Compositional Characterization of Cliff-Type Trans-Neptunian Objects

Introduction:  Trans-Neptunian Objects (TNOs) harbor materials from the early solar nebula (e.g., Gladman et al. 2008; Tegler et al. 2016). Observations from the NIRSpec instrument onboard the James Webb Space Telescope (JWST) included >50 TNOs during Cycle 1 for the large program “Discovering the Compositions of TNOs” (DiSCo-TNOs; #2418; PI: N. Pinilla-Alonso). DiSCo-TNO observations built a spectral taxonomic system consisting of three major classes. “Bowl” and Double-Dip” TNOs show abundances of water and CO/CO₂, respectively, which place their theoretical formation distance interior to the “Cliff” TNOs, which show spectral features potentially associated with methanol and a variety of complex hydrocarbons (Pinilla-Alonso et al. 2024). Cliff-type TNOs further separate spectrally into Cliff1 & Cliff2. Notably, all cold-classical KBOs exhibit Cliff2 spectra, though not all Cliff2s are cold-classical KBOs. Spectrally, Cliff1 spectra show deeper methanol features from 2 – 2.6 μm, redder near-infrared slopes, and deeper CO₂ combination bands near 2.7 μm than Cliff2 spectra (Brunetto et al. 2025). Additionally, Cliff1 spectra show slightly wider 3-μm bands than Cliff2 spectra (Figure 1).

Figure 1: Spectra of Cliff-subclasses in the 3-μm region, highlighting their spectral differences, normalized to 2.6 μm.

Motivation & Methodology:  Spectral separation among the Cliff-type TNOs highlights the need to compositionally constrain this population by quantifying  abundances of methanol, water, and light hydrocarbons. We tested the overarching hypothesis from the aforementioned literature: Cliff-type TNO compositions encompass the materials found within the other TNO classes in addition to volatile materials that are indicative of distant formation. We performed targeted spectral band comparisons and employed Hapke spectral modeling of the 3-μm region for all DiSCo Cliff-type TNOs.

Results & Analysis:  We broadly find that mixtures of methanol, water, and tholins adequately reflect the general structure of the 3-μm band of Cliff-types. Both Titan and ice tholins serve as suitable substitutes for the variety of complex hydrocarbons that (possibly) reside on their surfaces.  Some Cliff-type spectra present features associated with a mixture of complex aliphatic molecules. We discuss the possible dynamical scenarios if some or all of these materials are contained on Cliff-type surfaces. (Pinilla-Alonso et al. 2024b; Licandro, J. et al. 2024).

Hapke models of the Cliff-type spectra further suggest two main compositional groups – one dominated by methanol and another dominated by tholins. These compositional groups align with the spectral subclasses (Figure 2). Further, some Cliff2 spectra are potentially best modeled when tholins include embedded amorphous carbon (AC) grains. The AC grains lower albedo enough to match Cliff-type spectra, and their contribution also maintains a physically reasonable surface for a distant TNO. Higher-albedo Cliff2 spectra are potentially best modeled when mixed with relatively higher amounts of (crystalline) water and/or methanol, though their ice abundances are still lower than those of Cliff1 spectra. We also present specific results related to the Mors-Somnus binary system (Souza-Feliciano et al. 2024) and the “outlier” Cliff-type, 2004 PG₁₁₅ (Pinilla-Alonso et al. 2024a; Brunetto et al. 2025).

Figure 2: (a)  The collection of  spectra for each object in the Cliff1 (purple) and Cliff2 (gold) subgroups, along with (b) their corresponding best-fit models. Also shown are the average abundances of the primary components between the Cliff subclasses.

 

References:

Gladman, B. et al. (2008). The Solar System Beyond Neptune.

Tegler, S. C. et al. (2016) Astron. J. 152

Pinilla-Alonso, N. et al. (2024a). Nat. Astron., 9.

Brunetto, R. et al. (2025). AJL, 982.

Pinilla-Alonso, N. et al. (2024b). Astron. Astrophys., 692.

Licandro, J. et al. (2024). Nat. Astron., 9.

Souza-Feliciano, A. C. et al. (2024). Astron. Astrophys., 681.