arXiv subjects

Gerhard Wurm

Publications and source records attributed to Gerhard Wurm.

At least 37 records · Page 2Linked to original sources

arXiv2020

Destruction of eccentric planetesimals by ram pressure and erosion

Small, pebble-sized objects and large bodies of planetesimal size both play important roles in planet formation. They form the evolutionary steps of dust growth in their own respect. However, at later times, they are also thought to provide background populations of mass that larger bodies might feed upon. What we suggest in this work is that starting at times of viscous stirring, planetesimals on eccentric orbits could simply explode as they become supersonic in comparison to small, porous planetary bodies entering Earth's atmosphere. We present a toy model of planetesimal motion and destruction to show the key aspects of this process. The consequences are quite severe. At all times, it is shown that only planetesimals on more or less circular orbits exist in the inner disk. After the destruction of a planetesimal, the remaining matter is continuously redistributed to the pebble reservoir of the protoplanetary disk. Since destruction typically occurs at small stellar distances due to supersonic speeds, it is expected to boost pebble accretion in the inner protoplanetary disk as one of its main effects.

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arXiv2020

Cosmic radiation does not prevent collisional charging in (Pre)-Planetary Atmospheres

In (pre)-planetary environments, dust and sand grains regularly collide. They electrically charge and discharge during these events. In this work, we study if cosmic radiation has any influence on the equilibrium charge state on timescales of minutes. We developed an experiment that was carried out during the ascent of a stratospheric balloon. With increasing altitude, the radiation activity increases by a factor of 54. However, we found only a very minor decrease in grain charges of up to 30%. This implies that charge-moderated processes from thunderstorms on Earth, over early phases of planet formation to particle motion on the Martian surface on short timescales essentially proceed unhindered from a direct influence of cosmic radiation.

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arXiv2020

Accretion of eroding pebbles and planetesimals in planetary envelopes

Wind erosion is a destructive mechanism that completely dissolves a weakly bound object like a planetesimal into its constituent particles, if the velocity relative to the ambient gas and the local gas pressure are sufficiently high. In numerical simulations we study the influence of such wind erosion on pebble and planetesimal accretion by a planetary body up to $10 R_\mathrm{Earth}$. Due to the rapid size reduction of an in-falling small body, the accretion outcome changes significantly. Erosion leads to a strong decrease in the accretion efficiency below a threshold size of the small body on the order of 10 m. This slows down pebble accretion significantly for a given size distribution of small bodies. The threshold radius of the small body increases with increasing planet radius and decreases with increasing semi-major axis. Within the parameters studied, an additional planetary atmosphere (up to 1 bar) is of minor importance.

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arXiv2020

Laboratory Impact Splash Experiments to Simulate Asteroid Surfaces

Granular material that is bound by the low gravity of a small asteroid is mobilized by slow velocity impacts. These splashes generated by impacts might play an important role in sculpting the asteroid's surface. In laboratory experiments we characterize the ejecta generated by spherical 150 $\rm μm$ diameter basalt grains impacting a granular bed at 0.8 m/s. We find that such an impact typically leads to less than 10 particles being ejected from the granular bed, with typical ejecta trajectories rising to less than one particle diameter above the surface. That is, the observed impacts are highly dissipative and only a small fraction of the impact energy is imparted onto the ejecta. While the impactor itself still rebounds, it typically slows down significantly to an average of about 20 % of its impact velocity. Scaled to asteroids, impactor and ejecta generated from impacts of sand sized grains are not able to spread over the asteroid's surface but will stay close to the impact site. Therefore these highly inelastic impacts into soft granular beds efficiently trap grains, in contrast to more elastic impacts on bare, rocky surfaces confirming suggestions by Shinbrot (2017). This is also in agreement to observed features on asteroids as this topological elasticity bias suggests that redistribution of grains leads to a size segregation.

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arXiv2020

Composition and Size Dependent Sorting in Preplanetary Growth: Seeding the Formation of Mercury-like Planets

In an earlier work, we found that large metallic iron fractions in dust aggregates and strong magnetic fields boost preplanetary growth. This sets an initial bias for the formation of Mercury-like planets in the inner part of protoplanetary disks. We extended these experiments here by adding pure quartz aggregates to the iron-rich aggregates. Magnetic boost still leads to the formation of larger clusters of aggregates. These clusters now include silicate aggregates, which can also be connecting bridges between chains. However, at least a certain fraction of iron-rich aggregates are needed to trigger magnetic boost. Without a magnetic field, the sticking properties of the aggregates and their constituents determine the composition of clusters of a given size. This introduces a new fractionation and sorting mechanism by cluster formation at the bouncing barrier.

