Evolution of porosity in small icy bodies

Leliwa-Kopystyński J, Kossacki KJ

Planetary and Space Science

48(7-8), 2000, 727-745, 10.1016/S0032-0633(00)00038-6

n this paper we consider self-compaction of icy bodies with radii from 60 to 200 km. They could be some of the icy satellites of the giant planets and some of the Kuiper belt objects. It is assumed that the considered globes were formed as porous bodies by the process of homogeneous accretion. They are not products of disruptive collisions. The evolution is considered from an early stage of formation (the embryo stage), through the stage when accretion is completed (present-day mass is settled), until the time when the present state (present-day radius, as well as the moment of inertia, if available) is reached.

The model we use to calculate the evolution of the distributions of density (or porosity) and of temperature is based on that presented by Kossacki and Leliwa-Kopystyński (1993. Planet. Space Sci. 41(10), 729–741). The model can be applied with various rheological formulae for pressure- and temperature-dependent compaction of granular, initially porous, icy-mineral medium. The bodies under consideration are assumed to be composed of water ice with an admixture of ammonia and of silicates. The components are uniformly distributed with a mass ratio C of silicates to total being fixed. An abundance x of ammonia relative to water is one of the crucial parameters of the model. The internal sources of energy leading to the evolution are: (i) the gravitational energy of initially porous globes; and (ii) the energy of radioactive decay of radionuclides dispersed within the mineral component. The amount of long lived isotopes is that corresponding to chondritic meteorites. Moreover, an initial presence of short lived Al26 is not excluded a priori. Its initial abundance is another parameter of the model.

The particular examples of calculations concern bodies with the sizes and densities of Mimas, Janus and Epimetheus. The three-dimensional (3-D) presentation of the results has allowed us to estimate physically reasonable ranges for ammonia and Al26 contents.