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

拟议的项目涉及对67 P/Churyumov-Gerasimenko彗星表面的研究。可见光和红外热成像光谱仪最近获得的~（67）P的光谱测量（VIRTIS，0.25-5.1微米）将用于此目的。表面材料的性质，如成分，粗糙度和热惯性是区分不同类型的表面区域和分类彗星的关键。而表面温度决定了挥发物的升华状态，从而影响彗星的活动。在67 P的文献中，通常要么彗星表面的反射和发射特性以简化的方式建模，以便能够检索表面温度或热惯性，要么放弃受热发射影响的>3.5 µm的光谱范围，然后进行光度和成分分析。拟议的工作将统一和超越这两种方法。双向反射率和定向发射率、表面成分和纹理、表面特性的亚VIRTIS像素变化、热扩散、挥发物升华以及彗星核的复杂形状和由此产生的阴影和多次反射辐射贡献都被考虑在内，以模拟物理上逼真的辐射光谱。上述表面特性，然后检索VIRTIS数据的模拟辐射光谱进行比较。这是一项具有挑战性的任务，因为对于这种逆问题，不同的参数组合通常可以同样好地再现相同的测量光谱。为了仍然区分这些参数组合，将采用申请人最近开发的贝叶斯多光谱检索算法MSR。MSR允许，例如，检索时间常数表面特性作为参数，是共同的许多单一的测量。这种方法可以得到更可靠的结果，其可靠性将通过详细的误差分析进行量化。该研究方案将对67 P的表面性质进行物理上彻底和数学上一致的描述，并将能够获得热影响光谱范围>3.5 µm的重要信息，例如氮化合物的存在。该项目将为67 P的热物理状态和表面成分和纹理提供独特的新见解，67 P是行星系统早期阶段物质状态的代表。这样，它将有助于扩大我们对太阳系演化的了解。

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.