The footprint of cometary dust analogues – II. Morphology as a tracer of tensile strength and application to dust collection by the Rosetta spacecraft

Abstract

The structure of cometary dust is a tracer of growth processes in the formation of planetesimals. Instrumentation on board the Rosetta mission to comet 67P/Churyumov-Gerasimenko captured dust particles and analysed them in situ. However, these deposits are a product of a collision within the instrument. We conducted laboratory experiments with cometary dust analogues, simulating the collection process by Rosetta instruments (specifically COSIMA, MIDAS). In Paper I we reported that velocity is a key driver in determining the appearance of deposits. Here in Paper II we use materials with different monomer sizes, and study the effect of tensile strength on the appearance of deposits. We find that mass transfer efficiency increases from ∼ 1 up to ∼ 10% with increasing monomer diameter from 0.3 µm to 1.5 µm (i.e. tensile strength decreasing from ∼ 12 to ∼ 3 kPa), and velocities increasing from 0.5 to 6 m s −1 . Also, the relative abundance of small fragments after impact is higher for material with higher tensile strength. The degeneracy between the effects of velocity and material strength may be lifted by performing a closer study of the deposits. This experimental method makes it possible to estimate the mass transfer efficiency in the COSIMA instrument. Extrapolating these results implies that more than half of the dust collected during the Rosetta mission has not been imaged. We analysed two COSIMA targets containing deposits from single collisions. The collision that occurred closest to perihelion passage led to more small fragments on the target.

D/H in the refractory organics of comet 67P/Churyumov-Gerasimenko measured by Rosetta/COSIMA

“…However, the wide variation in the D/H ratios in particles of the R type casts some doubt upon this idea; the mean D/H for type R particles is within 1.2 σ of the mean D/H of every other particle type. A lack of variation is understandable if the different particle types do not result mainly from a difference in composition, but rather are due to a difference in a particle's initial size and incident speed, as laboratory simulations suggest (Ellerbroek et al 2017(Ellerbroek et al , 2019.…”

Dust-to-Gas and Refractory-to-Ice Mass Ratios of Comet 67P/Churyumov-Gerasimenko from Rosetta Observations

“…COSIMA might have collected particles from one or two potential families, one with a fractal dimension of 2.5-3 and a spread in impact velocity, and the other <2 if all particles impacts with the same velocity . A large fraction of the dust entering COSIMA is reflected, owing to a mass transfer efficiency estimated to 1 to 10% depending on size and velocity of incoming particles (Ellerbroek et al 2019). This potential collection bias of COSIMA based on a collection or detection area suggests that a fractionation of the dust populations might be applicable.…”