Posts Tagged water ice

Recent Postings from water ice

Multi-Epoch Detections of Water Ice Absorption in Edge-on Disks around Herbig Ae Stars: PDS 144N and PDS 453

We report the multi-epoch detections of the water ice in 2.8-4.2 micron spectra of two Herbig Ae stars, PDS 144N (A2 IVe) and PDS 453 (F2 Ve), which have an edge-on circumstellar disk. The detected water ice absorption is found to originate from their protoplanetary disks. The spectra show a relatively shallow absorption of water ice around 3.1 micron for both objects. The optical depths of the water ice absorption are ~0.1 and ~0.2 for PDS 144N and PDS 453, respectively. Compared to the water ice previously detected in low-mass young stellar objects with an edge-on disk with a similar inclination angle, these optical depths are significantly lower. It suggests that stronger UV radiation from the central stars effectively decreases the water ice abundance around the Herbig Ae stars through photodesorption. The water ice absorption in PDS 453 shows a possible variation of the feature among the six observing epochs. This variation could be due to a change of absorption materials passing through our line-of-sight to the central star.

A laboratory study of water ice erosion by low-energy ions

Water ice covers the surface of various objects in the outer Solar system. Within the heliopause, surface ice is constantly bombarded and sputtered by energetic particles from the solar wind and magnetospheres. We report a laboratory investigation of the sputtering yield of water ice when irradiated at 10 K by 4 keV singly (13C+, N+, O+, Ar+) and doubly charged ions (13C2+, N2+, O2+). The experimental values for the sputtering yields are in good agreement with the prediction of a theoretical model. There is no significant difference in the yield for singly and doubly charged ions. Using these yields, we estimate the rate of water ice erosion in the outer Solar system objects due to solar wind sputtering. Temperature-programmed desorption of the ice after irradiation with 13C+ and 13C2+ demonstrated the formation of 13CO and 13CO2, with 13CO being the dominant formed species.

Diffusion and Clustering of Carbon Dioxide on non-porous Amorphous Solid Water

Observations by ISO and Spitzer towards young stellar objects (YSOs) showed that CO$_2$ segregates in the icy mantles covering dust grains. Thermal processing of ice mixture was proposed as responsible for the segregation. Although several laboratory studied thermally induced segregation, a satisfying quantification is still missing. We propose that the diffusion of CO$_2$ along pores inside water ice is the key to quantify segregation. We combined Temperature Programmed Desorption (TPD) and Reflection Absorption InfraRed Spectroscopy (RAIRS) to study how CO$_2$ molecules interact on a non-porous amorphous solid water (np-ASW) surface. We found that CO$_2$ diffuses significantly on a np-ASW surface above 65~K and clusters are formed at well below one monolayer. A simple rate equation simulation finds that the diffusion energy barrier of CO$_2$ on np-ASW is 2150$\pm$50 K, assuming a diffusion pre-exponential factor of 10$^{12}$ s$^{-1}$. This energy should also apply to the diffusion of CO$_2$ on wall of pores. The binding energy of CO$_2$ from CO$_2$ clusters and CO$_2$ from H$_2$O ice have been found to be $2415\pm20$ and $2250\pm20$~K, respectively, assuming the same prefactor for desorption. CO$_2$-CO$_2$ interaction is stronger than CO$_2$-H$_2$O interaction, in agreement with the experimental finding that CO$_2$ does not wet np-ASW surface. For comparison, we carried out similar experiments with CO on np-ASW, and found that the CO-CO interaction is always weaker than CO-H$_2$O. As a result, CO wets np-ASW surface. This study should be of help to uncover the thermal history of CO$_2$ on the icy mantles of dust grains.

FU Orionis outbursts, preferential recondensation of water ice, and the formation of giant planets

Ices, including water ice, prefer to recondense onto pre-existing nuclei rather than spontaneously forming grains from a cloud of vapor. Interestingly, different potential recondensation nuclei have very different propensities to actually nucleate water ice at the temperatures associated with freeze-out in protoplanetary discs. Therefore, if a region in a disc is warmed and then recooled, water vapor should not be expected to refreeze evenly onto all available grains. Instead it will preferentially recondense onto the most favorable grains. When the recooling is slow enough, only the most favorable grains will nucleate ice, allowing them to recondense thick ice mantles. We quantify the conditions for preferential recondensation to rapidly create pebble-sized grains in protoplanetary discs and show that FU Orionis type outbursts have the appropriate cooling rates to drive pebble creation in a band about 5 astronomical units wide outside of the quiescent frost line from approximately Jupiter's orbit to Saturn's (about 4 to 10 au). Those pebbles could be of the appropriate size to proceed to planetesimal formation via the Streaming Instability, or to contribute to the growth of planetesimals through pebble accretion. We suggest that this phenomenon contributed to the formation of the gas giants in our own Solar System.

Water delivery from cores to disks: deuteration as a probe of the prestellar inheritance of H2O

We investigate the delivery of regular and deuterated forms of water from prestellar cores to circumstellar disks. We adopt a semi-analytical axisymmetric two-dimensional collapsing core model with post-processing gas-ice astrochemical simulations, in which a layered ice structure is considered. The physical and chemical evolutions are followed until the end of the main accretion phase. When mass averaged over the whole disk, a forming disk has a similar H2O abundance and HDO/H2O abundance ratio as their precollapse values (within a factor of 2), regardless of time in our models. Consistent with previous studies, our models suggest that interstellar water ice is delivered to forming disks without significant alteration. On the other hand, the local vertically averaged H2O ice abundance and HDO/H2O ice ratio can differ more, by up to a factor of several, depending on time and distance from a central star. Key parameters for the local variations are the fluence of stellar UV photons en route into the disk and the ice layered structure, the latter of which is mostly established in the prestellar stages. We also find that even if interstellar water ice is destroyed by stellar UV and (partly) reformed prior to disk entry, the HDO/H2O ratio in reformed water ice is similar to the original value. This finding indicates that some caution is needed in discussions on the prestellar inheritance of H2O based on comparisons between the observationally derived HDO/H2O ratio in clouds/cores and that in disks/comets. Alternatively, we propose that the ratio of D2O/HDO to HDO/H2O better probes the prestellar inheritance of H2O. It is also found that icy organics are more enriched in deuterium than water ice in forming disks. The differential deuterium fractionation in water and organics is inherited from the prestellar stages.

Tracing water vapor and ice during dust growth

The processes that govern the evolution of dust and water (in the form of vapor or ice) in protoplanetary disks are intimately connected. We have developed a model that simulates dust coagulation, dust dynamics (settling, turbulent mixing), vapor diffusion, and condensation/sublimation of volatiles onto grains in a vertical column of a protoplanetary disk. We employ the model to study how dust growth and dynamics influence the vertical distribution of water vapor and water ice in the region just outside the radial snowline. Our main finding is that coagulation (boosted by the enhanced stickiness of icy grains) and the ensuing vertical settling of solids results in water vapor being depleted, but not totally removed, from the region above the snowline on a timescale commensurate with the vertical turbulent mixing timescale. Depending on the strength of the turbulence and the temperature, the depletion can reach factors of up to ${\sim}50$ in the disk atmosphere. In our isothermal column, this vapor depletion results in the vertical snowline moving closer to the midplane (by up to 2 gas scale heights) and the gas-phase C/O ratio above the vertical snowline increasing. Our findings illustrate the importance of dynamical effects and the need for understanding coevolutionary dynamics of gas and solids in planet-forming environments.

