Planetesimals in rarefied gas: wind erosion in slip flow

Abstract

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 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.

Rapid destruction of planetary debris around white dwarfs through aeolian erosion

“…Recently, we showed that aeolian erosion can play an important role in planet formation by setting a new growth-barrier for pebbles/boulders in protoplanetary disks, and affecting pebble accretion and streaming instability (Grishin et al 2020;Rozner et al 2020). Aeolian erosion in protoplanetary disks is rapid and efficient, as also verified in lab experiments and numerical simulations of the conditions of protoplanetary disks (Paraskov et al 2006;Demirci et al 2020b;Demirci & Wurm 2020;Demirci et al 2020a;Schaffer et al 2020). Aeolian erosion leaves signatures on the dynamics of objects in the disk, e.g.…”

Accretion of eroding pebbles and planetesimals in planetary envelopes

“…The shear stress τ erosion , which is needed to erode a protoplanetary surface, was determined experimentally for a wide range of gas pressure and gravitational acceleration by Demirci et al (2019Demirci et al ( , 2020. It is where the first term describes the cohesion and the second term describes the gravitational acceleration.…”

“…The gravity term only becomes important for larger objects, like planetesimals. The effective surface energy γ eff (Johnson et al 1971) of the constituent particles is assumed to be about 10 −4 N m −1 and β = 0.67 is an empirical scaling factor for the Knudsen number in the Cunningham correction for d particle (Demirci et al 2019(Demirci et al , 2020. If the wall shear stress exceeds the erosion shear stress (τ wall > τ erosion ), the radius of the pebble will be decreased with a certain erosion rate = ∆R pebble /∆t.…”

“…The stability of small weak bodies against wind erosion has been studied experimentally by Demirci et al (2019Demirci et al ( , 2020. They found that small bodies on a regular circular orbit in the inner protoplanetary disk are already affected by erosion up to complete destruction.…”

Lifting of Tribocharged Grains by Martian Winds

“…However, one further ingredient in the physics of soil grain entrainment on Mars is the rarified Martian atmosphere. Recently, Demirci et al (2020) showed that the transition from hydrodynamic flow to molecular flow around an individual grain increases the necessary shear stress for smaller particles further. For a less penetrable ground of small grains, lifting might be easier owing to a pressure difference between subsoil and above-ground pressure as vortices travel the surface (Greeley et al 2003;Balme & Hagermann 2006;Neakrase et al 2016;Bila et al 2020).…”