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EXPERIMENTAL CONSTRAINTS ON THE ISOTOPE SIGNATURES OF THE EARLY SOLAR SYSTEM

Periodic Reporting for period 2 - COSMOKEMS (EXPERIMENTAL CONSTRAINTS ON THE ISOTOPE SIGNATURES OF THE EARLY SOLAR SYSTEM)

Reporting period: 2018-04-01 to 2019-09-30

This project aims at understanding better how the planets of the Solar System acquired their chemical and isotope compositions and how these characteristics can be used to identify the processes that transformed the initial dust of the early Solar System to typical planetary materials.
The environment around the Sun was very different from what it is today and this has greatly influenced the shaping of solid materials forming the terrestrial planet region. High temperatures, coupled with significant irradiation stemming from the young Sun could have transformed the gas and dust part of the protoplanetary disk. Similarly, the formation of the Moon resulting from a giant impact with the Earth could also have created high temperature conditions that would have modified the composition of the Moon when it finally accreted. In this project, we will attempt to reproduce experimentally some of these conditions and investigate how such environments could have left specific isotope signatures that will be used as fingerprints for reconstructing those past environments. We will specifically study the conditions of evaporation and condensation by investigating isotope fractionation in equilibrium between condensed phase and vapor. Second we will investigate the potential role of UV and particle irradiation on the isotope composition of dust or of volatile species present in the protoplanetary disk. Altogether our experiments will provide quantitative constraints on how terrestrial planets were shaped chemically.
The first part of this project was dedicated to designing and constructing a new mass spectrometer with the ability to measure the isotope composition of a vapor in chemical and isotope equilibrium with a condensed phase contained in a Knudsen cell. For various reasons, it was decided that the new mass spectrometer would be fitted to a currently existing double focusing instrument equipped with a multicollection system, the Neptune Plus produced by THERMO Fisher Instruments. A key issue was to ensure that the sensitivity and stability of the instrument will be sufficient for precise isotope measurements. For this purpose, we have modeled the molecular beam transmission, the electron beam emitted by filaments facing each other and the ion beam extracted into the mass spectrometer with the aim of optimizing the molecular beam transmission, improving the electron focusing onto the molecular beam and enhancing the ion extraction and focussing. An optimization of these parameters was achieved and should provide better performances compared with existing instruments. Our design should result in an enhancement in the ion production and transmission by a factor of approximately 100. This means that it should be possible to obtain precise measurements of isotope ratios, given that the maximum gas pressure in the Knudsen cell is 10-4 bar. Several additional improvements were also planned for the design of the instrument including in the cooling system, in the oven regulation and the pumping system. In parallel, a new mechanical design of the source housingand electronics has been initiated together with a specific software for controlling the instrument.
This project consists of three main subprojects: (1) we will simulate the effect of particle irradiation on solids to examine how isotopes can be fractionated by these processes to identify whether this can explain chemical variations in meteorites. We will examine whether particle irradiation can cause mass independent fractionation, (2) the novel KEMS instrument will be used to determine the equilibrium isotope fractionation associated with reactions between gas and condensed phases at high temperature. It will also be used to determine the kinetic isotope fractionation associated with evaporation and condensation of solids. This will provide new constraints on the thermodynamic conditions, T, P and fO2 during heating events that have modified the chemical composition of planetary materials. These constraints will also help identify the processes that cause the depletion in volatile elements and the fractionation in refractory elements observed in planetesimals and planets, (3) we will examine the effect of UV irradiation on chemical species in the vapour phase as an attempt to reproduce observed isotope compositions found in meteorites or their components. These results may radically change our view on how the protoplanetary disk evolved and how solids were transported and mixed.