Particle tracking analysis of ejected water-ice grains from pure ice cometary analogue samples

Sublimation of volatile materials causes a pressure build-up in the surface layers of comets, which then leads to outgassing and to the ejection of particles. To investigate this behavior in more detail, an experimental setup was developed as part of the CoPhyLab campaign [1]. With this setup different comet analogue samples are exposed to an artificial Sun to simulate cometary activity. The first experiments are conducted with pure water ice samples only since water ice comprises the bulk of volatile materials in most comets [2] and pure samples provide a well comprehensible starting point to model the development of the experiments. Thanks to our experiments we were able to observe, for the first time, that the sublimation of pure water ice (ice without any additional components, or impurities) can eject solid grains. Hence, the ejection of water ice grains does not require the presence of a more volatile species as suggested by [3]. In our experiments, the trajectories of the grains were recorded with a high-speed camera and subsequently evaluated with respect to their motion behavior using a particle tracking routine (Fig. 1). Fig.1: Superimposed image of all “well-tracked” particle trajectories of one experiment over the observed sample surface. The ejected particles possess terminal speeds of about 1 m/s. Further, we found that the terminal particle velocity, as well as the total activity, depends directly on the insolation. Moreover, the observed trajectories were modeled by using gas-drag laws to derive an estimate for the sublimation pressure inside the samples [4]. The knowledge gained from these experiments will be used both in a larger experimental setup [5] and in further experiments including mixtures of water ice with dust and with super-volatiles such as CO2. References [1] Gundlach, B., “CoPhyLab: recent and future experiments - an overview”, 2020. doi:10.5194/epsc2020-218. [2] Bockelée-Morvan, D., “An Overview of Comet Composition”, in The Molecular Universe, 2011, vol. 280, pp. 261–274. doi:10.1017/S1743921311025038. [3] A'Hearn, M. F., “EPOXI at Comet Hartley 2”, Science, vol. 332, no. 6036, p. 1396, 2011. doi:10.1126/science.1204054. [4] Capelo, H. L., “Observation of aerodynamic instability in the flow of a particle stream in a dilute gas”, Astronomy and Astrophysics, vol. 622, 2019. doi:10.1051/0004-6361/201833702. [5] Kreuzig, C., “The CoPhyLab comet-simulation chamber”, Review of Scientific Instruments, vol. 92, no. 11, 2021. doi:10.1063/5.0057030.