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Register a new userMicrometer-sized ice particles for planetary-science experiments - I. Preparation, critical rolling friction force, and specific surface energy
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Coagulation models assume a higher sticking threshold for micrometer-sized ice particles than for micrometer-sized silicate particles. However, in contrast to silicates, laboratory investigations of the collision properties of micrometer-sized ice particles (in particular, of the most abundant water ice) have not been conducted yet. Thus, we used two different experimental methods to produce micrometer-sized water ice particles, i. e. by spraying water droplets into liquid nitrogen and by spraying water droplets into a cold nitrogen atmosphere. The mean particle radii of the ice particles produced with these experimental methods are $(1.49 pm 0.79) , mathrm{mu m}$ and $(1.45 pm 0.65) , mathrm{mu m}$. Ice aggregates composed of the micrometer-sized ice particles are highly porous (volume filling factor: $phi = 0.11 pm 0.01$) or rather compact (volume filling factor: $phi = 0.72 pm 0.04$), depending on the method of production. Furthermore, the critical rolling friction force of $F_{Roll,ice}=(114.8 pm 23.8) times 10^{-10}, mathrm{N}$ was measured for micrometer-sized ice particles, which exceeds the critical rolling friction force of micrometer-sized $mathrm{SiO_2}$ particles ($F_{Roll,SiO_2}=(12.1 pm 3.6) times 10^{-10}, mathrm{N}$). This result implies that the adhesive bonding between micrometer-sized ice particles is stronger than the bonding strength between $mathrm{SiO_2}$ particles. An estimation of the specific surface energy of micrometer-sized ice particles, derived from the measured critical rolling friction forces and the surface energy of micrometer-sized $mathrm{SiO_2}$ particles, results in $gamma_{ice} = 0.190 , mathrm{J , m^{-2}}$.
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Models and observations suggest that ice-particle aggregation at and beyond the snowline dominates the earliest stages of planet-formation, which therefore is subject to many laboratory studies. However, the pressure-temperature gradients in proto-planetary disks mean that the ices are constantly processed, undergoing phase changes between different solid phases and the gas phase. Open questions remain as to whether the properties of the icy particles themselves dictate collision outcomes and therefore how effectively collision experiments reproduce conditions in pro- toplanetary environments. Previous experiments often yielded apparently contradictory results on collision outcomes, only agreeing in a temperature dependence setting in above $approx$ 210 K. By exploiting the unique capabilities of the NIMROD neutron scattering instrument, we characterized the bulk and surface structure of icy particles used in collision experiments, and studied how these structures alter as a function of temperature at a constant pressure of around 30 mbar. Our icy grains, formed under liquid nitrogen, undergo changes in the crystalline ice-phase, sublimation, sintering and surface pre-melting as they are heated from 103 to 247 K. An increase in the thickness of the diffuse surface layer from $approx$ 10 to $approx$ 30 {AA} ($approx$ 2.5 to 12 bilayers) proves increased molecular mobility at temperatures above $approx$ 210 K. As none of the other changes tie-in with the temperature trends in collisional outcomes, we conclude that the surface pre-melting phenomenon plays a key role in collision experiments at these temperatures. Consequently, the pressure-temperature environment, may have a larger influence on collision outcomes than previously thought.