Observational Constraints on Water Sublimation from 24 Themis and 1 Ceres

Recent observations have suggested that there is water ice present on the surfaces of 24 Themis and 1 Ceres. We present upper limits on the H$_2$O production rate on these bodies derived using a search for [OI]6300 Angstrom emission. For Themis, the water production is less than 4.5 $\times$ 10$^{27}$ mol s$^{-1}$, while for Ceres our derived upper limit is 4.6 $\times$ 10$^{28}$ mol s$^{-1}$. The derived limits imply a very low fraction of the surface area of each asteroid is active ($<$ 2 $\times$ 10$^{-4}$), though this estimate varies by as much as an order of magnitude depending on thermal properties of the surface. This is much lower than seen for comets, which have active areas of 10$^{-2}$ - 10$^{-1}$. We discuss possible implications for our findings on the nature of water ice on Themis and Ceres.

Ring Formation around Giant Planets by Tidal Disruption of a Single Passing Large Kuiper Belt Object

The origin of rings around giant planets remains elusive. Saturn's rings are massive and made of 90-95% of water ice. In contrast, the much less massive rings of Uranus and Neptune are dark and likely to have higher rock fraction. Here we investigate, for the first time, the tidal disruption of a passing object, including the subsequent formation of planetary rings. First, we perform SPH simulations of the tidal destruction of big differentiated objects ($M_{\rm body}=10^{21-23}$) that experience close encounters with Saturn or Uranus. We find that about $0.1-10$% of the mass of the passing body is gravitationally captured around the planet. However, these fragments are initially big chunks and have highly eccentric orbits around the planet. Then, we perform N-body simulations including the planet's oblateness, starting with data obtained from the SPH simulations. Our N-body simulations show that the chunks are tidally destroyed during their next several orbits. Their individual orbits then start to precess incoherently around the planet's equator, which enhances their encounter velocities on longer-term evolution, resulting in more destructive impacts. These collisions would damp their eccentricities resulting in a progressive collapse of the debris cloud into a thin equatorial and low-eccentricity ring. These high energy impacts are expected to be catastrophic enough to produce small particles. Our numerical results also show that the mass of formed rings is large enough to explain current rings including inner regular satellites around Saturn and Uranus. In the case of Uranus, a body can go deeper inside the planet's Roche limit resulting in a more efficient capture of rocky material compared to Saturn's case in which mostly ice is captured. Thus, our results can naturally explain the compositional difference between the rings of Saturn, Uranus and Neptune.

Ice Grain Collisions in Comparison: CO$_2$, H$_2$O and their Mixtures

Collisions of ice particles play an important role in the formation of planetesimals and comets. In recent work we showed, that CO$_2$ ice behaves like silicates in collisions. The resulting assumption was that it should therefore stick less efficiently than H$_2$O ice. Within this paper a quantification of the latter is presented. We used the same experimental setup to study collisions of pure CO$_2$ ice, pure water ice and 50\% mixtures by mass between CO$_2$ and water at 80K, 1 mbar and an average particle size of $\sim 90 \mu$m. The results show a strong increase of the threshold velocity between sticking and bouncing with increasing water content. This supports the idea that water ice is favorable for early growth phases of planets in a zone within the H$_2$O and the CO$_2$ iceline.

Hubble Space Telescope Observations of Active Asteroid 324P/La Sagra

Hubble Space Telescope observations of active asteroid 324P/La Sagra near perihelion show continued mass loss consistent with the sublimation of near-surface ice. Isophotes of the coma measured from a vantage point below the orbital plane are best matched by steady emission of particles having a nominal size $a \sim$ 100 $\mu$m. The inferred rate of mass loss, $dM_d/dt \sim$0.2 kg s$^{-1}$, can be supplied by sublimation of water ice in thermal equilibrium with sunlight from an area as small as 930 m$^2$, corresponding to about 0.2\% of the nucleus surface. Observations taken from a vantage point only 0.6\degr~from the orbital plane of 324P set a limit to the velocity of ejection of dust in the direction perpendicular to the plane, $V_{\perp} <$ 1 m s$^{-1}$. Short-term photometric variations of the near-nucleus region, if related to rotation of the underlying nucleus, rule out periods $\le$ 3.8 hr and suggest that rotation probably does not play a central role in driving the observed mass loss. We estimate that, in the previous orbit, 324P lost about 4$\times$10$^7$ kg in dust particles, corresponding to 6$\times$10$^{-5}$ of the mass of a 550 m spherical nucleus of assumed density $\rho$ = 1000 kg m$^{-3}$. If continued, mass loss at this rate would limit the lifetime of 324P to $\sim$1.6$\times$10$^4$ orbits (about 10$^5$ yr). To survive for the 100 Myr to 400 Myr timescales corresponding, respectively, to dynamical and collisional stability requires a duty cycle $2\times 10^{-4} \le f_d \le 8\times 10^{-4}$. Unless its time in orbit is over-estimated by many orders of magnitude, 324P is revealed as a briefly-active member of a vast population of otherwise dormant ice-containing asteroids.

The abundance and thermal history of water ice in the disk surrounding HD142527 from the DIGIT Herschel Key Program

The presence or absence of ice in protoplanetary disks is of great importance for the formation of planets. By enhancing the solid surface density and increasing the sticking efficiency, ice catalyzes the rapid formation of planetesimals and decreases the time scale for giant planet core accretion. Aims: In this paper we analyse the composition of the outer disk around the Herbig star HD~142527. We focus on the composition of the water ice, but also analyse the abundances of previously proposed minerals. Methods: We present new Herschel far infrared spectra and a re-reduction of archival data from the Infrared Space Observatory (ISO). We model the disk using full 3D radiative transfer to obtain the disk structure. Also, we use an optically thin analysis of the outer disk spectrum to obtain firm constraints on the composition of the dust component. Results: The water ice in the disk around HD~142527 contains a large reservoir of crystalline water ice. We determine the local abundance of water ice in the outer disk (i.e. beyond 130\,AU). The re-reduced ISO spectrum differs significantly from that previously published, but matches the new Herschel spectrum at their common wavelength range. In particular, we do not detect any significant contribution from carbonates or hydrous silicates, in contrast to earlier claims. Conclusions: The amount of water ice detected in the outer disk requires $\sim80\,$\% of the oxygen atoms. This is comparable to the water ice abundance in the outer solar system, in comets and in dense interstellar clouds. The water ice is highly crystalline while the temperatures where we detect it are too low to crystallize the water on relevant time scales. We discuss the implications of this finding.

Triggering Sublimation-Driven Activity of Main Belt Comets

It has been suggested that the comet-like activity of Main Belt Comets are due to the sublimation of sub-surface water-ice that has been exposed as a result of their surfaces being impacted by m-sized bodies. We have examined the viability of this scenario by simulating impacts between m-sized and km-sized objects using a smooth particle hydrodynamics approach. Simulations have been carried out for different values of the impact velocity and impact angle as well as different target material and water-mass fraction. Results indicate that for the range of impact velocities corresponding to those in the asteroid belt, the depth of an impact crater is slightly larger than 10 m suggesting that if the activation of MBCs is due to the sublimation of sub-surface water-ice, this ice has to exist no deeper than a few meters from the surface. Results also show that ice-exposure occurs in the bottom and on the interior surface of impact craters as well as the surface of the target where some of the ejected icy inclusions are re-accreted. While our results demonstrate that the impact scenario is indeed a viable mechanism to expose ice and trigger the activity of MBCs, they also indicate that the activity of the current MBCs is likely due to ice sublimation from multiple impact sites and/or the water contents of these objects (and other asteroids in the outer asteroid belt) is larger than the 5% that is traditionally considered in models of terrestrial planet formation providing more ice for sublimation. We present details of our simulations and discuss their results and implications.

