Water Ice Particles Research Articles

Evolution of Saturn’s north polar color and cloud structure between 2012 and 2017 inferred from Cassini VIMS and ISS observations

Cassini/ISS imagery and Cassini/VIMS spectral imaging observations from 0.35 to 5.12 μm show that Saturn’s north polar region (70°–90° N) evolved significantly between 2012 and 2017, with the region poleward of the hexagon changing from dark blue/green to a moderately brighter gold color, except for the inner eye region (88.2°–90° N), which remained relatively unchanged. These and even more dramatic near-IR changes can be reproduced by an aerosol model of four compact layers consisting of a stratospheric haze at an effective pressure near 50 mbar, a deeper haze of putative diphosphine particles typically near 300 mbar, an ammonia cloud layer with a base pressure between 0.4 bar and 1.3 bar, and a deeper cloud of a possible mix of NH4SH and water ice particles within the 2.7 to 4.5 bar region. Between the eye and the hexagon boundary near 75° N were many small discrete bright cloud features that VIMS spectra indicate have increased opacity in the ammonia cloud layer. Our analysis of the background clouds between the discrete features shows that between 2013 and 2016 the effective pressures of most layers changed very little, except for the ammonia ice layer, which decreased from about 1 bar to 0.4 bar near the edge of the eye, but increased to 1 bar inside the eye. Inside the hexagon there were large increases in optical depth, by up to a factor of 10 near the eye for the putative diphosphine layer and by a factor of four over most of the hexagon interior. Inside the eye, aerosol optical depths were very low, suggesting downwelling motions. The high contrast between eye and surroundings in 2016 was due to substantial increases in optical depths outside the eye. The color change from blue/green to gold inside most of the hexagon region can be explained in our model almost entirely by changes in the stratospheric haze, which increased between 2013 and 2016 by a factor of four in optical depth and by almost a factor of three in the short-wavelength peak of its wavelength-dependent imaginary index. A plausible mechanism for increasing aerosol opacity with time is the action of photochemistry as the north polar region became increasingly exposed to solar UV radiation. For 2013 we found an ammonia mixing ratio of about 50×10−6 in the depleted region between 4 bars and the NH3 condensation level (∼ 1 bar), but the NH3 results for 2016 are unclear due to very high retrieval uncertainties associated with increased aerosol opacity. We retrieved a deep abundance of about 5×10−6 for PH3 and a pressure breakpoint (where the PH3 abundance begins to decline with altitude) that coincided with the main cloud top near 300 mbar, except when that cloud opacity was very low, at which point the PH3 breakpoint pressure generally increased substantially, consistent with prior suggestions that the cloud layers shield PH3 from destruction by UV radiation above the clouds. We found an average AsH3 mixing ratio of 2×10−9 with some evidence for a decline with altitude above the main cloud layer.

Effect of Variations in the Extended Hydrogen Corona of Mars on the Efficiency of Charge Exchange with Solar Wind Protons

This paper presents the results of calculating the efficiency of solar wind proton charge exchange as a function of variations in the column density of hydrogen atoms in the extended corona of Mars. The presence of regular density variations has been known for a long time and has been associated with the change of seasons on Mars. However, sporadic density variations in the upper Martian atmosphere were detected recently as well. They were caused by various processes in the underlying atmosphere, such as ejection of water vapor and ice particles at altitudes up to 100 km due to global dust storms, which was discovered in the Mars Express and MAVEN observations. Taking into account the variations in the column density with the observed amplitude up to one order of magnitude is absolutely necessary to correctly consider the interaction of solar wind protons with the Martian atmosphere. The calculations using the kinetic Monte Carlo model revealed that with a 2- and 5-fold increase in the column density of H atoms in the corona of Mars, the charge exchange efficiency also increases and reaches 6% and 8%, respectively, as compared to the base value of 4%. The energy spectrum of hydrogen atoms penetrating into the Martian atmosphere does not change and remains identical in structure to the spectrum of undisturbed solar wind protons. These estimates, combined with the previously developed kinetic model of the proton and hydrogen atom precipitation into the planetary atmosphere, make it possible to do the following: to trace all the stages of the penetration of the undisturbed solar wind protons into the dense layers of the atmosphere, as well as interpret the observed characteristics of proton auroras depending on the variations of atomic hydrogen content in the Martian corona.

