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tokens: we averaged spectra from 2004 October to 2016 December with the same criteria, using 57,000 spectra for S(0) and 86,000 spectra for S(1). Although these large averages were necessary to improve the signal-to-noise, the resulting atmospheric profiles (temperature and para-H 2 ) are averaged over a broad region of the planet, and in the case of Saturn, over different seasons from southern summer to northern spring. As shown in Section 4 and Figure 4 , these spectra reveal dimeric structure around S(0) on both planets, and tentatively around S(1) on Saturn. Figure 4. Spectral fits to the S(0) and S(1) lines detected on all four giant planets, using Cassini data for Jupiter and Saturn and Spitzer data for Uranus and Neptune. Uranus and Neptune. Spitzer /IRS acquired 7–36 μ m disk-integrated spectra of Uranus during Directors Discretionary Time on 2007 December 16–17, shortly after Uranus’ equinox (program 467; full details of the data reduction process are provided by Orton et al. 2014a ). Disk-integrated spectra of Neptune were acquired on 2005 November 19–20 during Spitzer ’s Cycle 2 (program 20500), and reduced using the same process as Orton et al. ( 2014a ). For both planets, the observations were designed to sample multiple longitudes during a complete rotation, but we averaged all longitudes to form a single spectrum. Here, we focus on high-resolution ( R ∼ 600) observations in the “Short-High” (SH) 9.95–19.30 μ m and “Long High” (LH) 19.27–35.97 μ m ranges, revealing the S(0) and S(1) lines in Figure 4 , respectively. Note that Orton et al. ( 2014a ) identified offsets between low-resolution and high-resolution Uranus spectra related to flux losses from different slit sizes, and ultimately scaled the SH data to match the low-resolution modes, and abandoned the LH data from their analysis entirely. In the present work, where only narrow spectral ranges are considered to identify the dimer features, we do not find it necessary to perform such a scaling and present good fits to both SH and LH data. Voyager/IRIS: to supplement the high-resolution Cassini and Spitzer observations, we also reanalyze the low-resolution (4.3 cm −1 ) IRIS observations of all four giant planets. These 180–2500 cm −1 spectra were acquired by a Michelson interferometer in a series of north–south scans executed during the close flybys between 1979 and 1989. The selection criteria for the spectra and zonal averaging techniques have been previously described for Jupiter (Fletcher et al. 2017 ), Saturn (Fletcher et al. 2016 ), Uranus (Orton et al. 2015 ), and Neptune (Fletcher et al. 2014 ). Dimer features were previously identified only in the Jupiter and Saturn spectra (Frommhold et al. 1984 ), with the Uranus and Neptune data being too low in signal to reveal the dimer absorptions. The zonally averaged 300–550 cm −1 regions of these spectra are re-analyzed with the new opacity database in Section 4 . 3.2. Spectral Retrieval Model The NEMESIS spectral retrieval algorithm