Icy Planets Research Articles

The anatomy of fresh complex craters on the mid‐sized icy moons of Saturn and self‐secondary cratering at the rayed crater Inktomi (Rhea)

AbstractCassini mapping of Saturn’s mid‐sized icy moons of well‐preserved complex craters in the 45–95 km size range provides insight into cratering processes at lower surface gravity and on icy targets. These craters are characterized by steep rim scarps, rugged hummocky floor deposits of curvilinear ridges and scarps, and rugged conical central peaks. Ponded impact melt or related deposits are not observed on any floor or ejecta units of any of these complex craters, indicating that melt production may be much lower than predicted. Mantling ejecta units drape over pre‐existing topography, grading into concentric zones of secondary craters at ~1 crater diameter from the rim, demonstrating that secondaries occur on mid‐sized icy planetary bodies. The largest and youngest bright‐ray system is the 49 km central peak crater Inktomi, target of a mapping campaign down to 34 m pixel scales. In addition to classical secondary craters up to 3 km, several hundred small craters <1 km in size form an unusual densely spaced cluster across the eastern floor and ejecta deposit. The terrain‐indiscriminate distribution of these eastern floor craters indicates they are not explosive volatile release pits, such as mapped on Vesta or Mars, but self‐secondary craters formed by the fallback of ejected blocks back into the crater itself during crater formation. Self‐secondaries likely formed by an irregular and stochastic but poorly understood impact process, such as spallation influenced by irregular surface topography. Self‐secondary craters of this type could influence the interpretation of crater counts on large fresh impact craters and basins elsewhere.

Stress-activated electric currents in icy planetary bodies: H2O2-doped H2O ices

Planetary satellites such as the Jovian moon Europa and several Kuiper Belt objects (including Pluto) have surfaces consisting largely of H2O ice, which is brittle and behaves as a rock at the low temperatures that prevail in the outer Solar System. Several of those ice-crusted bodies show evidence of tectonic activity indicating high levels of stress. This paper reports on laboratory experiments with pure H2O ice and H2O2–doped H2O ices with H2O2 concentration levels comparable to those in the ices of Europa. Elongated rectangular ice blocks at T = −81 °C [192 K] were stressed at one end to test whether electric currents are generated and capable of flowing down the stress gradient. Pure H2O ice was found to not produce currents above the 10−10 A background level except for occasional transients in the 10−9–10−8 A range during fracture or rapid plastic deformation due to electrons, e’. By contrast, stressing H2O2-doped H2O ices consistently produced electric currents in the 10−7–10−5 A range, due to holes, h•, propagating from the stressed end to the unstressed end. The h• charge carriers are generated by the break-up of peroxy bonds of H2O2 molecules, leading to O−, equivalent to defect electrons or holes h• in the O2− matrix.

Emissivity and reflectance spectra at different temperatures of hydrated and anhydrous sulphates: A contribution to investigate the composition and dynamic of icy planetary bodies

