Jupiter's hot spots as observed by JIRAM-Juno: limb-darkening in thermal infrared

The Jupiter InfraRed Auroral Mapper (JIRAM) instrument on board the Juno spacecraft performed multiple observations of the Jupiter North Equatorial Belt (NEB) around the time of 12th Juno pericenter passage on April 1st 2018. The data consist in thermal infrared images (the JIRAM filter has a band pass centred around 4.8 μm) and show, among other atmospheric features, two bright hot-spots.Images of the same areas at different emission angles were used to constraint the trend of the limb darkening function.Comparison against simulated observations computed for different emission angles, total opacities, single scattering albedo ω0 and asymmetry parameter g suggest that ω0 ~ 0.9 and g ~ 0.32 provide best match with data, with the latter parameter only weakly constrained by JIRAM observations. Then, we computed the ω0 and g resulting from different size distributions (exploring the effective radius reff and variance v space), taking into account the complex refractive indices of ammonium hydrosulphide by [1] and [2].Our analysis suggests that neither sets of refractive indices are consistent with JIRAM observations. A more reasonable agreement is found once tholines are adopted, with an effective radius of 0.6 μm. This value is broadly consistent with the mean radius of Hot Spot's particles estimated by [3] on the basis of Galileo Entry Probe data. While a composition of pure tholine is not realistic for Jupiter conditions, our results indicate that scattering properties of clouds are largely dominated by optical properties of contaminants, as already suggested in [4]. Indeed, a thin (0.01 of total radius) coating of such compound over a NH4SH particle can effectively mask the optical properties of the latter. An effective radius of 0.4 μm for these coated particles produces the ω0 and g derived from JIRAM data. [1] Howett C. J. A. et al., (2007) J. Opt. Soc. Am. B 24, 126-136.[2] Ferraro J. R et al. (1980) Applied Spectroscopy, 34 (5), 525-533.[3] Ragent B. et al, (1998), J. Geophys. Res., 103 (E10), 22891- 22909.[4] Grassi D. et al. (2021) MNRAS, 503(4), 4892-4907. This work was supported by the Italian Space Agency through ASI-INAF contract 2016-23-H.1-2018.