Where does the matter in our solar system come from? How did it form? To answer these questions, scientists focus their studies on the Sun. In fact over 99% of the matter currently present in the solar system is concentrated within the Sun. More importantly, the Sun has conserved the initial composition of the protosolar nebula, the cloud of gas and dust that gave rise to the solar system. This is not the case with most of the other bodies in the solar system, such as Earth, Mars or the meteorites, which -- because they formed at high temperature -- have lost their primitive volatile elements. Consequently, their present composition does not reflect the composition of the protosolar nebula.
The chemical composition of the Sun is known from the analysis of the light that it emits. However, it is impossible to determine from a distance its isotopic abundance. Indeed, there can be different isotopes (14N and 15N for nitrogen; 16O, 17O and 18O for oxygen, etc.) for a single element: while they have the same number of electrons and protons, they differ by their number of neutrons. Determining the nitrogen and oxygen isotopic compositions of the Sun was one of the main objectives of the Genesis mission. The reason? The isotopic ratios of these elements (15N/14N for nitrogen(1), 17O/16O and 18O/16O for oxygen(2)) are very dissimilar between different objects of the solar system, namely Earth, the Moon, Mars, the meteorites, the comets and the giant planets. To explain these variations, it was essential to determine the isotopic composition of the protosolar nebula, in other words the Sun's composition today.
During the Genesis mission, which took place from December 2001 to April 2004, targets were irradiated by solar wind for 27 months. Bernard Marty's team at CNRS's Centre de Recherches Pétrographiques et Géochimiques (CRPG) was then selected by NASA to determine the abundance of nitrogen isotopes in the samples collected. All their analyses(3) point to the same result: solar nitrogen different to nitrogen from Earth. The Sun has 60% less 15N isotope than Earth. In other words, Earth and the meteorites have 60% more 15N, whereas the comets have 300% more 15N. In parallel, an American team has revealed that solar oxygen is also deficient in rare isotopes (17O and 18O) compared to the oxygen on Earth. Their study was also published in the journal Science two weeks ago. Furthermore, the 15N/14N ratio of the Sun is similar to that of Jupiter's atmosphere, analyzed ten years ago by an American space probe. This similarity demonstrates that the giant planets, including Jupiter, captured part of the gas present in the primitive nebula within their atmospheres.
All bodies in the solar system (with the exception of gaseous planets, such as Jupiter) contain an "abnormally" higher quantity of rare nitrogen and oxygen isotopes than the Sun. Such disparities are not observed in the case of non-volatile elements. Characterizing the origin of these enrichments could provide a better understanding of the phenomena that triggered the emergence of our solar system. One of the leads currently being followed is that these variations could result from an intense irradiation of the nebula's residual gas by the young Sun, which at the time had much more energy than today. Photochemical reactions may have enriched the compounds resulting from these reactions with rare isotopes. These compounds would then have been incorporated into meteorites and terrestrial planets. A hypothesis that still needs to be verified…
1 -- 14N is the most abundant nitrogen isotope on Earth.
2 -- 16O is the most abundant oxygen isotope on Earth.
3 -- The initial analyses were conducted on samples that were considerably polluted before the flight by 15N nitrogen. They gave rise to results with an uncertainty of +/-20%. NASA then provided fragments from a much less polluted target containing more solar particles. The isotopic composition of the nitrogen was then determined using the new ion probe installed in CRPG in late 2009 (uncertainty of +/- 0.7%).