Accurate analyses of existing spacecraft data and telescopic observations are of fundamental importance to describe in detail the surface composition of icy planetary bodies, such as the icy galilean moons. However, the spectral library data to compare the remote data with planetary observations are usually restricted to small spectral ranges and collected only at room temperature.In this study, selected hydrated Mg-sulphates were studied. Emissivity and reflectance spectra were collected in the 3–20 μm and 0.25–16 μm range, respectively, with emissivity collected in the temperature range 300–673 K, and reflectance from 300 to 193 K. All samples were recovered after the heating and cooling cycles and were characterized by means X-ray powder diffraction. Rietveld refinements of the collected data were performed to evaluate the mineralogical composition of the samples before and after the thermal treatment.Both reflectance and emissivity measurements gave us information about the vibrational modes and overtones of SO4 and H2O. Moreover, the careful analysis of the collected data allowed us to study the influence of the cation substitution in the sulphate's crystal structures on the wavelength position of the SO4 vibrational modes. In particular, in simple salts [kieserite MgSO4·(H2O); hexahydrite MgSO4·6(H2O); gypsum CaSO4·2(H2O); thenardite Na2SO4; arcanite K2SO4; anhydrite CaSO4; barite BaSO4], the increase of the cation's radius gives a shift of the ν3 overtone towards higher wavenumbers, varying in the range 1880–2300 cm−1. On the other hand, it was observed that, in hydrated sulphates, that the increase of the strength of the hydrogen bond gives a shift of the ν3(SO4) overtones towards lower wavenumbers.At high temperature, the depth of several absorption bands in the emissivity spectra increases; however, when total or partial de-hydration occurs, a discontinuity in the deepening is observed due to the endothermic character of the dehydration phenomena.Among the investigated samples, several hydrated and anhydrous sulphates undergo de-hydration and/or phase transition at specific temperature conditions. This leads to the stabilization of new crystal structures with higher density compared to the low temperature hydrated ones. The likely occurrence of minerals dehydration will strongly affect the availability of free water in planetary depths and, as a consequence, the thickness of the icy crust. Likewise, the density changes between the different polymorphs will affect the buoyancy. This means that the structural behavior of the “non-icy” components of the icy crust have a significant impact on the structure and dynamics of the planetary bodies and have to be considered in planetological models.

Reactivity of Cyanide at Water-Ice Interfaces: Implications for the Search for Organics on Icy Worlds

Icy moons in our solar system are targets of interest to search for signs of life and habitability beyond Earth. They are the astrobiological aim for several developing space missions (JUICE, Europa Clipper and Europa Lander). A further understanding of the processes that force a planetary object to develop a biological system and to develop habitability conditions could be obtained by adopting a prebiotic chemistry point of view. In this regard, it is highly interesting to search for organic compounds synthesized from cyanide since it has been proposed that polymerization of NH4CN would be applicable to icy planetary environments. Thus, the effect of water freeze-thaw cycles, UV-radiation and salts on the polymerization of NH4CN was explored as an approach to understand the likely rich organic chemistry of icy worlds. As a result, insoluble and soluble NH4CN polymers, synthesized under multiple conditions, were analyzed by GC-MS. A diverse set of amino acids, carboxylic acids and several N-heterocycles were identified. Glyoxylic acid was detected under particular reaction conditions, extending the plausible favorable conditions for the emergence of this molecule, referred to as the “glyoxylate scenario”. In addition, the GC-MS results were studied by multivariate analysis, showing that the ice-water interfaces may be ideal places to develop complex organic chemistry from a carbon source as simple as cyanide.

Exploration of Extreme Environments with Currentand Emerging Robot Systems

The discovery of living organisms under extreme environmental conditions of pressure, temperature, and chemical composition on Earth has opened up the possibility of existence and persistence of life in extreme environment pockets across the solar system. These environments range from the many intriguing moons, to the deep atmospheres of Venus and even the giant gas planets, to the small icy worlds of comets and Kuiper Belt Objects (KBOs). Exploring these environments can ascertain the range of conditions that can support life and can also identify planetary processes that are responsible for generating and sustaining habitable worlds. These environments are also time capsules into early formation of the solar system and will provide vital clues of how our early solar system gave way to the current planets and moons. Over the last few decades, numerous missions started with flyby spacecraft, followed by orbiting satellites and missions with orbiter/lander capabilities. Since then, there have been numerous missions that have utilized rovers of ever-increasing size and complexity, equipped with state-of-the-art laboratories on wheels. Although current generations of rovers achieve mobility through wheels, there are fundamental limitations that prevent these rovers from accessing rugged environments, cliffs, canyons, and caves. These rugged environments are often the first places geologist look to observe stratification from geohistorical processes. There is an important need for new robot mobility solutions, like hopping, rolling, crawling, and walking that can access these rugged environments like cliffs, canyons, and caves. These new generations of rovers have some extraordinary capabilities including being able to grip onto rocks like NASA/JPL LEMUR 2, operate in swarms such as MIT’s microbots, or have high-specific energy fuel cell power supply that is approximately 40-fold higher than conventional lithium ion batteries to Stanford/NASA JPL’s Hedgehog which is able to hop and somersault in low-gravity environments such asteroids. All of these mobility options and supporting technologies have been proposed and developed to explore these hard-to-reach unconventional environments. This article provides a review of the robotic systems developed over the past few decades, in addition to new state-of-the-art concepts that are leading contenders for future missions to explore extreme environments on Earth and off-world.

