Spectral Image Observations of Uranus’ Near-IR H2 Emission Spectrum using iSHELL

Observations of Uranus’ Near-IR emission spectrum are of interest because they show that the upper atmosphere – the ionosphere and thermosphere – has been cooling since at least the early 1990s [1-4]; yet, like for the other major planets, the corresponding temperature is higher than can be explained by EUV solar heating alone [5-8]. The source of the excess heat has been an open question since the Voyager epoch [9,10]. This heating is manifest by the high rovibrational temperatures of H₂ and H₃⁺  in Uranus’ thermosphere and ionosphere, respectively, as separately probed by Uranus’ K- and L-band emission spectra. Continued observation and study may illuminate this excess heating process; e.g., by characterizing the response time to seasonal changes by determining the epoch of the temperature downtrend reversal [11,12], or correlation with secular changes in the solar wind [13]. Moreover, Uranus is of interest as a candidate for a potential flagship mission.

 

We report on the ongoing processing of, and preliminary results from, recent K-band spectral image observations of Uranus’ near-IR H₂ emission lines taken at the IRTF through the 4” slit of iSHELL, the facility high-resolution spectrograph [14]. This 32-pixel-wide slit admits Uranus’ whole 3.”8 dia. disk at opposition. This has the advantage of collecting photons from the whole disk, helping to offset the weak emission-line signal, which has been secularly declining. The high resolution of iSHELL, even with the 4” slit (R ~7000), helps to suppress both the sky background and Uranus’ continuum spectrum. This and the low signal level can lead to incomplete or excessive sky subtraction, with night sky emission from the telluric water lines partially filling in their absorption profiles, which are weak to begin with owing to Uranus’ suppressed continuum. This complicates correcting for the telluric absorption. The slit is aligned with Uranus’ central meridian, so the different lengths of the disk chords parallel to the dispersion variously affect the spectral resolution, as does - to a lesser extent - the planetary rotation. The monochromatic line spread function of the extracted 1-D emission-line spectrum for uniform intensity over the disk in the absence of significant seeing is a semi-ellipse, twice as high as wide. It is feasible to combine H₂ emission lines that appear in overlapping orders and thereby offset the reduced S/N due to the strong order blaze attenuation there. The short 5” dekker of iSHELL in the K band requires nodding to sky. Since Uranus’ H₂ emission extends beyond its disk, xspextool’s background sky subtraction at the slit ends for each wavelength pixel [15] must not be set, or the emission spectrum will be partly or fully subtracted out. With Uranus’ spectrum not pegged to zero, the residual continuum has to be fitted and subtracted out, leaving only the emission spectrum of interest. Once the emission lines are extracted and their variance determined, they can then be analyzed by conventional techniques to extract the rovibrational temperature and column abundance.

 

Observing runs at the IRTF with iSHELL during the 2023 and 2024 apparitions to obtain Uranus’ K-band H₂ spectrum were successful. We present preliminary results and illustrate some of the phenomena and issues mentioned above.

 

 

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