Tensile strength of dust-ice mixtures and their relevance as cometary analog material

Abstract

Aims. The tensile strength of granular matter is of great importance to our understanding of the evolution of comets and to our attempts to reproduce processes on cometary surfaces in laboratory experiments. In this work, we investigate the tensile strength of three different materials and their mixtures, which can be used as cometary analog materials in the laboratory. Methods. We used two types of siliceous dusts and granular water ice whose polydisperse particles were either angular or spherical. Our samples were cooled to below 150 K to better simulate the conditions of a cometary surface and to avoid thermal alteration of the material. We used the Brazilian disk test method to exert stress on the cooled samples and determine the tensile strength at the moment the samples broke. Results. We find that the tensile strength of two component mixtures is strongly dominated by the component with the higher tensile strength. The materials made of mostly angular dust particles have a lower filling fraction, but a higher tensile strength compared to materials made of spherical particles. Furthermore, the tensile strength of the cooled components is substantially lower than the tensile strength of the same components at room temperature. This implies that the surface energy of the investigated materials at low temperatures is significantly lower than previously assumed.

Sublimation of ice-dust mixtures in cooled vacuum environments to reproduce cometary morphologies

“…Previous experimental work has shown that at temperatures of 150 K and below, the tensile strength of water ice is much lower than previously expected (Gundlach et al 2018a;Musiolik & Wurm 2019). Haack et al (2020) extended these experiments and investigated different mixtures of dust and water ice for their suitability as comet analogs and found that the tensile strength of a mixture is dominated by the component with the highest tensile strength. In this work, we use these mixtures to study the morphological evolution of samples in a vacuum sublimation chamber with the aim of reproducing basic morphological structures observed at the surface of 67P.…”

“…1). Additionally, it is possible to produce samples with a very low tensile strength with these spherical particles (Haack et al 2020) to simulate the low tensile strength of cometary materials (Groussin et al 2015;Attree et al 2018). The filling fraction of the samples describes how much free space is in a volume filled with particles and is with ∼0.4 (Fig.…”

“…In the experiment, the higher filling fraction would lead to more interparticle bonds and thus a higher tensile strength. However, this effect may be reduced by the spherical shape of the particles that reduces interlocking of realistically angular particles and would compensate for their limited freedom of movement (Haack et al 2020;Kappel et al 2020). Although olivine and pyroxene may be more suitable cometary refractory analog materials, the difficulties in acquiring these materials in a micrometer-sized spherical shape A35, page 2 of 14 D. Haack et al: Sublimation of insolated ice-dust mixtures Fig.…”

“…Electron-microscopy images and size distribution of the particles used in our experiments. The images of spherical fly ash in panel A and water ice particles in panel B are at the same scale (modified from Haack et al 2020). Panel C presents the comparable cumulative size distributions of fly ash (red dots) and water ice (blue diamonds).…”

“…In our experiments, we insolated samples, which were composed of water ice and nonvolatile dust particles in different ratios. We took advantage of the very low tensile strength of spherical water ice and fly ash particles that are mixed together and cooled below 150 K (Haack et al 2020;Kappel et al 2020). We also allowed vertical and horizontal mass transport of the sample material by the omission of lateral sample boundaries.…”