Towards additively manufacturing excavating tools for future robotic space exploration

AbstractMultiple metal alloys, that is, Ti‐6Al‐4 V, 316 L stainless steel, MS1 maraging steel, A2 tool steel, Inconel 625 with TiC and TiB2 reinforcement, and AMZ4 bulk metallic glass, were additively manufactured through laser powder bed fusion and tested as potential excavating tools for future robotic spacecraft landing on icy planetary bodies. Mechanical specific energy as a function of blade hardness was measured for each excavating tool as it trenched through soft and hard salt, where the salt is a regolith simulant for extraterrestrial ice. A2 tool steel, MS1 maraging steel, and bulk metallic glass cutting tools were shown to perform well in the experiments. A method for using the cutting tool as a sensor was also demonstrated.

Relict Ocean Worlds: Ceres

The aim of this chapter is to describe available evidence for the existence of relict ocean worlds. The focus is on Ceres as a clear example of such a world as indicated by the results from the Dawn mission. The sections of this chapter will also reflect the differences and commonalities with other recognized ocean worlds. We focus on Ceres as an example of relict ocean world and the implications it holds for the broader solar system. The presence of a large amount of water in the outer Solar System, the presence of low eutectic salts, and the similar pressure and temperature regimes at depth in icy moons and other dwarf planets suggest that many of these large icy worlds harbor or harbored deep oceans. Ceres is an example of relict Ocean world.

3-D Printed Multifunctional Sensor Platform for Space Applications

Nanomaterials offer a unique set of advantages as sensors for space applications. For instance, one and two dimensional materials such as carbon nanotube, graphene and molybdenum disulfide have high surface-to-volume ratio and superior electrical properties, resulting in highly sensitive environmental sensors. In addition, great mechanical, thermal, and electrical stability, as well as radiation hardness due to minute cross-sectional area, make them useful in space applications. Furthermore, lightweight, low power consumption and small size make nanomaterial-based devices suitable for targeted space missions with limited resources. Nanomaterial-based environmental sensors can be used for the in situ detection of neutrals, trace gases, volatile organics and environmental parameters in a wide range of mission architectures, including atmospheric probes, plume flybys on icy or ocean worlds such as Enceladus or Europa, and landed missions. For example, the multifunctional sensor platform can detect trace gases that can fingerprint various biological and abiotic processes on outer planetary bodies. It can be used to simultaneously detect methane, ammonia and other gases difficult to detect using mass spectrometry due to mass interference issues, and thus can be used in tandem with mass spectrometers. In addition, such a platform can be used for quick screening of samples in sample return missions without adding significantly to the mass and power of the payload. Furthermore, Human exploration missions can benefit from these highly sensitive sensors to detect leaks of toxic gases that are commonly onboard such as hydrazine, ensuring human safety and mission success. This talk will discuss the development of the multifunctional sensor platform with a suite of environmental nanosensors fabricated with additive manufacturing techniques, the performance of the sensors and their applicability to space missions.

