Icy Planets Research Articles

Calorimetric Investigation of Hydrogen-Atom Sublattice Transitions in the Ice VI/XV/XIX Trio.

Ice XIX represents the latest discovery of ice polymorphs and exists in the medium pressure range near 1–2 GPa. Ice XIX is a partially hydrogen-ordered phase, by contrast to its disordered mother phase ice VI, which shares the same oxygen-atom network with ice XIX. Ice XIX differs in terms of the ordering of the hydrogen-atom sublattice, and hence the space group, from its hydrogen-ordered sibling ice XV, which also features the same type of oxygen network. Together, ice VI, XV, and XIX form the only known trio of ice polymorphs, where polymorphic transformations from order to order, order to disorder, and disorder to order are possible, which also compete with each other depending on the thermodynamic path taken and the cooling/heating rates employed. These transitions in the H-sublattice have barely been investigated, so we study here the unique triangular relation in the ice VI/XV/XIX trio based on calorimetry experiments. We reveal the following key features for H-sublattice transitions: (i) upon cooling ice VI, domains of ice XV and XIX develop simultaneously, where pure ice XV forms at ≤0.85 GPa and pure ice XIX forms at ≥1.60 GPa, (ii) ice XIX transforms into ice XV via a transient disordered state, (iii) ice XV recooled at ambient pressure features a complex domain structure, possibly containing an unknown H-ordered polymorph, (iv) recooled ice XV partly transforms back into ice XIX at 1.80 GPa, and (v) partial deuteration slows down domain reordering strongly. These findings not only are of interest in understanding possible hydrogen-ordering and -disordering processes in the interior of icy moons and planets but, more importantly, also provide a challenging benchmark for our understanding and parameterizing many-body interactions in H-bonded networks.

Modeling the formation of Menrva impact crater on Titan: Implications for habitability

Titan is unique in the solar system: it is an ocean world, an icy world, an organic world, and has a dense atmosphere. It is a geologically active world as well, with ongoing exogenic processes, such as rainfall, sediment transportation and deposition, erosion, and possible endogenic processes, such as tectonism and cryovolcanism. This combination of an organic and an ocean world makes Titan a prime target for astrobiological research, as biosignatures may be present in its surface, in impact melt deposits and in cryovolcanic flows, as well as in deep ice and water ocean underneath the outer ice shell. Impact craters are important sites in this context, as they may have allowed an exchange of materials between Titan's layers, in particular between the surface, composed of organic sediments over icy bedrock, and the subsurface ocean. It is also possible that impacts may have favored the advance of prebiotic chemical reactions themselves, by providing thermal energy that would allow these reactions to proceed. To investigate possible exchange pathways between the subsurface water ocean and the organic-rich surface, we modeled the formation of the largest crater on Titan, Menrva, with a diameter of ca. 425 km. The premise is that, given a large enough impact event, the resulting crater could breach into Titan's ice shell and reach the subsurface ocean, creating pathways connecting the surface and the ocean. Materials from the deep subsurface ocean, including salts and potential biosignatures of putative subsurface biota, could be transported to the surface. Likewise, atmospherically derived organic surface materials could be directly inserted into the ocean, where they could undergo aqueous hydrolysis to form potential astrobiological building blocks, such as amino acids. To study the formation of a Menrva-like impact crater, we staged numerical simulations using the iSALE-2D shock physics code. We varied assumed ice shell thickness from 50 to 125 km and assumed thermal structure over a range consistent with geophysical data. We analyze the implications and potential contributions of impact cratering as a process that can facilitate the exchange of surface organics with liquid water. Our findings indicate that melt and partial melt of ice took place in the central zone, reaching ca. 65 km depth and ca. 60 km away from the center of the structure. Furthermore, a volume of ca. 102 km3 of ocean water could be traced to depths as shallow as 10 km. These results highlight the potential for a significant exchange of materials from the surface (organics and ice) and the subsurface (water ocean), particularly in the crater's central area. Our studies suggest that large hypervelocity impacts are a viable and likely key mechanism to create pathways between the underground water ocean and Titan's organic-rich surface layer and atmosphere.

Complex Brines and Their Implications for Habitability.

