Subseasonal Variation in Neptune’s Mid-infrared Emission

Abstract

We present an analysis of all currently available ground-based imaging of Neptune in the mid-infrared. Dating between 2003 and 2020, the images reveal changes in Neptune’s mid-infrared (∼8–25 μm) emission over time in the years surrounding Neptune’s 2005 southern summer solstice. Images sensitive to stratospheric ethane (∼12 μm), methane (∼8 μm), and CH3D (∼9 μm) display significant subseasonal temporal variation on regional and global scales. Comparison with H2 S(1) hydrogen quadrupole (∼17.035 μm) spectra suggests that these changes are primarily related to stratospheric temperature changes. The stratosphere appears to have cooled between 2003 and 2009 across multiple filtered wavelengths, followed by a dramatic warming of the south pole between 2018 and 2020. Conversely, upper-tropospheric temperatures—inferred from ∼17 to 25 μm imaging—appear invariant during this period, except for the south pole, which appeared warmest between 2003 and 2006. We discuss the observed variability in the context of seasonal forcing, tropospheric meteorology, and the solar cycle. Collectively, these data provide the strongest evidence to date that processes produce subseasonal variation on both global and regional scales in Neptune’s stratosphere.

Mid-Infrared Observations of the Giant Planets

“…N-band (8-14 µm) spectroscopy and imaging have been used in numerous investigations to infer temperatures, chemistry, and aerosol abundances in the troposphere and stratosphere of Jupiter (e.g., [49][50][51][52][53][54]) and Saturn (e.g., [55][56][57][58][59][60][61][62][63][64][65]). For Uranus and Neptune, the N band has been used to measure stratospheric emission associated with hydrocarbons (e.g., [33,[66][67][68][69][70][71][72][73][74]), but interpretations have been limited by larger uncertainties in both temperatures and chemical abundances.…”

“…Since the abundance of hydrogen and helium are homogeneous and relatively well constrained by the overall atmospheric density, this collision-induced absorption (CIA) provides a powerful, unambiguous indicator of the atmospheric thermal structure. Q-band observations (17-25 µm) are dominated by this absorption, and have thus successfully been used to infer atmospheric temperatures in the upper tropospheres of all the Solar System giant planets (e.g., [33,50,51,61,65,72,[74][75][76][77][78][79]). Hydrogen emission can also be found in the Q band, and this can additionally serve as a remote sensing thermometer of the stratosphere, as discussed in Section 3.1.…”

“…By the mid-2000s, numerous mid-infrared instruments were in operation on 8-m class telescopes, including: Long Wavelength Spectrometer (LWS) [280] at Keck; Michelle [281] at Gemini North; The VLT Imager and Spectrometer for Mid-Infrared (VISIR) [282] at the Very Large Telescope (VLT); the Thermal-Region Camera Spectrograph (T-ReCS) at Gemini South, [283]; and the Cooled Mid-Infrared Camera and Spectrometer (COMICS) [284] at Subaru. Typically, planetary observations with these instruments have applied narrowband filters covering spectral ranges between 8 and 13 µm (the N-band) and 17 to 25 µm (the Q-band), from which chemistry and/or temperatures were retrieved [33,51,53,65,74,77]. Additionally, such observations were frequently used to complement contemporaneous spacecraft observation, providing greater spatial or temporal coverage than possible from orbit [46,62,78,285].…”

“…In terms of spectroscopy, a notable workhorse of ground-based remote sensing at mid-infrared wavelengths over the past two decades is the Texas Echelon Cross Echelle Spectrograph (TEXES) [286]. Capable of the spectral resolving power of 15,000 to 100,000 (λ/∆λ) in windows between 5 and 25 µm, TEXES has been used to great effect on IRTF and Gemini North to map chemistry and temperatures in Jupiter [287][288][289][290][291][292], Saturn [293][294][295][296], and to a lesser extent Uranus [297,298] and Neptune [70,74], with the exceptionally high spectral resolution needed to resolve fine lines. The resulting quality of retrieved maps of temperature, composition, and aerosols have been noted to even surpass previous spacecraft results for Jupiter [52].…”

“…Figures 9 and 10 compare the observed mid-infrared spectra of the giant planets derived from ISO-SWS [266] and Cassini-CIRS [141,237] for Jupiter and Saturn, and Spitzer-IRS [74,76,275] for Uranus and Neptune. Figure 11 compares ground-based images in three key mid-infrared windows.…”