On the movement of the Chaplygin sleigh on a horizontal plane with dry friction

The study of dynamics, as well as the design of autonomous planetary rovers, is an urgent and important task. Recently, in the context of the development of robotic systems and mobile vehicles, the relevance of such systems has increased. Various rover designs are proposed and studied: wheeled, walking, jumping, etc., depending on the objectives of the study, the environment, the surface of the planet, etc. Reliability of such autonomous vehicles is the key element for the safety of planetary missions.This work is devoted to the study of the dynamics of a mechanical system called the Chaplygin sleigh, which is of independent interest, since it has a relatively simple mathematical model. At the same time, the proposed model is in some ways a simplification of a wide class of vehicles. The paper proposes a study of the influence of the Coulomb friction forces on the dynamics of such a system. A simple mathematical model makes it possible to qualitatively study the motion of such systems on surfaces under the condition of sliding of contact points. The results of the work can be useful for understanding the dynamics, as well as for the design of planetary rovers with a similar design, as well as for the design of planetary rovers moving on the surfaces of ice planets.

Coexistence of plastic and partially diffusive phases in a helium-methane compound.

Helium and methane are major components of giant icy planets and are abundant in the universe. However, helium is the most inert element in the periodic table and methane is one of the most hydrophobic molecules, thus whether they can react with each other is of fundamental importance. Here, our crystal structure searches and first-principles calculations predict that a He3CH4 compound is stable over a wide range of pressures from 55 to 155 GPa and a HeCH4 compound becomes stable around 105 GPa. As nice examples of pure van der Waals crystals, the insertion of helium atoms changes the original packing of pure methane molecules and also largely hinders the polymerization of methane at higher pressures. After analyzing the diffusive properties during the melting of He3CH4 at high pressure and high temperature, in addition to a plastic methane phase, we have discovered an unusual phase which exhibits coexistence of diffusive helium and plastic methane. In addition, the range of the diffusive behavior within the helium-methane phase diagram is found to be much narrower compared to that of previously predicted helium-water compounds. This may be due to the weaker van der Waals interactions between methane molecules compared to those in helium-water compounds, and that the helium-methane compound melts more easily.

Towards Determining Biosignature Retention in Icy World Plumes.

With the discovery of the persistent jets of water being ejected to space from Enceladus, an understanding of the effect of the space environment on potential organisms and biosignatures in them is necessary for planning life detection missions. We experimentally determine the survivability of microbial cells in liquid medium when ejected into vacuum. Epifluorescence microscopy, using a lipid stain, and SEM imaging were used to interrogate the cellular integrity of E. coli after ejected through a pressurized nozzle into a vacuum chamber. The experimental samples showed a 94% decrease in visible intact E. coli cells but showed a fluorescence residue in the shape of the sublimated droplets that indicated the presence of lipids. The differences in the experimental conditions versus those expected on Enceladus should not change the analog value because the process a sample would undergo when ejected into space was representative. E. coli was selected for testing although other cell types could vary physiologically which would affect their response to a vacuum environment. More testing is needed to determine the dynamic range in concentration of cells expected to survive the plume environment. However, these results suggest that lipids may be directly detectable evidence of life in icy world plumes.

Three Helium-Ammonia Compounds Found for Icy Planets

Three Helium-Ammonia Compounds Found for Icy Planets

Pluto's early oceans hint the icy world formed rapidly

Pluto's early oceans hint the icy world formed rapidly

Tides in subsurface oceans with meridional varying thickness

Tidal heating can play an important role in the formation and evolution of subsurface oceans of outer-planet moons. Up until now tidal heating has only been studied in subsurface oceans of spatially uniform thickness. We develop a numerical model to consider oceans of spatially variable thickness. We use the Laplace Tidal Equations for the ocean and model the ice shell using membrane theory. The problem is solved using the commercial Finite Element software Comsol Multiphysics®. We use this new model to study the tidal response of Enceladus' ocean with a twofold objective: to understand how ocean thickness variations modify the tidal response of a subsurface ocean and to assess if tidal dissipation in an Enceladan ocean with varying ocean thickness can explain the high heat flux emanating from Enceladus' South Polar Terrain and the perdurance of a subsurface ocean. We consider the effect of meridional ocean thickness changes of spherical harmonic degree two and three as suggested by topography and gravimetry data. We observe that an ocean with degree two topography responds with the same eigenmodes as an ocean of constant thickness but resonances occur for thicker oceans. However, resonant ocean thicknesses are still thin compared to current estimates for Enceladus ocean thickness. Rossby-Haurwitz waves, excited by the obliquity tide for thick oceans of constant thickness, are not excited at the tidal frequency when oceans of variable thickness are considered. This result implies that the role of the obliquity tide in ocean tidal-dissipation might have been overestimated for Enceladus and other icy worlds. An antisymmetric, degree-three ocean thickness variation mixes the ocean modes excited in a constant thickness ocean by the eccentricity and obliquity tide.

