[Paper Review] Sequestration of noble gases by H3+ in protoplanetary disks and outer solar system composition

O. Mousis, F. Pauzat+2 more|ArXiv.org|Sep 23, 2007

Atmospheric Ozone and Climate65 references20 citations

TL;DR

This paper proposes that noble gases (Ar, Kr, Xe) were sequestered in the form of XH₃⁺ complexes by H₃⁺ ions in the gas phase of the early solar nebula, preventing their incorporation into icy planetesimals. Using high-level quantum calculations, the authors show these complexes are highly stable, explaining the observed deficiencies of Kr and Xe in Titan and comets, with H₃⁺ from cosmic rays or young Sun activity enabling efficient trapping.

ABSTRACT

We study the efficiency of the noble gases sequestration by the ion H3+ in the form of XH3+ complexes (with X = argon, krypton or xenon) in gas phase conditions similar to those encountered during the cooling of protoplanetary disks, at the epoch of icy planetesimals formation. We show that XH3+ 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 presence of H3+ ions, argon, krypton or xenon are likely to remain sequestrated in the form of XH3+ 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 H3+ is available and efficient in capturing noble gases. In the dense gas of the mid-plane of solar nebula, H3+ is formed by the ionization of H2 from energetic particles, as those in cosmic rays or those ejected by the young Sun. Even using the largest estimate of the cosmic rays ionization rate, we compute that the H3+ abundance is two and three 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 to have enough H3+ to make krypton and xenon trapping efficent. Finally, additional source of H3+ formation may be provided by the presence of a nearby supernova, as discussed in the literature. Recent solar system observations show a deficiency of Ar, and, even more, of Kr and Xe in Titan and in comets. In this article, we consider the possibility that this deficiency is caused by the afore-mentioned process, namely trapping of those noble gases by H3+ ions in the solar nebula.

Motivation & Objective

• To explain the observed deficiency of krypton and xenon in Titan and cometary bodies, despite their expected abundance in outer solar system formation.
• To investigate whether noble gases could be sequestered in the gas phase via stable XH₃⁺ complexes during the cooling of protoplanetary disks.
• To assess the viability of H₃⁺-mediated noble gas trapping under conditions relevant to the solar nebula’s mid-plane.
• To evaluate the role of ionization sources—cosmic rays, young Sun activity, and nearby supernovae—in sustaining sufficient H₃⁺ for efficient noble gas capture.
• To provide theoretical spectroscopic signatures for future detection of XH₃⁺ complexes in space and laboratory validation.

Proposed method

• Employed high-level quantum chemical calculations at the CCSD(T)/cc-pVQZ level to determine the structure, binding energy, and vibrational frequencies of XH₃⁺ complexes (X = Ne, Ar, Kr, Xe).
• Used rotational and vibrational spectroscopic modeling to predict microwave and infrared signatures for experimental and observational detection.
• Compared theoretical rotational constants with experimental data from ArD₃⁺ to validate the accuracy of the computational model.
• Assessed the stability of XH₃⁺ complexes by comparing their binding energies to those of X·H₂O hydrates and pure X₂ condensates.
• Evaluated ionization rates from cosmic rays and young Sun activity to estimate H₃⁺ abundance relative to noble gas abundances in the solar nebula.
• Modeled the potential contribution of nearby supernovae as an alternative H₃⁺ source to enhance noble gas sequestration.

Experimental results

Research questions

• RQ1 Can XH₃⁺ complexes form stable structures in the gas phase under protoplanetary disk conditions, sufficient to trap noble gases?
• RQ2 How does the binding energy of XH₃⁺ compare to that of noble gas hydrates or pure noble gas condensates?
• RQ3 Is the abundance of H₃⁺ in the solar nebula sufficient to account for the observed deficiencies of Kr and Xe in Titan and comets?
• RQ4 What role do ionization sources—cosmic rays, young Sun activity, or supernovae—play in sustaining H₃⁺ levels for effective noble gas sequestration?
• RQ5 Can the theoretical spectroscopic properties of XH₃⁺ complexes enable future detection in space or laboratory experiments?

Key findings

• XH₃⁺ complexes (X = Ar, Kr, Xe) form highly stable gas-phase structures with binding energies significantly higher than those of X·H₂O hydrates or pure X₂ condensates.
• Theoretical rotational constants for ArH₃⁺ from the flexible model (A = 1490 GHz) agree within ~2% of the best experimental values, validating the accuracy of the CCSD(T)/cc-pVQZ calculations.
• The calculated vibrational frequencies and intensities for XH₃⁺ complexes (X = Ne–Xe) show distinct high-frequency internal modes and low-frequency interfragment motions, enabling spectroscopic identification.
• H₃⁺ abundance from cosmic rays is insufficient to explain Kr and Xe deficiencies, being two and three orders of magnitude lower than Kr and Xe abundances, respectively.
• Ionization from the young Sun or nearby supernovae may provide sufficient H₃⁺ to enable efficient sequestration of Kr and Xe, offering a viable mechanism for noble gas deficiency.
• The sequestration mechanism via XH₃⁺ complexes can quantitatively explain the observed Ar, Kr, and Xe deficiencies in Titan and comets, consistent with Huygens and FUSE observations.