Near-infrared spatially resolved spectroscopy of (136108) Haumea's multiple system

The transneptunian region of the solar system is populated by a wide variety of icy bodies showing great diversity. The dwarf planet (136108) Haumea is among the largest TNOs and displays a highly elongated shape and hosts two moons, covered with crystalline water ice like Hamuea. Haumea is also the largest member of the sole TNO family known to date. A catastrophic collision is likely responsible for its unique characteristics. We report here on the analysis of a new set of observations of Haumea obtained with SINFONI at the ESO VLT. Combined with previous data, and using light-curve measurements in the optical and far infrared, we carry out a rotationally resolved spectroscopic study of the surface of Haumea. We describe the physical characteristics of the crystalline water ice present on the surface of Haumea for both regions, in and out of the Dark Red Spot (DRS), and analyze the differences obtained for each individual spectrum. The presence of crystalline water ice is confirmed over more than half of the surface of Haumea. Our measurements of the average spectral slope confirm the redder characteristic of the spot region. Detailed analysis of the crystalline water-ice absorption bands do not show significant differences between the DRS and the remaining part of the surface. We also present the results of applying Hapke modeling to our data set. The best spectral fit is obtained with a mixture of crystalline water ice (grain sizes smaller than 60 micron) with a few percent of amorphous carbon. Improvements to the fit are obtained by adding ~10% of amorphous water ice. Additionally, we used the IFU-reconstructed images to measure the relative astrometric position of the largest satellite Hi`iaka and determine its orbital elements. An orbital solution was computed with our genetic-based algorithm GENOID and our results are in full agreement with recent results.

Martian north polar cap summer water cycle

A key outstanding question in Martian science is 'are the polar caps gaining or losing mass and what are the implications for past, current and future climate?' To address this question, we use observations from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) of the north polar cap during late summer for multiple Martian years, to monitor the summertime water cycle in order to place quantitative limits on the amount of water ice deposited and sublimed in late summer. We establish here for the first time the summer cycle of water ice absorption band signatures on the north polar cap. We show that in a key region in the interior of the north polar cap, the absorption band depths grow until Ls=120, when they begin to shrink, until they are obscured at the end of summer by the north polar hood. This behavior is transferable over the entire north polar cap, where in late summer regions 'flip' from being net sublimating into net condensation mode. This transition or 'mode flip' happens earlier for regions closer to the pole, and later for regions close to the periphery of the cap. The observations and calculations presented herein estimate that on average a water ice layer ~70 microns thick is deposited during the Ls=135-164 period. This is far larger than the results of deposition on the south pole during summer, where an average layer 0.6-6 microns deep has been estimated by Brown et al. (2014).

Capillary Action in a Crack on the Surface of Asteroids with an Application to 433 Eros

Some asteroids contain water ice, and a space mission landing on an asteroid may take liquid to the surface of the asteroid. Gas pressure is very weak on the surface of asteroids. Here we consider the capillary action in a crack on the surface of irregular asteroids. The crack is modelled as a capillary which has a fixed radius. An asteroid s irregular gravitational potential influences the height of the liquid in the capillary. The height of the liquid in the capillary on the surface of such asteroids is derived from the asteroid s irregular gravitational potential. Capillary mechanisms are expected to produce an inhomogeneaous distribution of emergent liquid on the surface. This result is applied to asteroid 433 Eros, which has an irregular, elongated, and concave shape. Two cases are considered 1) we calculate the height of the liquid in the capillary when the direction of the capillary is perpendicular to the local surface of the asteroid; 2) we calculate the height of the liquid in the capillary when the direction of the capillary is parallel to the vector from the center of mass to the surface position. The projected height in the capillary on the local surface of the asteroid seems to depend on the assumed direction of the capillary.

The Occurrence of Additional Giant Planets Inside the Water-Ice Line in Systems with Hot Jupiters: Evidence Against High-Eccentricity Migration

The origin of Jupiter-mass planets with orbital periods of only a few days is still uncertain. It is widely believed that these planets formed near the water-ice line of the protoplanetary disk, and subsequently migrated into much smaller orbits. Most of the proposed migration mechanisms can be classified either as disk-driven migration, or as excitation of a very high eccentricity followed by tidal circularization. In the latter scenario, the giant planet that is destined to become a hot Jupiter spends billions of years on a highly-eccentric orbit, with apastron near the water-ice line. Eventually, tidal dissipation at periastron shrinks and circularizes the orbit. If this is correct, then it should be especially rare for hot Jupiters to be accompanied by another giant planet interior to the water-ice line. Using the current sample of giant planets discovered with the Doppler technique, we find that hot Jupiters with P_orb < 10 days are no more or less likely to have exterior Jupiter-mass companions than longer-period giant planets with P_orb >= 10 days. This result holds for exterior companions both inside and outside of the approximate location of the water-ice line. These results are difficult to reconcile with the high-eccentricity migration scenario for hot Jupiter formation.

Water ice at the surface of HD 100546 disk

We made near infrared multicolor imaging observations of a disk around Herbig Be star HD100546 using Gemini/NICI. K (2.2\,$\mu$m), H$_2$O ice (3.06\,$\mu$m), and L'(3.8\,$\mu$m) disk images were obtained and we found the 3.1\,$\mu$m absorption feature in the scattered light spectrum, likely due to water ice grains at the disk surface. We compared the observed depth of the ice absorption feature with the disk model based on \cite{Oka2012} including water ice photodesorption effect by stellar UV photons. The observed absorption depth can be explained by the both disk models with/without photodesorption effect within the measurement accuracy, but slightly favors the model with photodesorption effects, implying that the UV photons play an important role on the survival/destruction of ice grains at the Herbig Ae/Be disk surface. Further improvement on the accuracy of the observations of the water ice absorption depth is needed to constrain the disk models.

Rotational properties of the Haumea family members and candidates: Short-term variability

Haumea is one of the most interesting and intriguing transneptunian objects (TNOs). It is a large, bright, fast rotator, and its spectrum indicates nearly pure water ice on the surface. It has at least two satellites and a dynamically related family of more than ten TNOs with very similar proper orbital parameters and similar surface properties. The Haumean family is the only one currently known in the transneptunian belt. Various models have been proposed but the formation of the family remains poorly understood. In this work, we have investigated the rotational properties of the family members and unconfirmed family candidates with short-term variability studies, and report the most complete review to date. We present results based on five years of observations and report the short-term variability of five family members, and seven candidates. The mean rotational periods, from Maxwellian fits to the frequency distributions, are 6.27+/-1.19 h for the confirmed family members, 6.44+/-1.16 h for the candidates, and 7.65+/-0.54 h for other TNOs (without relation to the family). According to our study, there is a suggestion that Haumea family members rotate faster than other TNOs, however, the sample of family member is still too limited for a secure conclusion. We also highlight the fast rotation of 2002 GH32. This object has a 0.36+/-0.02 mag amplitude lightcurve and a rotational period of about 3.98 h. Assuming 2002 GH32 is a triaxial object in hydrostatic equilibrium, we derive a lower limit to the density of 2.56 g cm^-3. This density is similar to Haumea's and much more dense than other small TNO densities.