Plastic and superionic phases in ammonia–water mixtures at high pressures and temperatures

The interiors of giant icy planets depend on the properties of hot, dense mixtures of the molecular ices water, ammonia, and methane. Here, we discuss results from first-principles molecular dynamics simulations up to 500 GPa and 7000 K for four different ammonia–water mixtures that correspond to the stable stoichiometries found in solid ammonia hydrates. We show that all mixtures support the formation of plastic and superionic phases at elevated pressures and temperatures, before eventually melting into molecular or ionic liquids. All mixtures’ melting lines are found to be close to the isentropes of Uranus and Neptune. Through local structure analyses we trace and compare the evolution of chemical composition and longevity of chemical species across the thermally activated states. Under specific conditions we find that protons can be less mobile in the fluid state than in the (colder, solid) superionic regime.

Microbial Diversity and Biosignatures: An Icy Moons Perspective

The icy moons of the outer Solar System harbor potentially habitable environments for life, however, compared to the terrestrial biosphere, these environments are characterized by extremes in temperature, pressure, pH, and other physico-chemical conditions. Therefore, the search for life on these icy worlds is anchored on the study of terrestrial extreme environments (termed “analogue sites”), which harbor microorganisms at the frontiers of polyextremophily. These so-called extremophiles have been found in areas previously considered sterile: hot springs, hydrothermal vents, acidic or alkaline lakes, hypersaline environments, deep sea sediments, glaciers, and arid areas, amongst others. Such model systems and communities in extreme terrestrial environments may provide important information relevant to the astrobiology of icy bodies, including the composition of potential biological communities and the identification of biosignatures that they may produce.Extremophiles can use either sunlight (phototrophs) or chemical energy (chemotrophs) as energy sources, and different chemical compounds as electron donors or acceptors. Aerobic microorganisms use oxygen (O2) as a terminal electron acceptor, whereas anaerobic microorganisms may use nitrate (\(\mathrm{NO}_{3} ^{-}\)), sulfate (\(\mathrm{SO}_{4} ^{2-}\)), carbon dioxide (CO2), Fe(III), or other organic or inorganic molecules during respiration. The phylogenetic diversity of extremophiles is very high, leading to their broad dispersal across the phylogenetic tree of life together with a wide variety in metabolic diversity.Some metabolisms are specific to archaea, for example, methanogenesis, an anaerobic respiration during which methane (CH4) is produced. Also sulfur-reduction performed by some bacteria and archaea is considered as a primitive metabolism which is restricted to anoxic sulfur-rich habitats in nature.Methanogenesis and sulfur reduction are of specific interest for icy moon research as it might be one of the few known terrestrial metabolisms possible on these celestial bodies.Therefore, the adaptation of these intriguing microorganisms to extreme conditions will be highlighted within this review.

