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Modeling the surface properties of small bodies of the inner and outer Solar system

Final Report Summary - ASTROSURFACE (Modeling the surface properties of small bodies of the inner and outer Solar system)

Discovery of a vast population of transneptunian objects, called also Kuiper belt objects, has resulted in a completely new view of the Solar system beyond Neptune and in developing new models of the formation and evolution of our planetary system. Two belts of minor bodies in the inner and outer parts of the Solar system are believed to contain fossil remnants from the early stage of the formation of Solar system. Study of these bodies formed in different parts of our Solar system provides essential information on the primordial processes that governed the evolution of the early solar nebula.

The project was aimed to investigate surface properties of selected objects from the inner and outer parts of the Solar system, including asteroids, Trojans, Centaurs, and transneptunian objects. To retrieve surface properties we combined photometric, polarimetric and spectroscopic remote sensing methods. We used a unique possibility for testing our approach by comparing surface characteristics derived from ground-based and space observations for two inner belt asteroids (2867) Steins and (21) Lutetia. These asteroids were intensively studied in support of the Rosetta space mission of the European Space Agency. Asteroid (2867) Steins was found to have homogeneous high-albedo surface with composition similar to enstatite achondrite meteorites. Asteroid (21) Lutetia was found to have a non-convex shape, probably due to a large crater, heterogeneous surface properties with a presence of fine-grained regolith. These findings were confirmed during the successful flybys of the Rosetta spacecraft to (2867) Steins and (21) Lutetia proving the efficiency of our approach in the study of surface properties of atmosphereless bodies.

To better investigate the surface characteristics of minor bodies from outer part of the Solar system we carried out spectroscopic, photometric and polarimetric observations of selected objects belonging to different dynamical groups. We found that surfaces of all observed bodies revealed a so-called negative polarisation, which is the specific case of linearly polarised light, with the polarisation plane coinciding with the scattering plane. It is a characteristic feature for surfaces with a complex structure, as observed for the majority of planetary surfaces. However, the phase angle behaviours of the linear polarisation degree were found to be unique among other Solar system bodies observed so far. The modelling of the surface properties of the largest transneptunian objects with diameters greater 1000 km suggests that their topmost surface layer consists of large (compared to the wavelength) transparent inhomogeneous particles. The surface properties of smaller size transneptunian objects and Centaurs can be explained by a presence of a thin frost layer of submicron ice crystals on a dark surface. The surfaces of Centaurs should be more heterogeneous as compared to transneptunian objects. Jupiter Trojans were found to exhibit surface properties more similar to dark primitive asteroids than to Centaurs.

The main conclusion of our study is that surface microscopic properties of inner and outer belt small Solar system bodies are quite different. Using asteroid-like model to interpret observations of transneptunian objects can result in wrong conclusions. Our results are important to improve the theoretical models of lightscattering by atmosphereless surfaces. The information derived during the project implementation can find implications for the calibration, analysis, and interpretation of the space mission data, for both recent (Rosetta, Dawn, Wise) and future space missions (Gaia, Marco Polo).