Icy Beginnings: How Growing Planetesimals Warmed, Melted, and Evolved

Kimura and collaborators investigate how icy planetesimals, the small, early building blocks of planets, heated up, melted, and reorganized internally during the first 100 million years of Solar System history. Their motivation comes from a puzzle: samples from the C-type asteroid Ryugu show that its parent body stayed extremely cold, while some iron meteorites with similar chemical fingerprints must have formed inside much hotter bodies. To understand how such different outcomes arise, the authors construct a numerical model that follows an icy planetesimal from an initial 1-km radius through radial growth, radioactive heating, impact heating, melting, aqueous alteration, and internal layering.

Modeling How Icy Planetesimals Change as They Grow

The model includes several key physical and chemical stages. As the planetesimal warms, ice begins to melt, allowing aqueous alteration to convert anhydrous minerals such as olivine into hydrous minerals like serpentine. If enough water is produced, dense hydrous minerals sink to form a rocky core, while excess liquid water forms a water mantle above it. At even higher temperatures, the hydrous minerals undergo dehydration, creating new anhydrous minerals and releasing additional water. In very large or early-forming bodies, temperatures can become high enough to melt metal, potentially producing differentiated cores similar to those inferred from iron meteorites.

How Size and Timing Control Thermal Evolution

The authors find that the planetesimal’s final size, growth timing, and growth duration are the dominant controls on its thermal evolution. For bodies that accrete early, around 1 Myr after the formation of CAIs, radioactive decay of short-lived isotopes such as ²⁶Al provides substantial internal heating. In these cases, planetesimals larger than ~35 km typically undergo full ice melting and extensive aqueous alteration, and those larger than ~200 km can even reach temperatures high enough for metallic melting. Later formation greatly reduces the energy available: if accretion begins at 2 Myr, much of the ²⁶Al has already decayed, preventing strong heating even in bodies hundreds of kilometers in radius.

Impact Heating and Growth Mode: Shallow vs. Deep Heating

Impact heating adds another layer of complexity. Because later-stage accretion involves higher-velocity collisions, the outermost layers of rapidly growing planetesimals can heat significantly, especially when the final radius exceeds several hundred kilometers. However, this heating is mostly shallow, affecting only the outer ~10% of the radius and not dramatically altering deeper thermal evolution. Instead, differences in growth mode, linear versus runaway, change when heating happens: in runaway growth, most of the radius is added late, after radioactive decay, which keeps internal temperatures relatively low.

Implications for Ryugu’s Parent Body

One major application the authors explore is the origin of Ryugu’s parent body. Ryugu’s minerals record extremely low alteration temperatures (roughly 20–40 °C), meaning its parent body must have avoided strong heating. The model shows that such mild conditions can occur either in small bodies (<30–70 km in radius) or in the shallow outer regions of larger bodies that grew late or slowly. For example, if accretion begins 2.0 Myr after CAIs and finishes quickly (within 0.4 Myr), a wide region of a several-hundred-kilometer planetesimal can maintain the cool conditions needed to reproduce Ryugu’s mineralogy, thanks in part to efficient convective heat transport in the water layer.

Connections to Enceladus and Other Icy Worlds

The authors also identify conditions that could create objects similar to Enceladus, the active icy moon of Saturn. A planetesimal that grows to ~250 km with the correct timing may develop a hydrated core 170–200 km wide, topped by a stable liquid water mantle, an internal structure reminiscent of Enceladus’ ocean. At the opposite extreme, early and rapid accretion in bodies larger than ~200 km can yield temperatures above the Fe–FeS eutectic, producing metal melts that could be the source of iron meteorites with carbonaceous-chondrite-like isotopic signatures.

A Unified Framework for Icy Planetesimal Evolution

Overall, Kimura et al. present one of the most comprehensive models to date of icy planetesimal evolution, integrating growth, melting, alteration, dehydration, and internal differentiation. Their results unify many previously disconnected observations, from Ryugu’s gentle thermal history to the intense heating required for iron meteorite formation, by showing that small differences in formation time and accretion rate can dramatically reshape a planetesimal’s interior. This work provides a vital framework for interpreting meteorite records and for understanding how the diverse icy worlds of our Solar System first emerged.

Source: Kimura