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Abstract:

<jats:title>Abstract</jats:title> <jats:p>In this work, we present for the first time infrared spectra of Titan from the Spitzer Space Telescope (2004–2009). The data are from both the short wavelength–low resolution (SL; 5.13–14.29 <jats:italic>μ</jats:italic>m, <jats:italic>R</jats:italic> ∼ 60–127) and short wavelength–high resolution (SH; 9.89–19.51 <jats:italic>μ</jats:italic>m, <jats:italic>R</jats:italic> ∼ 600) channels showing the emissions of CH<jats:sub>4</jats:sub>, C<jats:sub>2</jats:sub>H<jats:sub>2</jats:sub>, C<jats:sub>2</jats:sub>H<jats:sub>4</jats:sub>, C<jats:sub>2</jats:sub>H<jats:sub>6</jats:sub>, C<jats:sub>3</jats:sub>H<jats:sub>4</jats:sub>, C<jats:sub>3</jats:sub>H<jats:sub>6</jats:sub>, C<jats:sub>3</jats:sub>H<jats:sub>8</jats:sub>, C<jats:sub>4</jats:sub>H<jats:sub>2</jats:sub>, HCN, HC<jats:sub>3</jats:sub>N, and CO<jats:sub>2</jats:sub>. We compare the results obtained for Titan from Spitzer to those of the Cassini Composite Infrared Spectrometer (CIRS) for the same time period, focusing on the 16.35–19.35 <jats:italic>μ</jats:italic>m wavelength range observed by the SH channel but impacted by higher noise levels in the CIRS observations. We use the SH data to provide estimated haze extinction cross sections for the 16.67–17.54 <jats:italic>μ</jats:italic>m range that are missing in previous studies. We conclude by identifying spectral features in the 16.35–19.35 <jats:italic>μ</jats:italic>m wavelength range that could be analyzed further through upcoming James Webb Space Telescope Cycle 1 observations with the Mid-Infrared Instrument (5.0–28.3 <jats:italic>μ</jats:italic>m, <jats:italic>R</jats:italic> ∼ 1500–3500). We also highlight gaps in the current spectroscopic knowledge of molecular bands, including candidate trace species such as C<jats:sub>60</jats:sub> and detected trace species such as C<jats:sub>3</jats:sub>H<jats:sub>6</jats:sub>, that could be addressed by theoretical and laboratory study.</jats:p>

Abstract:

Close-in giant exoplanets with temperatures greater than 2,000 K (''ultra-hot Jupiters'') have been the subject of extensive efforts to determine their atmospheric properties using thermal emission measurements from the Hubble and Spitzer Space Telescopes. However, previous studies have yielded inconsistent results because the small sizes of the spectral features and the limited information content of the data resulted in high sensitivity to the varying assumptions made in the treatment of instrument systematics and the atmospheric retrieval analysis. Here we present a dayside thermal emission spectrum of the ultra-hot Jupiter WASP-18b obtained with the NIRISS instrument on JWST. The data span 0.85 to 2.85 $\mu$m in wavelength at an average resolving power of 400 and exhibit minimal systematics. The spectrum shows three water emission features (at $>$6$\sigma$ confidence) and evidence for optical opacity, possibly due to H$^-$, TiO, and VO (combined significance of 3.8$\sigma$). Models that fit the data require a thermal inversion, molecular dissociation as predicted by chemical equilibrium, a solar heavy element abundance (''metallicity'', M/H = 1.03$_{-0.51}^{+1.11}$ $\times$ solar), and a carbon-to-oxygen (C/O) ratio less than unity. The data also yield a dayside brightness temperature map, which shows a peak in temperature near the sub-stellar point that decreases steeply and symmetrically with longitude toward the terminators.Comment: JWST ERS bright star observations. Uploaded to inform JWST Cycle 2 proposals. Manuscript under review. 50 pages, 14 figures, 2 table