Doubly-charged ions in the planetary ionospheres: a review

Abstract

This paper presents a review of the current knowledge on the doubly-charged atomic and molecular positive ions in the planetary atmospheres of the Solar System. It is focused on the terrestrial planets which have a dense atmosphere of N(2) or CO(2), i.e. Venus, the Earth and Mars, but also includes Titan, the largest satellite of Saturn, which has a dense atmosphere composed mainly of N(2) and a few percent of methane. Given the composition of these neutral atmospheres, the following species are considered: C(++), N(++), O(++), CH(4)(++), CO(++), N(2)(++), NO(++), O(2)(++), Ar(++) and CO(2)(++). We first discuss the status of their detection in the atmospheres of planets. Then, we provide a comprehensive review of their complex and original photochemistry, production and loss processes. Synthesis tables are provided for those ions, while a discussion on individual species is also provided. Methods for detecting doubly-charged ions in planetary atmospheres are presented, namely with mass-spectrometry, remote sensing and fine plasma density measurements. A section covers some original applications, like the possible effect of the presence of doubly-charged ions on the escape of an atmosphere, which is a key topic of ongoing planetary exploration, related to the evolution of a planet. The results of models, displayed in a comparative way for Venus, Earth, Mars and Titan, are discussed, as they can predict the presence of doubly-charged ions and will certainly trigger new investigations. Finally we give our view concerning next steps, challenges and needs for future studies, hoping that new scientific results will be achieved in the coming years and feed the necessary interdisciplinary exchanges amongst different scientific communities.

The Escape of O^+ and CO^+ Ions from Mars and Titan Atmospheres by Coulomb Explosion of CO22+ Molecular Dications

“…In the case of O + formation by VUV double photoionization of carbon dioxide molecules, this energy is large enough for the atmosphere of Mars and Titan to allow these fragments to reach enough velocity to escape into space contributing to the continuous erosion of these atmospheres. In Table I there is reported the range of measured KER values for fragments ions produced by VUV dissociative double photoionization of CO 2 in comparison with the typical escape energy for various ions at the exobase in the atmosphere of Earth, Venus, Mars, and Titan [23]. In particular, considering the ionosphere of Mars, the dication (by UV light double photoionization of CO 2 ) followed by its Coulomb explosion and subsequent production of O + fragments having a high translational energy (ranging between 1.0 and 5.0 eV -see Fig.…”

“…Finally, single or double ionization can occurs also by absorption of X-rays. In this case the ejection of a core electron followed by the Auger emission of another electron, produces molecular dications, which have been suggested to play a role in the envelope of young stellar objects [22] and upper planetary atmospheres [23][24][25][26].…”

Molecular Fragmentation of Acetylene by VUV Double Photoionization

“…Finally, it has to be noted that the translational energy content of each H + , C + , CH + and CH2 + product ion is quite big, having an average value ranging between 4.0, 2.2, 2.5 and 2.0 eV, respectively. This kinetic energy is sufficient to allow this species participating in the atmospheric escape from Mars and Titan where they are characterized by a typical escape energy of 0.13 and 0.02 eV (in the case of H + ), 1.5 and 0.28 eV (in the case of C + ), 1.6 and 0.30 eV (for CH + ), 1.8 and 0,32 eV (for CH2 + ), respectively [17,21].…”

“…Such doubly charged ionic species can be produced also by cosmic rays, which are significant since they are ubiquitous in space, carrying a large energy content (up to 100 GeV), and consisting of protons, alpha particles, electrons, γ-and X-rays, and (to a small extent) also heavier nuclei (such as C 6+ ). For such reasons, molecular dications have been suggested to play a role in the envelope of young stellar objects [16] and upper planetary atmospheres [17][18][19][20].…”

First detection of the O III 495.8911 and 500.6843 nm lines in the Earth's upper atmosphere

“…These species are difficult to measure in the atmosphere because of their low density and their charge to mass ratio which is often equal to another singly-charged ion. They are interesting to study for their exotic or unexpected photo-chemistry and their high reactivity (Thissen et al, 2011). This paper focuses on the O ++ ion, which was detected in the terrestrial atmosphere in 1967 by mass spectrometry (Hoffman, 1967).…”