The 2.5-5.0 micrometers spectra of Io: evidence for H2S and H2O frozen in SO2

Infrared spectra of Io in the region 2.5-5.0 micrometers, including new observational data, are analyzed using detailed laboratory studies of plausible surface ices. Besides the absorption bands attributable to sulfur dioxide frosts, four infrared spectral features of Io are shown to be unidentified. These unidentified features show spatial and temporal band strength variations. One pair is centered around 3.9 micrometers (3.85 and 3.91 micrometers) and the second pair is centered around 3.0 micrometers (2.97 and 3.15 micrometers). These absorptions fall close to the fundamental stretching modes in H2S and H2O, respectively. The infrared absorption spectra of an extensive set of laboratory ices ranging from pure materials, to binary mixtures of H2S and H2O (either mixed at different concentrations or layered), to H2O:H2S:SO2 mixtures are discussed. The effects of ultraviolet irradiation (120 and 160 nm) and temperature variation (from 9 to 130 K) on the infrared spectra of the ices are examined. This comparative study of Io reflectance spectra with the laboratory mixed ice transmission data shows the following: (1) Io's surface most likely contains H2S and H2O mixed with SO2. The 3.85- and 3.91-micrometers bands in the Io spectra can be accounted for by the absorption of the S-H stretching vibration (nu 1) in H2S clusters and isolated molecules in an SO2-dominated ice. The weak 2.97- and 3.15-micrometers bands which vary spatially and temporally in the Io spectra coincide with the nu 3 and nu 1 O-H stretching vibrations of clusters of H2O molecules complexed, through hydrogen bonding and charge transfer interactions, with SO2. (2) The observations are well matched qualitatively by the transmission spectra of SO2 ices containing about 3% H2S and 0.1% H2O which have been formed by the condensation of a mixture of the gases onto a 100 K surface. (3) No new features are produced in the region 2.5 to 5.0 micrometers in the spectrum of these ices under prolonged ultraviolet irradiation or temperature variation up to 120 K. (4) Comparison of the Io spectra to transmission spectra of both mixed molecular ices and layered ices indicates that only the former can explain the shifts and splitting of the absorption bands seen in the Io spectrum and additionally can account for the fact that solid H2S is observed in the surface material of Io at temperature and pressure conditions above the sublimation point of pure H2S.     

Condensation and vaporization studies of CH3OH and NH3 ices: major implications for astrochemistry

In an extension of previously reported work on ices containing H2O, CO, CO2, SO2, H2S, and H2, we present measurements of the physical and infrared spectral properties of ices containing CH3OH and NH3. The condensation and sublimation behavior of these ice systems is discussed and surface binding energies are presented for all of these molecules. The surface binding energies can be used to calculate the residence times of the molecules on grain surfaces as a function of temperature. It is demonstrated that many of the molecules used to generate radio maps of and probe conditions in dense clouds, for example CO and NH3, will be significantly depleted from the gas phase by condensation onto dust grains. Attempts to derive total column densities solely from radio maps that do not take condensation effects into account may vastly underestimate the true column densities of any given species. Simple CO condensation onto and vaporization off of grains appears to be capable of explaining the observed depletion of gas phase CO in cold, dense molecular cores. This is not the case for NH3, however, where thermal considerations alone predict that all of the NH3 should be condensed onto grains. The fact that some gas phase NH3 is observed indicates that additional desorption processes must be involved. The surface binding energies of CH3OH, in conjunction with this molecule's observed behavior during warm up in H2O-rich ices, is shown to provide an explanation of the large excess of CH3OH seen in many warm, dense molecular cores. The near-infrared spectrum and associated integrated band strengths of CH3OH-containing ice are given, as are middle infrared absorption band strengths for both CH3OH and NH3.     

H2 in interstellar and extragalactic ices: infrared characteristics, ultraviolet production, and implications

H2 is the most abundant molecule in the universe. We demonstrate that this molecule may be an important component of interstellar and possibly intergalactic ices, both because it can be formed in situ, within the ices, and because gas phase H2 can freeze out onto dust grains in some astrophysical environments. The condensation-sublimation and infrared spectral properties of ices containing H2 are presented. We show that solid H2 in H20-rich ices can be detected by an infrared absorption band at 4137 cm-1 (2.417 micrometers). The surface binding energy of H2 to H2O ice was measured to the delta Hs/k = 555 +/- 35 K. Surface binding energies can be used to calculate the residence times of H2 on grain surfaces as a function of temperature. Some of the implications of these results are considered.     

