Dielectric Properties of Magnesium and Calcium Perchlorate Solutions: Implications for Subglacial Liquid Water on Mars

Dielectric Properties of Magnesium and Calcium Perchlorate Solutions: Implications for Subglacial Liquid Water on Mars 

Gabriele Turchetti¹, Barbara Cosciotti¹, Sebastian Emanuel Lauro¹, Elisabetta Mattei¹, Elena Pettinelli¹ 

¹ Mathematics and Physics Dept., Roma Tre University 

*Corresponding author: Gabriele.turchetti@uniroma3.it

Introduction The presence of perchlorates on Mars has been a significant focus of planetary exploration and astrobiology research due to the implications for both potential habitability and geological history of the planet. Calcium perchlorate Ca(ClO₄)₂ and magnesium perchlorate Mg(ClO₄)₂, have been detected through various missions highlighting their widespread presence across the planet [1][2]. The ability of perchlorates to lower the freezing point of water could allow the existence of liquid water solutions under Martian surface. An interesting site is the subglacial liquid water body detected by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS [3]) [4][5]. The analysis of perchlorates properties is fundamental to assess the physical-chemical conditions that allow the water to remain liquid in Martian subsurface. This work aims to measure electromagnetic properties of Magnesium and Calcium perchlorates solutions to understand the characteristics of possible stable brines in the Martian subsurface and to explain the conditions of the subglacial lake detected by MARSIS. The general behaviour of a water solution is explained by the eutectic diagram (fig.1 [6]). Key parameters are the eutectic concentration C_e and the eutectic temperature T_e. At the eutectic temperature water solutions with eutectic concentration of perchlorates remain liquid even below the freezing point of water. Between the eutectic temperature and the freezing point of pure water, a solution with a lower concentration than the eutectic is in a mixture of ice and brine (mush). Below the eutectic temperature all the solution is frozen. Eutectic parameters change with the salt, then different salts low differently the freezing point of water.   

 

Figure1: Eutectic diagram [5]. 

Methods  We measured the complex dielectric permittivity ε (eq.1) in the temperature range 195K-290K to include the eutectic temperatures of Calcium and Magnesium perchlorates (respectively 198K [7] and 216K [8]) in a radar sounder frequency spectrum (1MHz - 100MHz). At radar frequencies the expected real part ε' of an ice solid solution is around 3.1 and that of liquid water solutions is 80 or above, for the mushy mixture we expected an intermediate value. We computed also the conductivity σ (eq.2) and the apparent permittivity ε_a (eq.3), indicative of the reflection coefficient Γ₁₂ of an interface between two media with two different permittivity ε₁, ε₂ [9]. To reproduce MARSIS data consistent with the subglacial water body it should be larger than 30 [4]. 

Sample preparation  The samples are prepared by mixing double distilled water with granular perchlorates and pouring the solution into a measurement cell with a coaxial cage transmission line inside. The cell is placed in an ultra-freezer at the temperature of 193K with a pt100 sensor inserted in the sample to monitor the temperature. After some days, we take out the sample (fig.2) and measure the electromagnetic properties using a Vector Network Analyzer (VNA) increasing the temperature from 195K to the room temperature of 292K. The VNA measures scattering parameters, and the complex permittivity is estimated by applying the Nicholson-Ross-Weir algorithm [10][11]. Further details can be found in [12]. 

 

Figure 2: Mushy sample just taken out from the ultra-freezer. 

Results  We performed measurements with different setups changing the cell length, the concentration, the time inside the ultra-freezer. Fig.3 and 4 show the trends of ε′ and σ with temperature, measured at 4MHz (working frequency of MARSIS), of 5wt% and 10wt% solutions of Mg(ClO₄)₂ and Ca(ClO₄)₂ inside the 150mm cell, after 4 days in the ultra-freezer. The general trend is similar with lower values of both parameters for the lower concentration. The Mg(ClO₄)₂ 10wt% solution did not freeze completely, even below the T_e; its ε′ is coherent with a mush. The solution 5wt% froze. Due to its lower T_e, Ca(ClO₄)₂ samples did not freeze completely even at lower concentrations. Approaching 273K samples are almost melted and both parameters rise to expected values. For Ca(ClO₄)₂ the ε_a (fig.5) reaches the critical value to reproduce MARSIS data at 205K. 

Figure3: ε' and  σ of 2 measurements of different concentration of Mg(ClO₄)₂ solutions. 

Figure4: ε' and  σ of different measurements of Ca(ClO₄)₂ solutions. 

 

Figure5: ε_a of Ca(ClO₄)₂ solutions for different measurements. The black dashed line is the critical value of 30 to reproduce MARSIS data, the red dashed line represents the eutectic.

Conclusions  These results are important for the comprehension of the stability of perchlorates mushy solutions beneath Martian surface and give information about the possible composition of the subglacial lake detected by MARSIS. Further measurements with other concentrations and perchlorates mixtures will allow a complete understanding of the possible habitability of these environments.

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