There is evidence that life on Earth originated in cold saline waters around scorching hydrothermal vents, and that similar conditions might exist or have existed on Mars, Europa, Ganymede, Enceladus, and other worlds. Could potentially habitable complex brines with extremely low freezing temperatures exist in the shallow subsurface of these frigid worlds? Earth, Mars, and carbonaceous chondrites have similar bulk elemental abundances, but while the Earth is depleted in the most volatile elements, the Icy Worlds of the outer solar system are expected to be rich in them. The cooling of ionic solutions containing substances that likely exist in the Icy Worlds could form complex brines with the lowest eutectic temperature possible for the compounds available in them. Indeed, here, we show observational and theoretical evidence that even elements present in trace amounts in nature are concentrated by freeze–thaw cycles, and therefore contribute significantly to the formation of brine reservoirs that remain liquid throughout the year in some of the coldest places on Earth. This is interesting because the eutectic temperature of water–ammonia solutions can be as low as ~160 K, and significant fractions of the mass of the Icy Worlds are estimated to be water substance and ammonia. Thus, briny solutions with eutectic temperature of at least ~160 K could have formed where, historically, temperature have oscillated above and below ~160 K. We conclude that complex brines must exist in the shallow subsurface of Mars and the Icy Worlds, and that liquid saline water should be present where ice has existed, the temperature is above ~160 K, and evaporation and sublimation have been inhibited.

Exploration of Icy Ocean Worlds Using Geophysical Approaches

Abstract Geophysics-focused missions and improved geophysical data sets are critical for the future exploration of icy ocean worlds. Of particular interest is the exploration of the Galilean moon, Europa, and the Saturnian moons, Titan and Enceladus. These bodies likely have geologically active surfaces and may harbor habitable subsurface environments. Placing any candidate signatures for life in context requires further knowledge of the interior of these worlds. While the surfaces of these bodies have been mapped, their interiors remain poorly understood. Geophysical approaches such as geodesy, seismology, ice-penetrating radar and altimetry measurements, and electromagnetometry would provide critical information regarding the interior of these icy ocean worlds. The approaches described below would answer key science questions regarding ice shell and ocean dynamics, internal structure and interior layer thicknesses, near-surface structure, and how material from the deep interior might be exchanged with the surface. Here we outline the geophysical environments of Enceladus, Europa, and Titan; some outstanding science questions that remain to be addressed; and examples of the geophysical approaches that can provide the context to better understand icy ocean worlds.

Charon’s Far Side Geomorphology

Abstract New Horizons images of the illuminated nonencounter hemisphere (far side) of Charon display geomorphic features that are consistent with features observed in the highest-resolution images of the encounter hemisphere. Scarps, ridges, craters, and one area of smooth plains are identified. These features support previous hypotheses of global expansion and large-scale faulting across this icy world.

Constraining Europa's ice shell thickness with fundamental mode surface wave dispersion

Determining the thickness of Europa's outer ice shell is a key factor for understanding Europa's internal dynamics, evolution, and potential habitability. As such, one of the primary goals of any future lander mission to Europa would be to constrain the thickness of the ice shell and confirm the presence of a global subsurface ocean. Tidally induced ice fracturing events provide a natural source of seismic energy to illuminate the subsurface, thus a seismic instrument onboard a future lander mission could provide a promising means to probe ice shell thickness. A variety of seismic techniques could be used to constrain Europa's interior structure and dynamics, including body wave, surface wave, and normal mode seismology. Here, we use numerical simulations of seismic wave propagation on Europa in order to investigate the potential of using long period dispersion measurements of Rayleigh and flexural waves to constrain the ice shell thickness. Since the sensitivity kernels of group velocity dispersion measurements depend strongly on the structure of the ice shell, inverting for ice shell thickness is a non-linear problem.To address this, we use either a grid search or Markov chain Monte Carlo inversion approach, and test the method on a variety of plausible models of Europa's interior. Additionally, we demonstrate our approach in a “blind” inversion using the 1 week long synthetic catalogs of Europa's seismicity from Panning et al. (2018). We find that under most scenarios, group velocity dispersion measurements between periods of 25–250 s can constrain Europa's ice shell thickness to within several km uncertainty, although the method becomes increasingly inaccurate for thicker ice shells and at large epicentral distances. Our results, which suggest that surface waves from naturally occurring ice fracturing events on Europa can be used to help determine ice shell thickness, may help set instrument requirements for spaceflight capable seismometers aimed at exploring icy ocean worlds.

