GAS-PHASE SEQUESTRATION OF NOBLE GASES IN THE PROTOSOLAR NEBULA: POSSIBLE CONSEQUENCES ON THE OUTER SOLAR SYSTEM COMPOSITION
We address the problem of the sequestration of Ar, Kr, and Xe by H{sub 3}{sup +} in the gas-phase conditions encountered during the cooling of protoplanetary disks when H{sub 3}{sup +} is competing with other species present in the same environment. Using high-level ab initio simulations, we try to quantify other sequestration possibilities involving He, H{sub 5}{sup +}, H{sub 2}O, and H{sub 3}O{sup +} present in the protosolar nebula. Apart from the fact that H{sub 3}{sup +} complexes formed with heavy noble gases are found to be by far much more stable than those formed with He or H{sub 2}O, we show that H{sub 2}D{sup +} and H{sub 3}O{sup +}, both products of the reactions of H{sub 3}{sup +} with HD and H{sub 2}O, can also be efficient trapping agents for Ar, Kr, and Xe. Meanwhile, the abundance profile of H{sub 3}{sup +} in the outer part of the nebula is revisited with the use of an evolutionary accretion disk model that allows us to investigate the possibility that heavy noble gases can be sequestered by H{sub 3}{sup +} at earlier epochs than those corresponding to their trapping in planetesimals. We find that H{sub 3}{sup +} might be abundant enough in the outer protosolar nebula to trap Xe and Kr prior their condensation epochs, implying that their abundances should be solar in Saturn's current atmosphere and below the observational limit in Titan. The same scenario predicts that comets formed at high heliocentric distances should also be depleted in Kr and Xe. In situ measurements, such as those planed with the Rosetta mission on 67P/Churyumov-Gerasimenko, will be critical to check the validity of our hypotheses.