Sputtering Of Ice Research Articles

High-resolution Study of the Outflow Activity and Chemical Environment of Chamaeleon-MMS1

Abstract The earliest stages of low-mass star formation are unclear, with the first hydrostatic core (FHSC) as the transition stage between a prestellar and protostellar core. This work describes the local (∼4000 au) outflow activity associated with candidate FHSC Chamaeleon-MMS1 and its effect on the surrounding material to determine the evolutionary state of this young low-mass source. We observed Chamaeleon-MMS1 with the Atacama Large Millimeter/submillimeter Array at 220 GHz at high spatial (∼75 au) and spectral resolutions (0.1–0.3 km s−1). A low-energy outflow is detected, consisting of two components, a broad spectral feature (Δv ∼ 8 km s−1) to the northeast and narrow spectral features (Δv ∼ 1 km s−1) to both the northeast and southwest. The molecular tracers CS, formaldehyde (H2CO), and methanol (CH3OH) were used to analyze the effect of the outflows on the surrounding gas and determine its rotational temperature. The rotational temperature of H2CO is 40 K toward the continuum source with similarly low temperatures (10–75 K) toward clumps affected by the outflow. CH3OH is only detected toward gas clumps located away from the continuum source, where the methanol is expected to have been released by the energy of the outflow through ice sputtering. While molecular emission and high outflow speeds rule Cha-MMS1 out as an FHSC, its outflow is less energetic than those of other Class 0 objects and its physical properties are within the range covered by other low-luminosity protostars. The inferred gas temperatures toward the continuum source are also relatively low, indicating that Cha-MMS1 is one of the youngest known sources.

Efficiency of non-thermal desorptions in cold-core conditions

Context.Under cold conditions in dense cores, gas-phase molecules and atoms are depleted from the gas-phase to the surface of interstellar grains. Considering the time scales and physical conditions within these cores, a portion of these molecules has to be brought back into the gas-phase to explain their observation by milimeter telescopes.Aims.We tested the respective efficiencies of the different mechanisms commonly included in the models (photo-desorption, chemical desorption, and cosmic-ray-induced whole-grain heating). We also tested the addition of sputtering of ice grain mantles via a collision with cosmic rays in the electronic stopping power regime, leading to a localized thermal spike desorption that was measured in the laboratory.Methods.The ice sputtering induced by cosmic rays has been added to the Nautilus gas-grain model while the other processes were already present. Each of these processes were tested on a 1D physical structure determined by observations in TMC1 cold cores. We focused the discussion on the main ice components, simple molecules usually observed in cold cores (CO, CN, CS, SO, HCN, HC3N, and HCO+), and complex organic molecules (COMs such as CH3OH, CH3CHO, CH3OCH3, and HCOOCH3). The resulting 1D chemical structure was also compared to methanol gas-phase abundances observed in these cores.Results.We found that all species are not sensitive in the same way to the non-thermal desorption mechanisms, and the sensitivity also depends on the physical conditions. Thus, it is mandatory to include all of them. Chemical desorption seems to be essential in reproducing the observations for H densities smaller than 4 × 104cm−3, whereas sputtering is essential above this density. The models are, however, systematically below the observed methanol abundances. A more efficient chemical desorption and a more efficient sputtering could better reproduce the observations.Conclusions.In conclusion, the sputtering of ices by cosmic-rays collisions may be the most efficient desorption mechanism at high density (a few 104cm−3under the conditions studied here) in cold cores, whereas chemical desorption is still required at smaller densities. Additional works are needed on both mechanisms to assess their efficiency with respect to the main ice composition.

Water in star-forming regions: physics and chemistry from clouds to disks as probed by Herschel spectroscopy

Water in star-forming regions: physics and chemistry from clouds to disks as probed by <i>Herschel</i> spectroscopy

Cosmic ray sputtering yield of interstellar ice mantles

Aims. Cosmic-ray-induced sputtering is one of the important desorption mechanisms at work in astrophysical environments. The chemical evolution observed in high-density regions, from dense clouds to protoplanetary disks, and the release of species condensed on dust grains, is one key parameter to be taken into account in interpretations of both observations and models. Methods. This study is part of an ongoing systematic experimental determination of the parameters to consider in astrophysical cosmic ray sputtering. As was already done for water ice, we investigated the sputtering yield as a function of ice mantle thickness for the two next most abundant species of ice mantles, carbon monoxide and carbon dioxide, which were exposed to several ion beams to explore the dependence with deposited energy. Results. These ice sputtering yields are constant for thick films. It decreases rapidly for thin ice films when reaching the impinging ion sputtering desorption depth. An ice mantle thickness dependence constraint can be implemented in the astrophysical modelling of the sputtering process, in particular close to the onset of ice mantle formation at low visual extinctions.