Binding Energy of Molecules on Water Ice: Laboratory Measurements and Modeling

We measured the binding energy of N$_2$, CO, O$_2$, CH$_4$, and CO$_2$ on non-porous (compact) amorphous solid water (np-ASW), of N$_2$ and CO on porous amorphous solid water (p-ASW), and of NH$_3$ on crystalline water ice. We were able to measure binding energies down to a fraction of 1\% of a layer, thus making these measurements more appropriate for astrochemistry than the existing values. We found that CO$_2$ forms clusters on np-ASW surface even at very low coverages. The binding energies of N$_2$, CO, O$_2$, and CH$_4$ decrease with coverage in the submonolayer regime. Their values at the low coverage limit are much higher than what is commonly used in gas-grain models. An empirical formula was used to describe the coverage dependence of the binding energies. We used the newly determined binding energy distributions in a simulation of gas-grain chemistry for cold cloud and hot core models. We found that owing to the higher value of desorption energy in the sub-monlayer regime a fraction of all these ices stays much longer and up to higher temperature on the grain surface compared to the single value energies currently used in the astrochemical models.

Sticking of molecules on non-porous amorphous water ice

Accurate modeling of physical and chemical processes in the interstellar medium requires detailed knowledge of how atoms and molecule adsorb on dust grains. However, the sticking coefficient, a number between 0 and 1 that measures the first step in the interaction of a particle with a surface, is usually assumed in simulations of ISM environments to be either 0.5 or 1. Here we report on the determination of the sticking coefficient of H$_2$, D$_2$, N$_2$, O$_2$, CO, CH$_4$, and CO$_2$ on non-porous amorphous solid water (np-ASW). The sticking coefficient was measured over a wide range of surface temperatures using a highly collimated molecular beam. We showed that the standard way of measuring the sticking coefficient --- the King-Wells method --- leads to the underestimation of trapping events in which there is incomplete energy accommodation of the molecule on the surface. Surface scattering experiments with the use of a pulsed molecular beam are used instead to measure the sticking coefficient. Based on the values of the measured sticking coefficient we suggest a useful general formula of the sticking coefficient as a function of grain temperature and molecule-surface binding energy. We use this formula in a simulation of ISM gas-grain chemistry to find the effect of sticking on the abundance of key molecules both on grains and in the gas-phase.

Sticking of molecules on non-porous amorphous water ice [Replacement]

Accurate modeling of physical and chemical processes in the interstellar medium requires detailed knowledge of how atoms and molecule adsorb on dust grains. However, the sticking coefficient, a number between 0 and 1 that measures the first step in the interaction of a particle with a surface, is usually assumed in simulations of ISM environments to be either 0.5 or 1. Here we report on the determination of the sticking coefficient of H$_2$, D$_2$, N$_2$, O$_2$, CO, CH$_4$, and CO$_2$ on non-porous amorphous solid water (np-ASW). The sticking coefficient was measured over a wide range of surface temperatures using a highly collimated molecular beam. We showed that the standard way of measuring the sticking coefficient --- the King-Wells method --- leads to the underestimation of trapping events in which there is incomplete energy accommodation of the molecule on the surface. Surface scattering experiments with the use of a pulsed molecular beam are used instead to measure the sticking coefficient. Based on the values of the measured sticking coefficient we suggest a useful general formula of the sticking coefficient as a function of grain temperature and molecule-surface binding energy. We use this formula in a simulation of ISM gas-grain chemistry to find the effect of sticking on the abundance of key molecules both on grains and in the gas-phase.

Saturn's icy satellites investigated by Cassini - VIMS. IV. Daytime temperature maps

The spectral position of the 3.6 micron continuum peak measured on Cassini-VIMS I/F spectra is used as a marker to infer the temperature of the regolith particles covering the surfaces of Saturn's icy satellites. This feature is characterizing the crystalline water ice spectrum which is the dominant compositional endmember of the satellites' surfaces. Laboratory measurements indicate that the position of the 3.6 micron peak of pure water ice is temperature-dependent, shifting towards shorter wavelengths when the sample is cooled, from about 3.65 micron at T=123 K to about 3.55 micron at T=88 K. A similar method was already applied to VIMS Saturn's rings mosaics to retrieve ring particles temperature (Filacchione et al., 2014). We report here about the daytime temperature variations observed on the icy satellites as derived from three different VIMS observation types. Temperature maps are built by mining the complete VIMS dataset collected in years 2004-2009 (pre-equinox) and in 2009-2012 (post equinox) by selecting pixels with max 150 km/pixel resolution. VIMS-derived temperature maps allow to identify thermal anomalies across the equatorial lens of Mimas and Tethys.

Absorption at 11 microns in the interstellar medium and embedded sources: evidence for crystalline silicates

An absorption feature is occasionally reported around 11 ?microns in astronomical spectra, including those of forming stars. Candidate carriers include water ice, polycyclic aromatic hydrocarbons (PAHs), silicon carbide, crystalline silicates or even carbonates. All are known constituents of cosmic dust in one or more types of environments, though not necessarily together. In this paper we present new ground-based 8-13 ?micron spectra of one evolved star, several embedded young stellar objects (YSOs) and a background source lying behind a large column of the interstellar medium (ISM) toward the Galactic Centre. Our observations, obtained at a spectral resolution of ?approximately 100, are compared with previous lower resolution data, as well as data obtained with the Infrared Space Observatory (ISO) on these and other targets. By presenting a subset of a larger sample our aim is to establish the reality of the feature and subsequently speculate on its carrier. All evidence points toward crystalline silicate. For instance, the 11 ?micron band profile is well matched with the emissivity of crystalline olivine. Furthermore, the apparent association of the absorption feature with a sharp polarisation signature in the spectrum of two previously reported cases suggests a carrier with a relatively high band strength compared to amorphous silicates. If true, this would either set back the evolutionary stage in which silicates are crystallised, either to the embedded phase or even before within the ISM, or else the silicates ejected from the outflows of evolved stars retain some of their crystalline identity during their long residence in the ISM.

CO and N$_2$ desorption energies from water ice

The relative desorption energies of CO and N$_2$ are key to interpretations of observed interstellar CO and N$_2$ abundance patterns, including the well-documented CO and N$_2$H$^+$ anti-correlations in disks, protostars and molecular cloud cores. Based on laboratory experiments on pure CO and N$_2$ ice desorption, the difference between CO and N$_2$ desorption energies is small; the N$_2$-to-CO desorption energy ratio is 0.93$\pm$0.03. Interstellar ices are not pure, however, and in this study we explore the effect of water ice on the desorption energy ratio of the two molecules. We present temperature programmed desorption experiments of different coverages of $^{13}$CO and $^{15}$N$_2$ on porous and compact amorphous water ices and, for reference, of pure ices. In all experiments, $^{15}$N$_2$ desorption begins a few degrees before the onset of $^{13}$CO desorption. The $^{15}$N$_2$ and $^{13}$CO energy barriers are 770 and 866 K for the pure ices, 1034-1143 K and 1155-1298 K for different sub-monolayer coverages on compact water ice, and 1435 and 1575 K for $\sim$1 ML of ice on top of porous water ice. For all equivalent experiments, the N$_2$-to-CO desorption energy ratio is consistently 0.9. Whenever CO and N$_2$ ice reside in similar ice environments (e.g. experience a similar degree of interaction with water ice) their desorption temperatures should thus be within a few degrees of one another. A smaller N$_2$-to-CO desorption energy ratio may be present in interstellar and circumstellar environments if the average CO ice molecules interacts more with water ice compared to the average N$_2$ molecules.