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arXiv2020

Planetesimals in Rarefied Gas: Wind Erosion in Slip Flow

A planetesimal moves through the gas of its protoplanetary disc where it experiences a head wind. Though the ambient pressure is low, this wind can erode and ultimately destroy the planetesimal if the flow is strong enough. For the first time, we observe wind erosion in ground based and microgravity experiments at pressures relevant in protoplanetary discs, i.e. down to $10^{-1}\, \rm mbar$. We find that the required shear stress for erosion depends on the Knudsen number related to the grains at the surface. The critical shear stress to initiate erosion increases as particles become comparable to or larger than the mean free path of the gas molecules. This makes pebble pile planetesimals more stable at lower pressure. However, it does not save them as the experiments also show that the critical shear stress to initiate erosion is very low for sub-millimetre sized grains.

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arXiv2020

Lifting Grains by the Transient Low Pressure in a Martian Dust Devil

Lifting dust and sand into the thin Martian atmosphere is a challenging problem. Atmospheric pressure excursions within dust devils have been proposed to support lifting. We verify this idea in laboratory experiments. Pressure differences up to a few Pa are applied along particle layers of 50 to 400 $\rm μm$. As samples we used glass beads of $\sim$50 $\rm μm$ diameter and irregular basalt grains of $\sim$20 $\rm μm$ in size. The total ambient pressure of air was set to 600 Pa. Particles are ejected at pressure differences as low as 2.0 $\rm \pm$ 0.8 Pa. In the case of glass beads, the ejected grains returning to the particle bed can trigger new particle ejections as they reduce cohesion and release the tension from other grains. Therefore, few impacting grains might be sufficient to sustain dust lifting in a dust devil at even lower pressure differences. Particle lift requires a very thin, $\sim$100 $\rm μm$, low permeability particle layer on top of supporting ground with larger pore space. Assuming this, our experiments support the idea that pressure excursions in Martian dust devils release grains from the ground.

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arXiv2019

Laboratory Experiments on the Motion of Dense Dust Clouds

In laboratory experiments, we study the motion of levitated, sedimenting clouds of sub-mm grains at low ambient pressure and at high solid-to-gas ratios $ε$. The experiments show a collective behavior of particles, i.e. grains in clouds settle faster than an isolated grain. In collective particle clouds, the sedimentation velocity linearly depends on $ε$ and linearly depends on the particle closeness $C$. However, collective behavior only sets in at a critical value $ε_{\rm crit}$ which linearly increases with the experiment Stokes number St. For $\rm St <0.003 $ particles always behave collectively. For large Stokes numbers, large solid-to-gas ratios are needed to trigger collective behavior, e.g. $ε_{\rm crit} = 0.04$ at $\rm St = 0.01$. Applied to protoplanetary disks, particles in dense environments will settle faster. In balance with upward gas motions (turbulent diffusion, convection) the thickness of the midplane particle layer will be smaller than calculated based on individual grains, especially for dust. For pebbles, large solid-to-gas ratios are needed to trigger instabilities based on back-reaction.

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arXiv2019

ARISE: A granular matter experiment on the International Space Station

We developed an experiment to study different aspects of granular matter under microgravity. The 1.5U small experiment was carried out on the International Space Station. About 3500 almost identical spherical glass particles with 856 um diameter were placed in a container of 50*50 mm cross section. Adjusting the height between 5 and 50 mm, the filling factor can be varied. The sample was vibrated under different frequencies and amplitudes. The majority of the data are video images of the particles motion. Here, we first give an overview of the general setup and a first qualitative account of different phenomena observed in about 700 experimental runs. These phenomena include collisional cooling, collective motion via gas-cluster coupling, and the influence of electrostatic forces on particle-particle interactions.

