Impacts of Comet Siding Spring in the Middle Atmosphere of Mars

The meteoroid fluence at Mars during the passage of comet C/2013 A1 is predicted by Moorhead et al. (Icarus, 2014) to be 3 - 4 orders of magnitude higher than the sporadic background over a period of about 5400 s. We have applied a Chemical Meteoroid Ablation Model (CABMOD) to explore the impacts of this huge perturbation. Sporadic meteoroids enter the Martian atmosphere at low speeds (< 14 km s^-1), which results in incomplete ablation around 75 km and small levels of ionization. In contrast, the Siding Spring particles will enter at 56 km s^-1 and completely ablate around 100 km. Most of the ablated metal atoms from the cometary particles will ionize through hyperthermal collisions with CO₂ molecules, leading to an enhancement of the electron density around 100 km by a factor of at least 500 which should be easily observable by radio occultation.

The major meteoric metals - Fe and Mg - have emission lines falling within the wavelength range of MAVEN’s Imaging UV spectrometer. We have estimated the dayglow intensities of these species by scaling the dayglow radiances measured by terrestrial satellite spectrometers to the reduced solar irradiance at Mars, and using metal atom and ion concentration profiles in the Martian atmosphere from our 1D model (Whalley and Plane, Faraday Disc. 2010). The background intensities of Mg⁺ (280 nm), Mg (285 nm), Fe (248 nm) and Fe⁺ (260 nm) are then predicted to be 16.3, 2.5, 1.9 and 0.9 kR, respectively. These atomic lines should be readily observable against the strong molecular emissions from CO (185 and 270 nm) and CO₂⁺ (280 - 300 nm), even before the cometary input produces patches of dayglow orders of magnitude higher in intensity. The conversion of the cometary metal ions into neutral atoms should occur on a timescale of around 10 minutes because of the elevated electron densities, so time-resolved measurements of these emissions would provide a unique test of the chemistry of the lower ionosphere.

Finally, the cometary metal vapors will recondense into meteor smoke particles. We will explore the role of these particles in condensing CO₂ ice clouds in the Martian atmosphere, using results from a novel laboratory ion-trap experiment in which the nucleation and growth of CO₂ on nm-sized iron oxide and silicate particles has been observed for the first time.