Identification of Mars gully activity types associated with ice composition

The detection of geologically recent channels at the end of the twentieth century rapidly suggested that liquid water could have been present on Mars up to recent times. A mechanism involving melting of water ice during ice ages in the last several million years progressively emerged during years following the first observations of these gullies. However, the recent discovery of current activity within gullies now suggests a paradigm shift where a contemporary CO2 ice-based and liquid water-free mechanism may form all gullies. Here we perform a survey of near-infrared observations and construct time sequences of water and CO2 ice formation and sublimation at active gully sites. We observe that all major new erosive features such as channel development or lengthening systematically occur where and, if applicable, when CO2 ice is observed or probable. CO2 ice layers are however estimated to be only 1 mm to 1 cm thick for low-latitude sites, which may have implication for potential formation mechanisms. We also observe that part of current gully activity, notably the formation of some new deposits, is poorly compatible with the presence of CO2 ice. In particular, all new bright deposits reported in the literature have a low CO2 ice probability while water ice should be present at most sites. Our results confirm that CO2 ice is a key factor controlling present-day channel development on Mars and show that other mechanisms, potentially involving sublimation or melting of water ice, are also contributing to current gully activity.

Reconstructing the history of water ice formation from HDO/H2O and D2O/HDO ratios in protostellar cores

Recent interferometer observations have found that the D2O/HDO abundance ratio is higher than that of HDO/H2O by about one order of magnitude in the vicinity of low-mass protostar NGC 1333-IRAS 2A, where water ice has sublimated. Previous laboratory and theoretical studies show that the D2O/HDO ice ratio should be lower than the HDO/H2O ice ratio, if HDO and D2O ices are formed simultaneously with H2O ice. In this work, we propose that the observed feature, D2O/HDO > HDO/H2O, is a natural consequence of chemical evolution in the early cold stages of low-mass star formation: 1) majority of oxygen is locked up in water ice and other molecules in molecular clouds, where water deuteration is not efficient, and 2) water ice formation continues with much reduced efficiency in cold prestellar/protostellar cores, where deuteration processes are highly enhanced due to the drop of the ortho-para ratio of H2, the weaker UV radiation field, etc. Using a simple analytical model and gas-ice astrochemical simulations tracing the evolution from the formation of molecular clouds to protostellar cores, we show that the proposed scenario can quantitatively explain the observed HDO/H2O and D2O/HDO ratios. We also find that the majority of HDO and D2O ices are likely formed in cold prestellar/protostellar cores rather than in molecular clouds, where the majority of H2O ice is formed. This work demonstrates the power of the combination of the HDO/H2O and D2O/HDO ratios as a tool to reveal the past history of water ice formation in the early cold stages of star formation and when the enrichment of deuterium in the bulk of water occurred. Further observations are needed to explore if the relation, D2O/HDO > HDO/H2O, is common in low-mass protostellar sources.

CO2 hydrate dissociation at low temperatures - formation and annealing of ice Ic [Cross-Listing]

Dissociation of gas hydrates below 240 K leads to the formation of a metastable form of water ice, so called cubic ice (Ic). Through its defective nature and small particle size the surface film composed of such material is incapable of creating any significant diffusion barrier. Above 160 K, cubic ice gradually transforms to the stable hexagonal (Ih) form on laboratory time scales. The annealing, coupled with a parallel decomposition of gas hydrates, accelerates as temperature rises but already above 190 K the first process prevails, transforming cubic stacking sequences in-to ordinary Ih ice within a few minutes. Remaining stacking faults are removed through very slow isothermal annealing or after heating up above 240 K. The role of the proportion of cubic stacking on the decomposition rate is discussed. A better understanding of the dissociation kinetics at low temperatures is particularly im-portant for the critical evaluation of existing hypotheses that consider clathrates as a potential medium that actively participate in geological processes or is able to store gases (e.g. CH4, CO2 or Xe) in environments like comets, icy moons (i. e. Titan, Europa, Enceladus) or on Mars. Here, we present kinetics studies on the dissociation of CO2 clathrates at isothermal and isobaric conditions between 170 and 190K and mean Martian surface pressure. We place special attention to the formed ice and demonstrate its influence on the dissociation rates with a combination of neutron diffraction studies (performed on D20 at ILL/Grenoble) and cryo-SEM. More detailed crystallo-graphic information has been acquired via a flexible stacking-fault model capable of revealing the time evolution of the defect structure of ice Ic in terms of stacking probabilities and crystal size.

Spatially Resolved Spectroscopy of Europa: The Distinct Spectrum of Large-scale Chaos

We present a comprehensive analysis of spatially resolved moderate spectral resolution near infrared spectra obtained with the adaptive optics system at the Keck Observatory. We identify three compositionally distinct end member regions: the trailing hemisphere bullseye, the leading hemisphere upper latitudes, and a third component associated with leading hemisphere chaos units. We interpret the composition of the three end member regions to be dominated by irradiation products, water ice, and evaporite deposits or salt brines, respectively. The third component is associated with geological features and distinct from the geography of irradiation, suggesting an endogenous identity. Identifying the endogenous composition is of particular interest for revealing the subsurface composition. However, its spectrum is not consistent with linear mixtures of the salt minerals previously considered relevant to Europa. The spectrum of this component is distinguished by distorted hydration features rather than distinct spectral features, indicating hydrated minerals but making unique identification difficult. In particular, it lacks features common to hydrated sulfate minerals, challenging the traditional view of an endogenous salty component dominated by Mg-sulfates. Chloride evaporite deposits are one possible alternative.

Cometary Science with the James Webb Space Telescope

The James Webb Space Telescope (JWST), as the largest space-based astronomical observatory with near- and mid-infrared instrumentation, will elucidate many mysterious aspects of comets. We summarize four cometary science themes especially suited for this telescope and its instrumentation: the drivers of cometary activity, comet nucleus heterogeneity, water ice in comae and on surfaces, and activity in faint comets and main-belt asteroids. With JWST, we can expect the most distant detections of gas, especially CO2, in what we now consider to be only moderately bright comets. For nearby comets, coma dust properties can be studied with their driving gases, measured simultaneously with the same instrument or contemporaneously with another. Studies of water ice and gas in the distant Solar System will help us test our understanding of cometary interiors and coma evolution. The question of cometary activity in main-belt comets will be further explored with the possibility of a direct detection of coma gas. We explore the technical approaches to these science cases and provide simple tools for estimating comet dust and gas brightness. Finally, we consider the effects of the observatory's non-sidereal tracking limits, and provide a list of potential comet targets during the first 5 years of the mission.

Water deuteration and ortho-to-para nuclear spin ratio of H2 in molecular clouds formed via the accumulation of HI gas [Replacement]

We investigate the water deuteration ratio and ortho-to-para nuclear spin ratio of H2 (OPR(H2)) during the formation and early evolution of a molecular cloud, following the scenario that accretion flows sweep and accumulate HI gas to form molecular clouds. We follow the physical evolution of post-shock materials using a one-dimensional shock model, with post-processing gas-ice chemistry simulations. This approach allows us to study the evolution of the OPR(H2) and water deuteration ratio without an arbitrary assumption concerning the initial molecular abundances, including the initial OPR(H2). When the conversion of hydrogen into H2 is almost complete, the OPR(H2) is already much smaller than the statistical value of three due to the spin conversion in the gas phase. As the gas accumulates, the OPR(H2) decreases in a non-equilibrium manner. We find that water ice can be deuterium-poor at the end of its main formation stage in the cloud, compared to water vapor observed in the vicinity of low-mass protostars where water ice is likely sublimated. If this is the case, the enrichment of deuterium in water should mostly occur at somewhat later evolutionary stages of star formation, i.e., cold prestellar/protostellar cores. The main mechanism to suppress water ice deuteration in the cloud is the cycle of photodissociation and reformation of water ice, which efficiently removes deuterium from water ice chemistry. The removal efficiency depends on the main formation pathway of water ice. The OPR(H2) plays a minor role in water ice deuteration at the main formation stage of water ice.

