Enhanced Martian metal layers during a close encounter of Comet C/2013 A1 Siding Spring: Evidence from observations and model simulations

 

 

Abstract

Comet siding Spring (C/2013 A1) is an Oort cloud comet discovered on 3January 2013 by Robert McNaught at Siding Spring observatory and passed extremely close to Mars (the close encounter occurred at 134000 km from the centre of Mars at a relative speed of 56 km/s)on 19 October 2014 around 18:28 UTC. The input of around 80 t of dust from the coma during a 3-hour period resulted in significant perturbation to the meteoric metal atom/ion layers, revealed by the measurements from the IUVS (Imaging UV Spectrometer) and NGIMS (Neutral Gas Ion Mass Spectrometer)) instruments on NASA’s Mars Atmosphere and Volatile Evolution Mission (MAVEN) spacecraft. This extremely event has also provided a unique nature case and a great opportunity to study how cometary dust deposit into the Mars atmosphere and investigate the impact of the significant perturbation by this record meteor shower   on the changes in the Mars whole atmosphere including the mesosphere and ionosphere. It will also help us to test the Mars’ model performance and validate the model under this unprecedent scenario.  

In order to better understanding the chemistry/dynamic/physical processes controlling the metal layers in this extreme event, we have implemented the chemistry of four metals (Mg, Na, Fe and Si) in the  Mars Planetary Climate Model (PCM-Mars-metals). The model has been developed by combining three components: the PCM 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, Na and Si 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 as a result of the ablation of cosmic dust particles under normal and extreme conditions. The Mars PCM 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. The Mg chemistry has 7 neutral and 8 ionized Mg-containing species, connected by 44 photo-ionization, neutral and ion-molecule reactions. The corresponding Fe chemistry has 39 reactions with 15 Fe-containing species; Na chemistry has 10 neutral and only 2 ionized Na-containing species, with 38 reactions; and Si chemistry has 8 species with 20 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. We have also generated meteoric injection rates based on a dust-flux model used for  Siding Spring (Moorhead et al., 2014). The PCM simulations are evaluated against observations of Mg⁺, Mg, Na⁺,Fe⁺ and Fe above 80 km from IUVS and NGIMS measurements. The enhancement of metals ions in the upper atmosphere and the other changes in atmospheric chemical species/electron density and possible atmospheric dynamic changes due to Siding spring will be reported.