Different origins or different evolutions? Decoding the spectral diversity among C-type asteroids

TL;DR

This study investigates the spectral diversity among C-type asteroids, revealing evidence of exogenous enstatite contamination on Ceres and Eugenia, challenging previous assumptions about their origins and suggesting complex formation histories involving late accretion.

Contribution

It provides new spectral evidence of enstatite on Ceres and Eugenia, proposing exogenous contamination and diverse origins for C-type asteroids.

Findings

Enstatite detected on Ceres and Eugenia surfaces.

Ceres may have formed in the outer solar system.

Small C-types likely accreted from chondritic porous IDPs.

Abstract

Anhydrous pyroxene-rich interplanetary dust particles (IDPs) have been proposed as surface analogs for about two-thirds of all C-complex asteroids. However, this suggestion appears to be inconsistent with the presence of hydrated silicates on the surfaces of some of these asteroids including Ceres. Here we report the presence of enstatite (pyroxene) on the surface of two C-type asteroids (Ceres and Eugenia) based on their spectral properties in the mid-infrared range. The presence of this component is particularly unexpected in the case of Ceres because most thermal evolution models predict a surface consisting of hydrated compounds only. The most plausible scenario is that Ceres' surface has been partially contaminated by exogenous enstatite-rich material, possibly coming from the Beagle asteroid family. This scenario questions a similar origin for Ceres and the remaining C-types, and it possibly supports recent results obtained by the Dawn mission (NASA) that Ceres may have formed in the very outer solar system. Concerning the smaller C-types such as Eugenia, both their derived surface composition (enstatite and amorphous silicates) and low density suggest that these bodies accreted from the same building blocks, namely chondritic porous, pyroxene-rich IDPs and volatiles (mostly water ice), and that a significant volume fraction of these bodies has remained unaffected by hydrothermal activity likely implying a late accretion. In addition, their current heliocentric distance may best explain the presence or absence of water ice at their surfaces. Finally, we raise the possibility that CI chondrites, Tagish Lake-like material, or hydrated IDPs may be representative samples of the cores of these bodies.