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arXiv2019

Wind erosion on Mars and other small terrestrial planets

We carried out wind tunnel experiments on parabolic flights with 100 $μ$m Mojave Mars simulant sand. The experiments result in shear stress thresholds and erosion rates for varying g-levels at 600 Pa pressure. Our data confirm former results on JSC Mars 1A simulant where the threshold shear stress is lower under Martian gravity than extrapolated from earlier ground-based studies which fits observations of Martian sand activity. The data are consistent with a model by Shao and Lu (2000) and can also be applied to other small terrestrial (exo)-planets with low pressure atmospheres.

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arXiv2019

Onset of planet formation in the warm inner disk -- Colliding dust aggregates at high temperatures

Collisional growth of dust occurs in all regions of protoplanetary disks with certain materials dominating between various condensation lines. The sticking properties of the prevalent dust species depend on the specific temperatures. The inner disk is the realm of silicates spanning a wide range of temperatures from room temperature up to sublimation beyond $1500\,\mathrm{K}$. For the first time, we carried out laboratory collision experiments with hot levitated basalt dust aggregates of $1\, \rm mm$ in size. The aggregates are compact with a filling factor of $0.37 \pm 0.06$. The constituent grains have a wide size distribution that peaks at about $0.6\,μ\mathrm{m}$. Temperatures in the experiments are varied between approximately $600\,\mathrm{K}$ and $1100\,\mathrm{K}$. Collisions are slow with velocities between $0.002\,\mathrm{m}\,\mathrm{s}^{-1}$ and $0.15\,\mathrm{m}\,\mathrm{s}^{-1}$, i.e., relevant for protoplanetary disks. Aside from variations of the coefficients of restitution due to varying collision velocities, the experiments show low sticking probability below $900\,\mathrm{K}$ and an increasing sticking probability starting at $900\,\mathrm{K}$. This implies that dust can grow to larger size in hot regions, which might change planet formation. One scenario is an enhanced probability for local planetesimal formation. Another scenario is a reduction of planetesimal formation as larger grains are more readily removed as a consequence of radial drift. However, the increased growth at high temperatures likely changes planetesimal formation one way or the other.

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arXiv2019

Sticking Properties of Silicates in Planetesimal Formation Revisited

In the past, laboratory experiments and theoretical calculations showed a mismatch in derived sticking properties of silicates in the context of planetesimal formation. It has been proposed by Kimura et al. (2015) that this mismatch is due to the value of the surface energy assumed, supposedly correlated to the presence or lack of water layers of different thickness on a grain's surface. We present tensile strength measurements of dust aggregates with different water content here. The results are in support of the suggestion by Kimura et al. (2015). Dry samples show increased strengths by a factor of up to 10 over wet samples. A high value of $γ= 0.2 J/m^2$ likely applies to the dry low pressure conditions of protoplanetary disks and should be used in the future.

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arXiv2019

A challenge for Martian lightning: Limits of collisional charging at low pressure

Collisional charging is one potential initial step in generating lightning. In this work, we study the charging of colliding monodisperse, spherical basalt grains depending on ambient pressure. We used grains of 1.0 to 1.2 mm in one set and 2.0 to 2.4 mm in another set. We varied the ambient pressure between 0.03 mbar and 80 mbar. This especially includes Martian pressure being 6 mbar on average. At a few mbar the net charge gathering on colliding grains has a minimum. A smooth incline in charging occurs for larger pressures. Toward lower pressure the charge increases steeply. The pressure dependence is in agreement to a model where the maximum charge is limited by a gas discharge occurring between two charged colliding grains shortly after or before a collision. The capability of building up charge is at a minimum exactly in the range of Martian pressures. The charges on grains are at least a factor 5 smaller than at the highest pressure tested and still smaller compared to ambient pressure on Earth. This implies that on Mars collisional charging and the potential of subsequent generation of lightning or other large scale discharges are strongly reduced compared to Earth. This might result in less frequent and less energetic lightning on Mars.

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arXiv2019

Contacts of Water Ice in Protoplanetary Disks - Laboratory Experiments

Water ice is abundant in protoplanetary disks. Its sticking properties are therefore important during phases of collisional growth. In this work, we study the sticking and rolling of 1.1 mm ice grains at different temperatures. We find a strong increase in sticking between 175 K to 200 K which levels off at higher temperatures. In terms of surface energy this is an increase with a factor of 63.4, e.g. from $γ= 0.0029 \rm J/m^2$ to $γ= \rm 0.19 J/m^2$, respectively. We also measured critical forces for inelastic rolling. The critical rolling distance is constant with a value of 0.19 mm. In view of planetesimal formation at low temperatures in protoplanetary disks, the surface energy is not larger than for silicate dust and ice aggregation will share the same shortcommings. In general, water ice has no advantage over silicates for sticking and collisional growth might not favor ice over silicates.