The physical and infrared spectral properties of CO2 in astrophysical ice analogs

Both laboratory measurements and theory indicate that CO2 should be a common component in interstellar ices. We show that the exact band position, width, and profile of the solid-state 12CO2 infrared bands near 3705, 3600, 2340, and 660 cm-1 (2.70, 2.78, 4.27, and 15.2 micrometers) and the 13CO2 band near 2280 cm-1 (4.39 micrometers) are dependent on the matrix in which the CO2 is frozen. Measurements of these bands in astronomical spectra can be used to determine column densities of solid-state CO2 and provide important information on the physical conditions present in the ice grains of which the CO2 is a part. Depending on the composition of the ice, the CO2 asymmetric stretching band was observed to vary from 2328.7 to 2346.0 cm-1 and have full widths at half-maxima (FWHMs) ranging from 4.7 to 29.9 cm-1. The other CO2 bands showed similar variations. Both position and width are also concentration dependent. Absorption coefficients were determined for the five CO2 bands. These were found to be temperature independent for CO2 in CO and CO2 matrices but varied slightly with temperature for CO2 in H2O-rich ices. For all five bands this variation was found to be less than 15% from 10 to 150 K, the temperature at which H2O ice sublimes. A number of parameters associated with the physical behavior of CO2 in CO2- and H2O-rich ices were also determined. The CO2-CO2 surface binding energy in pure CO2 ices is found to be (delta Hs/k) = 2690 +/- 50 K. CO2-H2O and CO-H2O surface binding energies were determined to be (delta Hs/k) = 2860 +/- 200 K and 1740 +/- 100 K, respectively. Under our experimental conditions, CO2 condenses in measurable quantities into H2O-rich ices at temperatures up to 100 K, only slightly higher than the temperature at which pure CO2 condenses. Once frozen into an H2O-rich ice, the subsequent loss of CO2 upon warming is highly dependent on concentration. For ices with H2O/CO2 > 20, the CO is physically trapped within the H2O lattice, and little CO2 is lost until the sublimation temperature of the H2O matrix is reached. In contrast, in ices having H2O/CO2 < 5, the CO2 remains only to temperatures of about 90 K. Above this point the CO2 readily diffuses out of the H2O matrix. These results suggest that two different forms of H2O lattice are produced. The implications of these data for cometary models and our understanding of cometary formation are considered.     

Infrared spectroscopy of Triton and Pluto ice analogs: the case for saturated hydrocarbons

The infrared transmission spectra and photochemical behavior of various organic compounds isolated in solid N2 ices, appropriate for applications to Triton and Pluto, are presented. It is shown that excess absorption in the surface spectra of Triton and Pluto, i.e., absorption not explained by present models incorporating molecules already identified on these bodies (N2, CH4, CO, and CO2), that starts near 4450 cm-1 (2.25 micrometers) and extends to lower frequencies, may be due to alkanes (C(n)H2n+2) and related molecules frozen in the nitrogen. Branched and linear alkanes may be responsible. Experiments in which the photochemistry of N2:CH4 and N(2):CH4:CO ices was explored demonstrate that the surface ices of Triton and Pluto may contain a wide variety of additional species containing H, C, O, and N. Of these, the reactive molecule diazomethane, CH2N2, is particularly important since it may be largely responsible for the synthesis of larger alkanes from CH4 and other small alkanes. Diazomethane would also be expected to drive chemical reactions involving organics in the surface ices of Triton and Pluto toward saturation, i.e., to reduce multiple CC bonds. The positions and intrinsic strengths (A values) of many of the infrared absorption bands of N2 matrix-isolated molecules of relevance to Triton and Pluto have also been determined. These can be used to aid in their search and to place constraints on their abundances. For example, using these A values the abundance ratios CH4/N2 approximately 1.3 x 10(-3), C2H4/N2 < or = 9.5 x 10(-7) and H2CO/N2 < or = 7.8 x 10(-7) are deduced for Triton and CH4/N2 approximately 3.1 x 10(-3), C2H4/N2 < or = 4.1 x 10(-6), and H2CO/N2 < or = 5.2 x 10(-6) deduced for Pluto. The small amounts of C2H4 and H2CO in the surface ices of these bodies are in disagreement with the large abundances expected from many theoretical models.     

