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Because Europa is embedded in Jupiter's magnetosphere , Europa's crust, which is mainly made of water ice, is continuously bombarded by energetic ions and electrons. The bombardment of the energetic particles not only sputters and dissociates water molecules on Europa's surface but also introduces impurities to its surface ice. Especially, sulfur ions deliver sulfur to the ice, which induces further chemical reactions in the ice and leads to the formation of sulfur-containing species. Observational and experimental studies have shown that sulfuric acid is formed from the sulfur ion implantation on ice. However, the sulfur chemistry on the surface ice of Europa is still poorly understood. In this study, we use a chemical-transport model to simulate chemical processes occurring in the surface ice of Europa during irradiation by ions and electrons. We show that sulfuric acid is the dominant species on the surface ice of Europa, whose mixing ratio may be as high as several percent in the first 100 μm of the ice. We also find that sulfur chemistry may play an important role in the formation of O 2 on the surface ice of Europa. The results of our model can be used to estimate the transport rate of the key species (e.g., O2, H2O2, H2SO4) from Europa's surface ice to its subsurface ocean. Introduction The surface of Europa is bombarded by ions (primarily sulfur and oxygen ions) and energetic electrons with energy from keV to tens of MeV (Cooper et al., 2001). During the ion sputtering process, H2O and H2O products (H2, O2, etc.) are sputtered out of the surface ice and contributed to the tenuous atmosphere of Europa (Brown et al., 1982, Brown et al., 1984; Bar-Nun et al., 1985; Ip, 1996; Ip et al., 1998; Smyth and Marconi, 2006; Johnson et al., 2009; Roth et al., 2016; Teolis et al., 2017). Ion and electron irradiation on ice can dissociate water molecules, which contributes to the release of gases from the ice and the chemical alteration of the ice (Baragiola et al., 2002; Orlando and Sieger, 2003; Zheng et al., 2006; Galli et al., 2018; Nordheim et al., 2018; Vorburger and Wurz, 2018; Meier and Loeffler, 2020; Davis et al., 2021; Li et al., 2022). Observations have shown that a considerable amount of oxidant (e.g., H2O2 and O2) can be found on the surface of Europa (Carlson et al., 1999a; Spencer and Calvin, 2002; Hand et al., 2006; Hand and Brown, 2013). The existence of H2O2 is supported by the laboratory experiment performed by Hand and Carlson (2011). Meanwhile, the sulfur ions from the magnetosphere bring sulfur to the surface of Europa, which leads to the formation of sulfur-containing species (e.g., SO2, H2SO4) in the ice (Carlson et al., 1999b, Carlson et al., 2002; Gudipati et al., 2021; Becker et al., 2022). Experimental studies on the implantation of sulfur ions in water ice show that sulfuric acid and other sulfur-bearing species are generated in the ice (Strazzulla et al., 2007, Strazzulla et al., 2009; Loeffler et al., 2011; Ding et al., 2013). Galileo Near-Infrared Mapping Spectrometer observations on the surface of Europa confirm the existence of sulfuric acid in the surface ice and show that sulfuric acid is likely to be found in the trailing hemisphere rather than the leading hemisphere due to the negligible sulfur ion flux at the leading hemisphere (Dalton III et al., 2013). Studies also suggest that sulfuric acid hydrates, including monohydrate, tetrahydrate, hexahydrate, etc., could be abundant on the surface of Europa (Carlson et al., 1999a, Carlson et al., 1999b; Loeffler et al., 2011; Loeffler and Hudson, 2012; Maynard-Casely et al., 2013). However, the sulfur chemistry on Europa and the pathways of the formation of the sulfur-containing species are still poorly understood. In order to fill these gaps, we use a comprehensive one-dimensional chemical-transport model to simulate the chemical process occurring in the surface ice of Europa. This model includes the irradiation on the ice and the implantation of sulfur and oxygen ions in the ice. It aims to resolve the formation and distribution of both sulfur-containing and non‑sulfur-containing (e.g., O2, H2O2) species. In the next section, we will present the model setup. Section 3 shows the simulation results and analyzes the pathways of the formation of the key species in the ice. We will also estimate the transport rate of the key species from Europa's surface ice to its subsurface ocean in this section. In Section 4, we will present our conclusions. We will also discuss the limitations of our model and the prospect of future studies. Section snippets Model setup Our one-dimensional chemistry-transport model is developed based on the Caltech/JPL chemistry-transport model KINETICS (Allen et al., 1981; Yung