Cracking in Ceres’ Core as an Opportunity for Late Hydrothermal

Liquid Water Inside Ceres: The icy dwarf planet Ceres (radius ≈475 km, density 2.1 g.cm−3, semi-major axis 2.8 AU) is the only body besides the Earth and Mars where carbonates have been observed along with brucite, another product of aqueous alteration [1]. Ceres will be visited by the Dawn spacecraft in 2015, which will offer an opportunity to constrain the origin of this large icy body. For example, it could have formed along with main-belt objects, or with Kuiper belt objects (KBOs) [2]. Models of Ceres’ thermal evolution have predicted the existence of liquid water throughout most of its history [3,4], provided that it accreted one to a few percent ammonia (NH3) acting as antifreeze with respect to water. Ammonia has been predicted to condense within the snow line [5] and observed on a few outer Solar system bodies [6,7,8]. The eutectic point of a H2O-NH3 mixture is around 175 K [9], much lower than that of pure water (273 K) and brines (<210 to 250 K [10]). We have applied to Ceres thermal evolution models developed for KBOs [11] with NH3/H2O=1% and confirmed the long-term preservation of a deep liquid layer in Ceres [12], whereas in absence of ammonia our models cannot maintain liquid. Observations and models both indicate that (a) aqueous alteration played a role in Ceres’ history, and (b) Ceres’ interior may have been habitable, and could still be. This brings about the need to consider geochemical processes when modeling Ceres’ evolution. How Much Hydrothermal Activity? Shape data indicate that Ceres likely differentiated into a silicate core and a water-ice mantle [3,4,12,13]. At the core-mantle boundary, water-rock interactions can occur if a thermal gradient is high enough to initiate the circulation of fluid through hot, porous rock [14]. The core porosity and depth of fracturing determine the extent of hydrothermal activity as well as the water/rock ratio (W/R), a key geochemical parameter. Cracks develop as the core cools and contracts. At high enough pressure P and temperature T , cracks relax and seal. The balance of these two phenomena determines the depth of cracking z into the core. Previous models [14] have shown that Ceres-sized icy bodies should have a core fractured throughout (z > Rcore = 375 km), assuming a constant cooling rate Ṫ of 1 K/yr, i.e., that typical of Earth’s mid-Ocean ridges, and using T and P profiles with depth from static geophysical models [15]. Our geophysical evolution models have predicted that Ceres’ core has been cooling much more slowly (a few 100 K/Gyr) from 2 Gyr until today (4.56 Gyr), following a decrease in radiogenic heating [4,12]. We predict lower Ṫ because radiogenic heat is not removed as efficiently by conduction from the low thermal conductivity rocky core as it is removed at Earth’s mid-Ocean ridges through volcanism. Instead, heat builds up in the core, yielding a temperature gradient steeper than that predicted by [15]. Model Equations: Parameters and their values used in the following equations are further described in [14] and references therein. T anisotropy between square silicate grains result in a mean stress σ that depends on Ṫ . The threshold temperature at which stress starts accumulating is T ′, defined such that σ(T ′) = 0. An approximate analytical expression for T ′ is: