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JWST/NIRSpec Spectrum of the Inner Oort Cloud Dwarf Planet (90377) Sedna

Presentation #301.08 in the session TNOs and Centaurs as Viewed by JWST (Oral Presentation)

Published onOct 23, 2023
JWST/NIRSpec Spectrum of the Inner Oort Cloud Dwarf Planet (90377) Sedna

The dwarf planet (90377) Sedna is the largest of the three known members of the Inner Oort Cloud (IOC) population. Ground-based visible and near-infrared (NIR; 0.4 to 2.5 μm) spectra of Sedna showed absorption near 2.3 μm previously ascribed to CH4 and/or CH3OH ice, hints of H2O ice and a steep red spectral slope suggestive of complex organics. Photometry at 3.6 and 4.5 μm measured with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope indicated strong absorption bands relative to the 2-μm region and support spectral models that include H­2O ice and simple hydrocarbons. However, the low S/N of the NIR spectra and photometric nature of the IRAC data hinder detailed compositional analysis. With equilibrium temperatures of ~30 K and ~9 K at perihelion (~76AU) and aphelion (~937 AU), respectively, a wide range of ices should be stable on the surface. Sedna was observed by the James Webb Space Telescope (JWST) using the NIRSpec IFU in prism mode. The observations were part of GTO program 1272 (PI D. Hines) and occurred on Sept 13, 2022 (17:18-17:49 UT). The spectrum covers the wavelength range 0.7–5.3 μm with a spectral resolving power of ~100. H2O ice bands suggested in the ground-based spectra at 1.5 and 2.0 μm are confirmed, and distortions of the band shapes indicate significant contamination of the ice by other materials. The absorption band complex near 2.3 μm is revealed with significant substructure. At λ > 2.5 μm, the overall level of reflectance is consistent with the Spitzer/IRAC photometry. A broad and deep absorption from ~2.8 to 3.5 μm is likely due to H­2O ice, as is a gentle curvature at ~4–5 μm. Several narrower absorption bands throughout the NIRSpec prism wavelength range are consistent with ethane (C2H6) ice, including a very strong absorption complex near 3.4 μm. Interestingly, the strong absorption from methane (CH4) at 3.3 μm is not apparent in the spectrum, nor does CH3OH appear to be present. We will present the JWST spectrum of Sedna in the context of the previous ground-based and Spitzer data and will present new spectral analyses. Eventually, comparison of the composition of Sedna to other dynamical classes of Trans-Neptunian Objects (TNOs) will address the context of IOC objects within the TNO population.

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