Icy Surfaces Research Articles

Laboratory Studies of Icy Regoliths in Relation to Observations of Minor Bodies in the Outer Solar System

Observing the properties of solar light scattered by TNOs is (up to now) the only way to obtain information on the physical properties of their surfaces. As such observations, performed near backscattering, become available, it is important to stress the significance of the phase angle and wavelength dependences of the linear polarization of the scattered light. At small phase angles, a narrow spike in brightness and a significantly negative polarization could be typical of icy regoliths, actually expected to be formed by alteration of icy bodies surfaces. Accurate experimental simulations of icy aggregates and regoliths formation that should take place with the ICAPS facility on board the ISS are presented, with emphasis on light scattering measurements providing a link between remote observations of TNOs and physical properties of their surfaces.

Proton Irradiation of Centaur, Kuiper Belt, and Oort Cloud Objects at Plasma to Cosmic Ray Energy

Times for accumulation of chemically significant dosages on icy surfaces of Centaur, Kuiper Belt, and Oort Cloud objects from plasma and energetic ions depend on irradiation position within or outside the heliosphere. Principal irradiation components include solar wind plasma ions, pickup ions from solar UV ionization of interstellar neutral gas, energetic ions accelerated by solar and interplanetary shocks, including the putative solar wind termination shock, and galactic cosmic ray ions from the Local Interstellar Medium (LISM). We present model flux spectra derived from spacecraft data and models for eV to GeV protons at 40 AU, a termination shock position at 85 AU, and in the LISM. Times in years to accumulate dosages ∼100 eV per molecule are computed from the spectra as functions of sensible surface depth less than one centimeter at unit density.The collisional resurfacing model of Luu and Jewitt is reconsidered in thecontext of depth-dependent dosage rates from plasma, suprathermal,and higher energy protons, and global exposure, by micrometeoroiddust grain impacts, of moderately irradiated red material below athin crust of heavily irradiated neutral material. This material should be more visible on dynamically `cold’ objects in the ∼40 AU region.

Physics and chemistry of icy particles in the universe: answers from microgravity

During the last century, the presence of icy particles throughout the universe has been confirmed by numerous ground and space based observations. Ultrathin icy layers are known to cover dust particles within the cold regions of the interstellar medium, and drive a rich chemistry in energetic star-forming regions. The polar caps of terrestrial planets, as well as most of the outer-solar-system satellites, are covered with an icy surface. Smaller solar system bodies, such as comets and Kuiper Belt Objects (KBOs), contain a significant fraction of icy materials. Icy particles are also present in planetary atmospheres and play an important role in determining the climate and the environmental conditions on our host planet, Earth. Water ice seems universal in space and is by far the most abundant condensed-phase species in our universe. Many research groups have focused their efforts on understanding the physical and chemical nature of water ice. However, open questions remain as to whether ices produced in Earth's laboratories are indeed good analogs for ices observed in space environments. Although temperature and pressure conditions can be very well controlled in the laboratory, it is very difficult to simulate the time-scales and gravity conditions of space environments. The bulk structure of ice, and the catalytic properties of the surface, could be rather different when formed in zero gravity in space. The author list comprises the members of the ESA Topical Team: Physico-chemistry of ices in space. In this paper we present recent results including ground-based experiments on ice and dust, models as well as related space experiments performed under microgravity conditions. We also investigate the possibilities of designing a new infrastructure, and /or making improvements to the existing hardware in order to study ices on the International Space Station (ISS). The type of multidisciplinary facility that we describe will support research in crystal growth of ices and other solid refractory materials, aerosol microphysics, light scattering properties of solid particles, the physics of icy particle aggregates, and radiation processing of molecular ices. Studying ices in microgravity conditions will provide us with fundamental data on the nature of extraterrestrial ices and allow us to enhance our knowledge on the physical and chemical processes prevailing in different space environments.

Catalytic Model Reactions for the HCN Isomerization. I. Theoretical Characterization of Some Water-Catalyzed Mechanisms

An ab initio study is presented for the HCN ⇌ HNC isomerization catalyzed by one, two, three, and four water molecules. Two mechanisms are proposed in the monohydrated case and one for each of the other cases. Four and six complexes have been determined on the mono- and the dihydrated potential energy surfaces, respectively, whereas only the reactant and product have been characterized for the tri- and tetrahydrated surfaces. For the monohydrated mechanisms, a reactant complex is connected to a product complex via the two determined saddle points, and these two complexes are separated by a reaction energy of 0.56 eV. A barrier lowering of 0.22 eV and a barrier increase of 0.17 eV are obtained compared to the monomolecular isomerization barrier T. For the dihydrated mechanism, the reaction energy between the reactant and product complexes is 0.55 eV, and this mechanism corresponds to a substantial barrier lowering of 0.95 eV compared to T. The corresponding further barrier lowerings for trihydration and tetrahydration are 0.58 and 0.42 eV, respectively. Reaction energy slightly decreases with the degree of hydration; the respective values are 0.51 and 0.48 eV. The existence of such hydrated isomerization pathways in water-dominated environments allows one to reconsider theoretical determination of the HNC/HCN ratio, for example, in the vicinity of icy surfaces.

