Editor's choiceThe escape of O+ ions from the atmosphere: An explanation of the observed ion density profiles on Mars
Graphical Abstract
Introduction
Gas phase ionization processes induced by energetic photons, electrons or by excited metastable neutral species (also called collisional autoionization reactions [1], [2], [3], [4]) play an important role in several phenomena occurring in low energy ionized plasmas and electric discharges [5], [6]. Furthermore, ionic species are extremely important in the upper atmosphere of planets, where they govern the chemistry of ionospheres [7], [8]. In particular, the chemistry of the ionosphere of Titan has recently been revealed to be extremely active by the instruments on board Cassini [9], [10]. Moreover, molecular ions have also been detected in comet tails [11].
In space, ions are formed in various ways, the importance of which depends on specific conditions of the extraterrestrial environment considered [12], [13], [14]. The interaction of neutral molecules with cosmic rays, UV photons, X-rays and other phenomena, such as shock waves, are all important processes for their production. In particular, the absorption of UV photons, with an energy content higher than the ionization potential of the absorbing species, can induce ionization with the formation of both singly and doubly charged ions. In the latter case we have the so called molecular dications. These ionic species can be produced by different techniques, as mass spectrometry [15], ion-molecule reactions [16], and double photoionization processes [17], [18], [19], [20]. They can be formed in stable or metastable states [14], [20], [21] and may be used, in principle, as energy storage at a molecular level [15], [16], [22]. Multiply charged ionic species can be produced also by cosmic rays, which are significant since they are ubiquitous and carry a large energy content (up to 100 GeV). They consist of protons, alpha particles, electrons, γ-rays and (to a small extent) also heavier nuclei (such as C6+) [8]. Cosmic rays are very penetrating and can induce ionization in objects that are completely opaque to UV photons. X-rays are relatively abundant in several regions, such as active galactic nuclei, young stellar objects, and planetary nebulae with hot central stars. 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 [23] and upper planetary atmospheres [24], [25], [26], [27].
In general, when we ionize a molecule we can modify deeply its chemical behavior. In fact, first of all, the removed electron may change sensibly the electronic configuration of the neutral species, modifying its chemical reactivity. In addition, the ion-molecule interaction is much more intense than the neutral-neutral one, making more probable any collision event, and increasing the chemical reactivity in rarefied gaseous environments as those of astrochemical interest. Finally, the double ionization producing a molecular dication can induce Coulomb explosion and fragment ions formation with a high kinetic energy content. For such reasons molecular dications are considered as exotic species and, when they are formed in planetary ionospheres, the possibility to generate dissociative products with a kinetic energy of several eV, allows these ionic fragments to reach sufficient velocity to escape into space. Therefore, double ionization processes can contribute to the continuous erosion of the atmosphere of some planets of the Solar System, like Mars and Titan (the largest satellite of Saturn), as we point out in this letter.
Section snippets
Experimental
The data here presented and discussed have been recorded by double photoionization experiments performed at the ELETTRA Synchrotron Light Laboratory (Trieste, Italy) using the ARPES (Angle-Resolved Photoemission Spectroscopy) end station of the Gas Phase Beamline. Details about the beamline and the end station have been already reported elsewhere [28], [29], and the apparatus used for the experiment discussed here has also been described previously [30], [32]. Therefore, only some features
Results and discussion
Carbon dioxide is a simple molecule of interest for interstellar medium (ISM) and planetary atmospheres (not only for the Earth but also for other planets of the Solar System, like Mars, Venus and also Titan). The presence of CO2 in the ISM has been demonstrated by microwave spectroscopy [37]. Recently, CO2 + CO2 system was chosen as representative of the role that intermolecular interaction plays in collision dynamics [38]. In particular, rotovibrational energy exchange, associated in rarefied
Conclusions
The dissociative double photoionization processes induced by VUV photons and producing fragment ions with a high kinetic energy content, could be in general an important mechanism for ionic species to escape from the atmosphere of some planets of the Solar System, like Venus, Mars and Titan. In fact, these processes occur via formation of intermediate molecular dications that could dissociate by Coulomb explosion towards the formation of two ionic fragment species having a kinetic energy
Acknowledgements
Financial contributions from the MIUR – Italy (Ministero dell’Istruzione, dell’Università e della Ricerca) through PRIN 2009 (Grant 2009W2W4YF_002) project is gratefully acknowledged. The authors also gratefully thank ‘Fondazione Cassa di Risparmio di Perugia’ for partial supports (Project code: 2014.0255.021).
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