and DeMore, 1999). The governing equation of KINETICS is the continuity equation for each species i : ∂ n i ∂ t + ∂ φ i ∂ z = P i − L i , in which n i is the number density, φ i is the vertical flux, P i is the production rate, and L i is the loss rate at depth z and time t . P i and L i of each species i are determined by its related production and destruction reactions. The Simulation results The density profiles of the compositions constructed by our model are shown in Fig. 1. The non‑sulfur-containing group and the sulfur-containing group are shown separately in Fig. 1a and b. The density of S3 and S4 are much lower than the densities of other species, so they do not appear in Fig. 1. There are some abrupt drops in the density profiles in Fig. 1, which are mainly due to the simplified irradiation used in our model. In the non‑sulfur-containing group, H2O2 and O2 are the most Conclusions and discussion In this study, we use a one-dimensional chemical-transport model to simulate chemical processes occurring in the surface ice of Europa during irradiation by ions and electrons. Both water dissociation and ion implantation are considered in this model. Our model shows that the sulfur and sulfur-containing species are very important to the chemical processes in the surface ice. We find that sulfuric acid is the dominant species in the ice, whose mixing ratio may be as high as 4.5% (compared to Declaration of Competing Interest None. Acknowledgement JL and CL are supported by the University of Michigan startup grant awarded to CL. References (90) X. Zhang et al. Sulfur chemistry in the middle atmosphere of Venus Icarus (2012) K. Zahnle et al. 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Material ejected from Io is known to contribute an exogenic flux of dark material to Europa’s trailing hemisphere, and hydrated salt compounds concentrated within chaos terrain, ridges, and pits may be endogenous to the subsurface ocean. Many of Europa’s impact craters also exhibit dark ejecta, the origins of which are unknown. Our study examines the ejecta of several large impact craters to determine possible origins for this dark material. We compared the dark material found in crater ejecta to other surface materials using Galileo Near-Infrared Mapping Spectrometer data to assess similarities in composition between ejecta material and other dark materials on Europa’s surface. Our analysis shows that dark material found in crater ejecta has similar composition to other dark features on Europa and may be the result of comparable sources or alteration processes. We also considered dark impactors as sources for the dark ejecta material. Using crater scaling laws, we estimated the impactor size for each crater and determined the impactor’s potential contribution of dark material to the ejecta. We then compared these quantities to those derived using a radiative transfer model and the measured reflectance values of each dark-ejecta crater. Our model results show that the lower albedo of the ejecta of these craters cannot be solely attributed to an intimate mixture of the impactor material with the target material. In contrast, modeled impact heating and vaporization suggest sufficient amounts of ice could be removed in order to explain the observed low-albedo patterns, if preexisting or impactor-derived dark material is just 0.1% more resistant to vaporization than the ice. Given the lack of spatial correlation, and the presence of similar-sized craters without dark ejecta, these results point to either localized differences in the concentration of dark non-ice materials in Europa’s shell, or variations in impact velocities and geometries leading to differences in the amount of vaporized ejecta. Research article Geological analysis of Monad Regio, Triton: Possible evidence of endogenic and exogenic processes Icarus, Volume 392, 2023, Article 115368 Show abstract The surface morphology of Triton is believed to be predominantly influenced by endogenic processes, such as tectonism, volcanism, diapirism and geyser-like eruptions. To understand how some of these processes have reshaped the surface of the satellite during its evolutionary history, we performed a geological analysis of an extended area located in Monad Regio, where several of these endogenic processes may have taken place. We produced a geomorphological map at 1:1,000,000 scale derived from the joint analysis of the Voyager images, a digital elevation model and a roughness map of the study area. Such approach allowed us to delimit with higher accuracy the geomorphological units already known in literature, as planitiae, cantaloupe and smooth terraced terrains, paterae, knobby, and smooth terraced terrains, but it also enabled us to identify new ones (hummocky terrain, inner