Electronic sputtering of solid N[formula omitted] by swift ions

Most sputtering yield measurements for solid N2 are reported for stopping powers lower than 10−13eV cm2/molecule. We measured the sputtering yield for solid N2 at stopping powers, in the electronic regime, above 10−12eV cm2/molecule, extending the range of such measurements by more than an order of magnitude, using a 33 MeV 58Ni9+ swift heavy ions beam. The evolution of the thin N2 ice films was monitored in-situ by mid-infrared spectroscopy (FTIR) during irradiation. As N2 is only weakly infrared active, and can be hardly monitored directly via an infrared absorption mode in such experiments, we use the Fabry–Perot interference fringes of the ice film to evaluate, via an optical model, the erosion of the N2 film as a function of ion fluence. A sputtering model including several sputtering crater shapes is developed and tested against experimental data. We derive the sputtering yield for a semi-infinite N2 ice film and its dependence with the ice thickness for thin film conditions, monitoring the N2 ice sputtering depth. We combine the results with previous measurements at lower stopping powers to derive the electronic sputtering of solid N2 over a large stopping power range.

Heavy ion irradiation of crystalline water ice

Under cosmic irradiation, the interstellar water ice mantles evolve towards a compact amorphous state. Crystalline ice amorphisation was previously monitored mainly in the keV to hundreds of keV ion energies. We experimentally investigate heavy ion irradiation amorphisation of crystalline ice, at high energies closer to true cosmic rays, and explore the water-ice sputtering yield. We irradiated thin crystalline ice films with MeV to GeV swift ion beams, produced at the GANIL accelerator. The ice infrared spectral evolution as a function of fluence is monitored with in-situ infrared spectroscopy (induced amorphisation of the initial crystalline state into a compact amorphous phase). The crystalline ice amorphisation cross-section is measured in the high electronic stopping-power range for different temperatures. At large fluence, the ice sputtering is measured on the infrared spectra, and the fitted sputtering-yield dependence, combined with previous measurements, is quadratic over three decades of electronic stopping power. The final state of cosmic ray irradiation for porous amorphous and crystalline ice, as monitored by infrared spectroscopy, is the same, but with a large difference in cross-section, hence in time scale in an astrophysical context. The cosmic ray water-ice sputtering rates compete with the UV photodesorption yields reported in the literature. The prevalence of direct cosmic ray sputtering over cosmic-ray induced photons photodesorption may be particularly true for ices strongly bonded to the ice mantles surfaces, such as hydrogen-bonded ice structures or more generally the so-called polar ices.

The origin of organic emission in NGC 2071

Context: The physical origin behind organic emission in embedded low-mass star formation has been fiercely debated in the last two decades. A multitude of scenarios have been proposed, from a hot corino to PDRs on cavity walls to shock excitation. Aims: The aim of this paper is to determine the location and the corresponding physical conditions of the gas responsible for organics emission lines. The outflows around the small protocluster NGC 2071 are an ideal testbed to differentiate between various scenarios. Methods: Using Herschel-HIFI and the SMA, observations of CH3OH, H2CO and CH3CN emission lines over a wide range of excitation energies were obtained. Comparisons to a grid of radiative transfer models provide constraints on the physical conditions. Comparison to H2O line shape is able to trace gas-phase synthesis versus a sputtered origin. Results: Emission of organics originates in three spots: the continuum sources IRS 1 ('B') and IRS 3 ('A') as well as a outflow position ('F'). Densities are above 10$^7$ cm$^{-3}$ and temperatures between 100 to 200 K. CH3OH emission observed with HIFI originates in all three regions and cannot be associated with a single region. Very little organic emission originates outside of these regions. Conclusions: Although the three regions are small (<1,500 AU), gas-phase organics likely originate from sputtering of ices due to outflow activity. The derived high densities (>10$^7$ cm$^{-3}$) are likely a requirement for organic molecules to survive from being destroyed by shock products. The lack of spatially extended emission confirms that organic molecules cannot (re)form through gas-phase synthesis, as opposed to H2O, which shows strong line wing emission. The lack of CH3CN emission at 'F' is evidence for a different history of ice processing due to the absence of a protostar at that location and recent ice mantle evaporation.

