Spectral properties of cloud clearance regions in the wake of Jupiter’s Great Red Spot from JIRAM-Juno data

Thermochemical models of Jupiter's troposphere predict liquid water clouds below the 4–5 bar level [1,2], with NH₄SH and NH₃ aerosols forming higher layers. Consequently, spectroscopic detection of deeper water clouds remains sparse [3,4]. This study examines cloud-clearance areas identified using JIRAM-Juno spectra near the Great Red Spot and the South Equatorial Belt, along with their implications for the planet's cloud structure.

We analyzed spectra from Juno’s first perijove in August 2016. JIRAM covers 2–5 μm with 13 nm resolution, capturing data along 256-pixel slits. The limited data volume and operational constraints often restrict spatial coverage. For λ > 4 μm, thermal emission dominates, with lower 5-μm signals corresponding to NH₄SH and NH₃ opaque cloud tops at 1–0.5 bars. Cloud-clearance regions appear brighter, revealing deeper levels at 4–5 bars, where absorption from H₂ (collision-induced) and other minor gases (NH₃, H₂O, PH₃, GeH₄) prevails. At λ < 3.2 μm, reflected solar radiation dominates, while vertical aerosol profiles and gas opacity—notably methane—shape the signal. A radiance maximum at 2.78 μm highlights transparent regions in methane and ammonia spectra, allowing detection of clouds down to 2–3 bars in the absence of upper aerosol layers.

JIRAM data confirm an anticorrelation between thermal (≥ 5 μm) and solar (2.73 μm) signals [5], consistent with a gray upper cloud deck. Among these trends, regions with high thermal flux but very low solar signals suggest an exceptionally thin upper cloud deck. Their locations align with deep-cloud structures previously described in [6] in the wake of the Great Red Spot.

We compared spectral properties of these clearance areas to neighboring regions of similar thermal intensity but higher solar signals. A notable feature in clearance areas is a broad peak at 4.55 μm in the spectral ratio, inconsistent with deep clouds. This peak indeed suggests an absence of opacity sources at the 3–4 bar level (where 4.55-μm contribution functions peak [7]). Furthermore, preliminary simulations assuming liquid water clouds at 3–8 bars (following [4]) for non-clearance regions show a spectral ratio peak at 4.7 μm, not 4.55 μm. Raising the water clouds to 1.5 bars yields similar results. Although further exploration of cloud altitude, density, particle composition (liquid/ice), and size distribution is required to rule out the occurrence of deep water clouds, a set of spectral retrievals suggests that the spectral properties of clearance areas are better explained by local depletions in phosphine content and exceptionally thin clouds at the 1-bar level.

JIRAM is supported by the Italian Space Agency (ASI). This work is funded by the Addendum n. 2016-23-H.3-2023 to the ASI–INAF Agreement n. 2016-23-H.0.

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