Pathway Study of Cl-cycle on Mars, Step-I & II: Oxychlorine Salts and Electrostatic Discharge Phenomenon in a Mars Chamber

Abstract:

The directly measured high ClO₄^-/Cl^- ratio (4.3-8.75) at Phoenix site and the implied ClO₄^- existences at Curiosity and Viking sites reminded Mars science community on the importance of (1) the global distribution of ClO₄^-/Cl^- ratio; (2) the mechanisms that are responsible for Cl^- to ClO_y^- (y=1,2,3,4) transformation; and (3) the current and historical Cl^- cycle on Mars.

Our goal is to study electrostatic discharge (ESD) in a Mars Chamber, as one of the four proposed mechanisms for the formation of Martian perchlorate. ESD was anticipated during dust storm/devil on Mars. A model estimated that ESD generated oxidants can be 200 times of those produced by photochemistry.

Our study is conducted in three steps. Firstly, oxychlorine salts, NaClO_y, Mg(ClO₄)₂.xH₂O (x=0,6), and Ca(ClO₄)₂.xH₂O (x=0,4), were analyzed at ambient conditions using MIR, NIR (1.4-2.6 µm), Raman spectroscopy, and in a Mars Chamber using in-situ NIR and Raman spectroscopy. Our purpose is to understand their phase transition and spectral change at Mars pressure (P) and temperature (T) conditions. We have found: (1) Under current surface/subsurface P-T conditions in mid-latitudes/equatorial regions on Mars, Mg(ClO₄)₂.6H₂O and Ca(ClO₄)₂.4H₂O are stable, while the hydration degree of NaClO_4.H₂O would increase at T<-30℃ and decrease in 5<T<21℃with dehydration rate increases at higher T; (2) Within the P-T range anticipated in above environments, Raman ν1 peak positions of hydrous and anhydrous oxychlorine species have no obvious change, but H₂O peak would down-shift when T decreases; (3) In Mars 2020 and ExoMars, the Raman ν1 peak can be used to characterize oxychlorine salts, e.g., the oxidation degree changes from Cl⁺ to Cl⁷⁺ in NaClO_y^- that causes over 243 cm^-1 shift, the cation changes from Na⁺ to Mg²⁺ to Ca²⁺ that cause over 33 cm^-1 shift; (4) The dehydration of NaClO₄.H₂O was quantified by in situ NIR spectroscopy, based on which the dehydration rate as function of T was derived.

We are currently conducting ESD in a Mars Chamber, to detect the oxidant species in ESD generated plasma, first in pure CO₂ atmosphere then in simulated Mars gas mixture. Results will be reported at conference. Our step-3 will study the production/yield from chlorides mixed with Mars-relevant materials to oxychlorine salts through ESD process.