Water and methanol in low-mass protostellar outflows: gas-phase synthesis, ice sputtering and destruction

Water in outflows from protostars originates either as a result of gas-phase synthesis from atomic oxygen at T > 200 K, or from sputtered ice mantles containing water ice. We aim to quantify the contribution of the two mechanisms that lead to water in outflows, by comparing observations of gas-phase water to methanol (a grain surface product) towards three low-mass protostars in NGC1333. In doing so, we also quantify the amount of methanol destroyed in outflows. To do this, we make use of JCMT and Herschel-HIFI data of H2O, CH3OH and CO emission lines and compare them to RADEX non-LTE excitation simulations. We find up to one order of magnitude decrease in the column density ratio of CH3OH over H2O as the velocity increases in the line wings up to ~15 km/s. An independent decrease in X(CH3OH) with respect to CO of up to one order of magnitude is also found in these objects. We conclude that gas-phase formation of H2O must be active at high velocities (above 10 km/s, relative to the source velocity) to re-form the water destroyed during sputtering. In addition, the transition from sputtered water at low velocities to formed water at high velocities must be gradual. We place an upper limit of two orders of magnitude on the destruction of methanol by sputtering effects.

Magnetospheric ion sputtering and water ice grain size at Europa

We present the first calculation of Europa's sputtering (ion erosion) rate as a function of position on Europa's surface. We find a global sputtering rate of 2×1027H2O s−1, some of which leaves the surface in the form of O2 and H2. The calculated O2 production rate is 1×1026O2 s−1, H2 production is twice that value. The total sputtering rate (including all species) peaks at the trailing hemisphere apex and decreases to about 1/3rd of the peak value at the leading hemisphere apex. O2 and H2 sputtering, by contrast, is confined almost entirely to the trailing hemisphere. Most sputtering is done by energetic sulfur ions (100s of keV to MeV), but most of the O2 and H2 production is done by cold oxygen ions (temperature ∼ 100 eV, total energy ∼ 500 eV). As a part of the sputtering rate calculation we compared experimental sputtering yields with analytic estimates. We found that the experimental data are well approximated by the expressions of Famá et al. for ions with energies less than 100keV (Famá, M., Shi, J., Baragiola, R.A., 2008. Sputtering of ice by low-energy ions. Surf. Sci. 602, 156–161), while the expressions from Johnson et al. fit the data best at higher energies (Johnson, R.E., Burger, M.H., Cassidy, T.A., Leblanc, F., Marconi, M., Smyth, W.H., 2009. Composition and Detection of Europa's Sputter-Induced Atmosphere, in: Pappalardo, R.T., McKinnon, W.B., Khurana, K.K. (Eds.), Europa. University of Arizona Press, Tucson.). We compare the calculated sputtering rate with estimates of water ice regolith grain size as estimated from Galileo Near-Infrared Mapping Spectrometer (NIMS) data, and find that they are strongly correlated as previously suggested by Clark et al. (Clark, R.N., Fanale, F.P., Zent, A.P., 1983. Frost grain size metamorphism: Implications for remote sensing of planetary surfaces. Icarus 56, 233–245.). The mechanism responsible for the sputtering rate/grain size link is uncertain. We also report a surface composition estimate using NIMS data from an area on the trailing hemisphere apex. We find a high abundance of sulfuric acid hydrate and radiation-resistant hydrated salts along with large water ice regolith grains, all of which are consistent with the high levels of magnetospheric bombardment at the trailing apex.

Sputtering of ice grains and icy satellites in Saturn's inner magnetosphere

Icy grains and satellites orbiting in Saturn's magnetosphere are immersed in a plasma that sputters their surfaces. This limits the lifetime of the E-ring grains and ejects neutrals that orbit Saturn until they are ionized and populate its magnetosphere. Here we re-evaluate the sputtering rate of ice in Saturn's inner magnetosphere using the recent Cassini data on the plasma ion density, temperature and composition [Sittler Jr., E.C., et al., 2007a. Ion and neutral sources and sinks within Saturn's inner magnetosphere: Cassini results. Planet. Space Sci. 56, 3–18.] and a recent summary of the relevant sputtering data for ice [Famá, M., Shi, J., Baragiola, R.A., 2008. Sputtering of ice by low-energy ions. Surf. Sci. 602, 156–161.]. Although the energetic (>10 keV) ion component at Saturn is much smaller than was assumed to be the case after Voyager [Jurac, S., Johnson, R.E., Richardson, J.D., Paranicas, C., 2001a. Satellite sputtering in Saturn's magnetosphere. Planet. Space Sci. 49, 319–326; Jurac, S., Johnson, R.E., Richardson, J.D., 2001b. Saturn's E ring and production of the neutral torus. Icarus 149, 384–396.], we show that the sputtering rates are sensitive to the temperature of the thermal plasma and are still robust, so that sputtering likely determines the lifetime of the grains in Saturn's tenuous E-ring.

