Evolution of Insoluble Organic Matter and H2O mixtures Under Ganymede and Titan’s Interior Conditions

Models for the internal structure of the icy satellites Ganymede and Titan, as derived from the data of the Galileo and Cassini-Huygens space missions, suggest that both moons are differentiated with a hydrosphere of ices and liquid water overlaying an inner rocky core. The presence of significant amounts of Insoluble Organic Matter (IOM) in this silicate layer (in quantities consistent with those found in chondrites) has recently been advanced to properly explain the density and moment of inertia of these moons [1]. Interestingly, laboratory experiments at room pressure have shown that the pyrolysis of IOM (starting from temperatures as low as 500 K) gradually releases volatiles such as H₂O, CO/CO₂, CH₄, H₂S, and SO₂, with possible N-bearing compounds such as N₂, NO_x and NH₃ [2, 3, 4]. This evolution of the IOM could have a defining impact on the habitability and chemical evolution of icy worlds, including the formation of an atmosphere. However, the effect on these thermal reactions of the high pressures found inside large icy worlds remain largely unknown. The purpose of this study is to analyze the chemical and physical evolution of the IOM under the combined pressure and temperature conditions expected inside Titan and Ganymede (pressures from 0.5 to 7 GPa and temperatures up to 1200 K).

Figure 1: Species produced by IOM dissociation at high pressure and high temperature (blue) compared to those produced by pyrolysis at ambient pressure in Kuga et al. (2014) (green) and Okumura and Mimura (2011) (red).

We conducted anvil cell experiments on mixtures of IOM with water at temperatures up to 773 K and pressure up to 8 GPa. The IOM, with a composition of C₁₀₀H₉₃N₆₅O₆₁, was synthetized at the Nebulotron (CRPG, France [3]), an ultra-high vacuum chamber using a radiofrequency plasma to ionize a N₂-CO gas mixture. Systematic pressure and temperature monitoring, and in situ Raman spectroscopy analyses, were conducted during the experiments to characterize the evolution of the samples. Additional infrared analyses were conducted to compare the initial organic matter (as loaded in the anvil cell) with the residual IOM collected at the end of some of the experiments.

During our high-pressure experiments, elevated temperatures led to the production of C- and N-bearing species, as was reported by others during the pyrolysis of dry IOM at room pressure. Our IOM-water mixtures, however, yielded NH₃ (rather than N₂) as the main N-bearing molecule. Furthermore, CO₂ was never observed in our samples; instead, CO₃ (as carbonic acid and/or carbonate ions) was identified as the main C-bearing species alongside CH₄ (Figure 1). Overall, the degradation of the IOM at high pressure appears to start at slightly higher temperature, although additional experiments are needed to confirm this result (in particular for the formation of CO₃ species). Evidence of the restructuration of the IOM appeared in both Raman and infrared spectroscopy.

Our results support that the thermal dissociation of the IOM inside Titan may have contributed to the formation of its atmosphere [5,6]. These results will also prove useful in assessing the chemical evolution of the hydrosphere of icy worlds, notably regarding the formation of gas hydrates inside their high-pressure ice layers.

Acknowledgements:

This research is founded by CNRS 80 PRIME program. This work also acknowledges the financial support from CNES (Centre National d’Etudes Spatiales, France) in preparation of the ESA JUICE mission.

References:

[1] Néri et al. (2020) A carbonaceous chondrite and cometary origin for icy moons of Jupiter and Saturn. Earth and Planetary Science Letters, 530 :115920.

[2] Okumura and Mimura (2011) Gradual and stepwise pyrolysis of insoluble organic matter from the Murchison meteorite revealing chemical structure and isotopic distribution. Geochimica et Cosmochimica Acta, 75(22) :7063–7080.

[3] Kuga et al. (2014) Nitrogen isotopic fractionation during abiotic synthesis of organic solid particles. Earth and Planetary Science Letters, 393:2–13.

[4] Franklin (1949) A study of the fine structure of carbonaceous solids by measurements of true and apparent densities. Part I. Coals. Transactions of the Faraday society, 45:274–286.

[5] Tobie et al. (2012) Titan’s bulk composition constrained by Cassini-Huygens: implication for internal outgassing. The Astrophysical Journal, 752(2):125.

[6] Miller et al. (2019) Contributions from accreted organics to Titan’s atmosphere: new insights from cometary and chondritic data. The Astrophysical Journal, 871(1):59.