Neutral atmosphere near the icy surface of Jupiter’s moon Ganymede

The paper discusses the formation and dynamics of the rarefied gas envelope near the icy surface of Jupiter’s moon Ganymede. Being the most massive icy moon, Ganymede can form a rarefied exosphere with a relatively dense near-surface layer. The main parent component of the gas shell is water vapor, which enters the atmosphere due to thermal degassing, nonthermal radiolysis, and other active processes and phenomena on the moon’s icy surface. A numerical kinetic simulation is performed to investigate, at the molecular level, the formation, chemical evolution, and dynamics of the mainly H₂O- and O₂-dominant rarefied gas envelopes. The ionization processes in these rarefied gas envelopes are due to exposure to ultraviolet radiation from the Sun and the magnetospheric plasma. The chemical diversity of the icy moon’s gas envelope is attributed to the primary action of ultraviolet solar photons and plasma electrons on the rarefied gas in the H₂O- or O₂-dominant atmosphere. The model is used to calculate the formation and development of the chemical diversity in the relatively dense near-surface envelope of Ganymede, where an important contribution comes from collisions between parent molecules and the products of their photolysis and radiolysis.     

Ionization chemistry in H<Subscript>2</Subscript>O-dominated atmospheres of icy moons

The processes of the formation and dynamics of tenuous gaseous envelopes of icy moons in giant-planet systems are considered. Tenuous exospheres with relatively dense surface layers are likely to form around more massive icy satellites, such as, for example, the Galilean satellites Europa and Ganymede in the Jovian system. Escaping exospheres are formed in the case of low-mass icy moons, as happens for the icy satellite Enceladus in the Saturnian system. The main parent component of such gaseous envelopes is water vapor, which enters into the atmosphere as a result of thermal degassing processes, nonthermal radiolysis, and other active processes and phenomena on the icy surface of a satellite. A numerical kinetic model has been developed to study on a molecular level the processes of the formation, chemical evolution, and dynamics of tenuous gaseous envelopes dominated mainly by H₂O. The ionization processes in such tenuous gaseous envelopes are caused by solar ultraviolet (UV) radiation and solar-wind and/or magnetospheric plasma. The primary processes when ultraviolet solar photons and plasma electrons affect the tenuous gas of the H₂O-dominated atmosphere are responsible for the chemical diversity of the gaseous envelopes of icy moons. Ionization chemistry, including ion-molecular reactions, dissociative recombination of molecular ions, and the reactions of the charge exchange with magnetospheric ions, is important for the formation of chemical diversity in gaseous envelopes of icy satellites. The model considered in the study was used to numerically simulate the formation and development of chemical diversity in the tenuous gaseous envelope of Enceladus. The numerical results were compared to the direct Cassini measurements during its close flyby near Enceladus.     

Synthesis of hydrogen peroxide in water ice by ion irradiation

We present infrared absorption studies on the effects of 50-100 keV Ar⁺ and 100 keV H⁺ ion irradiation of water ice films at 20-120 K. The results support the view that energetic ions can produce hydrogen peroxide on the surface of icy satellites and rings in the outer Solar System, and on ice mantles on interstellar grains. The ion energies are characteristic of magnetospheric ions at Jupiter, and therefore the results support the idea that radiolysis by ion impact is the source of the H₂O₂ detected on Europa by the Galileo infrared spectrometer. We found that Ar⁺ ions, used to mimic S⁺ impacts, are roughly as efficient as H⁺ ions in producing H₂O₂, and that 100 keV H⁺ ions can produce hydrogen peroxide at 120 K. The synthesized hydrogen peroxide remained stable while warming the ice film after irradiation; the column density of the formed H₂O₂ is constant until the ice film begins to desorb, but the concentration of H₂O₂ increases with time during desorption because the water sublimes at a faster rate. Comparing the shape of the 3.5-μm absorption feature of H₂O₂ to the one measured on Europa shows excellent agreement in both shape and position, further indicating that the H₂O₂ detected on Europa is likely caused by radiolysis of water ice.     

