UltraCarbonaceous Antarctic micrometeorites, probing the Solar System beyond the nitrogen snow-line

The current Solar System architecture is a heritage of the protoplanetary disk that surrounded the young Sun, 4.56 Gy ago. Primitive extraterrestrial objects provide means to trace back the primordial composition and radial distribution of matter in this disk. Here, we present a combined micro-IR, Raman, chemical and isotopic study of two ultracarbonaceous micrometeorites recovered from Antarctica (UCAMMs). This study reveals particles containing an unusually high nitrogen- and deuterium-rich organic matter analogous to a polyaromatic hydrogenated carbon nitride, characterized by nitrogen concentration with bulk atomic N/C ratios of 0.05 and 0.12 (locally exceeding 0.15). We propose that such nitrogen-rich carbonaceous material can be formed by energetic irradiations of nitrogen-rich ices in very low temperature regions of the Solar System. Such conditions are encountered at the surface of small objects beyond the trans-neptunian region. UCAMMs provide unique insights on physico-chemical processes that occurred beyond the nitrogen snow-line, revealing organic material from the extreme outer regions of the Solar System that cannot be investigated by remote sensing methods.