Dust as interstellar catalyst I. Quantifying the chemical desorption process

Context. The presence of dust in the interstellar medium has profound consequences on the chemical composition of regions where stars are forming. Recent observations show that many species formed onto dust are populating the gas phase, especially in cold environments where UV and CR induced photons do not account for such processes. Aims. The aim of this paper is to understand and quantify the process that releases solid species into the gas phase, the so-called chemical desorption process, so that an explicit formula can be derived that can be included into astrochemical models. Methods. We present a collection of experimental results of more than 10 reactive systems. For each reaction, different substrates such as oxidized graphite and compact amorphous water ice are used. We derive a formula to reproduce the efficiencies of the chemical desorption process, which considers the equipartition of the energy of newly formed products, followed by classical bounce on the surface. In part II we extend these results to astrophysical conditions. Results. The equipartition of energy describes correctly the chemical desorption process on bare surfaces. On icy surfaces, the chemical desorption process is much less efficient and a better description of the interaction with the surface is still needed. Conclusions. We show that the mechanism that directly transforms solid species to gas phase species is efficient for many reactions.

Experimental study of surface erosion processes of the icy moons of Jupiter

We use an existing laboratory facility for space hardware calibration in vacuum to study the impact of energetic ions on water ice. The experiment is intended to simulate the conditions on the surface of Jupiter's icy moons. We present first results of ion sputtering in a sample of porous ice, including the first experimental results for sulphur ion sputtering of ice. The results confirm theoretical predictions and extrapolations from previous sputtering experiments obtained at different impact angles for non-porous water ice.

Experimental study of surface erosion processes of the icy moons of Jupiter [Replacement]

We use an existing laboratory facility for space hardware calibration in vacuum to study the impact of energetic ions on water ice. The experiment is intended to simulate the conditions on the surface of Jupiter's icy moons. We present first results of ion sputtering in a sample of porous ice, including the first experimental results for sulphur ion sputtering of ice. The results confirm theoretical predictions and extrapolations from previous sputtering experiments obtained at different impact angles for non-porous water ice.

Widespread Excess Ice in Arcadia Planitia, Mars

The distribution of subsurface water ice on Mars is a key constraint on past climate, while the volumetric concentration of buried ice (pore-filling versus excess) provides information about the process that led to its deposition. We investigate the subsurface of Arcadia Planitia by measuring the depth of terraces in simple impact craters and mapping a widespread subsurface reflection in radar sounding data. Assuming that the contrast in material strengths responsible for the terracing is the same dielectric interface that causes the radar reflection, we can combine these data to estimate the dielectric constant of the overlying material. We compare these results to a three-component dielectric mixing model to constrain composition. Our results indicate a widespread, decameters-thick layer that is excess water ice ~10^4 km^3 in volume. The accumulation and long-term preservation of this ice is a challenge for current Martian climate models.

The unstable CO2 feedback cycle on ocean planets

Ocean planets are volatile rich planets, not present in our Solar System, which are thought to be dominated by deep, global oceans. This results in the formation of high-pressure water ice, separating the planetary crust from the liquid ocean and, thus, also from the atmosphere. Therefore, instead of a carbonate-silicate cycle like on the Earth, the atmospheric carbon dioxide concentration is governed by the capability of the ocean to dissolve carbon dioxide (CO2). In our study, we focus on the CO2 cycle between the atmosphere and the ocean which determines the atmospheric CO2 content. The atmospheric amount of CO2 is a fundamental quantity for assessing the potential habitability of the planet's surface because of its strong greenhouse effect, which determines the planetary surface temperature to a large degree. In contrast to the stabilising carbonate-silicate cycle regulating the long-term CO2 inventory of the Earth atmosphere, we find that the CO2 cycle feedback on ocean planets is negative and has strong destabilising effects on the planetary climate. By using a chemistry model for oceanic CO2 dissolution and an atmospheric model for exoplanets, we show that the CO2 feedback cycle can severely limit the extension of the habitable zone for ocean planets.

The unstable CO2 feedback cycle on ocean planets [Replacement]

Ocean planets are volatile rich planets, not present in our Solar System, which are thought to be dominated by deep, global oceans. This results in the formation of high-pressure water ice, separating the planetary crust from the liquid ocean and, thus, also from the atmosphere. Therefore, instead of a carbonate-silicate cycle like on the Earth, the atmospheric carbon dioxide concentration is governed by the capability of the ocean to dissolve carbon dioxide (CO2). In our study, we focus on the CO2 cycle between the atmosphere and the ocean which determines the atmospheric CO2 content. The atmospheric amount of CO2 is a fundamental quantity for assessing the potential habitability of the planet's surface because of its strong greenhouse effect, which determines the planetary surface temperature to a large degree. In contrast to the stabilising carbonate-silicate cycle regulating the long-term CO2 inventory of the Earth atmosphere, we find that the CO2 cycle feedback on ocean planets is negative and has strong destabilising effects on the planetary climate. By using a chemistry model for oceanic CO2 dissolution and an atmospheric model for exoplanets, we show that the CO2 feedback cycle can severely limit the extension of the habitable zone for ocean planets.

"Ice cubes" in the center of the Milky Way - Water ice and hydrocarbons in the central parsec

The close environment of the central supermassive black hole of our Galaxy is studied thoroughly since decades in order to shed light on the behavior of the central regions of galaxies in general and of active galaxies in particular. The Galactic Center has shown a wealth of structures on different scales with a complicated mixture of early- and late-type stars, ionized and molecular gas, dust and winds. Here we aim at studying the distribution of water ices and hydrocarbons in the central parsec as well as along the line of sight. This study is made possible thanks to L-band spectroscopy. This spectral band, from 2.8 to 4.2$\mu m$, hosts important signatures of the circumstellar medium and interstellar dense and diffuse media among which deep absorption features are attributed to water ices and hydrocarbons. We observed the Galactic Center in the L-band of ISAAC spectrograph located on UT1/VLT ESO telescope. By mapping the central half parsec using 27 slit positions, we were able to build the first data cube of the region in this wavelength domain. Thanks to a calibrator spectrum of the foreground extinction in the L-band derived in a previous paper, we corrected our data cube for the line of sight extinction and validated our calibrator spectrum. The data show that a residual absorption due to water ices and hydrocarbons is present in the corrected data cube. This suggests that the features are produced in the local environment of the Galactic center implying very low temperatures well below 80K. This is in agreement with our finding of local CO ices in the central parsec described in Moultaka et al. (2015).

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.

