arXiv subjects

Gerhard Wurm

Publications and source records attributed to Gerhard Wurm.

At least 55 records · Page 3Linked to original sources

arXiv2018

Selective Aggregation Experiments on Planetesimal Formation and Mercury-Like Planets

Much of a planet's composition could be determined right at the onset of formation. Laboratory experiments can constrain these early steps. This includes static tensile strength measurements or collisions carried out under Earth's gravity and on various microgravity platforms. Among the variety of extrasolar planets which eventually form are (Exo)-Mercury, terrestrial planets with high density. If they form in inner protoplanetary disks, high temperature experiments are mandatory but they are still rare. Beyond the initial process of hit-and-stick collisions, some additional selective processing might be needed to explain Mercury. In analogy to icy worlds, such planets might, e.g., form in environments which are enriched in iron. This requires methods to separate iron and silicate at early stages. Photophoresis might be one viable way. Mercury and Mercury-like planets might also form due to the ferromagnetic properties of iron and mechanisms like magnetic aggregation in disk magnetic fields might become important. This review highlights some of the mechanisms with the potential to trigger Mercury formation.

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arXiv2018

Saltation under Martian Gravity and its Influence on the Global Dust Distribution

Dust and sand motion are a common sight on Mars. Understanding the interaction of atmosphere and Martian soil is fundamental to describe the planet's weather, climate and surface morphology. We set up a wind tunnel to study the lift of a mixture between very fine sand and dust in a Mars simulant soil. The experiments were carried out under Martian gravity in a parabolic flight. The reduced gravity was provided by a centrifuge under external microgravity. The onset of saltation was measured for a fluid threshold shear velocity of 0.82$\pm$0.04 m/s. This is considerably lower than found under Earth gravity. In addition to a reduction in weight, this low threshold can be attributed to gravity dependent cohesive forces within the sand bed, which drop by 2/3 under Martian gravity. The new threshold for saltation leads to a simulation of the annual dust cycle with a Mars GCM that is in agreement with observations.

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arXiv2017

Is There a Temperature Limit in Planet Formation at 1000 K?

Dust drifting inward in protoplanetary disks is subject to increasing temperatures. In laboratory experiments, we tempered basaltic dust between 873 K and 1273 K and find that the dust grains change in size and composition. These modifications influence the outcome of self-consistent low speed aggregation experiments showing a transition temperature of 1000\,K. Dust tempered at lower temperatures grows to a maximum aggregate size of $2.02 \pm 0.06$ mm, which is $1.49 \pm 0.08$ times the value for dust tempered at higher temperatures. A similar size ratio of $1.75 \pm 0.16$ results for a different set of collision velocities. This transition temperature is in agreement with orbit temperatures deduced for observed extrasolar planets. Most terrestrial planets are observed at positions equivalent to less than 1000 K. Dust aggregation on the millimeter-scale at elevated temperatures might therefore be a key factor for terrestrial planet formation.

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arXiv2017

Analog Experiments on Tensile Strength of Dusty and Cometary Matter

The tensile strength of small dusty bodies in the solar system is determined by the interaction between the composing grains. In the transition regime between small and sticky dust ($\rm μm$) and non cohesive large grains (mm), particles still stick to each other but are easily separated. In laboratory experiments we find that thermal creep gas flow at low ambient pressure generates an overpressure sufficient to overcome the tensile strength. For the first time it allows a direct measurement of the tensile strength of individual, very small (sub)-mm aggregates which consist of only tens of grains in the (sub)-mm size range. We traced the disintegration of aggregates by optical imaging in ground based as well as microgravity experiments and present first results for basalt, palagonite and vitreous carbon samples with up to a few hundred Pa. These measurements show that low tensile strength can be the result of building loose aggregates with compact (sub)-mm units. This is in favour of a combined cometary formation scenario by aggregation to compact aggreates and gravitational instability of these units.

