Pergamon www.elsevier.com/locate/asr Adv. SpaceRes. Vol.27, No. 11, pp. 1801-1806,2001 © 2001 COSPAR.Published by ElsevierScienceLtd. All rightsreserved Printedin GreatBritain 0273-1177/01 $20.00+ 0.00 PII: S0273-1177(01)00342-8 A MODEL OF THE MARTIAN IONOSPHERE BELOW 70 KM G.J. Molina-Cuberos 1'2, J.J. L6pez-Moreno 2, R. Rodrigo2, H. LichteneggeP, and K. Schwingenschuh 1 1 Space Research Institute, Dept. of Experimental Space Research, Austrian Academy of Sciences, Inffeldgasse 12, A-8010 Graz, Austria. 2 Instituto Astroflsica Andalueia, CSIC, PO Box 3004, 18080, Granada, Spain ABSTRACT The ion-neutral chemistry of the atmosphere of Mars has been studied with a one-dimensionalmodel. Here we present the main reactions that lead to the formation of the most abundant positive and negative ions. Galactic cosmic rays ionize the atmospheric constituents, mainly C02, and reactions of charge transfer of CO + with oxygen produces O +. The further ion-neutral chemistry is quite similar to that of D-region on Earth. The relative high abundance of water and its higher proton affinity compared with other molecules, lead to the expectation that the most abundant positive ions are hydronium hydrates, H+O(H20),. The chemistry includes the production of negative ions, O- and O~-, by electron capture of oxygen-bearing molecules and sinks of negative ions by electron photodetachment and ion-ion re- combination. Oxygen negative ions are transformed through a chain of reactions into the carbon and nitrogen-bearing ions, COn and NOn. The formation of hydrated negative ions is also considered. © 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved. INTRODUCTION The upper ionosphere of Mars has been sounded by radio occultation during day and night-time flybys by Mariners 4, 6, 7, and 9, Mars 2, 4, 5, and 6, and Viking 1 and 2. Upper ionospheric models, which consider solar radiation during daytime (Winchester and Ress, 1995) and magnetospheric electrons during nighttime (Haider, 1997) explain the concentration of electrons obtained during these experiments and the concentration of the most abundant ions, O +, O +, and CO +, measured by the Retarding Potential Analyzer (RPA) instrument, carried aboard the Viking Landers (Hanson et al., 1977). Most of the work concerning the Martian ionosphere has been devoted to explaining the upper ionosphere, above 100 km. The most detailed study of the lower ionosphere of Mars data has been carried out since the beginning of the 70's. Whitten et al. (1971) considered the ionization by cosmic rays and solar radiation in an ion-neutral model and calculated the concentration of ions and electrons below 80 km. They concluded that Mars should have a D-region and a permanent low-lying electron layer that becomes quite distinct at night. Here we continue the study of Mars' lower ionosphere by making use of the actual knowledge of Mars' aeronomy. We calculate the ionization rate by cosmic rays and develop a detailed ion-neutral model which includes more neutral compounds and reactions rates than previous works. The knowledge of the ion composition and understanding of the ion chemistry below the main ionization peak is required to fully understand the atmospheric electricity problems and wave propagation. ION NEUTRAL MODEL The concentrations of each ionic species and electrons, are calculated from the continuity equation which includes production of cations and electrons by cosmic rays, anion production by electron attachment to neutral, positive and negative reactions with neutrals, photodissociation of positive and negative ions, electron photodetachment of anions, and electron-ion and ion-ion recombination. Assuming steady-state conditions and neglecting transport, the concentration of individual cation n +,, anion n~- and electron n~ are calculated from: kj , (1) n+ = Y] g~nk + v~iene+ ~iin- + F i k 1801