H2O-H2SO4 system in Venus' clouds and OCS, CO, and H2SO4 profiles in Venus' troposphere

A coupled problem of diffusion and condensation is solved for the H2SO4-H2O system in Venus' cloud layer. The position of the lower cloud boundary and profiles of the H2O and H2SO4 vapor mixing ratios and of the H2O/H2SO4 ratio of sulfuric acid aerosol and its flux are calculated as functions of the column photochemical production rate of sulfuric acid, phi H2SO4. Variations of the lower cloud boundary are considered. Our basic model, which is constrained to yield fH2O (30 km) = 30 ppm (Pollack et al. 1993), predicts the position of the lower cloud boundary at 48.4 km coinciding with the mean Pioneer Venus value, the peak H2SO4 mixing ratio of 5.4 ppm, and the H2SO4 production rate phi H2SO4 = 2.2 x 10(12) cm-2 sec-1. The sulfur to sulfuric acid mass flux ratio in the clouds is 1 : 27 in this model, and the mass loading ratio may be larger than this value if sulfur particles are smaller than those of sulfuric acid. The model suggests that the extinction coefficient of sulfuric acid particles with radius 3.7 micrometers (mode 3) is equal to 0.3 km-1 in the middle cloud layer. The downward flux of CO is equal to 1.7 x 10(12) cm-2 sec-1 in this model. Our second model, which is constrained to yield fH2SO4 = 10 ppm at the lower cloud boundary, close to the value measured by the Magellan radiooccultations, predicts the position of this boundary to be at 46.5 km, which agrees with the Magellan data; fH2O(30 km) = 90 ppm, close to the data of Moroz et al. (1983) at this altitude; phi H2SO4 = 6.4 x 10(12) cm-2 sec-1; and phi co = 4.2 x 10(12) cm-2 sec-1. The S/H2SO4 flux mass ratio is 1 : 18, and the extinction coefficient of the mode 3 sulfuric acid particles is equal to 0.9 km-1 in the middle cloud layer. A strong gradient of the H2SO4 vapor mixing ratio near the bottom of the cloud layer drives a large upward flux of H2SO4, which condenses and forms the excessive downward flux of liquid sulfuric acid, which is larger by a factor of 4-7 than the flux in the middle cloud layer. This is the mechanism of formation of the lower cloud layer. Variations of the lower cloud layer are discussed. Our modeling of the OCS and CO profiles in the lower atmosphere measured by Pollack et al. (1993) provides a reasonable explanation of these data and shows that the rate coefficient of the reaction SO3 + CO --> CO2 + SO2 is equal to 10(-11) exp(-(13,100 +/- 1000)/T) cm3/s. The main channel of the reaction between SO3 and OCS is CO2 + (SO)2, and its rate coefficient is equal to 10(-11) exp(-(8900 +/- 500)T)cm3/s. In the conditions of Venus' lower atmosphere, (SO)2 is removed by the reaction (SO)2 + OCS --> CO + S2 + SO2. The model predicts an OCS mixing ratio of 28 ppm near the surface.     

Radiation Products in Processed Ices Relevant to Edgeworth-Kuiper-Belt Objects

Near the inner edge of the Edgeworth-Kuiper Belt (EKB) are Pluto and Charon, which are known to have N₂- and H₂O-dominated surface ices, respectively. Such non-polar and polar ices, and perhaps mixtures of them, also may be present on other trans-Neptunian objects. Pluto, Charon, and all EKB objects reside in a weak, but constant UV-photon and energetic ion radiation environment that drives chemical reactions in their surface ices. Effects of photon and ion processing include changes in ice composition, volatility, spectra, and albedo, and these have been studied in a number of laboratories. This paper focuses on ice processing by ion irradiation and is aimed at understanding the volatiles, ions, and residues that may exist on outer solar system objects. We summarize radiation chemical products of N₂-rich and H₂O-rich ices containing CO or CH₄, including possible volatiles such as alcohols, acids, and bases. Less-volatile products that could accumulate on EKB objects are observed to form in the laboratory from acid-base reactions, reactions promoted by warming, or reactions due to radiation processing of a relatively pure ice (e.g., CO → C₃O₂). New IR spectra are reported for the 1–5 mu;m region, along with band strengths for the stronger features of carbon suboxide, carbonic acid, the ammonium and cyanate ions, polyoxymethylene, and ethylene glycol. These six materials are possible contributors to EKB surfaces, and will be of interest to observers and future missions.     

