Since the discovery of atmospheric Mg+ on Mars in 2015 by the Mars Atmosphere and Volatile Evolution mission, there have been almost continuous observations of this meteoric ion layer in a variety of seasons, local times, and latitudes. Here, we present the most comprehensive set of observations of the persistent metal ion layer at Mars, constructing the first grand composite maps from pooled medians of subsamples of a metallic ion species. These maps demonstrate that Mg+ appears in almost all conditions when illuminated, with peak density values varying between 100 and 500 cm(-3), dependent on season and local time. There exists significant latitudinal variation within a given season, indicating that Mg+ is not simply an inert tracer, but may instead be influenced by the meteoric input distribution and/or atmospheric dynamics and chemistry. Geographic maps of Mg+ density as a function of latitude and longitude indicate the influence of atmospheric tides, and there is no apparent correlation with remnant crustal magnetic fields. This work also presents counter-intuitive results, such as a reduction of Mg+ ions in the northern hemisphere during Northern Winter in an apparent correlation with dust aerosols.

It is evident that a variety of metals are deposited in the Earth’s mesosphere and lower thermosphere (MLT, ~70-120 km) through meteoric ablation when the cosmic dust particles enter the atmosphere at high entry velocity (11-72 km/s). However, it is still unclear how much and accuarate cosmic dust enters the atmosphere of Mars (though the estimation of global dust input would be a few tons per sol) and what is the difference comparing to Earth’s atmosphere (which has a 1-2 order global input range from different estimations). We have developed global atmospheric meteoric models of Na, Fe, K, Mg, Ni, Ca, Al, Si, P, S etc) into the Whole Atmosphere Community Climate Model (WACCM) and its vertical extensions to 600 km (WACCM-X) from US National Center for Atmospheric Research (NCAR, termed WACCM-metals), which simulate well the metal layers compared with the available lidar/rocket/satellite measurements. New observations of some metals for the Martian atmosphere (i.e., Mg+ observations from IUVS (Imaging UV Spectrometer) and Mg+, Na+ and Fe+ from NGIMS (Neutral Gas Ion Mass Spectrometer)) instruments on NASA’s Mars Atmosphere and Volatile Evolution Mission (MAVEN) spacecraft are available from 2014. Therefore, we have incorporated the chemistry of three metals (Mg, Na and Fe) in the Laboratoire de Météorologie Dynamique (LMD) Mars global circulation model (termed as LMD-Mars-Metals), following similar work we have done for the Earth’s atmosphere. The model has been developed by combining three components: the state-of-the-art LMD-Mars model covering the whole atmosphere from the surface to the upper thermosphere (up to ~ 2 x10-8 Pa or 240 km), a description of the neutral and ion-molecule chemistry of Mg, Fe and Na in the Martian atmosphere (where the high CO2 abundance produces a rather different chemistry from the terrestrial atmosphere), and a treatment of injection of the metals into the atmosphere from the ablation of cosmic dust particles. The LMD-Mars model contains a detailed treatment of atmospheric physics, dynamics and chemistry from the lower atmosphere to the ionosphere. The model also includes molecular diffusion and considers the chemistry of the C, O, H and N families and major photochemical ion species in the upper atmosphere, as well as improved treatments of the day-to-day variability of the UV solar flux and 15 mm CO2 cooling under non-local thermodynamic equilibrium conditions. We have incorporated the chemistries of Mg, Fe and Na into LMD-Mars because these metals have different chemistries which control the characteristic features of their ionized and neutral layers in the Martian atmosphere. The Mg chemistry adds 7 neutral and 8 ionized Mg-containing species, connected by 42 neutral and ion-molecule reactions. The corresponding Fe chemistry has 39 reactions with 14 Fe-containing species. Na chemistry adds 7 neutral and only 2 ionized Na-containing species, with 32 reactions. The injection rate of these metals as a function of latitude, solar longitude at different pressure levels is pre-calculated from the Leeds Chemical Ablation Model (CABMOD) combined with an astronomical model which predicts the dust from Jupiter Family and Long Period comets, as well as the asteroid belt, in the inner solar system. The model has been evaluated against by Mg+, Na+ and Fe+ observations from IUVS and NGIMS measurements. The comparison of these metal layers between Earth’s and Mar’s atmospheres will be discussed, which allows us to understand the meteor astronomy, chemistry and transport processes that control the different metal layers in the upper atmosphere on different planets.

<p class="Abstract">Since the discovery of atmospheric Mg<sup>+</sup> at Mars in 2015 by the Mars Atmosphere and Volatile Evolution (MAVEN) mission, there have been almost continuous observations of this meteoric ion layer in a variety of seasons, local times, and latitudes. Here we present the most comprehensive set of observations of the persistent metal ion layer at Mars, constructing the first grand composite maps of a metallic ion species. These maps demonstrate that Mg<sup>+</sup> appears in almost all conditions when illuminated, with peak values varying between 100 and 500 cm<sup>-3</sup>, dependent on season and local time. There exists significant latitudinal variation within a given season, indicating that Mg<sup>+</sup> is not simply an inert tracer, but instead may be influenced by the meteoric input distribution and/or atmospheric dynamics and chemistry. Geographic maps of latitude and longitude indicate that Mg<sup>+</sup> may be influenced by atmospheric tides, and there is no apparent correlation with remnant crustal magnetic fields. This work also presents counter-intuitive results, such as a reduction of Mg<sup>+</sup> ions in the northern hemisphere during Northern Winter in an apparent correlation with dust aerosols.</p>

Interplanetary dust particles, liberated from the surfaces of comets and asteroids, are ubiquitous in interplanetary space within the solar system. These particles travel at orbital velocities and ablate upon entry into planetary atmospheres, where they are the sole explanation for high altitude atmospheric metal layers. On Earth, such layers inform us of the dynamics of the upper atmosphere, and we use the abundance of relative species to investigate the origin of these particles from various potential sources (Jupiter family comets, asteroids, etc.). Since the discovery of atmospheric Mg+ at Mars in 2015, there have been almost continuous observations of this layer in a variety of seasons, local times, and latitudes. Here we present the most comprehensive set of observations of the persistent metal ion layer at Mars, constructing the first grand average maps of metal ions species. Such maps can be compared to current and future modeling efforts, which attempt to track mesospheric transport, chemistry and interplanetary dust particle sources. This work confirms some previous model predictions and observations, such as the relatively long lifetime of Mg+, but also presents counter-intuitive results, such as a paucity of Mg+ ions in the northern hemisphere during Northern Winter in an apparent correlation with dust aerosols. Previous discrepancies between model predictions and metal ion observations led to the development of a novel nucleation scheme for mesospheric clouds, and we revisit these ideas on a global and seasonally varying scale. Overall, this represents the broadest investigation of meteoric metal ions, summarizes the first order behavior and outlines new model challenges for the future.