Shock compaction of chondrule dust rims

Dust envelopes surrounding chondrules encode important data on the formation of planetesimals. It is widely accepted that these envelopes accreted onto chondrule surfaces while free-floating in the solar nebula. One of the dynamical processes under current scrutiny is the compaction that primordial chondrule dust envelopes experienced after accretion. While theoretical and experimental studies show that envelope accretion produces relatively porous structures, the dust envelopes observed in chondrite samples are generally compact. The compaction mechanism is not yet fully determined. The purpose of this work is to specify the degree of envelope compaction and overall morphology resulting from one specific process: high-speed collisions between a rimmed chondrule and a dust cloud.

We perform 2D numerical simulations with the iSALE2D shock physics code. A portion of a chondrule is modeled as a dunite slab, with an initial porous layer of dust on the slab surface. The dust particles are modeled as dunite disks with diameters between 2.4 and 3.6 μm. The combined chondrule/dust structure undergoes a collision with a dust cloud containing the same type of grains as the chondrule dust layer. In order to probe a variety of collision scenarios, we change the particle fraction, f_p, in the dust cloud (0.01 – 0.30), the collision speed (750 m/s – 3000 m/s), and the cohesion of the intact material (5 x 10⁵ Pa – 10⁸ Pa). Preliminary results show that the original dust layer on the chondrule surface undergoes significant compaction for all the examined collision speeds. However, the morphology of the resulting layer differs considerably for different dust cloud particle fractions. In the case of small f_p, filamentary dust structures are formed as a result of the collision, and remain through the duration of the simulation. In contrast, no filaments are observed for large f_p. Overall, the topography of the compacted dust layer is more complex for small f_p than for large f_p. In all cases, appreciable deformation of the chondrule surface occurs, with indentations of varying depths that resemble those observed in the Paris CM chondrite. Upon impact, temperatures in the dust layer rise markedly above the initial temperature of 300 K, at which the chondrule remains throughout the simulation. The dust temperature can reach ~700 K in a 3000-m/s collision with f_p = 0.01 (see figures).

Further work remains to compute the porosity of the compacted dust layer. The porosity will be compared to values measured in chondrite samples and in upcoming high-speed-collision experiments that use chondrule analogs.