planitia, inner patera). We reconstructed the relative ages between such terrains, and we found that after an endogenic phase, an exogenic phase could have followed, during which several geological events reshaped the area. These comprise depositional and erosional processes, including the movement of ice and the subglacial flowing of liquid nitrogen. Such observations imply that Monad Regio could be strongly influenced by exogenic processes, perhaps still active today. Research article Mid-IR and VUV spectroscopic characterisation of thermally processed and electron irradiated CO2 astrophysical ice analogues Journal of Molecular Spectroscopy, Volume 385, 2022, Article 111599 Show abstract The astrochemistry of CO2 ice analogues has been a topic of intensive investigation due to the prevalence of CO2 throughout the interstellar medium and the Solar System, as well as the possibility of it acting as a carbon feedstock for the synthesis of larger, more complex organic molecules. In order to accurately discern the physico-chemical processes in which CO2 plays a role, it is necessary to have laboratory-generated spectra to compare against observational data acquired by ground- and space-based telescopes. A key factor which is known to influence the appearance of such spectra is temperature, especially when the spectra are acquired in the infrared and ultraviolet. In this present study, we describe the results of a systematic investigation looking into: (i) the influence of thermal annealing on the mid-IR and VUV absorption spectra of pure, unirradiated CO2 astrophysical ice analogues prepared at various temperatures, and (ii) the influence of temperature on the chemical products of electron irradiation of similar ices. Our results indicate that both mid-IR and VUV spectra of pure CO2 ices are sensitive to the structural and chemical changes induced by thermal annealing. Furthermore, using mid-IR spectroscopy, we have successfully identified the production of radiolytic daughter molecules as a result of 1 keV electron irradiation and the influence of temperature over this chemistry. Such results are directly applicable to studies on the chemistry of interstellar ices, comets, and icy lunar objects and may also be useful as reference data for forthcoming observational missions. Research article Surface composition and properties of Ganymede: Updates from ground-based observations with the near-infrared imaging spectrometer SINFONI/VLT/ESO Icarus, Volume 333, 2019, pp. 496-515 Show abstract Ganymede's surface exhibits great geological diversity, with old dark terrains, expressed through the surface composition, which is known to be dominated by two constituents: H2O-ice and an unidentified darkening agent. In this paper, new investigations of the composition of Ganymede's surface at global scale are presented. The analyses are derived from the linear spectral modeling of a high spectral resolution dataset, acquired with the near-infrared (1.40–2.50 μm) ground-based integral field spectrometer SINFONI ( SIN gle F aint O bject N ear-IR I nvestigation) of the Very Large Telescope (VLT hereafter) located in Chile. We show that, unlike the neighboring moon Europa, photometric corrections cannot be performed using a simple Lambertian model. However, we find that the Oren-Nayar (1994) model, generalizing the Lambert's law for rough surfaces, produces excellent results. Spectral modeling confirms that Ganymede's surface composition is dominated by H2O-ice, which is predominantly crystalline, as well as a darkening agent, but it also clearly highlights the necessity of secondary species to better fit the measurements: sulfuric acid hydrate and salts, likely sulfates and chlorinated. A latitudinal gradient and a hemispherical dichotomy are the strongest spatial patterns observed for the darkening agent, the H2O-ice, and the sulfuric acid: the darkening agent is by far the major compound at the equator and mid-latitudes (≤ ± 35°N), especially on the trailing hemisphere, while the H2O-ice and the sulfuric acid are mostly located at high latitudes and on the leading hemisphere. This anti-correlation is likely a consequence of the bombardment of the constituents in the Jovian magnetosphere which are much more intense at latitudes higher than ±35°N. Furthermore, the modeling confirms that polar caps are enriched in small, fresh, H2O-ice grains (i.e. ≤50 μm) while equatorial regions are mostly composed of larger grains (i.e. ≥200 μm, up to 1 mm). Finally, the spatial distribution of the salts is neither related to the Jovian magnetospheric bombardment nor the craters. These species are mostly detected on bright grooved terrains surrounding darker areas. Endogenous processes, such as freezing of upwelling fluids going through the ice shell, may