Evaluation of polar stratospheric clouds in the global chemistry–climate model SOCOLv3.1 by comparison with CALIPSO spaceborne lidar measurements

Abstract. Polar stratospheric clouds (PSCs) contribute to catalytic ozone destruction by providing surfaces for the conversion of inert chlorine species into active forms and by denitrification. The latter describes the removal of HNO3 from the stratosphere by sedimenting PSC particles, which hinders chlorine deactivation by the formation of reservoir species. Therefore, an accurate representation of PSCs in chemistry–climate models (CCMs) is of great importance to correctly simulate polar ozone concentrations. Here, we evaluate PSCs as simulated by the CCM SOCOLv3.1 for the Antarctic winters 2006, 2007 and 2010 by comparison with backscatter measurements by CALIOP on board the CALIPSO satellite. The year 2007 represents a typical Antarctic winter, while 2006 and 2010 are characterized by above- and below-average PSC occurrence. The model considers supercooled ternary solution (STS) droplets, nitric acid trihydrate (NAT) particles, water ice particles and mixtures thereof. PSCs are parameterized in terms of temperature and partial pressures of HNO3 and H2O, assuming equilibrium between the gas and particulate phase. The PSC scheme involves a set of prescribed microphysical parameters, namely ice number density, NAT particle radius and maximum NAT number density. In this study, we test and optimize the parameter settings through several sensitivity simulations. The choice of the value for the ice number density affects simulated optical properties and dehydration, while modifying the NAT parameters impacts stratospheric composition via HNO3 uptake and denitrification. Depending on the NAT parameters, reasonable denitrification can be modeled. However, its impact on ozone loss is minor. The best agreement with the CALIOP optical properties and observed denitrification was for this case study found with the ice number density increased from the hitherto used value of 0.01 to 0.05 cm−3 and the maximum NAT number density from 5×10-4 to 1×10-3 cm−3. The NAT radius was kept at the original value of 5 µm. The new parameterization reflects the higher importance attributed to heterogeneous nucleation of ice and NAT particles following recent new data evaluations of the state-of-the-art CALIOP measurements. A cold temperature bias in the polar lower stratosphere results in an overestimated PSC areal coverage in SOCOLv3.1 by up to 40 %. Offsetting this cold bias by +3 K delays the onset of ozone depletion by about 2 weeks, which improves the agreement with observations. Furthermore, the occurrence of mountain-wave-induced ice, as observed mainly over the Antarctic Peninsula, is continuously underestimated in the model due to the coarse model resolution (T42L39) and the fixed ice number density. Nevertheless, we find overall good temporal and spatial agreement between modeled and observed PSC occurrence and composition. This work confirms previous studies indicating that simplified PSC schemes, which avoid nucleation and growth calculations in sophisticated but time-consuming microphysical process models, may also achieve good approximations of the fundamental properties of PSCs needed in CCMs.

Martian Lee-wave cloud near Ascraeus Mons during Martian years 33 and 34: a study based on the Mars color camera (MCC) images

The north-eastern slope of Ascraeus Mons in Mars shows evidence of gravity wave cloud formation, commonly known as Lee-wave cloud. Mars color camera, onboard India’s first Mars orbiter mission, has captured over 25 images of Lee-wave clouds during Martian years 33 and 34. In our present study, our main objective is to estimate the atmospheric parameter associated with wave cloud appears over the Lee side of Ascraeus Mons and to interpret our results physically for the observed events. We have analyzed all the images of Lee-wave clouds and estimated the physical parameter such as wavelength, wind speed, height, formation temperature, nature of the cloud particle, and other atmospheric parameters related to the events. The wavelength of the Lee-wave cloud varies from 25 to 35 km. The wind speed ranges from 36 to 52 m/s at the height of 25–37 km from the planet’s surface. We consulted the global circulation model to validate our initial results. The estimated AOD value varies from 0.9 to 2.3 for the blue channel. In contrast, the scale height of AOD varies from 3 to 5 km for the observed region indicating the presence of a non-homogeneous mixture of air and airborne dust near the Lee side of Ascraeus Mons. We find the central part of the Lee-wave cloud is considered to be composed of water–ice particles. The formation temperature for the Lee-wave cloud is estimated to be 160–180 K during solar longitude 73° to 136° indicating a temporal distribution of the clouds near the Lee side of the mountain during the observed period.