Elasticity of selected icy satellite candidate materials (CO2, C6H6, MgSO4·7H2O and CaSO4·2H2O) revisited by dispersion corrected density functional theory

Seismic studies are essential for accurate characterisation of planetary interior structures, but are dependent on modelling for interpretation, requiring data on the elastic properties of likely constituent minerals. With the potential deployment of seismic stations on icy worlds such as Europa and Titan envisioned for the near future, a campaign of study into the elasticity of potential icy ocean world minerals is of paramount importance.In the paper we assess the role of first-principles computer simulations to this problem, in particular focussing on the application of recent advances in simulating dispersion forces in loosely-bonded molecular solids, likely to be the main constituents of icy ocean worlds. This is of particular interest for these kinds of materials, since the complex sample handling, phase transitions and the difficulty of obtaining single crystals often greatly complicates the experimental determination of the full elastic tensor.We focus on CO2, C6H6, MgSO4·7H2O and CaSO4·2H2O as they allow us to benchmark the performance over a wide range of chemical space, structural topologies, crystal symmetries and bonding types, and moreover have accurate experimentally determined unit-cell dimensions, bulk moduli and full elastic tensors for benchmarking purposes.We demonstrate that the dispersion corrected approaches indeed perform better in modelling the experimental density profiles (mean unsigned differences of only 0.04 g/cm3 (CO2), 0.02 g/cm3 (C6H6), 0.003 g/cm3 (MgSO4·7H2O) and 0.013 g/cm3 (CaSO4·2H2O)) and may find application in exploring the compressive parameters of candidate materials, which could then be used in rheological models of icy ocean worlds.Moreover, we have assessed if the elastic constants computed by dispersion corrected density functional theory are accurate enough to be used in a reference data base for the seismic exploration of icy ocean worlds. Despite one approach having demonstrated good accuracy compared with the experimental values in modelling the elasticity of CO2, we instead find average differences from expected P and S wave velocities of around 10 to 25% for the elastically more complex title compounds. In part these differences are due to the large temperature difference between the experimental elasticity data (typically near 300 K) and our calculations, which were performed in the athermal limit.

First principles study of dense and metallic nitric sulfur hydrides

Studies of molecular mixtures containing hydrogen sulfide (H2S) could open up new routes towards hydrogen-rich high-temperature superconductors under pressure. H2S and ammonia (NH3) form hydrogen-bonded molecular mixtures at ambient conditions, but their phase behavior and propensity towards mixing under pressure is not well understood. Here, we show stable phases in the H2S–NH3 system under extreme pressure conditions to 4 Mbar from first-principles crystal structure prediction methods. We identify four stable compositions, two of which, (H2S) (NH3) and (H2S) (NH3)4, are stable in a sequence of structures to the Mbar regime. A re-entrant stabilization of (H2S) (NH3)4 above 300 GPa is driven by a marked reversal of sulfur-hydrogen chemistry. Several stable phases exhibit metallic character. Electron–phonon coupling calculations predict superconducting temperatures up to 50 K, in the Cmma phase of (H2S) (NH3) at 150 GPa. The present findings shed light on how sulfur hydride bonding and superconductivity are affected in molecular mixtures. They also suggest a reservoir for hydrogen sulfide in the upper mantle regions of icy planets in a potentially metallic mixture, which could have implications for their magnetic field formation.

Homogeneous model for the TRAPPIST-1e planet with an icy layer

In this work, we investigate whether a multilayered planet can be approximated as a homogeneous planet, and in particular how well the dissipation rate of a multilayered planet can be reproduced with a homogeneous rheology. We study the case of a stratified body with an icy crust that, according to recent studies, displays a double peak feature in the tidal response that cannot be reproduced with a homogeneous planet with an Andrade rheology. We revisit the problem with a slightly more complex rheology for the homogeneous body, the Sundberg–Cooper rheology, which naturally has a double peak feature, and apply the model to the TRAPPIST-1e planet. Our results compare very well with the results obtained when employing a multilayered model, showing that it is possible to approximate the behavior of a multilayer icy planet with a homogeneous planet using the Sundberg–Cooper rheology. This highlights the fact that we do not need the complexity of the multilayer planet model in order to estimate the tidal dissipation of an icy planet.