explain this distribution. In addition, a small spectral residue that might be related to brines and/or hydrated silica-bearing minerals are located in the same areas. Research article The challenges of driving Charon's cryovolcanism from a freezing ocean Icarus, Volume 392, 2023, Article 115391 Show abstract A combination of geological interpretations and thermal-orbital evolution models imply that Pluto's large moon, Charon, had a subsurface water (and possibly ammonia) ocean that eventually froze. Ocean freezing generates large tensile stresses in the upper part of the ice shell and pressurizes the ocean below, perhaps leading to the formation of Charon's large canyons and putative cryovolcanic flows. Here, we identify the conditions in which a freezing ocean could create fractures that fully penetrate its ice shell, linking Charon's surface with its ocean and facilitating ocean-sourced cryovolcanism. We find that current models of Charon's interior evolution predict ice shells that are far too thick to be fully cracked by the stresses associated with ocean freezing. Either Charon's ice shell was <10 km thick when the flows occurred (as opposed to >100 km) or the surface was not in direct communication with the ocean as part of the eruptive process. If Charon's ice shell had been thin enough to be fully cracked, it would imply substantially more ocean freezing than is indicated by the canyons, Serenity and Mandjet Chasma. Due to the low radiogenic heating within Charon and the loss of tidal heating early in its history, a thin ice shell should have been short-lived, implying that ocean-sourced cryovolcanic flows would have ceased relatively early in Charon's history, consistent with interpretations of its surface geology. An additional (and perhaps implausibly large) heat source would be required to generate the substantially larger ocean implied by through-going fractures. We also find that ocean freezing can easily generate deep fractures that do not fully penetrate to the ocean, which may be the foundation of Charon's canyons. When ocean-bearing moons begin to cool down, their oceans can freeze. As new ice accretes to the bottom of the existing ice shell, the added volume of the ice can stress the shell. Pluto's largest moon, Charon, has canyons and cryovolcanic flows that may have formed in response to a freezing ocean. Here, we model the formation of fractures within Charon's ice shell as the ocean underneath it freezes to explore the evolution of Charon's interior and surface. We find that an ocean source for cryovolcanic flows is unlikely because the ice shell would have had to be much thinner than current thermal evolution models imply. However, freezing the ocean may have produced the stresses that formed canyons later in Charon's history. Research article Color centers in salts - Evidence for the presence of sulfates on Europa Icarus, Volume 326, 2019, pp. 37-47 Show abstract We have conducted experiments to better understand the extent to which radiation-induced color changes observed at near-ultraviolet through near-infrared wavelengths for various salts could provide insight into the composition of the nonice material(s) on the surface of Europa's trailing hemisphere. Salts of NaCl, KCl, Na2SO4, partially hydrated MgSO4, partially hydrated FeSO4, Na2CO3, and CaCO3 were irradiated with 1020 electrons/cm2 at room temperature; the electron energy was 40 keV. This is equivalent to ~100,000 years of exposure to electrons at Europa in this energy range, or to an equivalent few thousand years of exposure to electrons of at least ~100 eV, a conservative estimate for the lowest energy electrons capable of producing color centers. Each salt either browned, darkened, and/or developed color centers. Most salts also developed a probable colloidal band near 600 nm that would be expected to be much weaker or non-existent in materials at the temperature of Europa's surface. The ultraviolet through near-infrared telescopic spectra of Europa's trailing hemisphere is most consistent with a linear mixture of irradiated MgSO4·nH2O with a few percent of another salt that exhibits a color center near 600 nm (either Na2SO4·nH2O, Na2CO3, CaCO3, or KCl). Other irradiated salts exhibit spectral features that limit their abundances in the nonice material on the trailing hemisphere, with the abundance of NaCl, because of its strong color centers, limited to not more than approximately 10%. View full text © 2023 Elsevier Inc. All rights reserved. About ScienceDirect Remote access Shopping cart Advertise Contact and support Terms and conditions Privacy policy We use cookies to help provide and enhance our service and tailor content and ads. By continuing you agree to the use of cookies . 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