Three-dimensional continuum radiative transfer of polarized radiation in exoplanetary atmospheres

Context.Polarimetry is about to become a powerful tool for determining the atmospheric properties of exoplanets. For example, recent observations of the WASP-18 system allowed the polarized flux resulting from scattering in the atmosphere of WASP-18b to be constrained. To provide the basis for the interpretation of such observational results and for predictive studies to guide future observations, sophisticated analysis tools are required.Aims.Our goal is to develop a radiative transfer tool that contains all the relevant continuum polarization mechanisms for the comprehensive analysis of the polarized flux resulting from the scattering in the atmosphere of, on the surface of, and in the local planetary environment (e.g., planetary rings, exomoons) of extra-solar planets. Furthermore, our goal is to avoid common simplifications such as locally plane-parallel planetary atmospheres, the missing cross-talk between latitudinal and longitudinal regions, or the assumption of either a point-like star or plane-parallel illumination.Methods.As a platform for the newly developed numerical algorithms, we use the 3D Monte Carlo radiative transfer code POLARIS. The code is extended and optimized for the radiative transfer in exoplanetary atmospheres. We investigate the reflected flux and its degree of polarization for different phase angles for a homogeneous cloud-free atmosphere and an inhomogeneous cloudy atmosphere. Our results are compared with already existing results to verify the implementations. To take advantage of the 3D radiative transfer and to demonstrate the potential of the code, the impact of an additional circumplanetary ring on the reflected polarized flux is studied. Therefore, a simple ring model with water-ice particles is used and various inclination angles, optical depths and viewing angles are investigated.Results.The considered test cases show a good agreement with already existing results. The presence of a circumplanetary ring consisting of small water-ice particles has a noticeable impact on the reflected polarized radiation. In particular, the reflected flux strongly increases at larger phase angles if the planetary orbit is seen edge-on because the considered particles tend to scatter forwards. In contrast, the degree of polarization decreases at these phase angles.Conclusions.We present a polarization radiative transfer tool in which all relevant contributions to the reflected polarized continuum flux are considered. In a case study, we investigated the impact of a planetary ring on the net polarization signal.

Tectonics of Enceladus’ South Pole: Block Rotation of the Tiger Stripes

AbstractThe South Polar Terrain (SPT) of Enceladus is a site with eruptions of gas and water ice particle plumes, which indicate internal geodynamic activity. These eruptions are located along a series of tectonic structures, that is, the Tiger Stripe Fractures (TSF), which are composed of regularly spaced, linear depressions. The SPT is surrounded by sinuous chains of ridges and troughs (the Marginal Zone). To unravel the tectonics that affect the region and its evolution, we performed specific structural mapping and quantitative analyses of brittle features from remotely sensed images. The results are consistent with a block rotation model, in which several tectonic regimes coexist. The TSF are left‐lateral strike‐slip faults that bound rigid elongated blocks. The blocks rotate clockwise and are enclosed in a regional scale right‐lateral kinematic framework expressed in the Marginal Zone. These two opposite and complementary kinematic regimes induce transtensional and transpressional regimes within the SPT. An evolutionary tectonic model is proposed for the past and future evolution of the SPT. This model confirms the role of tectonic‐related kinematics in icy satellites and contributes to preparations for future missions.

Properties of Water Ice and Dust Particles in the Atmosphere of Mars During the 2018 Global Dust Storm as Inferred From the Atmospheric Chemistry Suite

AbstractThe properties of Martian aerosols are an integral part of the planetary climatology. Global dust storms (GDS) significantly alter spatial and vertical distributions of dust and water ice aerosols and their microphysical properties. We explored the 2018/Martian year 34 GDS with the Atmospheric Chemistry Suite instrument onboard the ESA‐Roscosmos Trace Gas Orbiter mission. Solar occultation observations of thermal infrared and near infrared channels in the 0.7–6 μm spectral range with >103 signal‐to‐noise ratio are used to constrain the vertical dependence and the temporal evolution of the particle properties of water ice and dust (effective radius, effective variance, number density, and mass loading) before the 2018 GDS and during its onset and decay phases. In most of the observations, the particle size of dust and water ice decreases with altitude. The effective radius of dust and water ice particles ranges in 0.1−3.5 μm and 0.1–5.5 μm, respectively. The largest aerosol particles (> 2.5 μm for dust and > 3.5 μm for water ice) are present below 10 km before the onset and during the GDS decay phase. During the peak of the GDS, dust reached altitudes of 85 km; the most frequently observed effective radius is 1–2 μm with 0.1–1 cm−3 number density and 0.1 effective variance. Detached layers of water ice composed of 0.1–1 μm particles are systematically observed at 50–100 km during this period. Below, at 0–50 km, we see the dust mixed with the main water ice layer comprising 1–4 μm particles.

