Dielectric properties of CO2 clathrate hydrates for the exploration of the jovian icy moons

Introduction

CO₂ clathrate is one of the possible compounds in the crust of ice moons. The analysis of the data provided by the NASA Galileo mission revealed that the Jovian moons have both, the chemical and the physical conditions required for clathrate hydrates to be formed from the surface to deeper layers (Prieto-Ballesteros et al. 2005, Hand et al. 2006, Izquierdo-Ruiz et al. 2020).

The study of the dielectric properties of water-related minerals is key to understand the results of future missions to Jupiter and its satellites. The JUICE (ESA) and Europa Clipper (NASA) missions will try to determine the internal subsurface structure of these Jovian icy moons with the radar instruments onboard: RIME and REASON respectively. These instruments will operate both at a frequency of 9 MHz, with capacity to penetrate from 1 to 30 km depth. REASON has an extra antenna of 60 MHz to explore shallow depths (that is, 300 m to 4.5 km).

There are many works about the physical properties of clathrate hydrates (for instance: Sloan et al, 2007), but just a few of them are focused on their electrical properties in planetary conditions. Davidson (1973) and Davidson & Ripmeester (1978) determined the high frequency permittivity (ε_ꚙ) of several clathrates, finding a value of 2.85 at 233 K for clathrates of simple guest molecules such as argon and nitrogen. This value increases over 7.7 for other guest molecules and structures such as ethylene oxide and acetone. So far, only Stern et al. (2021) investigated CO₂- and CH₄- clathrate hydrates to determine their electrical conductivity, but not their permittivity.

In this work we present novel results for CO₂ clathrate including a combined electrical conductivity and permittivity study.

 

Methodology

We use a stainless-steel high pressure chamber with an internal volume of 60 ml for the formation of clathrate hydrates. In this cell we mixed distillated water with CO₂ gas at 30 bar of pressure.

The conditions of temperature and pressure are recorded at fixed time intervals by sensors inserted into the cell during all the testing period. For the electrical measurements we used a Teflon cell inside the pressure chamber with two polished stainless steel electrodes. Data were taken in isothermal conditions, stabilizing the sample during 10 minutes at each temperature step, when the conductance (G) and parallel capacitance (Cp) were measured, in the frequency range from 20 Hz to 10⁶ Hz in several steps using an LCR precision meter (IM3536-01 Hioki).

The electrical measurements were taken at different temperatures inside the boundaries of the CO₂ clathrate equilibrium curves, where several phases such as water, CO₂ dissolved, CO₂ gas, water ice, and/or CO₂ clathrate, can co-exist, following the phase diagram H₂O-CO₂. We have performed in situ Raman spectroscopy through the sapphire window of the chamber, in order to determine the specific phases stabilized at each temperature step before collecting the electrical data measurement..

 

Results

We measured the electrical conductivity and real permittivity values for samples of 1) CO₂ clathrate and, 2) water with CO₂. The results allowed us to discriminate the presence of the CO₂ clathrate phase respecting to other phases that could also be formed during the experiments.

When CO₂ clathrate is formed, the electrical characterization shows the conductivity and the real part of the permittivity constant with frequencies higher than 100 kHz, with smooth variation with the temperature. At 255 K the real permittivity (ε_ꚙ) was 2.5, and the conductivity (σ) was 1.5·10^-6 S/cm. Both obtained values are in good agreement with Stern et al. (2021).

The conductivity and real permittivity of the two phases sample H₂O+CO₂ with no clathrate formation showed a steeper dependence with frequency and temperature in comparison with the CO₂ clathrate. At 265 K, ε_ꚙ was 1.5, and σ was 1.04·10^-6 S/cm.

The attenuation of the signal was calculated from the experimental values of conductivity and real permittivity according to the next equation (Petinelli et al., 2015) at 10 kHz.

𝛼 ≅ 𝜎/(2𝑐𝜀₀( √𝜀´))

The attenuation showed clear differences between CO₂ clathrate, icy water with dissolved CO₂, and ice water itself, with attenuation obtained values of 10.99, 1.35 and 0.34 dB/km at 265 K, respectively.

 

Conclusions

The different phases of the H₂O-CO₂ system have appreciable differences in their electrical properties, which means that differences in the physical state could be identifiable by radar measurements. The possible relation with surface morphological features could eventually help to understand the geological activity of the moons, such as the mechanism of jets formation proposed for Europa (Shibley & Laughlin, 2021) or  the formation of the double ridge structures (Culberg et al., 2022).

 

Acknowledgments

This work was supported by the ESA contract 4000126441/19/ES/CM, and the MINECO Projects PID2019-107442RBC32, and PRE2020-093227.

 

References

Prieto-Ballesteros, O. et al. 2005 Icarus, 177, 491-505.// Hand, K. et al. 2006, Astrobiology Vol 6, 3. 463-482. // Izquierdo-Ruiz, F. et al. 2020 ACS Earth and Space Chemistry 2020 4 (11), 2121-2128 // Sloan Jr, E.D. et al. 2007. Clathrate Hydrates of Natural Gases (3rd ed.). CRC Press // Davidson, D.W. 1973 The Physical Chemistry of Water Vol2 115-234. // Davidson D.W. & Ripmeester J.A. 1978. Journal of Glaciology, 21, 33-49. // Stern L. A. et al. 2021, Geophysical Research Letters, 48 // Petinelli et al. 2015. Reviews of Geophysics 53, 593-641 // Shibley, N.C. & Laughlin, G. 2021. The Planetary Science Journal, 2:221 // Culberg et al. 2022. Nat Commun 13, 2007