Tardigrade Survival Limits in High-Speed Impacts-Implications for Panspermia and Collection of Samples from Plumes Emitted by Ice Worlds.

The ability of tardigrades to survive impact shocks in the kilometer per second and gigapascal range was investigated. When rocks impact planetary surfaces, the impact speeds and shock pressures are in the kilometer per second and gigapascal range. This investigation tested whether tardigrades can survive in impacts typical of those that occur naturally in the Solar System. We found that they can survive impacts up to 0.9 km s−1, which is equivalent to 1.14 GPa shock pressure, but cannot survive impacts above this. This is significantly less than the static pressure limit and has implications for tardigrade survival in panspermia models. The potential survival of tardigrades in impacts of terrestrial impact ejecta on the Moon is shown to be impossible for the average lunar impact speed of such ejecta. However, a notable fraction (around 40%) of such ejecta impact at vertical speeds low enough to permit survival. Similarly, martian impact ejecta striking Phobos, for example, at a typical impact speed will not permit viable transfer of tardigrade-like organisms, but if a fraction of such material had a lower impact speed, survival may be possible. We also consider the implications of this for the collection of viable samples by spacecraft transiting the plumes of icy water worlds such as Europa and Enceladus. We have found the limit on survival of shocks to be around 1 GPa, which is instrumental in determining appropriate mission scenarios and collection methods for the acquisition of viable materials.

Size controls on the crossover from normal to self-inhibited sintering of ice spheres

Anomalies in the sintering of ice spheres were observed in previous studies as a growth of protrusions that evolve into porous bonds which eventually retreat. These anomalies were discussed in terms of Mullins-Sekerka theory which predicts the self-amplified growth of small perturbations. This interpretation would imply that the anomalous (mechanically weakening) sintering should crossover to normal (mechanically strengthening) sintering if the sphere size is reduced below a critical size. Here we show that this is indeed the case with sintering experiments in random sphere packs. Six samples, varying in particle diameter from 1 to 2.3 mm were sintered over ten days at −10 °C and then imaged by 3D micro-computed tomography. For two additional samples with small (1 mm) and large (2.1 mm) particles, we monitored the sintering evolution. Surface and bond properties were quantified by image analyses, which revealed solidly sintered bonds in small, and porous bonds in large spheres. Microstructure-based finite element simulations of the temperature field revealed higher temperature gradients between the particles in the bond region, as a driver for the protrusion growth. The temperature gradients, as well as the bond porosity, decreased after an initial increase up to five days and signal the retreat of the bonds. Our work details microstructural controls on anomalous sintering in ice as a self-inhibited bond growth process if only slight deviations from isothermal conditions occur. Besides the insight into fundamental sintering mechanisms, the results may have practical consequences for piste preparation, food processing, or research on icy planetary bodies.

Single‐Crystal Elasticity of High‐Pressure Ice up to 98 GPa by Brillouin Scattering

AbstractKnowledge of the elasticity of high‐pressure ice is key to constructing the internal structure of various planetary bodies. Here, we present direct experimental constraints on the single‐crystal elasticity of ice with 0.5 mol/L (0.9 mol.%) NaCl up to 98 GPa at 300 K. The change from ice‐VII to the dynamically disordered ice‐VII is characterized by an obvious softening of the longitudinal and bulk moduli between 42.4 and 54 GPa. Further transition to ice X at ∼64 GPa results in an apparent change in the pressure dependence for the elasticity. Using the obtained elasticity, our modeling reveals that the boundary between the ice‐VII layer and the silicate mantle or core in moderate‐size icy planetary bodies would be associated with a sudden velocity jump. In large planetary bodies, the change from ice X to the silicate layer will be characterized by an abrupt reduction in velocity.