Visible and near-infrared reflectance of hyperfine and hyperporous particulate surfaces

The composition of Solar System surfaces can be inferred through reflectance and emission spectroscopy, by comparing these observations to laboratory measurements and radiative transfer models. While several populations of objects appear to be covered by sub-micrometre sized particles (D < 1 μm) (referred to as hyperfine), there are limited studies on reflectance and emission of particulate surfaces composed of particles smaller than the visible and infrared wavelengths. We have undertaken an effort to determine the reflectance of hyperfine particulate surfaces in conjunction with high-porosity, in order to simulate the physical state of cometary surfaces and their related asteroids (P- and D-types). In this work, we present a technique developed to produce hyperfine particles of astrophysical relevant materials (silicates, sulphides, macromolecular organics). This technique is used to prepare hyperfine powders that were measured in reflectance in the 0.4–2.6 μm range. These powders were then included in water ice particles, sublimated under vacuum, in order to produce a hyperporous sample of hyperfine material (refers as to sublimation residue). When grinded below one micrometre, the four materials studied (olivine, smectite, pyroxene and amorphous silica), show strong decrease of their absorption features together with a blueing of the spectra. This “small grain degeneracy” implies that surfaces covered by hyperfine grains should show only shallow absorption features if any (in the case of moderately absorbing particles as studied here). These two effects, decrease of band depth and spectral blueing, appear magnified when the grains are incorporated in the hyperporous residue. We interpret the distinct behaviour between hyperporous and more compact surfaces by the distancing of individual grains and a decrease in the size of the elemental scatterers. This work implies that hyperfine grains are unabundant at the surfaces of S- or V-type asteroids, and that the blue nature of B-type may be related to a physical effect rather than a compositional effect.

Molecular dynamics study of grain boundaries and triple junctions in ice.

We perform classical molecular dynamics simulations of polycrystalline ice at 250 K using the TIP4P/Ice model. The structures of polycrystalline ice are prepared by growing ice particles in supercooled water. An order parameter developed recently is used to characterize local structures in terms of the liquid-liquid phase transition scenario. It is shown that the grain boundaries and triple junctions in ice are structurally similar to low-density liquid water in which most water molecules form four hydrogen bonds and the O-O-O angles deviate from the tetrahedral angle of 109.47°. The thickness of the grain boundaries is ∼1 nm. The diffusion coefficient of water molecules along the grain boundaries calculated in this study, 5.0 × 10-13 m2 s-1, is in good agreement with experimental data. The diffusion along the triple junctions is 3.4 times faster than that along the grain boundaries. We model the grain size dependence of diffusivity of water molecules in polycrystalline ice using the simulation results and find that the impact of the grain boundaries and the triple junctions on the diffusivity is negligible for typical polycrystalline ice samples having grain sizes of the order of millimeters. We also demonstrate that the properties of the grain boundaries are quite different from those of the ice/vapor interface at the same temperature: the quasi-liquid layer at the ice/vapor interface is similar to high-density liquid water and the diffusion coefficient along the ice/vapor interface is two orders of magnitude larger than that along the grain boundaries.

Two methods for determining the brightness change of a comet during its outburst

The paper presents two models that allow us to determine the change in the brightness of a comet during its outburst. The first of them (model A) is associated with the impact of individual scattering cross-sections on the brightness jump of the comet. The second (model B) is associated with the increase of sublimation active surface during an outburst. Based on these two models, we determine the change in the brightness of comets during the outburst. We assume that cometary particles consist of ice, dust, or their interconnections. For water–ice particles, the key factor is determining the lifetime in a coma. We find that the cometary outburst phase lasts until a significant amount of water–ice in the coma runs out. The loss of water–ice due to sublimation reduces the surface on which the incident sunlight is scattered. Consequently, this causes a slow decrease in brightness to the level before the outburst.