Ammonia-water ice laboratory studies relevant to outer Solar System surfaces

Although water- and ammonia-ices have been observed or postulated as important components of the icy surfaces of planetary satellites in the outer Solar System, significant gaps exist in our knowledge of the spectra and behavior of such mixtures under astrophysical conditions. To that end, we have completed low-temperature spectroscopic studies (1-20 μm) of water-ammonia mixtures, with an emphasis on features in the near-IR, a region which is accessible to ground-based observations. The influences of composition, formation temperature, thermal- and radiation-processing, and phase (crystalline or amorphous) of the components were examined. Spectra of both pure NH₃ and H₂O-NH₃ icy mixtures with ratios from 0.7 to 57 were measured at temperatures from 10 to 120 K. Conditions for the formation and thermal stability of the ammonia hemihydrate (2NH₃⋅H₂O) and the ammonia monohydrate (NH₃⋅H₂O) have been examined. Band positions of NH₃ in different H₂O-ices and major band positions of the hydrates were measured. We report spectral shifts that depend on concentration and temperature. The radiation-induced amorphization of the hemihydrate was observed and the radiation destruction of NH₃ in H₂O-ices was measured. Implications of these results for the formation, stability, and detection of ammonia on outer satellite surfaces are discussed.     

Ion irradiation of H2O ice on top of sulfurous solid residues and its relevance to the Galilean satellites

In this paper we present the results of new experiments of ion irradiation of water ice deposited on top of a solid sulfurous residue to study the potential formation of SO₂ at the interface ice/refractory material and discuss the possibility that this mechanism accounts for the sulfur dioxide ice detected on the surfaces of the Galilean satellites. In situ infrared spectroscopy was the used experimental technique. We have irradiated a thin film of H₂O frost on a sulfurous layer with 200 keV of He⁺ at 80 K. The used sulfurous residue was obtained by irradiation of frozen SO₂ at 16 K and it is used as a template of sulfur bearing solid materials. We have not found evidences of the efficient formation of SO₂ after irradiation of H₂O ice on top of the sulfurous residue. An upper limit to the production yield of SO₂, of interface area for each 100 eV of energy absorbed in 1 cm³ of ice-covered residue, has been estimated. These results have relevance in the context of the surfaces of the icy Galilean satellites in which SO₂ was detected. Our results show that radiolysis of mixtures of water ice and refractory sulfurous materials is not the primary formation mechanism responsible for the SO₂ present on the surfaces of the Galilean satellites.     

Hydrogen peroxide dimers and the production of O2 in icy satellite surfaces

Oxygen is seen in the reflectance spectra of the icy surfaces of Ganymede and Europa via absorption bands at 627 and 573 nm. Here we show that the trapped O₂ associated with these spectral features can be produced via radiation-induced decomposition of hydrogen peroxide dimers contained in hydrogen peroxide inclusions in these icy satellite surfaces.     

Radiation chemistry of H2O + O2 ices

The chemistry and spectroscopy of proton-irradiated H₂O + O₂ ices have been investigated in relation to the production of oxidants in icy satellite surfaces. Hydrogen peroxide (H₂O₂), ozone (O₃), and the hydroperoxy (HO₂) and hydrogen trioxide (HO₃) radicals have all been observed, and their temperature and dose dependent production trends have been measured. We find that O₂ aggregates form during the growth of H₂O + O₂ ice films, and the presence of these aggregates greatly affects the HO₂ and H₂O₂ yields. In addition, we have found that the position of the spectral maximum of the ν₃ vibration of O₃ shifts with ice composition, giving an indication of the degree of dispersion of O₃ molecules within the ice. We discuss the relevance of these measurements to icy satellite surfaces.     

The role of sputtering and radiolysis in the generation of Europa exosphere

The exosphere of an atmosphereless icy moon is the result of different surface release processes and subsequent modification of the released particles. At Europa icy moon, water molecules are directly released, but photolysis and radiolysis due to solar UV and Jupiter’s magnetospheric plasma, respectively, can result in OH, H, O and (possibly) H₂ production. These molecules can recombine to reform water and/or new chemical species. As a consequence, Europa’s neutral environment becomes a mixture of different molecules, among which, H₂O dominates in the highest altitudes and O₂, formed mainly by radiolysis of ice and subsequent release of the produced molecules, prevails at lower altitudes. In this work, starting from a previously developed Monte Carlo model for the generation of Europa’s exosphere, where the only considered species was water, we make a first attempt to simulate also the H₂ and O₂ components of the neutral environment around Europa, already observed by the Hubble Space Telescope and the Ultraviolet Imaging Spectrograph on board Cassini, during its flyby of Jupiter. Considering a specific configuration where the leading hemisphere coincides with the sunlit hemisphere, we estimate along the Europa–Sun line an O₂ column density of about 1.5 × 10¹⁹ m^?2 at the dayside and 3 × 10¹⁸ m^?2 at the nightside. In this work we also improve our previous estimation of the sputtered H₂O exosphere of this moon, taking into consideration the trailing–leading asymmetry in the magnetospheric ion bombardment and the energy and temperature dependences of the process yields. We find that a density of 1.5 × 10¹² H₂O/m³ is expected at altitudes ～0.1R_E above the surface of the trailing hemisphere. Additionally, we calculate the escape of H₂O, O₂ and H₂. The total number of neutral atoms in Europa’s neutral torus, is estimated to be in the range 7.8 × 10³²–3.3 × 10³³.     

