The escape of O+ ions from the atmosphere: an explanation of the observed ion density profiles on Mars

Significance Statement

Gas phase ionization processes induced by energetic photons, electrons or by excited metastable neutral species play a key role in numerous phenomena occurring in low energy ionized plasmas and electric discharges. Ionic species are present in the upper atmosphere of planets; they govern the chemistry of ionospheres. The chemistry of the ionosphere of Titan has been revealed to be active by the instruments of beard Cassini. Above all, molecular ions have been detected in comet tails.

In space, ions are formed in various ways, the importance of which depends on specific conditions of the extraterrestrial environment considered. The interactions of neutral molecules with Ultra violet photons, cosmic rays, x-rays and other phenomena, for instance, shock waves, are important processes for their production. In specific, absorption of ultra violet photons with an energy higher than the ionization energy of the absorbing species, can induce ionization with the formation of both singly and doubly charged ions (also called dications).

Researchers led by Professor Stefano Falcinelli from the University of Perugia, proposed to investigate the escape of O+ ions from the atmosphere of Mars and Titan (the largest satellite of Saturn), giving an explanation of the observed ion density profiles on Mars. They realized that when they double ionized a molecule, they would modify deeply its chemical behavior. The removed electrons would change sensibly the electronic configuration of the neutral species, changing its chemical reactivity.  The double ionization can produce a molecular dication inducing Coulomb explosion and fragment ions formation with a high kinetic energy content of several eV. Once formed, these ionic species are able to reach sufficient velocity to escape into space, contributing to the continuous erosion of the atmosphere of some planets of the Solar System, like Mars and Titan. Their wok is now published in peer-reviewed journal, Chemical Physics letters.

In their experiments, they are able to simulate in laboratory the rarefied gaseous environmental conditions of astrochemical interest, as for example the upper planetary atmospheres. In such experiments, the monochromatic energy from the selected synchrotron light beam crosses at right angles an effusive molecular beam of carbon dioxide neutral precursors, and the product ions are then detected in coincidence with photoelectrons. The used coincidence electron-ion-ion extraction and detection system consists in a time of flight mass spectrometer equipped with ion position sensitive detector. Such a detector is designed to properly measure the spatial momentum components of the dissociation ion products.

Carbon dioxide is a simple molecule of great interest for interstellar medium and planetary atmospheres, not only for the Earth, but for other planets like Mars, Venus and Titan. In fact, carbon dioxide is a well-known greenhouse gas, and has been found to be the main component in the Mars, Venus, and together with acetylene molecules, has been detected as a minor component in Titan atmosphere. The presence of carbon dioxide in the interstellar medium has been demonstrated by microwave spectroscopy.

The main goal of such a research is to highlight that the dissociative double ionization photoionization process induced by VUV photons and producing fragment ions with a high kinetic energy content, would be in general an important mechanism for ionic species to escape from the atmosphere of some planets. The double charged species can be produced in the upper atmospheres of planets by the double ionization of the neutral precursor induced by cosmic rays, ultra violet photons and X-rays. After they are formed, by Coulomb explosion they can form fragment ions having a high kinetic energy content able to allow them to escape into space.

About The Author

Stefano Falcinelli (ORCID ID: – ResearcherID: A-7871-2012 – Scopus Author ID: 6701373) is a Professor of “Chemistry and Materials Technology” at the University of Perugia (Italy). He received his Ph.D. in Chemical Sciences in 1994 from the University of Perugia (Italy), and during 1994-1995 he was a Postdoc at Stanford University, CA (USA) with Prof. Richard N. Zare.

His research ranges from the dynamics of elementary chemical reactions of atmospheric and astrochemical interest to applications of chemistry to problems related to human health. He spent several periods of study and research as a visiting professor at several universities in the USA (Stanford University, CA, University of Rochester, NY) and France (Université de Paris Nord XIII). He is co-author of 1 book and 128 international peer-reviewed publications.

Journal Reference

Stefano Falcinelli1, Fernando Pirani2, Michele Alagia3, Luca Schio3, Robert Richter4, Stefano Stranges3,5, Franco Vecchiocattivi1. The escape of O+ ions from the atmosphere: an explanation of the observed ion density profiles on Mars. Chemical Physics Letters, volume 666 (2016), pages 1–6.

Show Affiliations
  1. Department of Civil and Environmental Engineering, University of Perugia, Via G. Duranti 93, 06125 Perugia, Italy
  2. Department of Chemistry, Biology and Biotechnologies, University of Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy.
  3. IOM-CNR Tasc, km 163.5, Area Science Park, 34149 Basovizza, Trieste, Italy.
  4. Elettra-Sincrotrone Trieste, Area Science Park, 34149 Basovizza, Trieste, Italy.
  5. Department of Chemistry and Drug Technology, University of Rome ‘‘La Sapienza’’, 00185 Rome, Italy.


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