The contribution of black carbon to global ice nucleating particle concentrations relevant to mixed-phase clouds

Black carbon (BC) aerosol plays an important role in the Earth's climate system because it absorbs solar radiation and therefore potentially warms the climate; however, BC can also act as a seed for cloud particles, which may offset much of its warming potential. If BC acts as an ice nucleating particle (INP), BC could affect the lifetime, albedo, and radiative properties of clouds containing both supercooled liquid water droplets and ice particles (mixed-phase clouds). Over 40% of global BC emissions are from biomass burning; however, the ability of biomass burning BC to act as an INP in mixed-phase cloud conditions is almost entirely unconstrained. To provide these observational constraints, we measured the contribution of BC to INP concentrations ([INP]) in real-world prescribed burns and wildfires. We found that BC contributes, at most, 10% to [INP] during these burns. From this, we developed a parameterization for biomass burning BC and combined it with a BC parameterization previously used for fossil fuel emissions. Applying these parameterizations to global model output, we find that the contribution of BC to potential [INP] relevant to mixed-phase clouds is ∼5% on a global average.

Is water ice an efficient facilitator for dust coagulation?

ABSTRACT Beyond the snow line of protoplanetary discs and inside the dense core of molecular clouds, the temperature of gas is low enough for water vapour to condense into amorphous ices on the surface of pre-existing refractory dust particles. Recent numerical simulations and laboratory experiments suggest that condensation of the vapour promotes dust coagulation in such a cold region. However, in the numerical simulations, cohesion of refractory materials is often underestimated, while in the laboratory experiments, water vapour collides with surfaces at more frequent intervals compared to the real conditions. Therefore, to re-examine the role of water ice in dust coagulation, we carry out systematic investigation of available data on coagulation of water-ice particles by making full use of appropriate theories in contact mechanics and tribology. We find that the majority of experimental data are reasonably well explained by lubrication theories, owing to the presence of a quasi-liquid layer (QLL). Only exceptions are the results of dynamic collisions between particles at low temperatures, which are, instead, consistent with the JKR theory, because QLLs are too thin to dissipate their kinetic energies. By considering the vacuum conditions in protoplanetary discs and molecular clouds, the formation of amorphous water ice on the surface of refractory particles does not necessarily aid their collisional growth as currently expected. While crystallization of water ice around but outside the snow line eases coagulation of ice-coated particles, sublimation of water ice inside the snow line is deemed to facilitate coagulation of bare refractory particles.

The Composition of ~96 keV W+ in Saturn's Magnetosphere

AbstractEnceladus produces plumes of water vapor and ice particles whose gaseous H2O molecules dissociate into OH and O. The gas ejecta form a large toroidal planetary cloud colocated with and partially sourced by a similarly large torus of ice grain ejecta. Gas torus neutrals are ionized by charge exchange, solar ultraviolet, and electron impacts, producing the thermal water group ions, W+ (O+, OH+, H2O+, and H3O+), which become energized in Saturn's magnetosphere. We study the components of energetic (~96 keV) W+ using Cassini Charge‐Energy‐Mass Spectrometer (CHEMS) data from 78 near‐equatorial main ring current passes (dipole L = 7–16, ±10° in latitude) in 2004–2010. W+ fractional abundances are ~53% O+, ~22% OH+, ~22% H2O+, and ~3% H3O+, when averaged over L = 7–16, resulting in a mean water group mass of 16.7 amu. This large energetic O+ fractional abundance, over twice that observed for thermal O+ at L &lt; 10, qualitatively agrees with the broad atomic O cloud observed by Cassini and predicted by some models. Fractional abundance ratios for O+/W+, OH+/W+, and H2O+/W+ vary little over L = 7–21. Our observation of energetic H2O+/W+ ≈ 22% out to L ~ 21 suggests that neutral H2O spreads throughout the magnetosphere rather than being confined to a narrow torus centered on Enceladus' orbit. Although nearly constant at &lt;2% for L &gt; 13, H3O+/W+ tends to increase significantly to ~4–5% at L ~ 7–8.

