Sequestration of Noble Gases by H⁺₃ in Protoplanetary Disks and Outer Solar System Composition

Abstract

We study the efficiency of the noble gas sequestration by the ion H⁺₃ in the form of X H⁺₃ complexes (with X = argon , krypton, or xenon) in gas-phase conditions similar to those encountered during the cooling of protoplanetary disks. We show that X H⁺₃ complexes form very stable structures in the gas phase and that their binding energies are much higher than those involved in the structures of X-H₂O hydrates or pure X-X condensates. This implies that in the presence of H⁺₃ ions, argon, krypton, or xenon are likely to remain sequestrated in the form of X H⁺₃ complexes embedded in the gas phase rather than forming ices during the cooling of protoplanetary disks. The amount of the deficiency depends on how much H⁺₃ is available and efficient in capturing noble gases. In the dense gas of the mid-plane of solar nebula H⁺₃ is formed by the ionization of H₂ from energetic particles such as those in cosmic rays or those ejected by the young Sun. Even using the largest estimate of the cosmic-ray ionization rate, we compute that the H⁺₃ abundance is 2 and 3 orders of magnitude lower than the xenon and krypton abundance, respectively. Estimating the ionization induced by the young Sun, on the other hand, is very uncertain but leaves the possibility of having enough H⁺₃ to make krypton and xenon trapping efficient. This may cause a deficiency of Kr, Xe, and to a lower extent of Ar, in the forming icy planetesimals. We then suggest that this sequestration mechanism may explain the deficiency of Titan in noble gases revealed by the Huygens probe measurements. Similarly, comets formed from crystalline water ice in the outer nebula should be also deficient in krypton and xenon, and to a lower extent in argon, in agreement with some recent observations.