The radiolysis of SO2 and H2S in water ice: Implications for the icy jovian satellites

Spectra of Europa, Ganymede, and Callisto reveal surfaces dominated by frozen water, hydrated materials, and minor amounts of SO₂, CO₂, and H₂O₂. These icy moons undergo significant bombardment by jovian magnetospheric radiation (protons, electrons, and sulfur and oxygen ions) which alters their surface compositions. In order to understand radiation-induced changes on icy moons, we have measured the mid-infrared spectra of 0.8 MeV proton-irradiated SO₂, H₂S, and H₂O-ice mixtures containing either SO₂ or H₂S. Samples with H₂O/SO₂ or H₂O/H₂S ratios in the 3-30 range have been irradiated at 86, 110, and 132 K, and the radiation half-lives of SO₂ and H₂S have been determined. New radiation products include the H₂S₂ molecule and HSO⁻₃, HSO⁻₄, and SO²⁻₄ ions, all with spectral features that make them candidates for future laboratory work and, perhaps, astronomical observations. Spectra of both unirradiated and irradiated ices have been recorded as a function of temperature, to examine thermal stability and phase changes. The formation of hydrated sulfuric acid in irradiated ice mixtures has been observed, along with the thermal evolution of hydrates to form pure sulfuric acid. These laboratory studies provide fundamental information on likely processes affecting the outer icy shells of Europa, Ganymede, and Callisto.     

Surface-bounded atmosphere of Europa

A 1-D collisional Monte Carlo model of Europa's atmosphere is described in which the sublimation and sputtering sources of H₂O molecules and their molecular fragments are accounted for as well as the radiolytically produced O₂. Dissociation and ionization of H₂O and O₂ by magnetospheric electron, solar UV-photon and photo-electron impact, and collisional ejection from the atmosphere by the low-energy plasma are taken into account. Reactions with the surface are discussed, but only adsorption and atomic oxygen recombination are included in this model. The size of the surface-bounded oxygen atmosphere of Europa is primarily determined by a balance between atmospheric sources from irradiation of the satellite's icy surface by the high-energy magnetospheric charged particles and atmospheric losses from collisional ejection by the low-energy plasma, photo- and electron-impact dissociation, and ionization and pick-up from the surface-bounded atmosphere. A range of sources rates for O₂ to H₂O are used with a larger oxygen-to-water ratio than suggested by laboratory measurements in order to account for differences in adsorption onto grains in the regolith. These calculations show that the atmospheric composition is determined by both the water and oxygen photochemistry in the near-surface region, escape of suprathermal oxygen and water into the jovian system, and the exchange of radiolytic water products with the porous regolith. For the electron impact ionization rates used, pick-up ionization is the dominant oxygen loss process, whereas photo-dissociation and atmospheric sputtering are the dominant sources of neutral oxygen for Europa's neutral torus. Including desorption and loss of water enhances the supply of oxygen species to the neutral torus, but hydrogen produced by radiolysis is the dominant source of neutrals for Europa's torus in these models.     

CO2 production by ion irradiation of H2O ice on top of carbonaceous materials and its relevance to the Galilean satellites

In this work we report on new experiments of ion irradiation of water ice deposited on top of solid carbonaceous materials to study the production of CO₂ at the interface ice/refractory material and discuss the possibility that this mechanism accounts for the quantity of CO₂ ice detected on the surfaces of the Galilean satellites. The used experimental technique has been in situ infrared spectroscopy. We have irradiated thin films of H₂O frost on carbonaceous layers with 200 keV of He⁺ and Ar⁺, and 30 keV of He⁺ at 16 and 80 K. The used carbonaceous layers have been asphaltite, a natural bitumen, and solid organic residues obtained by irradiation of frozen benzene. In both cases the results show that CO₂ is produced very efficiently after irradiation obtaining a maximum quantity of the order of . These results are, also quantitatively similar, to those recently obtained for water ice deposited on amorphous carbon films [Mennella, V., Palumbo, M.E., Baratta, G.A., 2004. Formation of CO and CO₂ molecules by ion irradiation of water ice covered hydrogenated carbon grains. Astrophys. J. 615, 1073-1080]. Thus we suggest that, whatever is the carbonaceous residue, CO₂ will be produced efficiently by the studied process. These results have interest in the context of the surfaces of the icy Galilean satellites in which CO₂ has been detected mainly trapped in the non-ice material, not in the pure water ice. We suggest that radiolysis of mixtures of water ice and refractory carbonaceous materials is the primary formation mechanism responsible for the CO₂ formation on the surfaces of the Galilean satellites.     