Photodesorption of H2O, HDO, and D2O ice and its impact on fractionation

The HDO/H2O ratio in interstellar gas is often used to draw conclusions on the origin of water in star-forming regions and on Earth. In cold cores and in the outer regions of protoplanetary disks, gas-phase water comes from photodesorption of water ice. We present fitting formulae for implementation in astrochemical models using photodesorption efficiencies for all water ice isotopologues obtained using classical molecular dynamics simulations. We investigate if the gas-phase HDO/H2O ratio reflects that present in the ice or whether fractionation can occur during photodesorption. Probabilities for the top four monolayers are presented for photodesorption of X (X=H,D) atoms, OX radicals, and X2O and HDO molecules following photodissociation of H2O, D2O, and HDO in H2O amorphous ice at temperatures from 10-100 K. Isotope effects are found for all products: (1) H atom photodesorption probabilities from H2O ice are larger than those for D atom photodesorption from D2O ice by a factor of 1.1; the ratio of H and D photodesorbed upon HDO photodissociation is a factor of 2. This process will enrich the ice in deuterium atoms over time; (2) the OD/OH photodesorption ratio upon D2O and H2O photodissociation is on average a factor of 2, but the ratio upon HDO photodissociation is almost constant at unity for all temperatures; (3) D atoms are more effective in kicking out neighbouring water molecules than H atoms. However, the ratio of the photodesorbed HDO and H2O molecules is equal to the HDO/H2O ratio in the ice, therefore, there is no isotope fractionation upon HDO and H2O photodesorption. Nevertheless, the enrichment of the ice in D atoms due to photodesorption can over time lead to an enhanced HDO/H2O ratio in the ice, and, when photodesorbed, also in the gas. The extent to which the ortho/para ratio of H2O can be modified by the photodesorption process is also discussed. (Abridged)

Saturn's Great Storm of 2010-2011: Evidence for ammonia and water ices from analysis of VIMS spectra

Our analysis of Cassini/VIMS near-infrared spectra of Saturn's Great Storm of 2010-2011 reveals a multi-component aerosol composition comprised primarily of ammonia ice, with a significant component of water ice. The most likely third component is ammonium hydrosulfide or some weakly absorbing material similar to what dominates visible clouds outside the storm region. Horizontally heterogeneous models favor ammonium hydrosulfide as the third component, while horizontally uniform models favor the weak absorber. Both models rely on water ice absorption to compensate for residual spectral gradients produced by ammonia ice from 3.0 microns to 3.1 microns and need the third component to fill in the sharp ammonia ice absorption peak near 2.96 microns. The best heterogeneous model has spatial coverage fractions of 55% ammonia ice, 22% water ice, and 23% ammonium hydrosulfide. The best homogeneous model has an optically thin layer of weakly absorbing particles above an optically thick layer of water ice particles coated by ammonia ice. This is the first spectroscopic evidence of water ice in Saturn's atmosphere, found near the level of Saturn's visible cloud deck where it could only be delivered by powerful convection originating from ~200 km deeper in the atmosphere.

Correlations of atmospheric water ice and dust in the Martian Polar regions

We report on the interannual variability of the atmospheric ice/dust cycle in the Martian polar regions for Mars Years 28-30. We used CRISM emission phase function measurements to derive atmospheric dust optical depths and data from the MARCI instrument to derive atmospheric water ice optical depths. We have used autocorrelation and cross correlation functions in order to quantify the degree to which dust and ice are correlated throughout both polar regions during Mars Years 28-29. We find that in the south polar region, dust has the tendency to "self clear", demonstrated by negative autocorrelation around the central peak. This does not occur in the north polar region. In the south polar region, dust and ice are temporally and spatially anti correlated. In the north polar region, this relationship is reversed, however temporal correlation of northern dust and ice clouds is weak - 6 times weaker than the anticorrelation in the south polar region. Our latitudinal autocorrelation functions allow us to put average spatial sizes of event cores and halos. Dust events in the south are largest, affecting almost the entire pole, whereas dust storms are smaller in the north. Ice clouds in north are similar in latitudinal extent to those in the south (both have halos < 10{\deg}). Using cross-correlation functions of water ice and dust, we find that dust events temporally lag ice events by 35-80 degrees of solar longitude in the north and south poles, which is likely due to seasonality of dust and ice events.

Quantification of summertime water ice deposition on the Martian north polar ice cap

We use observations from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) of the north polar cap during late summer for two Martian years, to monitor the complete summer cycle of albedo and water ice grain size in order to place quantitative limits of the amount of water ice deposited in late summer. We establish here for the first time the complete spring to summer cycle of water ice grain sizes on the north polar cap. The apparent grain sizes grow until Ls=132, when they appear to shrink again, until they are obscured at the end of summer by the north polar hood. Under the assumption that the shrinking of grain sizes is due to the deposition of find grained ice, we quantify the amount of water ice deposited per Martian boreal summer, and estimate the amount of water ice that must be transported equatorward. Interestingly, we find that the relative amount of water ice deposited in the north cap during boreal summer (0.7-7 microns) is roughly equivalent to the average amount of water ice deposited on the south polar cap during austral summer (0.6-6 microns).

Collisions of small ice particles under microgravity conditions (II): Does the chemical composition of the ice change the collisional properties?

Context: Understanding the collisional properties of ice is important for understanding both the early stages of planet formation and the evolution of planetary ring systems. Simple chemicals such as methanol and formic acid are known to be present in cold protostellar regions alongside the dominant water ice; they are also likely to be incorporated into planets which form in protoplanetary disks, and planetary ring systems. However, the effect of the chemical composition of the ice on its collisional properties has not yet been studied. Aims: Collisions of 1.5 cm ice spheres composed of pure crystalline water ice, water with 5% methanol, and water with 5% formic acid were investigated to determine the effect of the ice composition on the collisional outcomes. Methods: The collisions were conducted in a dedicated experimental instrument, operated under microgravity conditions, at relative particle impact velocities between 0.01 and 0.19 m s^-1, temperatures between 131 and 160 K and a pressure of around 10^-5 mbar. Results: A range of coefficients of restitution were found, with no correlation between this and the chemical composition, relative impact velocity, or temperature. Conclusions: We conclude that the chemical composition of the ice (at the level of 95% water ice and 5% methanol or formic acid) does not affect the collisional properties at these temperatures and pressures due to the inability of surface wetting to take place. At a level of 5% methanol or formic acid, the structure is likely to be dominated by crystalline water ice, leading to no change in collisional properties. The surface roughness of the particles is the dominant factor in explaining the range of coefficients of restitution.

Short-term variability on the surface of (1) Ceres. A changing amount of water ice? [Replacement]

Context: The dwarf planet (1) Ceres - next target of the NASA Dawn mission - is the largest body in the asteroid main belt; although several observations of this body have been performed so far, the presence of surface water ice is still questioned. Aims: Our goal is to better understand the surface composition of Ceres, and to constrain the presence of exposed water ice. Methods: We acquired new visible and near-infrared spectra at the Telescopio Nazionale Galileo (TNG, La Palma, Spain), and reanalyzed literature spectra in the 3-$\mu$m region. Results: We obtained the first rotationally-resolved spectroscopic observations of Ceres at visible wavelengths. Visible spectra taken one month apart at almost the same planetocentric coordinates show a significant slope variation (up to 3 %/10$^3\AA$). A faint absorption centered at 0.67 $\mu$m, possibly due to aqueous alteration, is detected in a subset of our spectra. The various explanations in the literature for the 3.06-$\mu$m feature can be interpreted as due to a variable amount of surface water ice at different epochs. Conclusions: The remarkable short-term temporal variability of the visible spectral slope, and the changing shape of the 3.06-$\mu$m band, can be hints of different amounts of water ice exposed on the surface of Ceres. This would be in agreement with the recent detection by the Herschel Space Observatory of localized and transient sources of water vapour over this dwarf planet.

Short-term variability over the surface of (1) Ceres. A changing amount of water ice?

