Investigating cryogenic space weathering processes for Europa and Ganymede

Abstract

Introduction:  The surfaces of Europa and Ganymede contain hydrated material(s) that are continually altered by bombardment with Jovian magnetospheric ions, electrons, and micrometeroids. This bombardment may affect the structures of the surface materials, altering their spectral signatures in the visible and infrared [1]. The physical and chemical alteration by ultraviolet (UV), particle, and micrometeroid impacts has been used to explain the spectral features observed on the surfaces of ocean worlds [ e.g. 2], as well as provide insight into discrepancies between the infrared (IR) characteristics of these surfaces with analogs [ e.g. 3]. Regardless of the challenges, some robust identifications have been made. NaCl has been identified on Europa through radiation-induced absorption bands near 460 nm and 230 nm [4] and MgSO4 has been tentatively identified through an infrared band near 3.7 microns [5].  The hydrated sulfate is also hypothesized to exist on Ganymede [6]. We further investigate the spectral nature of irradiated materials to understand the compositions of Europa and Ganymede, including effects of high-energy (1 to 40 keV) electron irradiation and laser-simulated micrometeroid bombardment on the visible (Vis) through infrared (IR) reflectance spectra of these materials and other cryogenic hydrated materials with implications for interpreting the surface compositions of Europa.We present visible – infrared spectra of compositional analogs (mainly hydrated salts) to the icy Galilean satellites that have been exposed to high energy electron irradiation (1 to 40keV) and/or subjected to simulation of micrometeroid bombardment by lasing at 1064 nm, 212 milliJoules, with a ~9 ns duration. Experiments were conducted in the ultrahigh vacuum chamber in the APL Laboratory for Spectroscopy under Planetary Environmental Conditions (LabSPEC). Bidirectional reflectance spectra at an incidence angle of 15 degree and emission angle of 45 degree were collected once vacuum reached < 1 microtorr during which (including during pump-down) the sample temperature was maintained between 130 K and 150 K during irradiation and sample collection. Reflectance spectra at wavelengths from 400 nm to 2400 nm spectra were collected with an SVC point spectrometer; spectra from 1500 nm to 8000 nm were collected using a Bruker Vertex 70 FTIR.  Electron irradiation was produced with a Kimball-Physics model 6104 gun operated at a flux on the sample of 16  to 500 nanoamps/cm2, which ranges from a few times Europa conditions to about 2 orders of magnitude higher than electron flux on the equatorial portion of the leading hemisphere Europa. Total fluence equated to only days to several months of exposure on Europa’s leading hemisphere.Hydrated halides, sulfates, and hydrated sulfuric acid were irradiated. Despite the relatively short Europa-surface equivalent time of electron irradiation, every irradiated sample exhibited significant spectral changes. Electron irradiation causes darkening in the visible to near-IR for all samples, with an F-type color center forming in NaCl and a possible M-center forming in partially hydrated MgCl2. In the infrared, the irradiation affects the ~ 3-mm Fresnel reflection peak and generally increases the reflectance in the 4-micron transparency region, likely by increasing porosity through microscopic damage/stimulated desorption. Additionally, in the one experiment that contained organics in a matrix of NaCl with adsorbed waters (see 7), the organic bands in the visible and infrared diminished with increasing irradiation and after the final irradiation, CO2 was present, likely a result of chemistry between the degraded organics and adsorbed water on the NaCl. Laser simulation of micrometeroids physically damages and disrupts the surface, altering the reflectance at 3-micron and also increasing reflectance in the 4-micron region. No other spectral changes were noted, including no evidence for desiccation of the hydrated material. Neither electron irradiation nor damage by simulated micrometeroid bombardment otherwise alters the infrared spectra of hydrated cryogenic materials. Acknowledgments: We would like to acknowledge the support of SSW Grant # 80NSSC20K1044.