Core cracking and hydrothermal circulation can profoundly affect Ceres' geophysical evolution

Abstract

Observations and models of Ceres suggest that its evolution was shaped by interactions between liquid water and silicate rock. Hydrothermal processes in a heated core require both fractured rock and liquid. Using a new core cracking model coupled to a thermal evolution code, we find volumes of fractured rock always large enough for significant interaction to occur. Therefore, liquid persistence is key. It is favored by antifreezes such as ammonia, by silicate dehydration which releases liquid, and by hydrothermal circulation itself, which enhances heat transport into the hydrosphere. The effect of heating from silicate hydration seems minor. Hydrothermal circulation can profoundly affect Ceres' evolution: it prevents core dehydration via "temperature resets, " core cooling events lasting ∼50 Myr during which Ceres' interior temperature profile becomes very shallow and its hydrosphere is largely liquid. Whether Ceres has experienced such extensive hydrothermalism may be determined through examination of its present-day structure. A large, fully hydrated core (radius 420 km) would suggest that extensive hydrothermal circulation prevented core dehydration. A small, dry core (radius 350 km) suggests early dehydration from short-lived radionuclides, with shallow hydrothermalism at best. Intermediate structures with a partially dehydrated core seem ambiguous, compatible both with late partial dehydration without hydrothermal circulation, and with early dehydration with extensive hydrothermal circulation. Thus, gravity measurements by the Dawn orbiter, whose arrival at Ceres is imminent, could help discriminate between scenarios for Ceres' evolution.

Geochemistry, thermal evolution, and cryovolcanism on Ceres with a muddy ice mantle

“…Rocky fines have lower thermal conductivity (∼1 W m −1 K −1 ) than ice (∼3 W m −1 K −1 ), making the mantle more insulating according to the formalism of Desch et al []; they also increase its viscosity by a factor (1 − ϕ / ϕ m ) −2 ≈3 or 4000 for volume fractions ϕ = 0.26 and 0.62, respectively [ Mueller et al , ]. Other assumed parameters (e.g., densities, heat capacities, and thermal conductivities) and formalisms are those of Desch et al [] and Neveu et al [].…”

“…Previous models of Ceres's geophysical and thermal structure have assumed either no differentiation at all [ Zolotov , ] or complete ice‐rock differentiation [ McCord and Sotin , ; Castillo‐Rogez and McCord , ; Neveu et al , ]. The latter models assume that Ceres's mantle has the rheological and thermal properties of pure water ice, in apparent conflict with the assumption that the surface represents a lag deposit of minerals.…”

The fluffy core of Enceladus

“…A similar interior structure may be expected for other icy satellites of similar dimensions such as Mimas and Miranda, and possibly for the dwarf planet Ceres. This latter case has recently been studied by Neveu et al (2015) in the context of hydrothermal circulation throughout a fractured core. However, because of the higher pressures in Ceres' interior and higher peak core temperatures than in Enceladus (Neveu et al, 2014), they consider relatively low core porosity ($1%) compared to the range investigated here.…”

Conditions for the Long‐Term Preservation of a Deep Brine Reservoir in Ceres

“…This process is promoted in presence of ammonium whose exchange with potassium and sodium is enhanced at temperature below 50 • C (Neveu et al, 2017). We account for the redistribution of potassium following leaching from the silicates upon aqueous alteration.…”