Growth and thermal evolution of icy planetesimals

TL;DR

The paper tackles why icy planetesimals display diverse thermal histories by integrating accretion, radiogenic and impact heating, water phase changes, aqueous alteration, and differentiation into a 1D, time-dependent model evaluated for up to 100 Myr. It conducts a parametric study across final radius, accretion onset, duration, and growth mode to map interior temperatures and structures, revealing that larger bodies and earlier accretion yield higher internal temperatures, with some regions reaching the Fe-FeS eutectic regime. The results explain Ryugu's 20–40 °C aqueous alteration as arising from shallow hydrated zones in late-accreted or outer-shell material, and they predict Enceladus-like interior configurations for certain parameter sets, unifying disparate meteoritic records under a common framework. The work has implications for interpreting meteorite provenance, guiding future spacecraft observations, and informing models of icy body evolution in the Solar System, including tests via missions like Lucy and future laboratory experiments on hydration and differentiation.

Abstract

Icy planetesimals are likely to supply volatiles to terrestrial planets and serve as building blocks of icy bodies in the outer Solar System. Samples from the C-type asteroid Ryugu, collected by the Hayabusa-2 spacecraft, indicate a low-temperature history with aqueous alteration and organic materials. In contrast, iron meteorites with isotopic ratios similar to carbonaceous chondrites suggest exposure to higher temperatures. These findings imply that the thermal evolution of icy planetesimals is highly diverse. Since direct exploration provides only localized data, understanding this diversity requires comparing observational results with model calculations incorporating key evolutionary processes. We develop a model including radial growth, impact heating, water phase changes, aqueous alteration, and structural differentiation, to re-evaluate the thermal evolution of icy planetesimals during the first 100 Myr after CAI formation. The model considers final radius (10-1000 km), growth onset (1.0 or 2.0 Myr after CAI), growth duration (0.4 or 4.0 Myr), and growth mode (linear or runaway). Our results show that larger planetesimals generally reach higher temperatures, but growth timing and mode significantly affect thermal evolution. Early accretion leads to higher temperatures, with some bodies reaching the Fe-FeS eutectic (1250 K), while delayed or prolonged growth reduces heating. Our results show that the constituent materials of Ryugu, which kept below 40 degC, likely formed near the surface of a hydrated mineral layer. This is possible even in planetesimals several hundred kilometers in size due to efficient heat transport via convection. If accretion begins 2.0 Myr after CAI and completes in 0.4 Myr, a wide region in such a body could yield Ryugu's material.

Figures (12)

• Figure 1: Schematic diagram of the interior evolution model in this work. Initial planetesimal with radius of 1 km grows by accretion. Eventually, six layered structures in maximum are formed according to its thermal state. Heat transfer rate due to the solid-state convection is calculated separately for the anhydrous mineral (olivine) layer, the hydrated mineral (serpentine) layer, and the ice and initial composition layer. The water mantle is assumed to be isothermal.
• Figure 2: Temperature profiles in the icy planetesimal in case that accretional growth begins from 1.0 Myr after CAI formation and accretional duration of 0.4 Myr to a final radius of 100 km (case#051). Numbers along lines indicate times after CAI formation.
• Figure 3: (Left) Temporal change of the central temperatures for different final radius of planetesimal, and (Right) internal temperature profiles at the end of accretional growth (at 1.4 Myr after CAI formation) with the distance from the center normalized by the final radius of planetesimal for same calculation as shown in Figure \ref{['fig_result_typical']} (case#051).
• Figure 4: Same as Figure \ref{['fig_051']} but with the growth mode of $\beta=2$ (case#061).
• Figure 5: Same as Figure \ref{['fig_051']} but the start timing of the accretional growth was changed from 1.0 Myr to 2.0 Myr after the formation of the CAI (case#054). Note that the internal temperature profiles (right) are at the end of accretion (at 2.4 Myr after CAI formation).