Photochemistry of a Volcanically Driven Atmosphere on Io: Sulfur and Oxygen Species from a Pele-Type Eruption

To determine how active volcanism might affect the standard picture of sulfur dioxide photochemistry on Io, we have developed a one-dimensional atmospheric model in which a variety of sulfur-, oxygen-, sodium-, potassium-, and chlorine-bearing volatiles are volcanically outgassed at Io's surface and then evolve due to photolysis, chemical kinetics, and diffusion. Thermochemical equilibrium calculations in combination with recent observations of gases in the Pele plume are used to help constrain the composition and physical properties of the exsolved volcanic vapors. Both thermochemical equilibrium calculations (Zolotov and Fegley 1999, Icarus141, 40-52) and the Pele plume observations of Spencer et al. (2000; Science288, 1208-1210) suggest that S₂ may be a common gas emitted in volcanic eruptions on Io. If so, our photochemical models indicate that the composition of Io's atmosphere could differ significantly from the case of an atmosphere in equilibrium with SO₂ frost. The major differences as they relate to oxygen and sulfur species are an increased abundance of S, S₂, S₃, S₄, SO, and S₂O and a decreased abundance of O and O₂ in the Pele-type volcanic models as compared with frost sublimation models. The high observed SO/SO₂ ratio on Io might reflect the importance of a contribution from volcanic SO rather than indicate low eddy diffusion coefficients in Io's atmosphere or low SO “sticking” probabilities at Io's surface; in that case, the SO/SO₂ ratio could be temporally and/or spatially variable as volcanic activity fluctuates. Many of the interesting volcanic species (e.g., S₂, S₃, S₄, and S₂O) are short lived and will be rapidly destroyed once the volcanic plumes shut off; condensation of these species near the source vent is also likely. The diffuse red deposits associated with active volcanic centers on Io may be caused by S₄ radicals that are created and temporarily preserved when sulfur vapor (predominantly S₂) condenses around the volcanic vent. Condensation of SO across the surface and, in particular, in the polar regions might also affect the surface spectral properties. We predict that the S/O ratio in the torus and neutral clouds might be correlated with volcanic activity—during periods when volcanic outgassing of S₂ (or other molecular sulfur vapors) is prevalent, we would expect the escape of sulfur to be enhanced relative to that of oxygen, and the S/O ratio in the torus and neutral clouds could be correspondingly increased.     

Near-infrared spectral monitoring of Triton with IRTF/SpeX II: Spatial distribution and evolution of ices

This report arises from an ongoing program to monitor Neptune’s largest moon Triton spectroscopically in the 0.8 to 2.4 μm range using IRTF/SpeX. Our objective is to search for changes on Triton’s surface as witnessed by changes in the infrared absorption bands of its surface ices N₂,CH₄,H₂O, CO, and CO₂. We have recorded infrared spectra of Triton on 53 nights over the ten apparitions from 2000 to 2009. The data generally confirm our previously reported diurnal spectral variations of the ice absorption bands (Grundy and Young, 2004). Nitrogen ice shows a large amplitude variation, with much stronger absorption on Triton’s Neptune-facing hemisphere. We present evidence for seasonal evolution of Triton’s N₂ ice: the 2.15 μm absorption band appears to be diminishing, especially on the Neptune-facing hemisphere. Although it is mostly dissolved in N₂ ice, Triton’s CH₄ ice shows a very different longitudinal variation from the N₂ ice, challenging assumptions of how the two ices behave. Unlike Triton’s CH₄ ice, the CO ice does exhibit longitudinal variation very similar to the N₂ ice, implying that CO and N₂ condense and sublimate together, maintaining a consistent mixing ratio. Absorptions by H₂O and CO₂ ices show negligible variation as Triton rotates, implying very uniform and/or high latitude spatial distributions for those two non-volatile ices.     

