JWST mid-infrared spectroscopy of Centaurs and small TNOs: Linking the inner and outer solar system

Outer solar system small bodies (Centaurs, Comets, small Trans-Neptunian objects (TNOs)) hold the residual ingredients from our young protosolar disk that are still observable today. Thus, by measuring their composition and distribution across the solar system, we have the potential to unravel the scenario of our planetary system’s formation and dynamical evolution, providing a benchmark to which other stellar systems can be compared. In particular, a fraction of the small bodies that formed in the outer solar system (beyond Uranus/Neptune) are predicted to have been implanted in the inner solar system (P/D-type asteroids, Jupiter Trojans) following the outward migration of Uranus and Neptune (Nice model).

The objective of this study is to further test the hypothesis of a common origin of small bodies in the inner and outer solar systems. This will be achieved by measuring the silicate composition of Centaurs and small TNOs. Silicates are thermally stable over the heliocentric range extending from the main asteroid belt to the Kuiper belt. At present, they appear to be the only robust tracer of a common genetic link.

Spectroscopic data were obtained with JWST/MIRI for 3 Centaurs (Chariklo, Bienor, 2020 VF1) and 3 scattered disk objects (1999 OX3, 2002 GG166, 2013 LU28) with a Centaur-like orbit as part of the Cycle 2 GO program (GO  2820 PI: Pierre Vernazza). We also analyzed the MIRI data of GTO target 2013XZ8, also a Centaur (Cycle 1 GTO 1272; PI: D. Hines) and of three Jupiter Trojans (targets of the Cycle 1 GO 2574 programme; PI: M. E. Brown).

Our albedo values for less red Bowl-type (<0.1) and red Cliff-type (>0.1) Centaurs and small TNOs are consistent with those derived from Herschel observations. The emissivity spectra of Centaurs and small TNOs, including the variability of the spectral contrast of the main 10 mm silicate feature, are remarkably similar to those of Jupiter Trojans, P/D main belt asteroids, and comets. In particular, the spectral contrast observed for Cliff-type target 1999 OX3 is close to that observed for comet comae, suggesting substantial surface porosity, probably related to a lower silicate/(ices+organics) ratio. Careful analysis of the emissivity spectra of Centaurs, small TNOs and Jupiter Trojans reveals the presence of a large number (9) of features. These features are consistent with crystalline olivine and pyroxene - both with Mg/(Mg+Fe)>0.7 - being major refractory components of the surfaces of these objects. This further supports a strong link with comets and chondritic porous interplanetary dust particles (CP IDPs) in terms of extraterrestrial analogues.

JWST/MIRI observations of Centaurs and small TNOs support the hypothesis that these bodies share a common origin with inner solar system P/D-type asteroids and Jupiter Trojans, as outlined in the Nice model. Our results, together with previous measurements for these populations and dynamical studies, raise the possibility of a genetic link between P-types and Bowl-types, and between D-types and Cliff-types.