Differentiation of Enceladus and Retention of a Porous Core

Abstract

Introduction:The Cassini mission revealed gas plumes associated with surface features called "tiger stripes" at the south pole of Saturn's moon Enceladus. The composition of plume particles and local cryovolcanism suggested as a possible cause for the activity are typically considered in the context of hydrothermal circulation in the rocky core within a differentiated core-ocean-ice crust structure. We model the internal evolution and differentiation of Enceladus heated by radioactive nuclides and tidal dissipation. Calculating the core formation, we investigate its compaction by modeling the evolution of porosity, thereby varying the rock rheology based on different assumptions on the composition, such as grain size, creep activation energy, degree of hydration, and oxygen fugacity. We obtained "successful" final structures with a core radius of 185-205 km, a porous core layer of 4-70 km, an ocean of ≈10-27 km, and an ice crust layer of ≈30-40 km, that are largely consistent with the current estimates for Enceladus. By fitting the model results to these observations, we determined an accretion time of 1.3-2.3 Ma after Ca-Al-rich inclusions for Enceladus. Our models produce a porous outer core for wet and dry olivine rock rheologies supporting the hypothesis of hydrothermal circulation of oceanic water in the core. No porosity is retained for an antigorite rheology, implying that the core of Enceladus is not dominated by this mineral.Model: For model calculations we adapted a 1D thermal evolution code used previously for Ceres [1] to Enceladus. The updated model [2] calculates heating by the short-and long-lived radioactive isotopes, latent heat, compaction of initially porous rock, continuous water-rock separation, redistribution of radionuclides, tidal heating, and convection in the water ocean.A crucial aspect of the model is the calculation of the melt porosity in the rocky core during its formation and subsequent evolution (i.e., the change of the pore space filled with water). The core forms by the redistribution of water and silicate rock particles in a two-phase system after melting of ice. The melting front proceeds from the center upward. The rock particles in the H 2 O-rich layer below the melting front settle and form an agglomerate with water-filled pores, while atop that a water layer forms. This agglomerate is a high-porosity core that grows along with the water layer while the melting front moves further upward. Deformation of the core can lead to a reduction of the

The subsurface habitability of small, icy exomoons

“…Similarly, if the body has a higher radionuclide content than Earth it should generate more endogenic heat. Comparing the values in Table 2 in Neumann & Kruse (2019) and Table 3 in McDonough et al (2019) shows that while Earth may be depleted in potassium compared to (what is modeled for the initial state of) Enceladus, it makes up for that in terms of uranium and thorium. In the end Equation 8 still yields a heat production in agreement with Czechowski (2004), so exactly what isotope produces this heat is not relevant to the results of this work.…”

“…In the end Equation 8 still yields a heat production in agreement with Czechowski (2004), so exactly what isotope produces this heat is not relevant to the results of this work. Important to note is that Neumann & Kruse (2019) assume Enceladus is primordial, which may not be the case (see Section 5.3 andĆuk et al 2016) and may influence their results for Enceladus' isotopic abundances today. We do not know whether Enceladus is representative for all icy moons in its modeled isotope abundances; regardless, until more data on this topic becomes available, we use Equation 8 as a baseline.…”

Compositions and Interior Structures of the Large Moons of Uranus and Implications for Future Spacecraft Observations

“…Hence, it is very unlikely that the core of Titania would contain significant porosity. Following compaction and cementation, hydrothermal circulation that could control the temperature in the core (e.g., Choblet et al, 2017) shuts down and conductive heat transfer takes over, driving compaction creep (Neumann and Kruse. 2019).…”

Science Objectives for Flagship-Class Mission Concepts for the Search for Evidence of Life at Enceladus

“…Understanding the structure and dynamics of the rocky interior via seismic and gravity-field investigations ( e.g ., Vance et al , 2018) provides key insights into the spatial extent and longevity of geochemical interactions. For example, a porous core provides a larger interface area for water–rock interactions and has implications for internal dissipation and the conditions necessary to sustain a global ocean ( e.g ., Neumann and Kruse, 2019 ).…”