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arXiv2017

Growing into and out of the bouncing barrier in planetesimal formation

In recent laboratory studies the robustness of a bouncing barrier in planetesimal formation was studied with an ensemble of preformed compact mm-sized aggregates. Here we show that a bouncing barrier indeed evolves self-consistently by hit-and-stick from an ensemble of smaller dust aggregates. In addition, we feed small aggregates to an ensemble of larger bouncing aggregates. The stickiness temporarily increases, but the final number of aggregates still bouncing remains the same. However, feeding on the small particle supply, the size of the bouncing aggregates increases. This suggests that in the presence of a dust reservoir aggregates grow into but also out of a bouncing barrier at larger size.

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arXiv2016

Self-charging of identical grains in the absence of an external field

We investigate the electrostatic charging of an agitated bed of identical grains using simulations, mathematical modeling, and experiments. We simulate charging with a discrete-element model including electrical multipoles and find that infinitesimally small initial charges can grow exponentially rapidly. We propose a mathematical Turing model that defines conditions for exponential charging to occur and provides insights into the mechanisms involved. Finally, we confirm the predicted exponential growth in experiments using vibrated grains under microgravity, and we describe novel predicted spatiotemporal states that merit further study.

physics.comp-ph↗

arXiv2016

The motion of chondrules and other particles in a protoplanetary disc with temperature fluctuations

We consider the mechanism of photophoretic transport in protoplanetary disks that are optically thick to radiation. Here, photophoresis is not caused by the central star but by temperature fluctuations that subject suspended solid particles, including chondrules, to non-isotropic thermal radiation within the disk. These short-lived temperature fluctuations can explain time-of-flight size sorting and general number density enhancements. The same mechanism will also lead to velocity fluctuations of dust aggregates beyond $100\,\mathrm{m\,s^{-1}}$ for mm-sized particles in protoplanetary disks. Applying this in future research will change our understanding of the early phases of collisional dust evolution and aggregate growth as particles cross the bouncing barrier and as mass transfer rates are altered.

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arXiv2016

Self-Sustained Recycling in the Inner Dust Ring of Pre-Transitional Disks

Observations of pre-transitional disks show a narrow inner dust ring and a larger outer one. They are separated by a cavity with no or only little dust. We propose an efficient recycling mechanism for the inner dust ring which keeps it in a steady-state. No major particle sources are needed for replenishment. Dust particles and pebbles drift outwards by radiation pressure and photophoresis. The pebbles grow during outward drift until they reach a balanced position where residual gravity compensates photophoresis. While still growing larger they reverse their motion and drift inwards. Eventually, their speed is fast enough that they get destroyed in collisions with other pebbles and drift outwards again. We quantify the force balance and drift velocities for the disks LkCa15 and HD135344B. We simulate single particle evolution and show that this scenario is viable. Growth and drift timescales are on the same order and a steady state can be established in the inner dust ring.

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arXiv2016

Failed Growth at the Bouncing Barrier in Planetesimal Formation

In laboratory experiments, we studied collisions of ensembles of compact (filling factor 0.33) millimeter dust aggregates composed of micrometer quartz grains. We used cylindrical aggregates, triangular aggregates, square aggregates, and rectangular aggregates. Ensembles of equal size aggregates as well as ensembles with embedded larger aggregates were studied. The typical collision velocities are 10-20 mm $\rm s^{-1}$. High spatial and temporal resolution imaging unambiguously shows that individual collisions lead to sticking with a high probability of 20 percent. This leads to connected clusters of aggregates. The contact areas between two aggregates increase with collision velocity. However, this cluster growth is only temporary, as subsequent collisions of aggregates and clusters eventually lead to the detachment of all aggregates from a cluster. The contacts are very fragile as aggregates cannot be compressed further or fragment under our experimental conditions to enhance the contact stability. Therefore, the evolution of the ensemble always leads back to a distribution of individual aggregates of initial size. This supports and extends earlier experiments showing that a bouncing barrier in planetesimal formation would be robust against shape and size variations.

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arXiv2016

Ice Grain Collisions in Comparison: CO$_2$, H$_2$O and their Mixtures

Collisions of ice particles play an important role in the formation of planetesimals and comets. In recent work we showed, that CO$_2$ ice behaves like silicates in collisions. The resulting assumption was that it should therefore stick less efficiently than H$_2$O ice. Within this paper a quantification of the latter is presented. We used the same experimental setup to study collisions of pure CO$_2$ ice, pure water ice and 50\% mixtures by mass between CO$_2$ and water at 80K, 1 mbar and an average particle size of $\sim 90 μ$m. The results show a strong increase of the threshold velocity between sticking and bouncing with increasing water content. This supports the idea that water ice is favorable for early growth phases of planets in a zone within the H$_2$O and the CO$_2$ iceline.

