Abstract EANA2024-46

Monohydrate ammonia (NH3-H2O) and dihydrate ammonia (NH3-2H2O) are two of the chemical mixtures identified at the South Pole of Enceladus. These compounds are found in small amounts on the surface from the material expelled by the water plumes. However, due to Enceladus' partial enclosure by Saturn's magnetosphere, these compounds are unstable on the surface. As a result, ammonia and its constituents are more frequently eliminated by sputtering under ion irradiation, leaving only a tiny concentration of them in some regions along the tiger stripes.

From this point, there are some uncertainties related to the influence of both compounds during the early geological evolution of the subsurface ocean of Enceladus and the strong tectonic activity before the formation of the fractures and fissures on surface.  Prior to the deformation of the ice shell at the South Pole, could those compounds have had a significant impact on the thermal evolution of the ocean and ice crust? This study investigates the effects of ammonia-water ice mixtures on thermodynamic processes within the ice shell, assuming that the percentage of NH3 is equal to the commonly accepted value of less than 33 wt% water ice.

In order to predict their impact during the first geological stages of Enceladus, we performed numerical geodynamic simulations of the thermal evolution and production of partial melt within the ice crust. To this, there were tested several experiments with geophysical variations such as the reference viscosity (10^13 Pa s, 3*10^13 Pa s, 10^14 Pa s, 10^15 Pa s), and the thickness of the ice crust (15 km, 25 km, 50 km), for 2d-spherical and cartesian geometries. It was also computed the influence of the tidal heating, strain deformation, and the temperature dependence of the viscosity for a Newtonian rheology with a grain size lower than 0.03 mm. The melting temperature, specific heat, and densities for each combination were adjusted based on their thermodynamic properties.

Results show how the combination of this volatile prevent the freezing of the ocean and delay the complete melting of the ice crust. For models with high thicknesses (25 km and 50 km) and a combination of salty ocean water + ammonia, the molten material production was higher than models with monohydrate and dihydrate ammonia at 50 km. While, for models with 25 km of thickness and a mixture of monohydrate and dihydrate ammonia it was not possible to find production of melt, allowing for it the maintaining of a liquid ocean beneath the ice crust. We also identified that the presence of salts contributes to the deformation of the ice crust. These results also give Astrobiology insights about the chemical conditions that could allow for the possible formation of hydrothermal vents at the bottom of the ocean layer, because it reveals the geophysical parameters that maintain a constant exchange of material among the ocean, ice crust, and surface.