  The formation of organic compounds is generally assumed to result from
abiotic processes in the Solar System, with the exception of biogenic organics
on Earth. Nitrogen-bearing organics are of particular interest, notably for
prebiotic perspectives but also for overall comprehension of organic formation
in the young solar system and in planetary atmospheres. We have investigated
abiotic synthesis of organics upon plasma discharge, with special attention to
N isotope fractionation. Organic aerosols were synthesized from N2-CH4 and
N2-CO gaseous mixtures using low-pressure plasma discharge experiments, aimed
at simulating chemistry occurring in Titan s atmosphere and in the protosolar
nebula, respectively. Nitrogen is efficiently incorporated into the synthesized
solids, independently of the oxidation degree, of the N2 content of the
starting gas mixture, and of the nitrogen speciation in the aerosols. The
aerosols are depleted in 15N by 15-25 permil relative to the initial N2 gas,
whatever the experimental setup is. Such an isotopic fractionation is
attributed to mass-dependent kinetic effect(s). Nitrogen isotope fractionation
upon electric discharge cannot account for the large N isotope variations
observed among solar system objects and reservoirs. Extreme N isotope
signatures in the solar system are more likely the result of self-shielding
during N2 photodissociation, exotic effect during photodissociation of N2
and/or low temperature ion-molecule isotope exchange. Kinetic N isotope
fractionation may play a significant role in the Titan s atmosphere. We also
suggest that the low delta15N values of Archaean organic matter are partly the
result of abiotic synthesis of organics that occurred at that time.
