Simulation of the isotopic fractionation of argon at Mars for current conditions

Introduction

Curiosity has measured the concentrations of three isotopes of argon in the Martian atmosphere: ³⁶Ar, ³⁸Ar and ⁴⁰Ar [1]. The main isotope, ⁴⁰Ar, is produced from internal radioactive decay of ⁴⁰K, but the two minor isotopes ³⁶Arand ³⁸Ar are primordial volatiles carried during the formation of the planets. The ³⁶Ar/³⁸Ar ratio measured on giant planets and Earth have a value of 5.5 equal to the solar value, showing that most of these species have been retained during the history of the planets. On Mars, the ratio measured by Curiosity is 4.2±0.1 [1], suggesting a fractionation due to a slightly more efficient escape of the light isotope ³⁶Ar compared to ³⁸Ar. The fractionation of argon can be explained by the molecular diffusion in the Martian thermosphere and by the different efficiency of the escape processes for the two isotopes [2].

Modeling

To simulate this vertical variation of the isotope abundance of argon in the Martian upper atmosphere, we used PCM-Mars, a 3D model of the Martian atmosphere from the surface to the exobase [3,4]. Recent simulations have shown that simulated density of the main isotope (⁴⁰Ar) near the exobase, is in good agreement with the density measured by MAVEN/NGIMS [5,6]. In the simulations presented here, we have added the two minor isotopes of argon (Fig.1), initialized uniformly in the atmosphere at their observed mixing ratio. After a few days of simulation, their vertical profiles reach a steady state. In Fig. 1, the simulated ³⁶Ar/³⁸Ar increases from 4.2 in the lower atmosphere to 10 at the top boundary and the simulated ³⁶Ar/⁴⁰Ar at the top of the model is ~5.0 times the value in the lower atmosphere (Fig. 1 right panel).

Fig. 1 Left: Examples of vertical profiles of the main atmospheric species in the Martian upper atmosphere, near Ls = 360° at dayside. Central panel: Vertical profiles of the mixing ratios of CO₂, O, He and the three Ar isotopes at the same time and location. Right panel :Vertical profiles of two argon isotopic ratios at the same time and location.

Several processes contribute to atmospheric escape but their relative efficiency is mass dependent [7, 8]. For argon, a heavy noble gas, Jeans escape, dominant for H, is negligible. The hot oxygen produced by the dissociative recombination of O₂⁺ (the main ion in the Martian ionosphere) can escape and transfer a part of its energy to the other atmospheric species. This is the main channel for helium escape [7] but the energy is not sufficient to induce an escape for argon, even if it can produce an extended hot argon component in the exosphere [8]. The atmospheric escape processes for argon are (1) ion escape driven by the solar wind interaction and (2) atmospheric sputtering by incident energetic O⁺ ions, detected by MAVEN [9]. These two processes and their consequences on the argon fractionation will be discussed.

References

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