Looking for life in unlikely places: reasons why planets may not be the best places to look for life

A new method is proposed to search for extraterrestrial life adapted to cold environments far from the Sun. To keep warm, using the light from a distant sun, any life-form must grow a system of optical concentrators, lenses or mirrors, to focus sunlight on to its vital parts. Any light not absorbed, or any heat radiation emitted from the vital parts, will be focused by the optical concentrators into a narrow beam pointing back towards the sun. To search for such life-forms, we should scan the sky with optical and infrared telescopes pointing directly away from the Sun. Any living vegetation will be seen as a bright patch in strong contrast to its dark surroundings, like the eyes of a nocturnal animal caught in the headlights of a car. This method of search may be used either with space-based or with ground-based telescopes. Examples of places where the method would work well are the surfaces of Europa, Trojan asteroids or Kuiper Belt objects. Any life-form that adapted successfully to a vacuum environment would be likely to spread widely over objects with icy surfaces in the outer regions of the solar system.

A review of possible optical absorption features of oxygen molecules in the icy surfaces of outer solar system bodies

In this review we provide the data needed to interpret remote spectroscopic studies of O 2 molecules embedded in the icy surfaces of outer solar system bodies. O 2 produced by radiolysis has been seen in the gas phase and as the so-called ‘solid O 2’ trapped in the icy surfaces of Ganymede, Europa and Callisto. It may also have been indirectly observed on a number of objects by its radiolysis product, O 3. These observations indicate the importance of O 2 for understanding the chemical processes occurring on icy outer solar system surfaces. Therefore, the published absorption spectra of gaseous, liquid and solid O 2 and of O 2 embedded in H 2O ice are reviewed in some detail. Particular emphasis has been placed on the presentation of transition probabilities for the various O 2 spectral series so that their relative importances can be assessed when they are used for modelling the radiation chemistry occurring in such environments.

Survivability of bacteria ejected from icy surfaces after hypervelocity impact.

Both the Saturnian and Jovian systems contain satellites with icy surfaces. If life exists on any of these icy bodies (in putative subsurface oceans for example) then the possibility exists for transfer of life from icy body to icy body. This is an application of the idea of Panspermia, wherein life migrates naturally through space. A possible mechanism would be that life, here taken as bacteria, could become frozen in the icy surface of one body. If a high-speed impact occurred on that surface, ejecta containing the bacteria could be thrown into space. It could then migrate around the local region of space until it arrived at a second icy body in another high-speed impact. In this paper we consider some of the necessary steps for such a process to occur, concentrating on the ejection of ice bearing bacteria in the initial impact, and on what happens when bacteria laden projectiles hit an icy surface. Laboratory experiments using high-speed impacts with a light gas gun show that obtaining icy ejecta with viable bacterial loads is straightforward. In addition to demonstrating the viability of the bacteria carried on the ejecta, we have also measured the angular and size distribution of the ejecta produced in hypervelocity impacts on ice. We have however been unsuccessful at transferring viable bacteria to icy surfaces from bacteria laden projectiles impacting at hypervelocities.

Nitrogen condensation on water ice

We studied, by infrared absorption spectroscopy, icy samples (16 K) of pure water, a mixture N2:H2O=100:1, and a sample made of N2 condensed on water ice and diffused in it after warm up to 30 K. We concentrated our efforts in two spectral regions around 3700 cm–1 where the feature due to the O–H dangling bonds in porous amorphous water falls and around 5000 cm–1 where a broad water band is present. We found that in the N2:H2=100:1 mixture the profile of the broad water feature at about 5000 cm–1 dramatically changed to a very narrow band at about 5300 cm–1. When N2 diffuses in water ice a feature at about 5300 cm–1 appears along with the broad 5000 cm–1 band. We also studied some of the effects of ion irradiation (Ar++, 60 keV ions) on these icy samples. We found that after processing the feature due to the O–H dangling bonds it reduced in intensity and eventually disappeared. Here we present the experimental results, discuss their astrophysical relevance and suggest that a band at about 5300 cm–1 (1.88 µm) should be searched for on icy surfaces in the outer Solar System, namely Pluto, Triton, Edgeworth–Kuiper Belt Objects, and Centaurs. PACS No.: 68.43Pg

