title: Experimental studies of the properties of dust, ice, and their associations with relevance for planet formation and characterization of planetary surfaces
Creators: name: Spadaccia, Stefano
Publisher: name: Universität Bern
Year: 2023
Subjects: subject: 520 Astronomy
subject: 530 Physics
subject: 620 Engineering

description: Planet formation processes in protoplanetary disks are still not completely understood. We can approach this problem through the study of both the physical processes that are crucial for formation of planetesimals in the disk, and by studying the most pristine and ancient bodies of our Solar System. Recently, the water ice line has attracted the attention as a potential preferred location where planet formation and growth can be triggered. To correctly simulate planetary formation at the ice line, it is important to understand the evolution processes involving millimeter-size icy aggregates (icy pebbles) that drift inward in the disk toward the central star, and they cross the ice line. The outcome of the ice sublimation due to the rising temperatures can determine the different evolutionary path of planet formation. In this context, it is important to study the properties of ice-dust pebbles to understand their physical processes in the disk. Stepping forward in time, we need to infer the properties of the available planetesimals in the Solar System (like asteroids and comets) to gather information on how, when, and where they formed in the disk. Observing their surface characteristics, such as the presence of water ice and the type of minerals mixed together, is precious for deriving their origin and accretion processes.
In this work, I present experiments that address the evolution of icy pebbles in the protoplanetary disk, the analysis of composition and textural properties of asteroid through reflected polarimetry, and finally the spectroscopic and spectropolarimetric analysis of water ice frost forming on regolith surfaces. I created experimental models of mm-size icy pebbles, and I observed their evolution under temperature-pressure conditions relevant for protoplanetary disks. I found that pebbles do not disrupt easily under sublimation of the ice, meaning that icy pebbles produce porous mm-sized dusty aggregates when they cross the ice line. The porous dusty pebbles can then be accreted or participate in planet formation processes close to the ice line. Moreover, I performed polarimetric measurements of pulverized mineral mixtures and aggregates, to understand how the negative polarization phase curve is influenced by the surface mineralogical composition and texture of asteroids. I found that while the presence of aggregates does not change the negative polarimetric phase curve, mixing different minerals can result in a deepening and broadening of the negative polarization. This phenomenon can be important in explaining the polarimetric characteristics of some classes of asteroids (for example, F-type and L-type asteroids). Finally, I studied the possibility of detecting thin layers of water ice frost condensing on regolith surfaces both through spectropolarimetry and spectrophotometry in the visible. I discovered that water frost can be well detected by its spectral slope or negative polarization changes over time, already when it is only 10 μm thick. Our spectrophotometric data agree with the inferred frost thickness measured on 67P/Churyumov-Gerasimenko, and the spectropolarimetric data are sensitive to the deposition of the fist micrometers of frost and to its crystalline structure.