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arXiv2019

Dense Particle Clouds in Laboratory Experiments in Context of Drafting and Streaming Instability

The streaming instability, as an example of instabilities driven by particle feedback on a gas flow, has been proven to have a major role in controlling the formation of planetesimals. Here, we present experiments to approach this situation in the laboratory for particles in the Knudsen flow regime. In these experiments, we observe a particle cloud trapped for about 30 s in a rotating system under Earth's gravity. For average dust-to-gas ratios up to 0.08, particles behave like individual test particles. Their sedimentation speed is identical to that of a single free-falling particle, even in locally denser regions. However, for higher dust-to-gas ratios, the motion of particles becomes sensitive to clumping. Particles in locally denser regions now sediment faster. Their sedimentation speed then depends linearly on the overall dust-to-gas ratio. This clearly shows a transition from tracerlike behavior to collective behavior. Beyond these findings, these types of experiments can now be used as a gauge to test the particle feedback models in astrophysical hydrocodes which are currently used for numerical simulations of streaming instabilities.

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arXiv2019

Are Pebble Pile Planetesimals Doomed?

In parabolic flight experiments we studied the wind induced erosion of granular beds composed of spherical glass beads at low gravity and low ambient pressure. Varying g-levels were set by centrifugal forces. Expanding existing parameter sets to a pressure range between $p=300-1200\,$Pa and to g-levels of $g=1.1-2.2\,\rm m\,s^{-2}$ erosion thresholds are still consistent with the existing model for wind erosion on planetary surfaces by Shao & Lu (2000). These parameters were the lowest values that could technically be reached by the experiment. The experiments decrease the necessary range of extrapolation of erosion thresholds from verified to currently still unknown values at the conditions of planetesimals in protoplanetary discs. We apply our results to the stability of planetesimals. In inner regions of protoplanetary discs, pebble pile planetesimals below a certain size are not stable but will be disassembled by a head wind.

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arXiv2018

Seeding the Formation of Mercurys: An Iron-sensitive Bouncing Barrier in Disk Magnetic Fields

The inner part of protoplanetary disks can be threaded by strong magnetic fields. In laboratory levitation experiments, we study how magnetic fields up to 7 mT influence the aggregation of dust by observing the self-consistent collisional evolution of particle ensembles. As dust samples we use mixtures of iron and quartz in different ratios. Without magnetic fields, particles in all samples grow into a bouncing barrier. These aggregates reversibly form larger clusters in the presence of magnetic fields. The size of these clusters depends on the strength of the magnetic field and the ratio between iron and quartz. The clustering increases the size of the largest entities by a factor of a few. If planetesimal formation is sensitive to the size of the largest aggregates, e.g., relying on streaming instabilities, then planetesimals will preferentially grow iron-rich in the inner region of protoplanetary disks. This might explain the iron gradient in the solar system and the formation of dense Mercury-like planets.

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arXiv2018

Constraints on Compound Chondrule Formation from Laboratory High-Temperature Collisions

In laboratory experiments, spherical 1 mm-wide glass and basalt particles are heated, and the hot particles collide at about 1m/s with a flat glass target that is at room temperature. When the particles are heated below 900 K, the collisions are essentially elastic with coefficients of restitution of about 0.9, but above 900K collisions become increasingly inelastic and the coefficient of restitution decreases with increasing temperature. At 1100K the glass particles approach sticking but, simultaneously, at the same temperature the particles melt on timescales of minutes. The basalt particles approach sticking at 1200 K. Only above 1400K do basalt grains in contact with each other fuse together, forming compounds on timescales of hours, and at 1500K basalt grains completely fuse together. Therefore, cooling basalt grains only have a 100K window for compound formation, and velocities very likely have to be below 1m/s for sticking in the first place. We predict that this puts constraints on compound chondrule formation and particle densities in the solar nebula.

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