Large Ocean Worlds with High-Pressure Ices

Pressures in the hydrospheres of large ocean worlds extend to ranges exceeding those in Earth deepest oceans. In this regime, dense water ices and other high-pressure phases become thermodynamically stable and can influence planetary processes at a global scale. The presence of high-pressure ices sets large icy worlds apart from other smaller water-rich worlds and complicates their study. Here we provide an overview of the unique physical states, thermodynamics, dynamic regimes, and evolution scenarios specific to large ocean worlds where high-pressure ice polymorphs form. We start by (i) describing the current state of knowledge for the interior states of large icy worlds in our solar system (i.e. Ganymede, Titan and Callisto). Then we (ii) discuss the thermodynamic and physical specifics of the relevant high–pressure materials, including ices, aqueous fluids and hydrates. While doing this we (iii) describe the current state of the art in modeling and understanding the dynamic regimes of high-pressure ice mantles. Based on these considerations we (iv) explore the different evolution scenarios for large icy worlds in our solar system. We (v) conclude by discussing the implications of what we know on chemical transport from the silicate core, extrapolation to exoplanetary candidate ocean worlds, limitations to habitability, differentiation diversity, and perspectives for future space exploration missions and experimental measurements.

Survivability of Anhydrobiotic Cyanobacteria in Salty Ice: Implications for the Habitability of Icy Worlds.

Two anhydrobiotic strains of the cyanobacterium Chroococcidiopsis, namely CCMEE 029 and CCMEE 171, isolated from the Negev Desert in Israel and from the Dry Valleys in Antarctica, were exposed to salty-ice simulations. The aim of the experiment was to investigate the cyanobacterial capability to survive under sub-freezing temperatures in samples simulating the environment of icy worlds. The two strains were mixed with liquid solutions having sub-eutectic concentration of Na2SO4, MgSO4 and NaCl, then frozen down to different final temperatures (258 K, 233 K and 203 K) in various experimental runs. Both strains survived the exposure to 258 K in NaCl solution, probably as they migrated in the liquid veins between ice grain boundaries. However, they also survived at 258 K in Na2SO4 and MgSO4-salty-ice samples—that is, a temperature well below the eutectic temperature of the solutions, where liquid veins should not exist anymore. Moreover, both strains survived the exposure at 233 K in each salty-ice sample, with CCMEE 171 showing an enhanced survivability, whereas there were no survivors at 203 K. The survival limit at low temperature was further extended when both strains were exposed to 193 K as air-dried cells. The results suggest that vitrification might be a strategy for microbial life forms to survive in potentially habitable icy moons, for example in Europa’s icy crust. By entering a dried, frozen state, they could be transported from niches, which became non-habitable to new habitable ones, and possibly return to metabolic activity.

The familiarity of icy worlds

The familiarity of icy worlds

Electrochemistry and the Origin of Life

There is considerable international interest in the origin of life, both on Earth and icy worlds such as Europa, Enceladus, Ceres, and Titan. These ocean worlds may contain hydrothermal vents at the interfaces between their salty oceans and rocky cores and have been prioritized by NASA in the search for evidence of life in our solar system.1 Hydrothermal vents on Earth have been shown to produce voltage and current in the field2,3 and may be related to the origin of life.4 Electrochemistry certainly plays a role here, with hot reductants flowing from the crust and interacting with oceans through porous, partially conductive minerals. Understanding the natural systems that exist on Earth and may have contributed to the emergence of life, and determining similarities to hydrothermal vents on ocean worlds, is a critical step in exploring these unknown environments. We have attempted to model these systems using both cyclic voltammetry and fuel cell experiments,5 with results presented here. While there is a plethora of potential redox couples which could be exploited in nature at hydrothermal vents, one of the most active reductants is H2S, which is produced at many black smoker chimneys and reacts with various metals to generate compounds such as FeS and FeS2, both of which are known electroactive materials. Studies on these materials indicate that they may produce energy which could be directly used by simple organisms on the outside of the chimneys and in turn feed larger organisms. This chain of energy utilization exists without solar irradiation, thus giving credence to the hypothesis that life may exist at the bottom of the ocean on the icy moons around Jupiter and Saturn.6 ACKNOWLEDGEMENT The work described here was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration (NASA) supported by JPL’s Research and Technology Development program.