Context: The dwarf planet (1) Ceres - next target of the NASA Dawn mission - is the largest body in the asteroid main belt; although several observations of this body have been performed so far, the presence of surface water ice is still questioned. Aims: Our goal is to better understand the surface composition of Ceres, and to constrain the presence of exposed water ice. Methods: We acquired new visible and near-infrared spectra at the Telescopio Nazionale Galileo (TNG, La Palma, Spain), and reanalyzed literature spectra in the 3-$\mu$m region. Results: We obtained the first rotationally-resolved spectroscopic observations of Ceres at visible wavelengths. Visible spectra taken one month apart at almost the same planetocentric coordinates show a significant slope variation (up to 3 %/10$^3\AA$). A faint absorption centered at 0.67 $\mu$m, possibly due to aqueous alteration, is detected in a subset of our spectra. The various explanations in the literature for the 3.06-$\mu$m feature can be interpreted as due to a variable amount of surface water ice at different epochs. Conclusions: The remarkable short-term temporal variability of the visible spectral slope, and the changing shape of the 3.06-$\mu$m band, can be hints of different amounts of water ice exposed on the surface of Ceres. This would be in agreement with the recent detection by the Herschel Space Observatory of localized and transient sources of water vapour over this dwarf planet.

Detections of trans-Neptunian ice in protoplanetary disks

We present Herschel Space Observatory PACS spectra of T Tauri stars, in which we detect amorphous and crystalline water ice features. Using irradiated accretion disk models, we determine the disk structure and ice abundance in each of the systems. Combining a model-independent comparison of the ice feature strength and disk size with a detailed analysis of the model ice location, we estimate that the ice emitting region is at disk radii >30AU, consistent with a proto-Kuiper belt. Vertically, the ice emits most below the photodesorption zone, consistent with Herschel observations of cold water vapor. The presence of crystallized water ice at a disk location a) colder than its crystallization temperature and b) where it should have been re-amorphized in ~1 Myr suggests that localized generation is occurring; the most likely cause appears to be micrometeorite impact or planetesimal collisions. Based on simple tests with UV models and different ice distributions, we suggest that the SED shape from 20 to 50 micron may probe the location of the water ice snow line in the disk upper layers. This project represents one of the first extra-solar probes of the spatial structure of the cometary ice reservoir thought to deliver water to terrestrial planets.

The D/H Ratio of Water Ice at Low Temperatures

We present the modeling results of deuterium fractionation of water ice, H2, and the primary deuterium isotopologues of H3+ adopting physical conditions associated with the star and planet formation process. We calculated the deuterium chemistry for a range of gas temperatures (T_gas ~ 10 - 30 K), molecular hydrogen density (n(H2)~ 10^4 - 10^7), and ortho/para ratio (opr) of H2 based on state-to-state reaction rates and explore the resulting fractionation including the formation of a water ice mantle coating grain surfaces. We find that the deuterium fractionation exhibits the expected temperature dependence of large enrichments at low gas temperature. More significantly the inclusion of water ice formation leads to large D/H ratios in water ice (>= 10^-2 at 10 K) but also alters the overall deuterium chemistry. For T < 20 K the implantation of deuterium into ices lowers the overall abundance of HD which reduces the efficiency of deuterium fractionation at high density. In agreement with an earlier study, under these conditions HD may not be the primary deuterium reservoir in the cold dense interstellar medium and H3+ will be the main charge carrier in the dense centers of pre-stellar cores and the protoplanetary disk midplane.

Collisions of small ice particles under microgravity conditions

Planetisimals are thought to be formed from the solid material of a protoplanetary disk by a process of dust aggregation. It is not known how growth proceeds to kilometre sizes, but it has been proposed that water ice beyond the snowline might affect this process. To better understand collisional processes in protoplanetary disks leading to planet formation, the individual low velocity collisions of small ice particles were investigated. The particles were collided under microgravity conditions on a parabolic flight campaign using a purpose-built, cryogenically cooled experimental setup. The setup was capable of colliding pairs of small ice particles (between 4.7 and 10.8 mm in diameter) together at relative collision velocities of between 0.27 and 0.51 m s ^-1 at temperatures between 131 and 160 K. Two types of ice particle were used: ice spheres and irregularly shaped ice fragments. Bouncing was observed in the majority of cases with a few cases of fragmentation. A full range of normalised impact parameters (b/R = 0.0-1.0) was realised with this apparatus. Coefficients of restitution were evenly spread between 0.08 and 0.65 with an average value of 0.36, leading to a minimum of 58% of translational energy being lost in the collision. The range of coefficients of restitution is attributed to the surface roughness of the particles used in the study. Analysis of particle rotation shows that up to 17% of the energy of the particles before the collision was converted into rotational energy. Temperature did not affect the coefficients of restitution over the range studied.

The stickiness of micrometer-sized water-ice particles

Water ice is one of the most abundant materials in dense molecular clouds and in the outer reaches of protoplanetary disks. In contrast to other materials (e.g., silicates) water ice is assumed to be stickier due to its higher specific surface energy, leading to faster or more efficient growth in mutual collisions. However, experiments investigating the stickiness of water ice have been scarce, particularly in the astrophysically relevant micrometer-size region and at low temperatures. In this work, we present an experimental setup to grow aggregates composed of $\mathrm{\mu}$m-sized water-ice particles, which we used to measure the sticking and erosion thresholds of the ice particles at different temperatures between $114 \, \mathrm{K}$ and $260 \, \mathrm{K}$. We show with our experiments that for low temperatures (below $\sim 210 \, \mathrm{K}$), $\mathrm{\mu}$m-sized water-ice particles stick below a threshold velocity of $9.6 \, \mathrm{m \, s^{-1}}$, which is approximately ten times higher than the sticking threshold of $\mathrm{\mu}$m-sized silica particles. Furthermore, erosion of the grown ice aggregates is observed for velocities above $15.3 \, \mathrm{m \, s^{-1}}$. A comparison of the experimentally derived sticking threshold with model predictions is performed to determine important material properties of water ice, i.e., the specific surface energy and the viscous relaxation time. Our experimental results indicate that the presence of water ice in the outer reaches of protoplanetary disks can enhance the growth of planetesimals by direct sticking of particles.

How to link the relative abundances of gas species in coma of comets to their initial chemical composition ?

The chemical composition of comets is frequently assumed to be directly provided by the observations of the abundances of volatile molecules in the coma. The present work aims to determine the relationship between the chemical composition of the coma, the outgassing profile of volatile molecules and the internal chemical composition, and water ice structure of the nucleus, and physical assumptions on comets. To do this, we have developed a quasi 3D model of a cometary nucleus which takes into account all phase changes and water ice structures (amorphous, crystalline, clathrate, and a mixture of them); we have applied this model to the comet 67P/Churyumov-Gerasimenko, the target of the Rosetta mission. We find that the outgassing profile of volatile molecules is a strong indicator of the physical and thermal properties (water ice structure, thermal inertia, abundances, distribution, physical differentiation) of the solid nucleus. Day/night variations of the rate of production of species helps to distinguish the clathrate structure from other water ice structures in nuclei, implying different thermodynamic conditions of cometary ice formation in the protoplanetary disc. The relative abundance (to H2O) of volatile molecules released from the nucleus interior varies by some orders of magnitude as a function of the distance to the sun, the volatility of species, their abundance and distribution between the trapped and condensed states, the structure of water ice, and the thermal inertia and other physical assumptions (dust mantle, ...) on the nucleus.

 

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