of Jupiter, Saturn, and Uranus are 110, 85, and 65 K, respectively.…”