Meteoric Metal Chemistry in the Martian Atmosphere

Abstract

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 90 km 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 0.06 t 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 CO 2 molecules. Dissociative recombination of MgO + .(CO 2 ) n cluster ions with electrons to produce MgCO 3 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 MgCO 3 and FeCO 3 ) 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 H 2 O 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 CO 2 ‐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.

The Impact of Comet Siding Spring's Meteors on the Martian Atmosphere and Ionosphere

“…Persistent layer observations of Mg + (Crismani et al, ) offer the only constraints on the meteoric chemistry of atomic Mg, which can be, but has not been, observed by IUVS. Figure demonstrates that the chemical model from Plane, Carrillo‐Sanchez, et al () does predict the abundance of Mg to a reasonable accuracy. However, as these are the only observations of Fe + and Fe in the atmosphere of Mars, it is not surprising that, though the shape of Fe + and Fe are roughly consistent, the model overestimates their abundances.…”

“…The model is then run for more than 60 hr with relevant vertical transport and chemistry (Figure ). Because the Siding Spring dust entered the atmosphere at a velocity of 56 km/s, the peak ablation altitudes are ~30 km higher than predicted for the persistent layer (Crismani et al, ; Plane, Carrillo‐Sanchez, et al, ). The correlation of dust velocity with ablation altitude is counterintuitive when compared to auroral or energetic particle processes.…”

“…These retrieved densities are averaged across the orbit; therefore, they do not represent spatial and vertical variability that exists for each of the species (see Figure ). The model profiles are scaled to the retrieved densities of Mg+, whose chemistry is most closely constrained by previous Imaging Ultraviolet Spectrograph observations (Plane, Carrillo‐Sanchez, et al, ).…”

“…In orbit 116, there is a 50% reduction of Mg abundance from western to eastern scans, and in orbit 117 this reduction is closer to 75% (Figure , right bottom row). This rapid loss of neutral Mg is not fully captured by the 1‐D model (sections 2.3 and 3.5), despite the new chemistry that was developed to account for the lack of observable Mg in the persistent layer (Crismani et al, ; Plane, Carrillo‐Sanchez, et al, ). The 1‐D model's absence of horizontal winds and diurnal variation may suggest the mechanisms for this rapid loss of the neutral species.…”

Ion‐Neutral Coupling in the Upper Atmosphere of Mars: A Dominant Driver of Topside Ionospheric Structure

“…The first region is the lower ionosphere. It lies below ~150 km, is photochemically driven, includes a main plasma layer and often one or more lower altitude plasma layers, and is produced by absorption of extreme ultraviolet, X‐ray photons, and meteor ablation (Fallows et al, ; Molina‐Cuberos et al, ; Plane et al, ; Rishbeth & Mendillo, ). Observations of the lower ionosphere of Mars have been adequately interpreted using photochemical equilibrium theory and subsequently well reproduced by numerical simulations (e.g., Fallows et al, ; Fox & Dalgarno, ; Fox et al, ; Fox & Weber, ; Fox & Yeager, , ; Krasnopolsky, ; Lollo et al, ; Martinis et al, ).…”