The molecular diversity of TNOs revealed by JWST and its links to protoplanetary composition and processes

Introduction: Bodies orbiting the trans-Neptunian regions (TNOs) are windows into Solar System formation, history and evolution. They are remnants from the ancient time of planetary formation in the protoplanetary disk, but they have all been affected to some extent by subsequent dynamical evolution of the Solar System. Many are probably very primitive, in the sense that they incorporated solids from the protoplanetary disk and never melted, and can provide precious information about conditions in the solar nebula. Others remelted and differentiated, and we can use them to learn about the processes of planet growth and evolution.

Methods: Before the launch of the James Webb Space Telescope (JWST), the detection of molecules on TNOs has long been limited by the terrestrial atmosphere and the sensitivity of the available instrumentation. We used the low spectral resolution PRISM grating on the Near-Infrared Spectrograph (NIRSpec) of JWST as part of the Cycle 1 Large Program “Discovering the Surface Composition of trans-Neptunian objects" (DiSCo-TNOs) to observe 54 medium-sized TNOs and 5 Centaurs. The sample contains objects spanning the diversity of the TNO population in terms of size, visible colors, geometric albedo, and dynamical properties, except for the volatile rich dwarf planets.

Results: JWST is providing an unprecedented view of the molecular diversity on the surfaces of TNOs [1]. We report the detection of several molecular ices throughout the TNO population, including H₂O, CO₂, ¹³CO₂, CO, CH₃OH, and complex molecules and refractory materials containing aliphatic C–H, C≡N, O–H, and N–H bonds. In particular, CO₂ is widespread in all TNO populations [2]. As a result of the imprint that these molecules leave on the spectra, three main compositional groups consistently emerge from multiple independent cluster analysis efforts: Bowl-type surfaces are water- and dust-rich and CH-poor, while Cliff and Double-dip surfaces are C-rich and water-poor, with Double-dip being rich in CO₂ and Cliff surfaces being rich in organics. The C/O and (CH+NH)/(C+O) ratios on the surface of TNOs are the primary indicators of the spectral differences among the three TNO groups that we observe today.

Discussion: Our results unlock the long-standing question of the interpretation of TNO colour diversity providing compositional information. The marked separation of the three spectral clusters reveals sharp variations in the surface molecular constituents. CO₂ and CH₃OH play a major role, probably because these ices were abundant in the protoplanetary disk. We propose that medium-sized TNOs are fossil remnants of icy planetesimals, and that the three compositional groups provide a picture of the ice retention lines in the Solar System that likely occurred in the outer protoplanetary disk, possibly just before a major planetary migration. Finally, Centaurs observed by DiSCo provide direct evidence of how the three TNO spectral groups evolve when surface activity is triggered by entering the region between the orbits of Jupiter and Neptune [3].

References: [1] Pinilla-Alonso N. et al. (2024). Nature Astronomy, under revision. [2] DePrà M. et al. (2024). Nature Astronomy, in press. [3] Licandro J. et al. (2024). Nature Astronomy, in press.