The future of Stardust science

Recent observations indicate that >99% of the small bodies in the solar system reside in its outer reaches—in the Kuiper Belt and Oort Cloud. Kuiper Belt bodies are probably the best‐preserved representatives of the icy planetesimals that dominated the bulk of the solid mass in the early solar system. They likely contain preserved materials inherited from the protosolar cloud, held in cryogenic storage since the formation of the solar system. Despite their importance, they are relatively underrepresented in our extraterrestrial sample collections by many orders of magnitude (~10¹³ by mass) as compared with the asteroids, represented by meteorites, which are composed of materials that have generally been strongly altered by thermal and aqueous processes. We have only begun to scratch the surface in understanding Kuiper Belt objects, but it is already clear that the very limited samples of them that we have in our laboratories hold the promise of dramatically expanding our understanding of the formation of the solar system. Stardust returned the first samples from a known small solar system body, the Jupiter‐family comet 81P/Wild 2, and, in a separate collector, the first solid samples from the local interstellar medium. The first decade of Stardust research resulted in more than 142 peer‐reviewed publications, including 15 papers in Science. Analyses of these amazing samples continue to yield unexpected discoveries and to raise new questions about the history of the early solar system. We identify nine high‐priority scientific objectives for future Stardust analyses that address important unsolved problems in planetary science.     

A mechanism for the production of hydrogen peroxide and the hydroperoxyl radical on icy satellites by low-energy electrons

Hydrogen peroxide (H₂O₂) is one of the minor constituents of the water ice covered surfaces of the jovian satellites Europa, Ganymede, and Callisto. Here we demonstrate that H₂O₂ production may be initiated by the dissociative electron attachment (DEA) of low-energy electrons (LEEs) to water molecules. Electronic excitation or ionization by electrons also contributes to H₂O₂ formation at higher electron energies. Finally, we show that hydroperoxyl (HO₂) radicals could be formed on the surfaces of icy satellites by LEE impact.     

The formation and stability of carbonic acid on outer Solar System bodies

The radiation chemistry, thermal stability, and vapor pressure of solid-phase carbonic acid (H₂CO₃) have been studied with mid-infrared spectroscopy. A new procedure for measuring this molecule’s radiation stability has been used to obtain intrinsic IR band strengths and half-lives for radiolytic destruction. We report, for the first time, measurements of carbonic acid’s vapor pressure (0.290-2.33 × 10⁻¹¹ bar for 240-255 K) and its enthalpy of sublimation (71 ± 9 kJ mol⁻¹). We also report the first observation of a chemical reaction involving solid-phase carbonic acid. Possible applications of these findings are discussed, with an emphasis on the outer Solar System icy surfaces.     

Evolution of density in solar system ICES

Pores present in ices in the solar system do not remain unchanged. In isothermal conditions they shrink while in a thermal gradient they migrate towards the higher temperature and escape so that the ice densifies. This motion has been investigated for pure H₂O- and CO₂-ices in a very simple one-dimensional model assuming uniform thermal conductivity and temperature gradient. The results indicate that the densification of H₂O-ice is so slow that it could be significant only for icy satellites having an internal heat source. On the other hand, CO₂-ice densifies orders of magnitude faster and the effect should be important for the CO₂ component of cometary nuclei. No effect is expected for icy planetary rings.     

Laboratory simulation experiments on the solid-state greenhouse effect in planetary ices

Icy surfaces like the polar caps of Mars, comets, Edgeworth-Kuiper belt objects or the surface areas of many moons in the outer Solar System behave different than rock and soil surfaces when irradiated by solar light. The latter ones absorb and reflect incoming solar radiation immediately at the surface. In contrast, ices are partially transparent in the visible spectral range and opaque in the infrared. Due to this fact it is possible for the solar radiation to reach a certain depth and increase the temperature of the sub-surface layers directly. This internal temperature rise is called “solid-state greenhouse effect,” in analogy to the classical greenhouse effect in an atmosphere. It may play an important role in the energy balance of various icy bodies in the Solar System. Within the scope of a project conducted at the Space Research Institute of the Austrian Academy of Sciences in Graz the solid-state greenhouse effect was investigated experimentally and theoretically. A number of experiments with diverse materials, focussing mainly on layered samples with a surface cover consisting of transparent H₂O-ice, were performed. The samples were irradiated under cryo-vacuum conditions by a solar simulator. The temperature distributions inside the samples were measured and compared with the results of numerical model calculations. We found that the predicted sub-surface temperature maximum is very clearly measurable in glass beads samples with various particle size distributions, but can also be detected in transparent compact surface ice layers. However, in the latter case it is less distinct than originally expected. Measuring the effect by laboratory methods turned out to be a difficult task due to the shallow depth where the temperature maximum occurs.     