Quantifying misidentification rates of Europa's microfeatures in low-resolution Galileo imaging

Europa is a compelling target because of its unique geology and potential for habitability. Efforts to map the small-scale global geology of the moon have been hampered by the availability of suitable imaging (≤ 230 m/pix), which covers only 15% of the surface. New data on the abundance of microfeatures (features ≤100 km2 in area) on a global scale and their locations would help put better constraints on the geophysics of the icy world. We mapped microfeatures in low-resolution images of the E15RegMap01 region of Europa and compared our low-resolution dataset to a validated dataset collected using regional mosaic imaging. We evaluated how many features were missed in low-resolution mapping, the feature types to which they belonged, how often misclassifications were made, and how many false-positive features there were in the low-resolution dataset. We found that microchaos, hybrids, and spots were found at relatively high completeness rates when compared to the regional mosaic dataset, but domes and especially pits were almost always missed. Microchaos, hybrids, and spots also had the highest rates of consistent classification between the low-resolution and regional mosaic datasets. Finally, 36% of our total potential feature count from the low-resolution dataset were revealed to be false positives, mostly caused by ridges and shadows that were mistaken for features in low-resolution imaging. These findings quantitatively estimate the likely errors in existing and future global geologic maps that note the presence of microfeatures, offer remedies to minimize these errors, and provide guidance for future Europa mappers.

Diamond formation from methane hydrate under the internal conditions of giant icy planets

Hydrocarbon chemistry in the C–O–H system at high pressure and high temperature is important for modelling the internal structure and evolution of giant icy planets, such as Uranus and Neptune, as their interiors are thought to be mainly composed of water and methane. In particular, the formation of diamond from the simplest hydrocarbon, i.e., methane, under the internal conditions of these planets has been discussed for nearly 40 years. Here, we demonstrate the formation of diamond from methane hydrate up to 3800 K and 45 GPa using a CO2 laser-heated diamond anvil cell combined with synchrotron X-ray diffraction, Raman spectroscopy, and scanning electron microscopy observations. The results show that the process of dissociation and polymerisation of methane molecules to produce heavier hydrocarbons while releasing hydrogen to ultimately form diamond proceeds at milder temperatures (~ 1600 K) and pressures (13–45 GPa) in the C–O–H system than in the C–H system due to the influence of water. Our findings suggest that diamond formation can also occur in the upper parts of the icy mantles of giant icy planets.

Subzero, saline incubations of Colwellia psychrerythraea reveal strategies and biomarkers for sustained life in extreme icy environments.

Colwellia psychrerythraea is a marine psychrophilic bacterium known for its remarkable ability to maintain activity during long-term exposure to extreme subzero temperatures and correspondingly high salinities in sea ice. These microorganisms must have adaptations to both high salinity and low temperature to survive, be metabolically active, or grow in the ice. Here, we report on an experimental design that allowed us to monitor culturability, cell abundance, activity and proteomic signatures of C. psychrerythraea strain 34H (Cp34H) in subzero brines and supercooled sea water through long-term incubations under eight conditions with varying subzero temperatures, salinities and nutrient additions. Shotgun proteomics found novel metabolic strategies used to maintain culturability in response to each independent experimental variable, particularly in pathways regulating carbon, nitrogen and fatty acid metabolism. Statistical analysis of abundances of proteins uniquely identified in isolated conditions provide metabolism-specific protein biosignatures indicative of growth or survival in either increased salinity, decreased temperature, or nutrient limitation. Additionally, to aid in the search for extant life on other icy worlds, analysis of detected short peptides in -10°C incubations after 4 months identified over 500 potential biosignatures that could indicate the presence of terrestrial-like cold-active or halophilic metabolisms on other icy worlds.

ALMA Observations of the Asymmetric Dust Disk around DM Tau

Abstract We report an analysis of the dust disk around DM Tau, newly observed with the Atacama Large Millimeter/submillimeter Array (ALMA) at 1.3 mm. The ALMA observations with high sensitivity (8.4 μJy beam–1) and high angular resolution (35 mas, 5.1 au) detect two asymmetries on the ring at r ∼ 20 au. They could be two vortices in early evolution, the destruction of a large-scale vortex, or double continuum emission peaks with different dust sizes. We also found millimeter emissions with ∼50 μJy (a lower-limit dust mass of 0.3 M Moon) inside the 3 au ring. To characterize these emissions, we modeled the spectral energy distribution (SED) of DM Tau using a Monte Carlo radiative transfer code. We found that an additional ring at r = 1 au could explain both the DM Tau SED and the central point source. The disk midplane temperature at the 1 au ring calculated in our modeling is less than the typical water sublimation temperature of 150 K, prompting the possibility of forming small icy planets there.

