Remote sensing observations at visible-infrared (VIS-IR) wavelengths of the nucleus of comet 67P/Churyumov-Gerasimenko performed by the Rosetta mission have revealed a surface ubiquitously covered by low-albedo material (Capaccioni et al., 2015; Ciarniello et al., 2015; Fornasier et al., 2015), characterized by the presence of refractory and semi-volatile organics and dark opaque phases (Capaccioni et al., 2015; Quirico et al., 2016). However, a quantitative determination of the physical properties (grain size, porosity) and composition of the surface regolith, from spectrophotometric analysis, is still missing. As comets are considered the most primitive objects in the Solar System (Weidenschilling, 2004; Mumma & Charnley, 2011; Davidsson et al., 2016), assessing these properties is of paramount importance to constrain the physical conditions of the early solar nebula. The characterization of the regolith is also crucial to investigate the physical processes active on the surface, such as sublimation of volatiles, dust deposition and transport, and thermal stresses. In this perspective, the comparison of nucleus multi- and hyper-spectral data with spectra derived from radiative transfer models and spectra measured in the laboratory is fundamental for an accurate assessment of the properties of the material. While laboratory reflectance spectra for samples with known composition provide a direct input for comparison with remote sensing observations for a limited set of available materials and mixtures, spectral inversion based on radiative transfer models (Hapke, 2012; Shkuratov et al., 1999) allows us exploring a wide range of combinations simultaneously. However, the latter method relies on the capability of the theoretical approach to account for and correctly describe the physical properties of the investigated materials.
Based on this rationale the aim of the research project we propose is twofold:

  • Investigation of 67P/Churyumov-Gerasimenko surface composition from the convergence of results from dedicated laboratory measurements of cometary analogues (by means of deriving the optical constants/reflectances/absorbances/ of minerals, organics and ices) and radiative transfer models (analytical and numerical), applied to Rosetta spectrophotometric observations of the nucleus.
  • Test and improvement of widely used radiative transfer models needed for spectral inversion (Hapke, 2012; Shkuratov et al., 1999), by comparing their output with laboratory spectrophotometric measurements of materials with known compositions made available for 67P/Churyumov-Gerasimenko spectral unmixing. In this context, the main objectives of the study will concern the capability of the models to constrain composition (endmember abundances and mixing modalities) and physical properties (material grain size distribution and porosity).

Such an effort will be addressed within an international team hosted by ISSI, composed of experts on radiative transfer models in particulate media, visible-infrared laboratory spectrophotometry and remote sensing imaging spectroscopy.  The presence within the team of members directly involved in the Rosetta mission, and/or having direct access to spectroscopy laboratories and measurements, represents an important asset for the maximization of the scientific outcome of the collaboration.

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