La Chimie du disque solaire T-Tauri – T-Tauri.Chem.

If one desired to identify the major discovery in cosmochemistry from the various research projects developed worldwide during the last few years, it would certainly be the understanding that the first hundred thousands to one million years of the solar system is the key period which controls most of the evolution of matter around, close or at distance, the accreting star. This idea does not come only from astrophysical observations of young stellar objects (YSOs) and of their disks but is the result of recent quantitative measurements made on some of the constituents of primitive meteorites, on the cometary grains from the StarDust mission and on the solar wind from the Genesis mission. During our previous ANR poject we have for instance shown that the first solids of the solar system condensed very close to the forming Sun (within 0.05 AU), were irradiated and were rapidly distributed through the accretion disk so that they participated to the formation of comets at 30 AU or more. We have established for some key elements (e.g. O or N) the isotopic composition of the Sun and demonstrated that most solar system solids had their composition strongly modified, probably during reactions taking place upon irradiation of the disk by UV light. This kind of reactions is most likely responsible for a number of key characteristics of the building blocks of planetesimals and planets such as for instance the existence of non mass-dependant isotopic anomalies, the complexity in structure and isotopic composition of organic polymers or the scale (from nanometer to astronomical units) of chemical and isotopic heterogeneities in the disk. Contrary to the "classic" view of the formation of the solar system we have shown that, most likely, protoplanets formed within this period of the first million years. To make now significant progress towards a better understanding of the formation of our solar system requires that some fundamental issues, raised notably by our previous work, be settled. These issues can be summarized as follows: how well do we know the physical state of the disk surrounding the early Sun and to what extent the geochemical markers (the isotopic, chemical and mineralogical compositions of the solids), commonly used to reconstruct these early solar system environments, are reliable? In Cosmochemistry, we have now clearly reached a point where the behaviour of these markers is difficult to predict because of the extremely unusual nature of the conditions that prevailed in the disk of gas and dust surrounding the early Sun. We believe that noticeable progresses can now be made through laboratory experiments designed with the aim of reproducing some of the most astounding observations that have no counterpart in any known natural of laboratory samples. As now observed around young stars, the intense activity of the young Sun had drastic consequences on the disk of gas and dust surrounding the young star as for example, an intense irradiation of the disk by MeV protons and alpha particles or a photo-evaporation of the surface of the disk on distances of up to 10s of AU away from the Star. Under these circumstances, it is unlikely that the condensation of the solids from the gas took place according to the so-called "classical" condensation sequence that considers only chemical reactions between reactants at thermal equilibrium. Our recent experimental approaches of these problems have shown the importance of kinetic processes on the mineralogy of condensed phases. In addition, photochemical reactions in gases or photo-evaporation of solids can result in the formation of minerals whose structures and isotopic compositions are mostly unknown. Similarly, MeV energetic particles can produce new chemical elements via the nuclear interactions between the solar MeV particles and the gas or the solids of the disk (the so-called spallation reactions). For example, during the course of our last ANR, we have discovered excesses of 10B and 7Li that were produced by the decay of the now extinct radioactive 10Be and 7Be, nuclei that can be only produced by spallation reactions. The present situation is favourable for French Cosmochemistry provided a substantial effort of funding is made. We believe that the only way to succeed is to help the development of our laboratories who are partners in this field since more than 15 years: l'IPG à Paris (ICP-Mass spectrometer), le LEME au Muséum (Nanosims 50 ion-probe), l'ENS-Lyon (ICP mass spectrometer with large radius NU 1700) et le CRPG-CNRS à Nancy (IMS 1270 ion-probe). The aim of the present proposal is thus to (i) better document the specific isotopic and mineralogical signatures of the extraterrestrial material and to (ii) interpret these signatures at the light of experimental and theoretical simulations.