Backscattering from snow and ice: Laboratory and field measurements

We present photometry of snow samples in the backscattering direction, i.e., at very small phase (light-source – target – observer) angles. Relative brightness measurements are taken to study the backscattering behaviour of snow and ice samples to be able to interpret the opposition effect of icy solar-system surfaces (for example, icy rings and satellites). Preliminary results of three separate experiments are presented, all of which exhibit a similar feature: a backscattering peak is exhibited by snow of different grain types and with different amounts of contamination. The peak grows stronger with increasing contamination. PACS No.: 96.35

Can redistribution of material by sputtering explain the hemispheric dichotomy of europa?

Sputtering and redeposition make up a significant geologic process on Europa and may contribute to the observed color differences between the leading and trailing hemispheres. Sputtering is particularly efficient on Europa as an erosive process, due to the high rate of ion bombardment from the Io plasma torus, together with Europa’s easily sputtered icy surface. We estimate the global average sputtering erosion rate on Europa at 14.7 mm Myr −1. However, 42% to 86% of sputtered water molecules survive to redeposit onto the surface again. Due to gravitational escape and removal by electron-impact ionization, the number of redepositing particles cannot overcome sputtering erosion, and the global average result of sputtering is net erosion. However, neither sputtering nor redeposition is globally uniform, and differences in the global distributions of the two processes can result locally in net deposition. We propose that Europa’s hemispheric color dichotomy might be explained by net deposition on the leading hemisphere, which may obscure the non-ice signature by covering it with a thin water frost. To test this hypothesis, we have created a simulated model of the sputtering erosion/redeposition process on Europa. Our objectives are to determine the conditions under which net deposition occurs on the leading hemisphere and to evaluate the effects on this process of Jupiter’s gravity, of Europa’s rotation, and of the loss of water molecules to the jovian magnetosphere. We have followed the trajectories of hundreds of thousands of simulated sputtered water molecules in a Monte Carlo process, evolving their orbits under the gravity of both Europa and Jupiter. We have performed this model multiple times, in order to explore the effects of different assumptions of the global distribution of impacting ions, as well as of the sputtered particle ejection velocity. Based upon our results, we conclude that net deposition occurs under a wide range of conditions, making sputtering erosion and redeposition a plausible explanation for the hemispheric color dichotomy of Europa.

The role of electron-stimulated production of O 2 from water ice in the radiation processing of outer solar system surfaces

We present measurements of the electron energy threshold, fluence, and temperature dependence for O 2 production during low-energy (5–100 eV) electron bombardment of thin (∼40 bilayer) amorphous and crystalline D 2O ice films deposited on a Pt(1 1 1) substrate in vacuum. The electron energy threshold (referenced to the vacuum level) for O 2 formation is approximately 10±2 eV. This threshold is due to valence level electronic excitations or ionization of condensed water molecules. Comparison of the fluence dependence with kinetic models shows that the formation of O 2 is dominated by direct excitation and dissociation of a stable precursor molecule, and not by diffusion and recombination of oxygen atoms. The O 2 yield is also strongly dependent upon the temperature of ice, is different for crystalline and amorphous ice films, and is indicative of surface and bulk structural transitions. We demonstrate that electron-stimulated production of O 2 is important in radiation processing of low-temperature icy surfaces present on outer solar system bodies. This is particularly true in regions with strong magnetospheres such as the Saturnine and Jovian systems.

Students’ Pre-Instructional Beliefs and Reasoning Strategies About Astrobiology Concepts

Students’ Pre-Instructional Beliefs and Reasoning Strategies About Astrobiology Concepts

The habitability of Europa: A cautionary note

Despite the lack of empirical evidence, the hypothesized existence of a liquid water ocean beneath the icy surface of Europa is supported by enough indirect data—photographic, spectroscopic, magnetic, and gravitometric—to make the proposal plausible [Gaidos and Nimmo, 2000; Greenberg, 2002; Kivelson et al., 2000; Prockter and Pappalardo, 2000]. A number of planetary scientists have suggested that were a Europan ocean to exist, it could be inhabited, if only by primitive microbial organisms [Chyba and Phillips, 2002; Gaidos and Nimmo, 2000; Greenberg, 2002; Kargel et al., 2000; McKollom, 1999]. We argue that speculation concerning potential habitability, unlike the Europan ocean hypothesis, is data‐deficient. Engaging as this speculation may seem, a greater measure of caution should be exercised when elaborating a supportive framework of arguments.