Strong Variability of Martian Water Ice Clouds During Dust Storms Revealed From ExoMars Trace Gas Orbiter/NOMAD

AbstractObservations of water ice clouds and aerosols on Mars can provide important insights into the complexity of the water cycle. Recent observations have indicated an important link between dust activity and the water cycle, as intense dust activity can significantly raise the hygropause, and subsequently increase the escape of water after dissociation in the upper atmosphere. Here present observations from Nadir and Occultation for MArs Discovery/Trace Gas Orbiter that investigate the variation of water ice clouds in the perihelion season of Mars year 34 (April 2018–2019), their diurnal and seasonal behavior, and the vertical structure and microphysical properties of water ice and dust. These observations reveal the recurrent presence of a layer of mesospheric water ice clouds subsequent to the 2018 global dust storm. We show that this layer rose from 45 to 80 km in altitude on a time scale of days from heating in the lower atmosphere due to the storm. In addition, we demonstrate that there is a strong dawn‐dusk asymmetry in water ice abundance, related to nighttime nucleation and subsequent daytime sublimation. Water ice particle sizes are retrieved consistently and exhibit sharp vertical gradients (from 0.1 to 4.0 μm), as well as mesospheric differences between the global dust storm (&lt;0.5 μm) and the 2019 regional dust storm (1.0 μm), which suggests differing water ice nucleation efficiencies. These results form the basis to advance our understanding of mesospheric water ice clouds on Mars, and further constrain the interactions between water ice and dust in the middle atmosphere.

Martian Water Ice Clouds During the 2018 Global Dust Storm as Observed by the ACS‐MIR Channel Onboard the Trace Gas Orbiter

AbstractThe Atmospheric Chemistry Suite (ACS) instrument onboard the ExoMars Trace Gas Orbiter (TGO) European Space Agency‐Roscosmos mission began science operations in March 2018. ACS Mid‐InfraRed (MIR) channel notably provides solar occultation observations of the Martian atmosphere in the 2.3‐ to 4.2‐ m spectral range. Here, we use these observations to characterize water ice clouds before and during the MY 34 Global Dust Storm (GDS). We developed a method to detect water ice clouds with mean particle size 2 m and applied it to observations gathered between and . We observe a shift in water ice cloud maximum altitudes from about 60 km before the GDS to above 90 km during the storm. These very high altitude, small‐sized ( m) water ice clouds are more frequent during MY 34 compared to non‐GDS years at the same season. Particle size frequently decreases with altitude, both locally within a given profile and globally in the whole data set. We observe that the maximum altitude at which a given size is observed can increase during the GDS by several tens of kilometers for certain sizes. We notably notice some large water ice particles ( m) at surprisingly high altitudes during the GDS (50–70 km). These results suggest that GDS can significantly impact the formation and properties of high‐altitude water ice clouds as compared to the usual perihelion dust activity.

Laboratory study of the properties of frazil ice particles and flocs in water of different salinities

Abstract. Measurements of the size and shape of frazil ice particles and flocs in saline water and of frazil ice flocs in freshwater are limited. This study consisted of a series of laboratory experiments producing frazil ice at salinities of 0 ‰, 15 ‰, 25 ‰ and 35 ‰ to address this lack of data. The experiments were conducted in a large tank in a cold room with bottom-mounted propellers to create turbulence. A high-resolution camera system was used to capture images of frazil ice particles and flocs passing through cross-polarizing lenses. The high-resolution images of the frazil ice were processed using a computer algorithm to differentiate particles from flocs and determine key properties including size, concentration and volume. The size and volume distributions of particles and flocs at all four salinities were found to fit log-normal distributions closely. The concentration, mean size, and standard deviation of flocs and particles were assessed at different times during the supercooling process to determine how these properties evolve with time. Comparisons were made to determine the effect of salinity on the properties of frazil ice particles and flocs. The overall mean size of frazil ice particles in saline water and freshwater was found to range between 0.52 and 0.45 mm, with particles sizes in freshwater ∼13 % larger than in saline water. However, qualitative observations showed that frazil ice particles in saline water tend to be more irregularly shaped. The overall mean size of flocs in freshwater was 2.57 mm compared to a mean size of 1.47 mm for flocs in saline water. The average growth rate of frazil particles was found to be 0.174, 0.070, 0.033, and 0.024 mm min−1 and the average floc growth rate was 0.408, 0.118, 0.089, and 0.072 mm min−1 for the 0 ‰, 15 ‰, 25 ‰, and 35 ‰, respectively. Estimates for the porosity of frazil ice flocs were made by equating the estimated volume of ice produced based on thermodynamic conditions to the estimated volume of ice determined from the digital images. The estimated porosities of frazil ice flocs were determined to be 0.86, 0.82, 0.8 and 0.75 for 0 ‰, 15 ‰, 25 ‰ and 35 ‰ saline water, respectively.