Io's 4-μm band and the role of adsorbed SO2

The role of adsorbed SO₂ on Io's surface particles in producing the observed spectral absorption band near 4 μm in Io's reflectance spectrum is explored. Calculations show that a modest 50% monolayer coating of adsorbed SO₂ molecules on submicron grains of sulfur of alkali sulfide, assumed to make up Io's uppermost optical surface (“radialith”), will result in a ν₁ + ν₃ absorption band near 4 μm with depth ～30% below the adjacent continuum, consistent with the observed strength of the Io band. The precise wavelength position of the ν₁ + ν₃ band of SO₂ in different phase states such as frost, ice, adsorbate, and gas are summarized from the experimental literature and compared with the available telescopic measurements of the Io band position. The results suggest that the 4-μm band in Io's full disk spectrum can best be explained by the presence on Io's surface of widespread SO₂ in the form of adsorbate rather than ice or frost.     

Saturn's Titan: Surface change, ammonia, and implications for atmospheric and tectonic activity

Titan is known to have a young surface. Here we present evidence from the Cassini Visual and Infrared Mapping Spectrometer that it is currently geologically active. We report that changes in the near-infrared reflectance of a 73,000 km² area on Titan (latitude 26° S, longitude 78° W) occurred between July 2004 and March of 2006. The reflectance of the area increased by a factor of two between July 2004 and March-April 2005; it then returned to the July 2004 level by November 2005. By late December 2005 the reflectance had surged upward again, establishing a new maximum. Thereafter, it trended downward for the next three months. Detailed spectrophotometric analyses suggest these changes happen at or very near the surface. The spectral differences between the region and its surroundings rule out changes in the distribution of the ices of reasonably expected materials such as H₂O, CO₂, and CH₄ as possible causes. Remarkably, the change is spectrally consistent with the deposition and removal of NH₃ frost over a water ice substrate. NH₃ has been proposed as a constituent of Titan's interior and has never been reported on the surface. The detection of NH₃ frost on the surface might possibly be explained by episodic effusive events occur which bring juvenile ammonia from the interior to the surface. If so, its decomposition would feed nitrogen to the atmosphere now and in the future. The lateral extent of the region exceeds that of active areas on the Earth (Hawaii) or Io (Loki).     

H-implantation in SO2 and CO2 ices

Ices in the solar system are observed on the surface of planets, satellites, comets and asteroids where they are continuously subordinate at particle fluxes (cosmic ions, solar wind and charged particles caught in the magnetosphere of the planets) that deeply modify their physical and structural properties. Each incoming ion destroys molecular bonds producing fragments that, by recombination, form new molecules also different from the original ones. Moreover, if the incoming ion is reactive (H⁺, Oⁿ⁺, Sⁿ⁺, etc.), it can concur to the formation of new molecules.Those effects can be studied by laboratory experiments where, with some limitation, it is possible to reproduce the astrophysical environments of planetary ices.In this work, we describe some experiments of 15-100 keV H⁺ and He⁺ implantation in pure sulfur dioxide (SO₂) at 16 and 80 K and carbon dioxide (CO₂) at 16 K ices aimed to search for the formation of new molecules. Among other results we confirm that carbonic acid (H₂CO₃) is formed after H-implantation in CO₂, vice versa H-implantation in SO₂ at both temperatures does not produce measurable quantity of sulfurous acid (H₂SO₃). The results are discussed in the light of their relevance to the chemistry of some solar system objects, particularly of Io, the innermost of Jupiter's Galilean satellites, that exhibits a surface very rich in frost SO₂ and it is continuously bombarded with H⁺ ions caught in Jupiter's magnetosphere.     

Evolution of Io's volatile inventory

Using Voyager results, we have made crude estimates of the rate at which Io loses volatiles by a variety of processes to the surrounding magnetosphere for both the current SO₂-dominated atmosphere as well as hypothetical paleoatmospheres in which other gases, such as N₂, may have been the dominant constituent. Loss rates are strongly influenced by the surface pressure on the night side, the relationship between the exobase and the Jovian magnetospheric boundary, the exospheric temperature, and the peak altitudes reached by volcanic plumes. Several mechanisms make significant contributions to the prodigious rate at which Io is currently losing volatiles. These include: interaction of the magnetospheric plasma with volcanic plume particles and the background atmosphere; sputtering of ices on the surface, if the nightside atmospheric pressure is low enough; and Jeans' escape of O, a dissociation product of SO₂ gas. For paleoatmospheres, only the first two of these mechanisms would have been effective. However, they are capable of eliminating large amounts of N₂ and other volatiles from Io over the satellite's lifetime. Io could have also lost large amounts of water over its lifetime due to the extensive recycling of water between its upper and lower crust, with the partial dissociation of water vapor in silicate magma chambers initiating this loss process. Significant amounts of water may also have been lost as a result of the interaction of the magnetospheric plasma with water ice particles in volcanic plumes. Once an SO₂-dominated atmosphere becomes established, much water may have also been lost through the sputtering of surface water ice.     

