Deciphering the silicate composition of Centaurs and small TNOs

Decades of observations have provided a preliminary understanding of the architecture of our solar system and of the compositional distribution among inner (<5AU; see Vernazza & Beck 2017 for a review) and outer solar system small bodies (e.g., Barucci & Merlin 2020). Previous observations have revealed a number of puzzling features in each dynamical population of small bodies including the compositional distribution and diversity of the asteroid belt, the inclination distribution of the Jupiter and Neptune Trojans, and the peculiar orbital distribution of TNOs. These findings have led the path to new models of the formation and evolution of the solar system, notably the Nice model (e.g., Morbidelli et al. 2005, Levison et al. 2009, Nesvorny et al. 2018). In turn, the Nice and other models make predictions about the small bodies that must be verified to validate or dismiss these models.

In order to probe the dynamical and chemical evolutionary scenarios that would explain their present-day compositions, we measured the mid-infrared spectral properties of 4 Centaurs (representing bodies formerly in the Kuiper Belt) with JWST/MIRI (GO 2820, PI: Pierre Vernazza) over the 5-28 micron range. These observations will provide elements of answers to the following fundamental yet still open questions:

Question 1:  What types of silicates are present among Centaurs and TNOs? The use of MIRI in spectroscopic mode over the 5–28-micron range allows us to characterize the composition of the silicates as done in the past for comets and Jupiter Trojans with the Spitzer space telescope (e.g., Emery et al. 2006, Vernazza et al. 2012, 2015). This is the ideal wavelength range for detecting fine-grained silicates and characterizing their composition as they display strong emissivity bands.

Question 2: Are there compositional differences between less red and very red objects that would inform us about a compositional heterogeneity in the protoplanetary disk at heliocentric distances greater than ~10 AU? Our proposed observations will shed light on possible compositional differences (in terms of silicate mineralogy, and in particular the amorphous/crystalline and the olivine/pyroxene ratios) between less red and very red objects and may thus provide unique constraints on the primordial heliocentric compositional gradient in the outer (~15-35 AU) protoplanetary disk.                           

Question 3:  Do inner solar system P/D-type asteroids and Jupiter Trojans share the same origin as Centaurs, hence TNOs?  The ways in which these object classes are potentially linked are explained in the Nice model. The most up-to-date version of that model (Nesvorny et al. 2018, Morbidelli & Nesvorny 2020) - invoking an outward migration of Uranus and Neptune during the first 100 My of solar system history - implies that the P/D-type main belt asteroids (MBAs) and the Trojans of Jupiter could be compositionally related to outer small bodies such as Centaurs, short-period comets and small (D<300km) TNOs. Spectroscopy with JWST/MIRI of Centaurs (former small TNOs)  is essential for establishing a definitive link between these presently dynamically isolated populations by probing the mineralogical composition of the silicates present at the objects’ surfaces. Unlike ices and organics, silicates are thermally stable over the considered heliocentric range that goes from the main belt to the Kuiper Belt and thus appear as the only robust tracers of a primordial common origin. It is actually the presence of similar silicate emission features in the mid-infrared spectra of P/D MBAs, Jupiter Trojans, and comets that previously succeeded in establishing a likely common origin for these bodies (e.g. Emery et al. 2006, Vernazza et al. 2015).

Here, we will present an overview of the results obtained from this JWST Cycle 2 observing program.