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arXiv2016

Amplification of dust loading in Martian dust devils by self-shadowing

Insolation of the Martian soil leads to a sub-surface overpressure due to thermal creep gas flow. This could support particle entrainment into the atmosphere. Short time shadowing e.g. by the traverse of a larger dust devil would enhance this effect. We find in microgravity experiments that mass ejection rates are increased by a factor of 10 for several seconds if a light source of 12.6 kW/$\rm m^2$ is turned off. Scaled to Mars this implies that self-shadowing of a partially opaque dust devil might lead to a strongly amplified flux of lifted material. We therefore suggest that self-shadowing might be a mechanism on Mars to increase the total dust loading of a dust devil and keep it self-sustained.

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arXiv2016

Collisions of CO$_2$ Ice Grains in Planet Formation

In protoplanetary disks, CO$_2$ is solid ice beyond its snow line at $\sim 10 \rm AU$. Due to its high abundance, it contributes heavily to the collisional evolution in this region of the disk. For the first time, we carried out laboratory collision experiments with CO$_2$ ice particles and a CO$_2$-covered wall at a temperature of 80 K. Collision velocities varied between 0 - 2.5 m/s. Particle sizes were on the order of $\sim$ 100 $μ$m. We find a threshold velocity between the sticking and the bouncing regime at 0.04 m/s. Particles with greater velocities but below 1 m/s bounce off the wall. For yet greater velocities, fragmentation occurs. We give analytical models for the coefficients of restitution and fragmentation strength consistent with the experimental data. Set in context, our data show that CO$_2$ ice and silicate dust resemble each other in the collisional behavior. Compared to water ice the sticking velocity is an order of magnitude smaller. One immediate consequence as example is that water ice particles mantled by CO$_2$ ice lose any "sticking advantage." In this case, preferential planetesimal growth attributed to the sticking properties of water ice will be limited to the region between the H$_2$O ice line and the CO$_2$ ice line.

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arXiv2015

Thermal creep assisted dust lifting on Mars: Wind tunnel experiments for the entrainment threshold velocity

In this work we present laboratory measurements on the reduction of the threshold friction velocity necessary for lifting dust if the dust bed is illuminated. Insolation of a porous soil establishes a temperature gradient. At low ambient pressure this gradient leads to thermal creep gas flow within the soil. This flow leads to a sub-surface overpressure which supports lift imposed by wind. The wind tunnel was run with Mojave Mars Simulant and air at 3, 6 and 9 mbar, to cover most of the pressure range at martian surface levels. Our first measurements imply that the insolation of the martian surface can reduce the entrainment threshold velocity between 4 % and 19 % for the conditions sampled with our experiments. An insolation activated soil might therefore provide additional support for aeolian particle transport at low wind speeds.

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arXiv2015

An Insolation Activated Dust Layer on Mars

The illuminated dusty surface of Mars acts like a gas pump. It is driven by thermal creep at low pressure within the soil. In the top soil layer this gas flow has to be sustained by a pressure gradient. This is equivalent to a lifting force on the dust grains. The top layer is therefore under tension which reduces the threshold wind speed for saltation. We carried out laboratory experiments to quantify the thickness of this activated layer. We use basalt with an average particle size of 67 $μ$m. We find a depth of the active layer of 100 to 200 $\rm μm$. Scaled to Mars the activation will reduce threshold wind speeds for saltation by about 10%.