A coupled microphysical/radiative transfer model of albedo and emissivity of planetary surfaces covered by volatile ices

A new model of albedo and emissivity of planetary surfaces covered by volatile ices in the form of porous slab-like deposits is described. In the model, a radiative transfer model is coupled with a microphysical model in order to link changes in albedo and emissivity to changes in porosity caused by ice metamorphism. Preliminary results for Triton, Pluto, and Io are presented (the martian CO₂ caps will be the subject of a separate publication). The coupled model will aid in the interpretation of ground-based and spacecraft observations and should lead to advances in surface and atmospheric modeling.     

Ice lines, planetesimal composition and solid surface density in the solar nebula

To date, there is no core accretion simulation that can successfully account for the formation of Uranus or Neptune within the observed 2–3 Myr lifetimes of protoplanetary disks. Since solid accretion rate is directly proportional to the available planetesimal surface density, one way to speed up planet formation is to take a full accounting of all the planetesimal-forming solids present in the solar nebula. By combining a viscously evolving protostellar disk with a kinetic model of ice formation, which includes not just water but methane, ammonia, CO and 54 minor ices, we calculate the solid surface density of a possible giant planet-forming solar nebula as a function of heliocentric distance and time. Our results can be used to provide the starting planetesimal surface density and evolving solar nebula conditions for core accretion simulations, or to predict the composition of planetesimals as a function of radius. We find three effects that favor giant planet formation by the core accretion mechanism: (1) a decretion flow that brings mass from the inner solar nebula to the giant planet-forming region, (2) the fact that the ammonia and water ice lines should coincide, according to recent lab results from Collings et al. [Collings, M.P., Anderson, M.A., Chen, R., Dever, J.W., Viti, S., Williams, D.A., McCoustra, M.R.S., 2004. Mon. Not. R. Astron. Soc. 354, 1133–1140], and (3) the presence of a substantial amount of methane ice in the trans-saturnian region. Our results show higher solid surface densities than assumed in the core accretion models of Pollack et al. [Pollack, J.B., Hubickyj, O., Bodenheimer, P., Lissauer, J.J., Podolak, M., Greenzweig, Y., 1996. Icarus 124, 62–85] by a factor of 3–4 throughout the trans-saturnian region. We also discuss the location of ice lines and their movement through the solar nebula, and provide new constraints on the possible initial disk configurations from gravitational stability arguments.     

Infrared study of ion-irradiated N2-dominated ices relevant to Triton and Pluto: formation of HCN and HNC

Infrared spectra and radiation chemical behavior of N₂-dominated ices relevant to the surfaces of Triton and Pluto are presented. This is the first systematic IR study of proton-irradiated N₂-rich ices containing CH₄ and CO. Experiments at 12 K show that HCN, HNC, and diazomethane (CH₂N₂) form in the solid phase, along with several radicals. NH₃ is also identified in irradiated N₂ + CH₄ and N₂ + CH₄ + CO. We show that HCN and HNC are made in irradiated binary ice mixtures having initial N₂/CH₄ ratios from 100 to 4, and in three-component mixtures have an initial N₂/(CH₄ + CO) ratio of 50. HCN and HNC are not detected in N₂-dominated ices when CH₄ is replaced with C₂H₆, C₂H₂, or CH₃OH.The intrinsic band strengths of HCN and HNC are measured and used to calculate G(HCN) and G(HNC) in irradiated N₂ + CH₄ and N₂ + CH₄ + CO ices. In addition, the HNC/HCN ratio is calculated to be ∼1 in both icy mixtures. These radiolysis results reveal, for the first time, solid-phase synthesis of both HCN and HNC in N₂-rich ices containing CH₄.We examine the evolution of spectral features due to acid-base reactions (acids such as HCN, HNC, and HNCO and a base, NH₃) triggered by warming irradiated ices from 12 K to 30-35 K. We identify anions (OCN⁻, CN⁻, and N3−) in ices warmed to 35 K. These ions are expected to form and survive on the surfaces of Triton and Pluto. Our results have astrobiological implications since many of these products (HCN, HNC, HNCO, NH₃, NH₄OCN, and NH₄CN) are involved in the syntheses of biomolecules such as amino acids and polypeptides.     

