Meteoric Metal Chemistry in the Martian Atmosphere

Recent measurements by the Imaging Ultraviolet Spectrograph (IUVS) instrument on NASA's Mars Atmosphere and Volatile EvolutioN mission show that a persistent layer of Mg+ ions occurs around 90km in the Martian atmosphere but that neutral Mg atoms are not detectable. These observations can be satisfactorily modeled with a global meteoric ablation rate of 0.06tsol(-1), out of a cosmic dust input of 2.71.6tsol(-1). The absence of detectable Mg at 90km 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 metalsconstrained by the IUVS measurementsenables 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 six H2O molecules at temperatures below 150K. 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 80km at temperatures below 95K and above 85km at temperatures about 5K colder. Plain Language Summary When interplanetary dust particles enter a planetary atmosphere, collisions with air molecules cause heating and evaporation, a process termed meteoric ablation. This results in the continuous injection of metal atoms and ions into the planet's atmosphere. In the case of Earth, layers of metals such as Na and Fe have been observed for over 40 years. However, only very recently has a metallic layer been observed around another planet: a spectrometer on NASA's MAVEN spacecraft has detected a layer of Mg+ ions around 95 km. The present study explores the unusual chemistry of metallic ions in a CO2 atmosphere, and then develops a model of magnesium chemistry to explain the observed layer of Mg+ and the surprising absence of a detectable Mg layer. The model predicts that metals like Mg and Fe form carbonates, which readily condense water to form dirty ice particles at the low temperatures of the Mars upper atmosphere. These particles provide the seeds on which CO2 can condense at temperatures below -180 degrees C, thus producing the clouds of CO2-ice particles that have been observed by rovers on the surface of the planet and from orbiting spacecraft.