Water ice is one of the most abundant materials in dense molecular clouds and in the outer reaches of protoplanetary disks. In contrast to other materials (e.g., silicates) water ice is assumed to be stickier due to its higher specific surface energy, leading to faster or more efficient growth in mutual collisions. However, experiments investigating the stickiness of water ice have been scarce, particularly in the astrophysically relevant micrometer-size region and at low temperatures. In this work, we present an experimental setup to grow aggregates composed of $mathrm{mu}$m-sized water-ice particles, which we used to measure the sticking and erosion thresholds of the ice particles at different temperatures between $114 , mathrm{K}$ and $260 , mathrm{K}$. We show with our experiments that for low temperatures (below $sim 210 , mathrm{K}$), $mathrm{mu}$m-sized water-ice particles stick below a threshold velocity of $9.6 , mathrm{m , s^{-1}}$, which is approximately ten times higher than the sticking threshold of $mathrm{mu}$m-sized silica particles. Furthermore, erosion of the grown ice aggregates is observed for velocities above $15.3 , mathrm{m , s^{-1}}$. A comparison of the experimentally derived sticking threshold with model predictions is performed to determine important material properties of water ice, i.e., the specific surface energy and the viscous relaxation time. Our experimental results indicate that the presence of water ice in the outer reaches of protoplanetary disks can enhance the growth of planetesimals by direct sticking of particles.
An atom moving in a vacuum at constant velocity and parallel to a surface experiences a frictional force induced by the dissipative interaction with the quantum fluctuations of the electromagnetic field. We show that the combination of nonequilibrium dynamics, anomalous Doppler effect and spin-momentum locking of light mediates an intriguing interplay between the atoms translational and rotational motion. In turn, this deeply affects the drag force in a way that is reminiscent of classical rolling friction. Our fully non-Markovian and nonequilibrium description reveals counterintuitive features characterizing the atoms velocity-dependent rotational dynamics. These results prompt interesting directions for tuning the interaction and for investigating nonequilibrium dynamics as well as the properties of confined light.
The geophysics of extrasolar planets is a scientific topic often regarded as standing largely beyond the reach of near-term observations. This reality in no way diminishes the central role of geophysical phenomena in shaping planetary outcomes, from formation, to thermal and chemical evolution, to numerous issues of surface and near-surface habitability. We emphasize that for a balanced understanding of extrasolar planets, it is important to look beyond the natural biases of current observing tools, and actively seek unique pathways to understand exoplanet interiors as best as possible during the long interim prior to a time when internal components are more directly accessible. Such pathways include but are not limited to: (a) enhanced theoretical and numerical modeling, (b) laboratory research on critical material properties, (c) measurement of geophysical properties by indirect inference from imprints left on atmospheric and orbital properties, and (d) the purpose-driven use of Solar System object exploration expressly for its value in comparative planetology toward exoplanet-analogs. Breaking down barriers that envision local Solar System exploration, including the study of Earths own deep interior, as separate from and in financial competition with extrasolar planet research, may greatly improve the rate of needed scientific progress for exoplanet geophysics. As the number of known rocky and icy exoplanets grows in the years ahead, we expect demand for expertise in exogeoscience will expand at a commensurately intense pace. We highlight key topics, including: how water oceans below ice shells may dominate the total habitability of our galaxy by volume, how free-floating nomad planets may often attain habitable subsurface oceans supported by radionuclide decay, and how deep interiors may critically interact with atmospheric mass loss via dynamo-driven magnetic fields.
Planetary science beyond the boundaries of our Solar System is today in its infancy. Until a couple of decades ago, the detailed investigation of the planetary properties was restricted to objects orbiting inside the Kuiper Belt. Today, we cannot ignore that the number of known planets has increased by two orders of magnitude nor that these planets resemble anything but the objects present in our own Solar System. Whether this fact is the result of a selection bias induced by the kind of techniques used to discover new planets -mainly radial velocity and transit - or simply the proof that the Solar System is a rarity in the Milky Way, we do not know yet. What is clear, though, is that the Solar System has failed to be the paradigm not only in our Galaxy but even just in the solar neighbourhood. This finding, although unsettling, forces us to reconsider our knowledge of planets under a different light and perhaps question a few of the theoretical pillars on which we base our current understanding. The next decade will be critical to advance in what we should perhaps call Galactic planetary science. In this paper, we review highlights and pitfalls of our current knowledge of this topic and elaborate on how this knowledge might arguably evolve in the next decade.More critically, we identify what should be the mandatory scientific and technical steps to be taken in this fascinating journey of remote exploration of planets in our Galaxy.
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