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 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 six 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.

Keywords: Mars magnesium layer; Mars mesospheric clouds; ablation; cosmic dust; meteoric metals.

Figure 1

A schematic diagram of the neutral and ion‐molecule chemistry of magnesium in the Martian upper atmosphere. Ionized and neutral compounds are shown in blue and green boxes, respectively. The first building block of metal‐rich ice particles is shown in gray. Reactions with measured rate coefficients (see Table 1) are indicated with red arrows.

Figure 2

Injection rate of Na, Mg, Fe, and Si as a function of altitude, from the ablation of cosmic dust entering Mars atmosphere.

Figure 3

Trajectory calculation of Mg colliding with CO₂ at a relative velocity of 10.8 km s⁻¹ and producing MgO + CO. The change in potential energy as a function of time is shown, along with the molecular geometries at four points along the trajectory (Mg: yellow; oxygen: red; carbon: gray). Level of theory: b3lyp/6‐311+g(2d,p). An animation of the trajectory is included in movie S2.

Figure 4

Potential energy surface for the addition of CO₂ to the MgO⁺ ion, calculated at the CBS‐QB3 level of theory (Mg: yellow; O: red; C: gray).

Figure 5

Change in Gibbs free energy (ΔG) at 100 K for the formation of the nth H₂O cluster by addition of a single H₂O molecule to the (n − 1)th cluster, for MgCO₃ and FeCO₃.

Figure 6

Route from MgCO₃ to the stable ice nanoparticle. Initially up to four CO₂ molecules can bind to the MgCO₃, but these switch with the stronger H₂O ligand to form MgCO₃(H₂O)₆. These clusters can then coagulate with favorable free energies (ΔG, in kJ mol⁻¹).

Figure 7

Vertical profiles of Mg⁺, Mg, and MgCO₃ predicted by the 1‐D model for local noon at the equator, L _s = 85°. The symbols show Mg⁺ measurements by the Imaging Ultraviolet Spectrograph instrument on Mars Atmosphere and Volatile EvolutioN during two successive orbits.

Figure 8

Sensitivity of the 1‐D modeled Mg and Mg⁺ to the branching ratio β in reaction 20 and to the fraction of meteoric magnesium which ablates to neutral Mg atoms (see text for further details). The model base case is that illustrated in Figure 6. The blue cross marks the detection limit for Mg atoms at 90 km using Mars Atmosphere and Volatile EvolutioN's Imaging Ultraviolet Spectrograph instrument.

Figure 9

Size distribution of metal‐containing ice particles as a function of height, produced by a meteoric input of 2.7 t sol⁻¹.

Figure 10

(a) Threshold radius of a H₂O‐ice nucleating particles (NPs) that can be activated for nucleation and growth of CO₂‐ice, as a function of height in the Mars atmosphere, and for a selection of temperatures between 80 and 95 K. (b) Number density of NPs with a size equal to or greater than the threshold radius, as a function of temperature, and for a selection of altitudes between 65 and 95 km. The horizontal gray line indicates the number density required to form a CO₂‐ice cloud with the maximum optical thickness observed.