Thermal evolution of trans‐Neptunian objects, icy satellites, and minor icy planets in the early solar system

Abstract

Numerical simulations are performed to understand the early thermal evolution and planetary scale differentiation of icy bodies with the radii in the range of 100–2500 km. These icy bodies include trans‐Neptunian objects, minor icy planets (e.g., Ceres, Pluto); the icy satellites of Jupiter, Saturn, Uranus, and Neptune; and probably the icy‐rocky cores of these planets. The decay energy of the radionuclides, 26Al, 60Fe, 40K, 235U, 238U, and 232Th, along with the impact‐induced heating during the accretion of icy bodies were taken into account to thermally evolve these planetary bodies. The simulations were performed for a wide range of initial ice and rock (dust) mass fractions of the icy bodies. Three distinct accretion scenarios were used. The sinking of the rock mass fraction in primitive water oceans produced by the substantial melting of ice could lead to planetary scale differentiation with the formation of a rocky core that is surrounded by a water ocean and an icy crust within the initial tens of millions of years of the solar system in case the planetary bodies accreted prior to the substantial decay of 26Al. However, over the course of billions of years, the heat produced due to 40K, 235U, 238U, and 232Th could have raised the temperature of the interiors of the icy bodies to the melting point of iron and silicates, thereby leading to the formation of an iron core. Our simulations indicate the presence of an iron core even at the center of icy bodies with radii ≥500 km for different ice mass fractions.

Thermal evolution of the early Moon

“…The choice of any initial average temperature lower than this range appears to be formidable, whereas a higher choice will definitely result in the core–mantle differentiation with a comparatively large initial convective magma ocean. The impact‐induced efficiency parameter “ h ” in most of the simulations is confined within the range of 0.1–0.2 in order to be consistent with the thermal models of other planetary bodies (see e.g., Bhatia and Sahijpal , , ) that indicate lower values. However, we ran several simulations with a value of 0.5 for h .…”

“…), is not explicitly relevant for the thermal evolution of the Moon. However, the role of these short‐lived nuclides in the thermal evolution of planets and their planetary embryos is certainly relevant (Sahijpal and Bhatia ; Bhatia and Sahijpal , , ), and could be of relevance even for the early heating of Earth and the impactor responsible for the formation of the Moon. Hence, the implicit contribution of short‐lived radioactive heating at least in Earth and Theia cannot be rejected.…”

“…The heat conduction differential equation (Equation ) was numerically solved by using the finite difference equation (Sahijpal et al. ; Gupta and Sahijpal ; Sahijpal and Bhatia ; Bhatia and Sahijpal , , ). The final radius and the mass of the Moon are assumed to be 1740 km and 7.35 × 10 22 kg, respectively.…”

“…The impact energy generated during the accretion of the Moon was incorporated by following the criteria developed in the earlier works (Kaula ; Schubert et al. ; Bhatia and Sahijpal , , ). The energy generated during impacts is converted into an associated rise in the temperature (Equation ) above the prevailing temperature of a specific spatial grid defined by the finite difference method.…”

“…Here, “ M ( r )” is the mass of the body at a specific instant of accretion, “ c ” is the specific heat, and “ u ” is the relative approach velocity per unit mass of the incoming Moonlet. The parameter, “ h ” is an uncertain efficiency parameter which determines the extent of impact energy that is effectively translated in terms of the temperature rise against heat losses from the surface (Kaula ; Pritchard and Stevenson ; Barr ; Bhatia and Sahijpal , , ). The earlier works based on the thermal models of planetary bodies suggest that the contribution of the second term in the parentheses of Equation is negligible (Bhatia and Sahijpal ), and the efficiency parameter, “ h ,” has been generally considered to be of the order of 0.1.…”