Sputtering of ice by low-energy ions

We measured the total sputtering yield Y of amorphous water ice at 80 K for 0.35–4 keV He and Ar ions. We found that Y depends linearly on the elastic stopping cross section at low energies as predicted by the standard linear cascade theory of sputtering. As the energy increases, a quadratic dependence with the electronic stopping cross section arises due to cooperative effects between excitations produced by the projectile. We also studied how sputtering depends on the projectile incidence angle and found that Y follows a cos − f ( θ) dependence with f = 1.45 ± 0.05 for 2 keV He + and f = 1.78 ± 0.08 for 2 keV Ar +. Measurements of Y for 2 keV ions vs. ice temperature between 30 and 140 K show the same temperature dependence as that reported previously for high-energy ions. We introduce a general formula to calculate Y below 100 keV once the ice temperature, projectile type, energy and angle of incidence are known. This formula can be used for modeling the production of extended neutral atmospheres around astrophysical icy bodies subject to ion bombardment.

Monte Carlo model of sputtering and other ejection processes within a regolith

The ejection of atoms and molecules from a surface due to a flux of energetic particles has been studied for a variety of target materials and radiation types. We simulate these processes within a porous regolith using a Monte Carlo model. In doing so we assume that the surfaces of regolith grains behave like the surfaces of laboratory samples, where the results of an ejection event have been found to depend upon the energetic-particle angle of incidence and intrinsic properties of the target material. Total sputtering yields, sputter-product exit-angle distributions, and energy distributions are calculated for application to Solar System objects with emphasis on sodium ejection from the lunar surface. We find that the regolith yield is typically less than that from a laboratory surface made of the same material and depends on the sticking probability. The sputter-product exit-angle distribution from a porous regolith is found to be little altered by variation of the primary sputtering parameters, with the exception of nearly specular sputtering. The regolith grain shape has little effect on the total sputtering yield. The results are in rough agreement with an earlier analytic model [Johnson, R.E., 1989. Application of laboratory data to the sputtering of a planetary regolith. Icarus 78, 206–210]. The experimental results of Hapke and Cassidy [1978. Is the Moon really as smooth as a billiard ball? Remarks concerning recent models of sputter-fractionation on the lunar surface. Geophys. Res. Lett. 5 (4), 297–300] require a strongly forward-directed sputtering process, which may apply to a silicate with a low sputtering yield but not to the sputtering of ices by heavy ions.

Production of O2 on icy satellites by electronic excitation of low-temperature water ice.

The signature of condensed molecular oxygen has been reported in recent optical-reflectance measurements of the jovian moon Ganymede, and a tenuous oxygen atmosphere has been observed on Europa. The surfaces of these moons contain large amounts of water ice, and it is thought that O2 is formed by the sputtering of ice by energetic particles from the jovian magnetosphere. Understanding how O2 might be formed from low-temperature ice is crucial for theoretical and experimental simulations of the surfaces and atmospheres of icy bodies in the Solar System. Here we report laboratory measurements of the threshold energy, cross-section and temperature dependence of O2 production by electronic excitation of ice in vacuum, following electron-beam irradiation. Molecular oxygen is formed by direct excitation and dissociation of a stable precursor molecule, rather than (as has been previously thought) by diffusion and chemical recombination of precursor fragments. The large cross-section for O2 production suggests that electronic excitation plays an important part in the formation of O2 on Ganymede and Europa.

Sputtering of ices in the outer solar system

Exploration of the outer solar system has led to studies in a new area of physics: electronically induced sputtering of low-temperature, condensed-gas solids (ices). Many of the icy bodies in the outer solar system were found to be bombarded by relatively intense fluxes of ions and electrons, causing both changes in their optical reflectance and ejection (sputtering) of molecules from their surfaces. The small cohesive energies of the condensed-gas solids afford relatively large sputtering rates from the electronic excitations produced in the solid by fast ions and electrons. Such sputtering produces an ambient gas about an icy body, often the source of the local plasma. This colloquium outlines the physics of the sputtering of ices and its relevance to several outer-solar-system phenomena: the sputter-produced plasma trapped in Saturn{close_quote}s magnetosphere; the O{sub 2} atmosphere on Europa; and optical absorption features such as SO{sub 2} in the surface of Europa and O{sub 2} and, possibly, O{sub 3} in the surface of Ganymede. {copyright} {ital 1996 The American Physical Society.}