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arXiv2014

Photophoretic Strength on Chondrules. 2. Experiment

Photophoretic motion can transport illuminated particles in protoplanetary disks. In a previous paper we focused on the modeling of steady state photophoretic forces based on the compositions derived from tomography and heat transfer. Here, we present microgravity experiments which deviate significantly from the steady state calculations of the first paper. The experiments on average show a significantly smaller force than predicted with a large variation in absolute photophoretic force and in the direction of motion with respect to the illumination. Time-dependent modeling of photophoretic forces for heat-up and rotation show that the variations in strength and direction observed can be well explained by the particle reorientation in the limited experiment time of a drop tower experiment. In protoplanetary disks, random rotation subsides due to gas friction on short timescales and the results of our earlier paper hold. Rotation has a significant influence in short duration laboratory studies. Observing particle motion and rotation under the influence of photophoresis can be considered as a basic laboratory analog experiment to Yarkovsky and YORP effects.

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arXiv2014

Experimental Study on Bouncing Barriers in Protoplanetary Disks

For dust aggregates in protoplanetary discs a transition between sticking and bouncing in individual collisions at mm to cm size has been observed in the past. This lead to the notion of a bouncing barrier for which growth gets stalled. Here, we present long term laboratory experiments on the outcome of repeated aggregate collisions at the bouncing barrier. About 100 SiO$_2$ dust aggregates of 1 mm in size were observed interacting with each other. Collisions occured within a velocity range from below mm/s up to cm/s. Aggregates continuously interacted with each other over a period of 900 s. During this time more than $10^5$ collisions occured. Nearly 2000 collisions were analyzed in detail. No temporal stable net growth of larger aggregates was observed even though sticking collision occur. Larger ensembles of aggregates sticking together are formed but were disassembled again during the further collisional evolution. The concept of a bouncing barrier supports the formation of planetesimals by seeded collisional growth as well as by gravitational instability favouring a significant total mass being limited to certain size ranges. Within our parameter set the experiments confirm that bouncing barriers are one possible and likely evolutionary limit of a self consistent particle growth.

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arXiv2013

Preplanetary scavengers: Growing tall in dust collisions

Dust collisions in protoplanetary disks are one means to grow planetesimals, but the destructive or constructive nature of high speed collisions is still unsettled. In laboratory experiments, we study the self-consistent evolution of a target upon continuous impacts of submm dust aggregates at collision velocities of up to 71m/s. Earlier studies analyzed individual collisions, which were more speculative for high velocities and low projectile masses. Here, we confirm earlier findings that high speed collisions result in mass gain of the target. We also quantify the accretion efficiency for the used SiO2 (quartz) dust sample. For two different average masses of dust aggregates (0.29g and 2.67g) accretion efficiencies are decreasing with velocity from 58% to 18% and from 25% to 7% at 27m/s to 71m/s, respectively. The accretion efficiency decreases approximately as logarithmic with impact energy. At the impact velocity of 49m/s the target acquires a volume filling factor of 38%. These data extend earlier work that pointed to the filling factor leveling off at 8m/s to a value of 33%. Our results imply that high speed collisions are an important mode of particle evolution. It especially allows existing large bodies to grow further by scavenging smaller aggregates with high efficiency.

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arXiv2013

Photophoretic Strength on Chondrules 1: Modeling

Photophoresis is a physical process that transports particles in optical thin parts of protoplanetary disks, especially at the inner edge and at the optically surface. To model the transport and resulting effects in detail, it is necessary to quantify the strength of photophoresis for different particle classes as a fundamental input. Here, we explore photophoresis for a set of chondrules. The composition and surface morphology of these chondrules was measured by X-ray tomography. Based on the three-dimensional models, heat transfer through illuminated chondrules was calculated. The resulting surface temperature map was then used to calculate the photophoretic strength. We found that irregularities in particle shape and variations in composition induce variations in the photophoretic force. These depend on the orientation of a particle with respect to the light source. The variations of the absolute value of the photophoretic force on average over all chondrules is $4.17\%$. The deviation between the direction of the photophoretic force and illumination is $3.0^\circ \pm 1.5^\circ$. The average photophoretic force can be well approximated and calculated analytically assuming a homogeneous sphere with a volume equivalent mean radius and an effective thermal conductivity. We found an analytic expression for the effective thermal conductivity. The expression depends on the two main phases of a chondrule, and decreases with the amount of fine-grained devitrified, plagioclase-normative mesostasis up to factor of three. For the chondrule sample studied (Bjurböle chondrite), we found a dependence of the photophoretic force on chondrule size.

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