The assessment of the integration of slip resistance, thermal insulation and wearability of footwear on icy surfaces

Prevention of slip hazard in frozen environments is not paid much attention. Current winter and safety footwear does not provide sufficient slip resistance and appropriate wearability for use on icy surfaces. The objectives of this study were to assess the integration of slip resistance, thermal insulation, and wearability of footwear used on icy surfaces, and the anti-slip effect of materials spread on ice using outdoor walking trials. Twenty-five subjects wore four types of footwear walking on five different icy surfaces. A five-point rating scale was used recording wearer's perceptions of slipperiness, thermal comfort and wearability. The results showed that pure ice was perceived as very slippery. Spreading sand (180 g/m 2) greatly decreased the slipperiness. Slip resistance, thermal insulation and wearability of footwear tested were not properly integrated, and were ranked differently. The tested winter and safety footwear did not provide sufficient slip resistance and good wearability. In addition to thermal insulation, prevention of slip and fall hazard by improving anti-slip property and wearability must also be priorities for development of footwear for use in cold climate.

Radiation chemical alterations in solar system ices: An overview

Many solar system objects, such as planets, satellites, rings, and comets, are known to either be made of ices or to have icy surfaces. Because these ices are subjected to radiation processing by keV‐MeV ions and UV photons, their chemical and physical properties evolve over time. We have employed a laboratory approach to investigate these radiolytic and photolytic changes. Ices, either pure or mixtures, are subjected to ionizing radiation and UV photons and then probed by IR spectroscopy. It has been found that radiolysis and photolysis destroy reactant molecules, synthesize new molecules, cause changes of phase in pure materials, and eject molecules from ices. Although our laboratory approach initially focused on product identification, it has become increasingly necessary to work toward a comprehensive understanding of ice chemistry. To this end, we describe recent radiation experiments on some of the most important solar system and interstellar ice molecules, such as H2O, CO, CO2, and hydrocarbons. Chemical reactions are given to account for our observations, and these reactions include such general types as acid‐base, electron‐transfer, radical‐radical combination, and H‐atom addition. Comparisons of ion and UV processing are described for pure H2O and for two binary mixtures, H2O + CO2 and NH3 + CO. Predictions of molecular evolution in complex ices, both H2O‐ and N2‐dominated, are made. Applications are described for cometary organic chemistry, ion processing in interstellar ices, and the recent discovery of H2O2 on the surface of Europa.

Physical state of ices in the outer solar system

Comparison of the identity and abundances of ices observed around protostars and those associated with comets clearly suggests that comets preserve the heritage of the interstellar materials that aggregrated to form them. However, the ability to identify these same species on icy satellites in the outer solar system is a complex function of the composition of the original ices, their subsequent thermal histories, and their exposure to various radiation environments. Our ability to identify the ices currently present on objects in the outer solar system relies upon observational and laboratory, and theoretical efforts. To date there is ample observational evidence for crystalline water ice throughout the outer solar system. In addition, there is growing evidence that amorphous ice may be present on some bodies. More volatile ices, e.g., N2, CH4, and CO, and other species, e.g., ammonia hydrate, are identified on objects lying at and beyond Uranus. Both photolysis and radiolysis play important roles in altering the original surfaces due to chemical reactions and erosion of the surface. Ultraviolet photolysis appears to dominate alteration of the upper few hundred angstroms, although sputtering of the surface can sometimes be a significantly competitive process, dominating on icy surfaces embedded in a strong planetary magnetospheric field. There is growing observational evidence that the by‐products of photolysis and radiolysis, suggested on a theoretical basis, are present on icy surfaces.

Impacts into Ice–Silicate Mixtures: Crater Morphologies, Volumes, Depth-to-Diameter Ratios, and Yield