The eruptive evolution of the Galilean satellites: Implications for the ancient magnetic field of Jupiter

The hypothesis considering the Jupiter-Sun system as a limiting case of a close binary star implies the initial relative ice abundances in all the Galilean satellites to be essentially equal. The satellites move in the Jovian magnetosphere; thus the unipolar current flowing through their bodies subjected their ices to volumetric electrolysis. Explosions of the electrolysis products resulted in a loss of ices. While Callisto did not explode at all, Ganymede exploded once, Europa twice, and Io two or three times. An analysis of the magnetic field changes needed to create the modern ice abundances in the satellite shows:

1. the initial field of Jupiter was ～10² times stronger when compared with the present-day field, and
2. the field had to decrease exponentially with τ_2| ≈ (0.6?1), which means its relic nature.

     

Experimental impact shock chemistry on planetary icy satellites

We present new experimental results on impact shock chemistry into icy satellites of the outer planets. Icy mixtures of pure water ice with CO₂, Na₂CO₃, CH₃OH, and CH₃OH/(NH₄)₂SO₄ at 77 K were ablated with a powerful pulsed laser—a new technique used to simulate shock processes which can occur during impacts. New products were identified by GC-MS and FTIR analyses after laser ablation. Our results show that hydrogen peroxide is formed in irradiated H₂O/CO₂ ices with a final concentration of 0.23%. CO and CH₃OH were also detected as main products. The laser ablation of frozen H₂O/Na₂CO₃ generates only CO and CO₂ as destruction products from the salt. Pulsed irradiation of water ice containing methanol leads also to the formation of CO and CO₂, generates methane and more complex molecules containing carbonyl groups like acetaldehyde, acetone, methyl formate, and a diether, dimethyl formal. The last three compounds are also produced when adding ammonium sulfate to H₂O/CH₃OH ice, but acetone is more abundant. The formation of two hydrocarbons, CH₄ and C₂H₆ is observed as well as the production of three nitrogen compounds, nitrous oxide, hydrogen cyanide, and acetonitrile.     

Spatially resolved SO2 ice on Io, observed in the near IR

We present equivalent width maps of the 1.98 and 2.13 μm SO₂ ice absorption bands on the surface of Io. The data were taken on 17 April 2006 with the near-infrared mapping spectrometer, OSIRIS at the W.M. Keck Observatory, Hawaii. The maps show significant regional enhancements of SO₂ ice over the Bosphoros, Media, Tarsus and Chalybes Regiones.     

Ganymede,Europa, Callisto,and Saturn's rings: Compositional analysis from reflectance spectroscopy

The reflectance spectra of Ganymede, Europa, Callisto, and Saturn's rings are analyzed using recent laboratory reflectance studies of water frost, water ice, and water and mineral mixtures. It is found that the spectra of the icy Galilean satellites are characteristic of water ice (e.g., ice blocks or possibly very large ice crystals ? 1 cm) or frost on ice rather than pure water frost, and that the decrease in reflectance at visible wavelengths is caused by other mineral grains in the surface. The spectra of Saturn's rings are more characteristic of water frost with some other mineral grains mixed in the frost but not on the surface. The impurities on all these objects are not in spectrally isolated patches but appear to be intimately mixed with the water. The impurity grains appear to have reflectance spectra typical of minerals containing Fe³⁺. Some carbonaceous chondrite meteorite spectra show the necessary spectral shape. Ganymede is found to have more water ice on the surface than previously thought (～90 wt%), as is Callisto (30–90 wt%). The surface of Europa has a vast frozen water surface with only a few percent impurities. Saturn's rings also have only a few percent impurities. The amount of bound water or bound OH for these objects is 5 ± 5 wt% averaged over the entire surface. Thus with the small amount of nonicy material present on these objects, no hydrated minerals can be ruled out. A new absorption feature is identified in Ganymede, Callisto, and probably Europa at 1.5 μm which is also seen in the spectra of Io but not in Saturn's rings. This feature has not been seen in laboratory studies and its cause is unknown.