Mechanisms for the desorption of large organic molecules

Energizing mechanisms and models for the ejection of biomolecules due to electronic excitations by fast ions are compared and discussed. The energizing processes are also compared with those induced by UV photons in organic solids and by electronic sputtering of low-temperature condensed-gas solids by fast ions. The calculation of a yield for an ejected species is roughly divided into the determination of the probability of creation/survival of the species, an effective area, and an effective depth, which is determined by the radial dissipation of energy when ions are at normal incidence. Aspects of this calculation can be tested separately by experiment and the various models proposed in the literature often only address a part of the yield calculation. The measured low-temperature, condensed-gas sputtering yields for H 2O indicate that a much larger material volume is removed than that suggested by the recent neutral yields for the ejection of whole leucine molecules. It is seen that significant radial energy transport determines the yields for water ice sputtering at high excitation densities but probably not the leucine yields. The general nature of a model for the total yield is discussed and differences between this and the observed energy density dependence for neutral leucine yields are considered. A rough estimate of the neutral leucine yields is given based on the initial track parameters for the energy deposited and assuming the incident ion induces a local expansion of the solid. Further, differences in the dependencies of the biomolecular ion yields and neutral yields on energy density deposited suggest that the ions do not come uniformly from the ejected volume.

Sputtering of ices by high energy particle impact

Electronic excitation of thin films of molecular and rare gas solids by MeV He and H ions is efficient in causing sputtering. The sputtering yields vary widely for different species, even for those with very similar surface binding energy. The sputtering yields for CO, Ar, and N 2, for example, differ by a factor of 10. Even more striking for these three is the dependence of the sputtering yield on the electronic stopping power of the bombarding particle. The dependence is found to be quadratic for CO, linear for Ar and it exhibits a linear-to-quadratic transition in N 2. The experimental results indicate the importance of the specific routes for nonradiative electronic deexcitation and the importance of cooperative effects associated with neighboring excitations produced along the tracks of individual bombarding particles.

Sputtering of ices by keV ions

We have measured the sputtering yields of H 2O, CO 2 and N 2O ices at 78 K under Ar +, Ne +, N +, He + and e − bombardment in the keV energy range. The experimental technique involves the determination of the initial ice film thickness by optical interference and the fluence required to sputter through the film by SIMS. The measured yields were compared with predictions using Sigmund's linear cascade theory. The results indicate that processes in addition to the purely ballistic process contribute to the ejection.

Astrophysical implications of ice sputtering

Volatiles condensed on surfaces and condensed as major constituents of astronomical bodies (e.g., comets or the satellites of Saturn) are important constituents of the solar system and, likely, the interstellar medium. In most regions of space the charged particle environments such as the solar wind and planetary magnetospheres can interact directly with the ice-covered surfaces and the icy objects, producing physical and chemical alterations. Many of the recent studies of the effects of charged particles on solar system objects, using laboratory-based and spacecraft-measured, data are summarized here.

Sputtering of ices: a review

The sputtering of low temperature rare-gas and molecular-gas solids by ion bombardment occurs through two different routes. Collision cascades initiated by momentum transferring collisions of ions with atoms of the solid are effective as they are in metals. The apparent surface binding energy is, however, much lower than the sublimation energy and there is a substantial low energy component to the ejected energy distribution that is suggestive of a thermal or collisional spike. In the ices sputtering is also produced by electronic excitation by ion or electron bombardment. This arises from conversion of electronic energy to translational kinetic energy and at low excitation densities may involve individual excitons or repulsive recombination events. At higher densities, cooperative effects lead to nonlinear sputtering yields. The current status of both the collisional and the electronic regimes of ice sputtering is briefly reviewed.

Europan surface phenomena

The interaction of corotating iogenic plasma with the surface of Europa in light of recent ice sputtering, experimental results, and published Voyager data has been examined. It has been found that the residual atmosphere of Europa is made up of sputtered molecular oxygen and is exospheric from the surface outwards. It was also found that if sputtering, redistribution, and escape are considered and the sulfur dioxide/water mixing ratio is held constant over a UV observing depth, the observed sulfur dioxide density on the trailing hemisphere lends support to the hypothesis that liquid water from the interior of Europa is boiling out and being deposited as a frost layer on the surface at the rate of about 0.04 μm/year.