In this and a companion paper, we analyze the strength-dominated impact process into ice–silicate mixtures via experimental techniques, which is important in understanding the evolution of icy satellite surfaces as well as the dynamics of planetary rings. We used the plasmadynamic accelerator (PDA) and the electrothermal accelerator (ET) of the Fachbereich Raumfahrt of the Technische Universität München to perform impact experiments. The projectile velocities were varied between 0.9 and 11.8 km/s and projectile masses were between 1.7·10 −11 kg and 7.2·10 −10 kg at the plasma accelerator (projectile material was glass); at the electrothermal accelerator, projectile velocities were between 1.1 to 3.3 km/s and projectile masses between 6·10 −6 kg and 3·10 −5 kg (projectile material: Nylon). The kinetic energies ranged over 6 orders of magnitude, from 8.8·10 −5 J to 70 J. Silicate contents of the target were varied from 5% to 20% (mass), the rest was water ice. The target temperature was 250K±5K. Typical crater diameters were in the order of 0.1 to 1 mm at the PDA and several cm at the ET. In general, the crater morphology can be described by a conical spallation zone, in which a central crater, at the electrothermal accelerator with raised rim, was observed. In the center, the craters were much brighter than the undisturbed target surface. At the ET experiments, we measured the silicate content in different regions of the crater. It does not change significantly; actually, there is a slight tendency for the silicate content to be higher in the center of the crater as compared to the undisturbed surface. Together with data from Frisch (1990, Dissertation, TU München; and 1992, Proceedings of the Workshop on Hypervelocity Impacts in Space) for pure ice and Moore et al. (1963, Trans. Min. Eng. 229, 258–262) for basalt, an empirical relationship for the crater volume depending on the target and projectile parameters is found. Typically, crater volumes in pure ice are about two orders of magnitude larger than craters in pure silicates, for identical projectile mass and velocity. The crater yield, defined as the ratio of ejected mass to projectile mass, was found to be in the order of 1 to 10 3. We derived an empirical relationship for the yield as a function of the target and projectile properties. This relationship was compared to the necessary yields found by Durisen et al. (1992, Icarus 100, 364–393) for the description of mass transfer in Saturn's A- and B-ring. We find that their required yields are about one to two orders of magnitude larger than our experiments showed and conclude that more knowledge of the flux of incoming meteoroids and the ring's viscosity are necessary.

Sputtering of carbon monoxide ice by hydrogen ions

Carbon monoxide is an important constituent of comet comae, of icy surfaces of planetary bodies, and of interstellar grains. We present new laboratory studies of sputtering of frozen CO by hydrogen ions for energies below 10 keV. The sputtering yield turned out to depend critically on the energy and the electronic stopping power, (dE/dx)e, for the ions. The yield for a 9 keV H+ incident on solid CO was ∼34 CO/H+. For proton bombardment the yield is proportional to (dE/dx)e1.3, similar to the behavior of the sputtering yield for water ice. It means that the particle ejection occasionally requires species from two ionization/excitation events. For molecular ions the yield for CO ice increases with the square of the stopping power (dE/dx)e. The distribution of the sputtered CO molecules exhibits a maximum at 13 meV and falls off strongly with ejection energy E1. A complicating feature is the formation of a residue, possibly CO2 ice, during bombardment of solid CO. The sputtering yield depends slightly on the initial residue, similar to the case of sputtering by keV electrons. The sputtering yield and the chemical efficiency are high compared to similar properties of solid N2, which means that mixtures of these solids are preferentially depleted of CO.

The role of friction in the measurement of slipperiness, Part 1: Friction mechanisms and definition of test conditions

Friction has been widely used as a measure of slipperiness. However, controversies around friction measurements remain. The purposes of this paper are to summarize understanding about friction measurement related to slipperiness assessment of shoe and floor interface and to define test conditions based on biomechanical observations. In addition, friction mechanisms at shoe and floor interface on dry, liquid and solid contaminated, and on icy surfaces are discussed. It is concluded that static friction measurement, by the traditional use of a drag-type device, is only suitable for dry and clean surfaces, and dynamic and transition friction methods are needed to properly estimate the potential risk on contaminated surfaces. Furthermore, at least some of the conditions at the shoe/floor interface during actual slip accidents should be replicated as test conditions for friction measurements, such as sliding speed, contact pressure and normal force build-up rate.

The identification of factors in the systematic evaluation of slip prevention on icy surfaces

Slips and falls on icy roads often result in fractures or sprains and is a major problem in Nordic countries. Walking trials by 25 subjects wearing four types of winter shoes on five different icy walking surfaces provided subjective and objective measures of tendency to slip and number of slips, respectively. Since friction is a major determinant of a slip, the influence of material spread on icy surfaces, the surface temperatures and the shoe soling characteristics versus the Coefficient of Friction (COF) of the shoes were measured. Sand and gravel on icy roads had positive effects on improving COF. The study revealed that the aetiology of slips and falls is multi-faceted and attempts to solve the problem must adopt a systems approach. Perception of risk, aging, training, experience and postural balance are other factors to be considered in preventing slips and falls. Future research should concentrate on the degree of impact of each factor to the aetiology of slips and falls, which can help to decide priority action in preventing slips and falls. Relevance to industry The personal protective devices used by outdoor workers during winter season have to provide two types of protection, namely, protection from an occupational hazard and protection from the cold climate. Safety shoes used on snow or ice covered surfaces add a third type of protection, namely, an anti-slip quality of the shoe. The 3rd protection can be achieved from among other things, by improving the friction of the shoe soles.