Meteoric Metal Chemistry in the Martian Atmosphere

John M.C. Plane (1) , Juan Diego Carrillo-Sanchez (1) , T. P. Mangan (1) , Matteo M. J. Crismani (2) , Nicholas M. Schneider (2) , Anni Määttänen (3)

1 School of Chemistry [Leeds]
2 LASP - Laboratory for Atmospheric and Space Physics [Boulder]
3 PLANETO - LATMOS

Recent measurements by the Imaging Ultraviolet Spectrograph (IUVS) instrument on NASA's Mars Atmosphere and Volatile EvolutioN (MAVEN) mission show that a persistent layer of Mg⁺ ions occurs around 90 km in the Martian atmosphere, but that neutral Mg atoms are not detectable. These observations can be satisfactorily modeled with a global ablation rate of 0.06 metric tonnes of magnesium sol^-1, out of a cosmic dust input of 2.7 ± 1.6 t sol^-1. The absence of detectable Mg at 90 km requires that at least 50% of the ablating Mg atoms ionize through hyperthermal collisions with CO₂ molecules. Dissociative recombination of MgO⁺.(CO₂)n cluster ions with electrons to produce MgCO₃ directly, rather than MgO, also avoids a buildup of Mg to detectable levels. The meteoric injection rate of Mg, Fe and other metals – constrained by the IUVS measurements - enables the production rate of metal carbonate molecules (principally MgCO₃ and FeCO₃) to be determined. These molecules have very large electric dipole moments (11.6 and 9.2 Debye, respectively), and thus form clusters with up to 6 H₂O molecules at temperatures below 150 K. These clusters should then coagulate efficiently, building up metal carbonate-rich ice particles which can act as nucleating particles for the formation of CO₂-ice clouds. Observable mesospheric clouds are predicted to occur between 65 and 80 km at temperatures below 95 K, and above 85 km at temperatures about 5 K colder.

Mots clés: Cosmic dust; Ablation; Meteoric metals; Mars magnesium layer; Mars mesospheric clouds

Domaines: Astrophysique [astro-ph]; Autre

John M.C. Plane, Juan Diego Carrillo-Sanchez, T. P. Mangan, Matteo M. J. Crismani, Nicholas M. Schneider, et al.. Meteoric Metal Chemistry in the Martian Atmosphere. Journal of Geophysical Research. Planets, 2018, 123 (3), pp.695-707. ⟨10.1002/2017JE005510⟩. ⟨insu-01705753⟩