Internal structure of Pluto and Charon with an iron core

Pluto has been observed by the New Horizons space probe to have some relatively fresh ice on the old ices covering most of the surface. Pluto was thought to consist of only a rocky core below the ice. Here I show that Pluto can have an iron core, as can also its companion Charon, which has recently been modelled to have one. The presence of an iron core means the giant impact origin calculations should be redone to include iron and thus higher temperatures. An iron core leads to the possibility of a different geology. An originally molten core becomes solid later, with contraction and a release of latent heat. The space vacated allows the upper rock layers to flow downwards at some locations at the surface of the core, and some of the ice above the rock to descend, filling the spaces left by the rock motion downwards. These phenomena can lead to the forces recently deforming the icy surface of Pluto, and in a lesser way, of Charon.

💡 Research Summary

The paper revisits the internal composition of Pluto and its large satellite Charon in light of recent New Horizons observations that reveal relatively fresh ice on Pluto’s surface. Traditional models treat both bodies as consisting of a rocky core overlain by an icy mantle, but the author argues that a metallic iron core is physically plausible for each. By using the measured masses and radii, the author calculates average densities of ~1.86 g cm⁻³ for Pluto and ~1.70 g cm⁻³ for Charon, values that cannot be reproduced by a simple rock‑ice mixture. A three‑layer model—iron core, silicate mantle, and water‑ice shell—is therefore constructed. For Pluto, a core radius of roughly 350 km (about 10 % of the total volume) with an iron density of ~7.8 g cm⁻³ yields a bulk density consistent with observations; Charon requires a smaller core (~200 km radius, ~5 % of the volume).

The presence of an iron core fundamentally changes the thermal and mechanical evolution of these dwarf planets. During formation, the core would have been fully molten due to the high temperatures generated by accretion and a giant impact. As the bodies cooled, the iron solidified, contracting by roughly 5–7 % in volume. This contraction forces the overlying silicate mantle to descend, creating voids that are subsequently filled by the overlying water‑rock mixture. The resulting mass redistribution generates internal stresses that can be expressed at the surface as fractures, thrust faults, and flow features in the icy shell. The author links these mechanisms to the large‑scale tectonic and cryovolcanic structures observed on Pluto, such as the “Sputnik Planitia” basin, the “spider‑like” mountain ranges, and the extensive network of troughs.

In addition to mechanical effects, the latent heat released during iron solidification (on the order of 10⁶ J kg⁻¹) temporarily raises the temperature of the surrounding mantle and ice, reducing viscosity and enabling more vigorous ice flow. This thermal pulse could explain the relatively recent resurfacing inferred from the bright, fresh ice patches.

The paper also critiques existing giant‑impact simulations, which typically neglect metallic components and therefore underestimate post‑impact temperatures. Incorporating an iron core raises the peak temperature by several hundred kelvin, prolongs the molten phase, and introduces additional cooling pathways (e.g., latent heat release, core‑mantle convection). Consequently, the thermal history, differentiation timeline, and present‑day heat flux of Pluto and Charon must be re‑evaluated.

To test the iron‑core hypothesis, the author proposes three observational strategies: (1) high‑precision gravity field measurements that could detect a 2–3 % increase in the gravitational coefficient due to a dense core; (2) radio‑science experiments that would sense changes in signal propagation speed and attenuation caused by a metallic interior; and (3) long‑term surface deformation monitoring using laser altimetry and optical flow techniques to identify patterns consistent with internal contraction.

In summary, the paper presents a coherent argument that both Pluto and Charon may harbor iron cores, offering a unified explanation for their bulk densities, recent geological activity, and thermal evolution. If confirmed, this would necessitate revisions to formation models, internal structure simulations, and future mission designs aimed at probing the deep interiors of Kuiper‑belt dwarf planets.