Multi-spectrum retrieval of surface properties of comet 67P/Churyumov-Gerasimenko based on Rosetta/VIRTIS measurements

• Applicant: Dr. David Kappel
• Subject Area: Astrophysics and Astronomy ; Geodesy, Photogrammetry, Remote Sensing, Geoinformatics, Cartography
• Term: from 2018 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 395792296

The proposed project addresses the study of comet 67P/Churyumov-Gerasimenko's surface. The spectral measurements of 67P recently acquired by the Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS, 0.25-5.1 µm) aboard ESA's Rosetta spacecraft will be used for this purpose.Properties of the surface material like composition, roughness, and thermal inertia are key to distinguish different types of surface regions and to classify comets in general, while surface temperatures determine the sublimation state of volatiles, thereby affecting cometary activity. In the literature on 67P, usually either the reflectance and emission properties of the cometary surface are modeled in a simplified way to enable the retrieval of surface temperatures or thermal inertia, or the spectral range >3.5 µm affected by thermal emissions is discarded to then perform photometric and compositional analyses. The proposed work will unify and surpass both approaches. Bidirectional reflectance and directional emissivity, surface composition and texture, sub-VIRTIS-pixel variations of surface properties, heat diffusion, sublimation of volatiles as well as the complex shape of the comet nucleus and resulting shadows and multiply reflected radiation contributions are all taken into account to enable the simulation of physically realistic radiance spectra. The aforementioned surface properties are then retrieved from the VIRTIS data by comparisons to simulated radiance spectra. This is a challenging task, since for this kind of inverse problems, different parameter combinations can often reproduce the same measured spectrum equally well. To still discriminate between such parameter combinations, the Bayesian multi-spectrum retrieval algorithm MSR recently developed by the applicant will be employed. MSR allows, e.g., the retrieval of time-constant surface properties as parameters that are common to many single measurements. This approach leads to more reliable results, whose dependability will be quantified using a detailed error analysis.The research proposal will yield a physically thorough and mathematically consistent description of 67P's surface properties and will enable the access to the thermally affected spectral range >3.5 µm bearing important information on, for instance, the occurrence of nitrogen compounds. The project will provide unique new insights into the thermo-physical state and surface composition and texture of 67P, a proxy for the state of the matter in the early stages of the planetary system. This way, it will contribute to extend our knowledge of the solar system's evolution.

• DFG Programme: Research Grants
• International Connection: Italy
• Co-Investigators: Professorin Dr. Gabriele Arnold; Dr. Ekkehard Kührt; Dr. Liubov Moroz; Dr. Stefano Mottola
• Cooperation Partners: Dr. Mauro Ciarniello; Dr. Stubbe Hviid, Ph.D.; Anne-Kathrin Markus; Professor Dr. Frank Spahn, Ph.D.; Dr. Federico Tosi