Parametric study of water vapor and water ice particle plumes based on DSMC calculations: Application to the Enceladus geysers

The field parameters – number density, velocity components and temperature – for the Enceladus geysers, and possibly similar jets from other bodies, such as Europa, Ceres or comets, are expensive to obtain using physically correct simulation methods, such as the Direct Simulation Monte Carlo (DSMC) approach. It would be very useful to be able to correctly reproduce all the different states that the flow is undergoing while expanding into vacuum, from a high density and collisional state near the surface to free-molecular and collisionless at high altitude without resorting to expensive DSMC simulations in every case. In this work, we consider a two-phase water vapor/grain mixture exiting a circular vent, assuming a uniform radius of the water ice grains, and study how the field parameters can be fitted at an altitude of 10 km, where the flow is collisionless. To do so, we define simple functional forms for each of these fitted parameters, and we study how their coefficients vary as a function of the vent exit parameters, i.e. the vent radius, the water mixture mass flow, the water vapor/water ice mass ratio, the water ice grain radius, the water vapor and water ice exit speed, the vent exit angle and flow temperature. We define polynomial approximations to model these variations. We show that all the vent parameters have nearly-independent influences on the radial profiles at 10 km, except for the water vapor and water ice exit speed, for which we considered cross-correlations. We finally show that the geyser field parameters can be reconstructed using our parametric study for variations of the vent parameters within the range considered here, and in a time frame of a few milliseconds. The results of the parametrizations presented in this study can now be used to propagate the geyser field parameters using computationally inexpensive free molecular/ballistic codes up to higher altitudes. The DSMC results that have been run for this paper are available at an online repository.

Dynamics of nonspherical, fractal-like water-ice particles in a plasma environment

Plasmas containing small solid-state particles (also known as dust particles) are ubiquitous in nature and laboratories. Existing models typically assume that the dust particles are spherical but several observations and simulations indicate that a significant amount of dust particles are nonspherical. Because dust particles are not spherical they show different dynamics from spherical particles in a plasma environment namely, they align in the direction perpendicular to the force equilibrium line, rotate about their alignment axis due to the interaction between the dipole moment and the surrounding electric field, and show vortex motion while maintaining their alignment and rotation when they are exposed to a nonconservative drag force.

Polarimetry of Water Ice Particles Providing Insights on Grain Size and Degree of Sintering on Icy Planetary Surfaces

AbstractThe polarimetry of the light scattered by planetary surfaces is a powerful tool to provide constraints on their microstructure. To improve the interpretation of polarimetric data from icy surfaces, we have developed the POLarimeter for ICE Samples complementing the measurement facilities of the Ice Laboratory at the University of Bern. The new setup uses a high precision Stokes polarimeter to measure the degree of polarization in the visible light scattered by surfaces at moderate phase angles (from 1.5° to 30°). We present the photometric and polarimetric phase curves measured on various surfaces made of pure water ice particles having well‐controlled size and shape (spherical, crushed, and frost). The results show how the amplitude and the shape of the negative polarization branch change with the particles sizes and the degree of metamorphism of the ice. We found that fresh frost formed by water condensation on cold surfaces has a phase curve characterized by resonances (Mie oscillations) indicating that frost embryos are transparent micrometer‐sized particles with a narrow size distribution and spherical shape. Comparisons of these measurements with polarimetric observations of the icy satellites of the Solar System suggest that Europa is possibly covered by relatively coarser (~40–400 μm) and more sintered grains than Enceladus and Rhea, more likely covered by frost‐like particles of few micrometers in average. The great sensitivity of polarization to grain size and degree of sintering makes it an ideal tool to detect hints of ongoing processes on icy planetary surfaces, such as cryovolcanism.

OSIRIS-REx low-velocity particles during outbound cruise

We analyzed high-angular rate streaks first recorded by OSIRIS-REx’s MapCam during a 2017 search for Earth Trojan asteroids. We interpret them as water-ice particles that translated across the imager’s field of view, originating from the spacecraft itself. Their translation velocities approximated 0.1–1 m/s based on reasonable conclusions about their range. Pursuing several lines of investigation to seek a coherent hypothesis, we conclude that the episodic releases of the water ice particles are associated with spacecraft attitudes that resulted in solar illumination of previously shadowed regions. This correlation suggests that the OSIRIS-REx spacecraft itself possesses micro-climatic zones consisting of hot regions and cold traps that may temporarily potentially pass volatiles back and forth before losing most of them.