Water ice on outer solar system surfaces: Basic properties and radiation effects

This paper reviews the properties of vapor-deposited water ice in connection with icy surfaces in the outer solar system. The emphasis is on knowledge gained during the last decade, and on the properties of the amorphous phase, especially those affected by the presence of microporosity. The paper discusses the role played by the properties of different phases of ice and the effect of irradiation on the icy surfaces of satellites in the outer solar system: sputtering, phase transformation, the production and trapping of molecular radiation products, and stress induced cracking. The understanding of how growth and irradiation processes affect the optical properties of ice will lead to extract better information from optical remote sensing than is possible today. It is argued that cracks in ice induced by stresses are the main reason causing low-temperature ices to be strongly scattering.     

The search for other planets: Clues from the solar system

Studies of element abundances and values of D/H in the atmospheres of the outer planets and Titan support a two-step model for the formation of these bodies. This model suggests that the dimensions of Uranus provide a good index for the sensitivity required to detect planets around other stars. The high proportion of N₂ on the surfaces of Pluto and Triton indicates that this gas was the dominant reservoir of nitrogen in the early solar nebula. It should also be abundant on pristine comets. There is evidence that some of these comets may well have brought a large store of volatiles to the inner planets, while others were falling into the sun. In other systems, icy planetesimals falling into stars should reveal themselves through high values of D/H.Paper presented at the Conference on Planetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

Ion irradiation experiments and nitrogen bearing species on Jovian and Saturnian icy surfaces

Implantation of reactive ions into targets of planetary interest is a relevant subject to be studied in the laboratory. It could in fact produce new molecular species that are not native to those surfaces. Presented here are new laboratory results obtained by nitrogen implantation (15–30 keV N⁺) on frozen mixtures of H₂O:CH₄ (2:1). These species have been chosen in view of their possible presence on the surface of Jovian and Saturnian satellites and rings. In fact these surfaces are exposed to intense irradiation by magnetospheric and/or solar energetic particles. The laboratory investigation utilizes IR spectroscopy. The main objectives of the present study are to identify newly produced species and to verify if these (or at least if the profile of their IR bands) are different from those produced by unreactive ions impinging on targets in which nitrogen is already present, occurring in the form of frozen NH₃ (Strazzulla and Palumbo, 1998) or N₂ (Palumbo et al., 1999). I find that CN-bearing group is in fact formed and its IR feature has a profile (peak position and band profile) that differs from that obtained after irradiation or frozen gases containing nitrogen. The relevance the results might have to elucidate the origin of some species observed on Jovian icy moons or predicted to be observed on Saturnian satellites are outlined.     

The role of Fischer-Tropsch catalysis in the origin of methane-rich Titan

Fischer-Tropsch catalysis, which converts CO and H₂ into CH₄ on the surface of iron catalyst, has been proposed to produce the CH₄ on Titan during its formation process in a circum-planetary subnebula. However, Fischer-Tropsch reaction rate under the conditions of subnebula have not been measured quantitatively yet. In this study, we conduct laboratory experiments to determine CH₄ formation rate and also conduct theoretical calculation of clathrate formation to clarify the significance of Fischer-Tropsch catalysis in a subnebula. Our experimental result indicates that the range of conditions where Fischer-Tropsch catalysis proceeds efficiently is narrow (T∼500-600 K) in a subnebula because the catalysts are poisoned at temperatures above 600 K under the condition of subnebula (i.e., H₂/CO = 1000). This suggests that an entire subnebula may not become rich in CH₄ but rather that only limited region of a subnebula may enriched in CH₄ (i.e., CH₄-rich band formation). Our experimental result also suggests that both CO and CO₂ are converted into CH₄ within time significantly shorter than the lifetime of the solar nebula at the optimal temperatures around 550 K. The calculation result of clathration shows that CO₂-rich satellitesimals are formed in the catalytically inactive outer region of subnebula. In the catalytically active inner region, CH₄-rich satellitesimals are formed. The resulting CH₄-rich satellitesimals formed in this region play an important role in the origin of CH₄ on Titan. When our experimental data are applied to a high-pressure model for subnebula evolution, it would predict that there should be CO₂ underneath the Iapetus subsurface and no thick CO₂ ice layer on Titan's icy crust. Such surface and subsurface composition, which may be observed by Cassini-Huygens mission, would provide crucial information on the origin of icy satellites.