A Tale of 3 Dwarf Planets: Ices and Organics on Sedna, Gonggong, and Quaoar from JWST Spectroscopy

Abstract: We observed Sedna, Gonggong, and Quaoar with the NIRSpec instrument on the James Webb Space Telescope (JWST). All three bodies were observed in the low-resolution prism mode at wavelengths spanning 0.7 to 5.2 $\mu$m. Quaoar was also observed at 10x higher spectral resolution from 0.97 to 3.16 $\mu$m using medium-resolution gratings. Sedna's spectrum shows a large number of absorption features due to ethane (C$_2$H$_6$), as well as acetylene (C$_2$H$_2$), ethylene (C$_2$H$_4$), H$_2$O, and possibly minor CO$_2$. Gonggong's spectrum also shows several, but fewer and weaker, ethane features, along with stronger and cleaner H$_2$O features and CO$_2$ complexed with other molecules. Quaoar's prism spectrum shows even fewer and weaker ethane features, the deepest and cleanest H$_2$O features, a feature at 3.2 $\mu$m possibly due to HCN, and CO$_2$ ice. The higher-resolution medium grating spectrum of Quaoar reveals several overtone and combination bands of ethane and methane (CH$_4$). Spectra of all three objects show steep red spectral slopes and strong, broad absorptions between 2.7 and 3.6 $\mu$m indicative of complex organic molecules. The suite of light hydrocarbons and complex organic molecules are interpreted as the products of irradiation of methane. We infer that the differences in apparent abundances of irradiation products are likely due to their distinctive orbits, which lead to different timescales of methane retention and to different charged particle irradiation environments. In all cases, however, the continued presence of light hydrocarbons implies a resupply of methane to the surface. We suggest that these three bodies have undergone internal melting and geochemical evolution similar to the larger dwarf planets and distinct from all smaller KBOs.

• The paper uses JWST spectroscopy to analyze the surface composition of Sedna, Gonggong, and Quaoar, linking ice and organic distribution to their size and unique orbits.
• Analysis revealed distinct chemical signatures: Sedna shows significant ethane, Gonggong has less ethane and weak methane, and Quaoar displays weak ethane with tentative methane/CO₂ and a unique feature.
• Differences in ethane abundance likely result from varying methane retention and irradiation histories, suggesting these bodies undergo internal processes similar to larger dwarf planets.

Analytical Overview of Ice and Organic Composition on Dwarf Planets Sedna, Gonggong, and Quaoar via JWST Spectroscopy

This paper presents a comprehensive study on the surface compositions of three notable dwarf planets, Sedna, Gonggong, and Quaoar, utilizing data from the James Webb Space Telescope's (JWST) Near-Infrared Spectrograph (NIRSpec). These celestial bodies, situated on diverse orbits beyond the Kuiper Belt, offer intriguing insights into the chemical and physical processes shaped by their unique environments. The research focuses on deciphering the impact of their size and orbits on surface composition by employing high-resolution spectroscopy.

Key Observations and Findings

The paper provides a detailed spectral analysis, elucidating the presence of various light hydrocarbons and complex organics on the surfaces of Sedna, Gonggong, and Quaoar. Among the key findings:

1. Sedna's Composition: Sedna's spectrum reveals a significant presence of ethane (C₂H₆) alongside acetylene (C₂H₂) and ethylene (C₂H₄), indicating a robust irradiation chemistry driven primarily by methane decomposition. Notably, Sedna also shows potential CO₂ presence, albeit possibly mingled with complex organics.
2. Gonggong's Spectrum: Exhibiting features attributed to ethane and potentially complexed CO, Gonggong's spectral data suggest less abundant ethane compared to Sedna. Despite the noted features, Gonggong displays a lack of strong methane indicators, influencing interpretations of surface chemistry evolution.
3. Quaoar's Analysis: Medium-resolution spectra for Quaoar reveal weaker ethane signatures than Sedna and Gonggong, with methane and CO₂ detections remaining tentative. Additionally, the presence of a unique 3.2 μm feature suggests HCN, although lacking corroborative bands at longer wavelengths, rendering identification uncertain.

These chemical signatures confirm the spectra's prior ground-based attributions, and JWST’s enhanced resolution facilitates the identification of weaker absorption features that provide new compositional insights.

Interpretations and Implications

The detected features suggest that differences in ethane abundance—most prevalent on Sedna and least on Quaoar—primarily arise from varying methane retention capacities and irradiation histories, heavily influenced by each body's orbital parameters. The long-term stability of methane greatly affects the production of irradiation by-products, which are particularly pronounced in Sedna due to its extended sojourn beyond the heliopause. The researchers propose that internal geochemical processes could resupply methane, underpinning the production of observed hydrocarbons.

While pinpointing hydrocarbons such as ethane, acetylene, and possibly HCN indicates complex organic chemistry, it also raises questions about the mechanisms these planets employ to maintain volatile supplies. Moreover, sediments of irradiation products suggest analogous processes of internal differentiation and volatile evolution, aligning these bodies more closely to larger dwarfs like Pluto, Eris, and Makemake than to smaller KBOs, which lack such pronounced surface features.

Future Prospects

The study paves the way for further spectral modeling to clarify material abundances and their surface distributions. High-resolution follow-up observations, particularly aiming to resolve ambiguities surrounding methane's presence, are essential. The distinctions between residual methane and its by-products scatter light on size-orbit interdependencies, requiring nuanced quantitative models to unravel volatile dynamics further.

This investigation into Sedna, Gonggong, and Quaoar not only substantiates the influence of geophysical processes on surface compositions but underscores JWST's pivotal role in astronomical spectroscopy, setting the stage for deepened understanding of outer Solar System bodies.