Author(s): , ,
Institution(s): 1. German Aerospace Center
The early Solar system produced a variety of bodies with different properties. Among the small bodies, objects that contain notable amounts of water ice (Ceres, icy satellites) are of particular interest. Water-rock separation on such worlds is probable and has been confirmed in some cases. Heating by 26Al and 60Fe suffices to produce liquid water (T>273 K) even in km-sized seeds during the early accretion of icy worlds.
Assuming accretion of ice and dust, the rheology is dominated by one of the two components, depending on their proportions. Two differentiation regimes arise: (a) Upon melting of an icy matrix, the dust grains settle via Stokes flow; (b) Upon melting of ice in a rocky matrix, water ascends through the matrix via Darcy flow. Prior to ice melting, porosity is reduced by creep of ice. However, there are leftover pores filled with gas. For (a) the differentiation scheme is not affected. For (b) ice melting increases the porosity of the matrix. Only a part of the void space will be filled with water. Water will percolate if the matrix (1) deforms sufficiently to close the pores filled with gas and (2) deforms further to squeeze the water out of the matrix. Temperatures of up to 700 K are needed for this. Thus, water will first migrate downwards filling the pores, vacated previously by gas. After that, it will either remain in suspension until the matrix deforms and then percolate, or will vaporise first and then fill the pores with the vapour. Subsequent matrix deformation will mobilise the vapour. On its way to the surface water will form in the cooler layers.
The differentiation starts during the accretion. At the end of the accretion, a pre-differentiated structure around the centre is possible, leading to a reallocation of the heat sources and changing the temperature profile. The evolution path varies with the growth rate assumed.
We couple porosity loss and water-rock differentiation of an accreting icy object in an adaptive-grid 1D numerical model. The model is applicable to Ceres, icy satellites, and KBOs, and is suited to assess the thermal metamorphism of the interior and the present-day internal structures.