Did ²⁶Al and impact‐induced heating differentiate Mercury?

Abstract

Numerical models dealing with the planetary scale differentiation of Mercury are presented with the short‐lived nuclide, 26Al, as the major heat source along with the impact‐induced heating during the accretion of planets. These two heat sources are considered to have caused differentiation of Mars, a planet with size comparable to Mercury. The chronological records and the thermal modeling of Mars indicate an early differentiation during the initial ~1 million years (Ma) of the formation of the solar system. We theorize that in case Mercury also accreted over an identical time scale, the two heat sources could have differentiated the planets. Although unlike Mars there is no chronological record of Mercury's differentiation, the proposed mechanism is worth investigation. We demonstrate distinct viable scenarios for a wide range of planetary compositions that could have produced the internal structure of Mercury as deduced by the MESSENGER mission, with a metallic iron (Fe‐Ni‐FeS) core of radius ~2000 km and a silicate mantle thickness of ~400 km. The initial compositions were derived from the enstatite and CB (Bencubbin) chondrites that were formed in the reducing environments of the early solar system. We have also considered distinct planetary accretion scenarios to understand their influence on thermal processing. The majority of our models would require impact‐induced mantle stripping of Mercury by hit and run mechanism with a protoplanet subsequent to its differentiation in order to produce the right size of mantle. However, this can be avoided if we increase the Fe‐Ni‐FeS contents to ~71% by weight. Finally, the models presented here can be used to understand the differentiation of Mercury‐like exoplanets and the planetary embryos of Venus and Earth.

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

“…Thermal evolution of the icy planetary bodies was numerically simulated by solving the heat conduction partial differential Equation (Sahijpal et al. ; Sahijpal and Bhatia ; Bhatia and Sahijpal , ). In order to simplify the problem, satellites, minor planets, and TNOs were assumed to have acquired spherical shape during and subsequent to accretion.…”

“…In order to simplify the problem, satellites, minor planets, and TNOs were assumed to have acquired spherical shape during and subsequent to accretion. This reduces Equation to a one‐dimensional equation in the radial coordinate, “ r ” and time, “ t .” The resulting radial heat conduction equation was solved numerically by the finite difference method using classical explicit approximations (see e.g., Bhatia and Sahijpal , ).…”

“…The initial temperature of the seed body was assumed to be 170 K (Sahijpal ). The specific heat of rock before and after the melting of ice was assumed to be temperature‐dependent (Bhatia and Sahijpal , ). The specific heats of ice and water were assumed to be 2.1 × 10 3 and 4.2 × 10 3 J kg −1 K −1 , respectively (Sahijpal ).…”

“…The growth of mass with time t will occur according to the relation, m ( t ) ∝ t 3 n . A detailed account of the adopted approach is discussed recently by Bhatia and Sahijpal (). In the accretion scenario with an assumed value of 0.33 for n , a rapid radial growth of a body occurs during the initial stages of accretion.…”

“…Aside from the contributions in the heating, melting, and differentiation of the rocky planetary bodies (see e.g., Sahijpal et al. ; Bhatia and Sahijpal , , ), the role played by 26 Al in the early thermal evolution of planetary bodies formed by the accretion of icy planetesimals has also been explored (Prialnik and Bar‐Nun ; Schubert et al. ; Barr and Canup ; Guilbert‐Lepoutre et al.…”