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Composition of solar system small bodies

Periodic Reporting for period 2 - SOLARYS (Composition of solar system small bodies)

Reporting period: 2019-10-01 to 2021-03-31

The solar system is made of 1 star, 8 planets, several hundredth of satellites, but also of millions of so-called small bodies. These small solar system objects are thought to be primitive, because being small they should have escaped the profound thermal processing experienced by planets. They are also the building blocks of planets and giant planetary cores: understanding how they were formed and what they are made of provides information on how our planetary system was formed. The overall objectives of this project is to provide constrains on the composition of solar system small bodies, through laboratory measurements. We are using a multi-scale approach to understand theire composition. First, we look at true extra-terrestrial samples at the sub-µm scale, with infrared light (Task 1). This first task is dedicated to understanding the constituents of meteorites, micrometeorites and inter-planetary dust particles. Second, we try to understand how light propagates in a compact media made of sub-µm constituents. For that we prepare sub-µm grain of relevant materials (silicates, organics, metals) and we measure how they reflect and emit light. We also prepare highly-porous samples but using a sublimation protocol, analogue to what occurs at the surface of comets. Finally, we compare our laboratory results to space-based or ground-based observations of small bodies, to shed light on their composition.
The first half of the project has seen the developement of the various methologies used. We have setup an nano-infrared spectroscopy system at IPAG, assessed the performances of the system and optimized samples preparation. We have tested several preparation (microtome, FIB, diamond windows) for analogue materials as well as extra-terrestrial samples. In parallell we have also develop and tested approaches to quantify the structure of organic matter based on infrared spectroscopy.We have shown the strength of NanoIR for analizing organic sample (coals) by comparing to standard IR spectroscopy. We have also shown the capabilities to analize mixtures with minerals by studying phyllosilicates-rich coals. Last, for the first time we have measured NanoIR spectra of meteoritic insoluble organic matter.

We have designed a protocol that enables to produce sub-µm particles of silicates, sulfides and organic materials. We have also set-up a methodology that enables to incorporate these small grains inside water-ice sphere, and produce hyper-porous samples by sublimation of the ice. This work led to several major results. First we showed that there is a small grains degeneracy for weakly absorbing material. In other words, when small grains are present (<1 µm), as expected in comets and primitive small bodies, the diagnostic signature of minerals decrease significantly. We also show that the presence of elevated porosity maximize these effects. The incorporation of opaque grains to these mixture leads to a strong blueing of the samples. This blueing is caused by a Rayleigh-scattering like behavior. Such a phenomena was used to explain the blueing of fresh crater around ceres.

Our laboratory characterization have been used to compare and confront to available observations of the small bodies population. This is where the most striking results have been obtained. First we have used the sample preparation protocol designed to understand the presence of a peculiar absorption of comet 67P around 3-µm. We have shown that it is due to the presence of ammonium salts, with strong implications for the budget of nitrogen (a life-forming element) in the Solar System. We show that at the begining of the solar system nitrogen may have been mostly in refractory form, salts, rather than in the gas phase. This facilitates incorporation of nitrogen in small bodies and the forming planets. Second, we have assessed how much water is present in the main-asteroid belt, based on reflectance spectra obtained on primitive extra-terrestrial samples. From this work we shown that main-belt asteroids appear dehydrated when compared to their asteroidal counterpart, and that the mass of water in the main-belt is of the order of the mass of Enceladus. Last, we have started to shade light on the links between primitives meteorites families and dark asteroids types. We have shown that many carbonaceous chondrites are not related to C-type asteroids, and proposed associations of asteroids spectral types to primitive meteorites groups.
We expect to make significant progresses on the following issues by the end of the project:
-The nature and origin of extra-terrestrial organics. Now that the method is developped, by applying Nano-IR to extra-terrestrial dusts we will assess the variability in the composition of macromomecular organics phases within their petrographical context. We will determine whether the accreted organics compounds are homogeneous within a parent body and test the hypothesis that different pathways were present in the formation of E-T organics. This will tell us about their origin (are there N-rich grain?) and the level of mixing in the prosotosolar nebula. We will also investigate whether the putative variability in composition is related to the associated minerals, and then assess whether catalytics effects occurred.
-We hope to provide significant advances in the undertansing of the nature of D-type objetcs, to which cometary nuclei are related. The optical signature of cometary dust has been began to be studied by investigated reflectance spectra of pure phases with sub-µm grain sizes. We will continue by looking at mixture of hyperfine particles with contrasted optical constant and build a radiative transfer model to described our observations. This will finally be applied to understand the optical signatures of C-complex and D-type solar system small bodies and put constrain on theire composition and relation with available extra-terrestrial samples. We will investigate the optical signature of ammonium bearing cometary analogues to try to assess the carrier of the 3-µm band of D-type objects. This may help understanding the origin of the N enrichment in cometary dust and help understanding if the semi-volatile carrier is inherited from the ISM or formed in the solar nebula. We will also search for the nature of the "infrared brightener" needed to explain the signature of D-type objects in emission by testing organics compounds and salts.
-We will determine the spectrophotometric behavior of our hyperfine grained and porous analogues of the surface of small bodies. This will help building an appropriate radiative transfer model for such media, and provide new observables to confront to observations.
-We will determine the level of primitivity of a suite of carbonaceous and ordinary chondrites, in particular theire level of aqueous alteration. We will use conventional and NanoIR in bulk sample and matrices to investigate the carrier of volatiles phases, as well as to assess possible parent bodies within the small bodies population. This will be coupled to estimation of C and H in the samples as well as theire isotopic ratio to provide further constains on the parent bodies as well as pathways for incorporation of these elements.
Hyperporous and hyperfine grained cometary surface analogue