Fluid-like elastic response of superionic NH3 in Uranus and Neptune

Significance H 2 O and NH 3 are representative materials comprising the mantle of ice giant planets. While H 2 O probably exists as the superionic state, in which hydrogen rapidly diffuses through oxygen sublattices, in the deep mantle of Uranus and Neptune, the stability field and physical properties of the superionic phase of NH 3 are poorly understood. Our Brillouin study for NH 3 suggested a new superionic phase exhibiting a longitudinal wave velocity comparable to that of the liquid, which is stable in the mantle of Uranus and Neptune at pressures higher than ∼50 GPa. We suggest that the superionic NH 3 is nonviscous and thus cannot contribute to the internal stratification responsible for generation of the nondipolar magnetic fields observed at the icy planets.

Superionic iron oxide–hydroxide in Earth’s deep mantle

Water ice becomes a superionic phase under the high pressure and temperature conditions of deep planetary interiors of ice planets such as Neptune and Uranus, which affects interior structures and generates magnetic fields. The solid Earth, however, contains only hydrous minerals with a negligible amount of ice. Here we combine high pressure and temperature electrical conductivity experiments, Raman spectroscopy and first-principles simulations to investigate the state of hydrogen in the pyrite-type FeO2Hx (x ≤ 1), which is a potential H-bearing phase near the core–mantle boundary. We find that when the pressure increases beyond 73 GPa at room temperature, symmetric hydroxyl bonds are softened and the H+ (or proton) becomes diffusive within the vicinity of its crystallographic site. Increasing temperature under pressure, the diffusivity of hydrogen is extended beyond the individual unit cell to cover the entire solid, and the electrical conductivity soars, indicating a transition to the superionic state, which is characterized by freely moving protons and a solid FeO2 lattice. The highly diffusive hydrogen provides fresh transport mechanisms for charge and mass, which dictate the geophysical behaviours of electrical conductivity and magnetism, as well as geochemical processes of redox, hydrogen circulation and hydrogen isotopic mixing in Earth’s deep mantle.

Characterizing the ice-ocean interface of icy worlds: A theoretical approach

We investigate the structure and evolution of multiphase ice-ocean interfaces (‘mushy layers’) and the implications for the geophysics and habitability of ice-ocean worlds. Understanding the potential diversity of these multiphase layers across solar system bodies provides insight into the potential rates and mechanisms of heat and solute transport between their respective oceans and ice shells - which remain largely unconstrained. Additionally, variations in mushy layer properties may drive diverse geophysical processes unique to individual bodies or that may vary regionally on an individual icy world. We explore mushy layer evolution by analytically solving for the thickness of a simplified ice-ocean mushy layer system. We investigate two dynamic regimes, one driven by molecular diffusion and one driven by convection of brine within the mushy layer. We analyze the impact of gravity, thermal gradient, and ocean composition on the thickness of mushy layers. Additionally, a perturbation analysis is carried out to investigate the existence of mushy layer steady states. We show that stable mushy layers exist when ice shells are thickening, suggesting that mushy layers are likely persistent and common features of growing ice shells and accretionary regions of ice-ocean worlds.

Ionic Phases of Ammonia-Rich Hydrate at High Densities

Mixtures of ammonia and water are major components of the "hot ice" mantle regions of icy planets. The ammonia-rich ammonia hemihydrate (AHH) plays a pivotal role as it precipitates from water-rich mixtures under pressure. It has been predicted to form ionic high-pressure structures, with fully disintegrated water molecules. Utilizing Raman spectroscopy measurements up to 123GPa and first-principles calculations, we report the spontaneous ionization of AHH under compression. Spectroscopic measurements reveal that molecular AHH begins to transform into an ionic state at 26GPa and then above ∼69 GPa transforms into the fully ionic P3[over ¯]m1 phase, AHH-III, characterized as ammonium oxide (NH_{4}^{+})_{2}O^{2-}.