A comprehensive picture about Jovian clouds and hazes from Juno/JIRAM infrared spectral data

Jupiter, the largest planet in our solar system, is a vital reference point for understanding gaseous exoplanets and their atmospheres. While we know its upper tropospheric chemical composition well, the nature and structure of its clouds remain puzzling. We, therefore, rely on theoretical models and remote sensing data to address this.

While traditional equilibrium chemistry condensation models (ECCM) are sensitive to input parameters, advanced models [1] offer more realistic cloud property predictions. Remote sensing data can help determine cloud properties and test theoretical predictions thanks to the application of multiple scattering atmospheric retrieval. Still, the process is highly degenerate and, therefore, computationally demanding. The predicted tropospheric layers are upper ammonia ice (∼0.7 bar) and ammonium hydrosulfide (∼2 bar) clouds [2], but their spectral detection has been limited to small, dynamically active regions (<2% of the disk) [3, 4].

This study analyzes JIRAM/Juno data [5] to investigate Jovian clouds and aerosols, focusing on viewing conditions relevant to future exoplanet missions. Preliminary radiative transfer simulations (using PSG [6]) indicated aerosol signatures are most prominent in the 2.6-2.8 μm and 4.5-5 μm high gas transmittance windows. The low gas opacity makes the aerosol contributions observable by JIRAM in terms of reflection of the incident solar radiation (2.6-2.8 μm) and attenuation of thermal black body radiation from the planet (4.5-5 μm). In particular, the observed solar reflection necessitates a vertically extended tropospheric haze layer consistent with previous observations [7] and Titan-adapted microphysical models [8, 9], suggesting stratospheric formation via methane photochemistry [10].

Retrievals across the full 2-5 μm JIRAM range were performed using PSG [6], varying cloud/haze parameters and gas mixing ratios, employing a single haze layer and two cloud layers (main and deep). Three compositions were tested for the main cloud: pure ammonia ice, tholins [11], and “Jupiter-adapted” tholins ([11] data removed of the 4.7 μm feature). Haze and deep cloud were modeled as reflecting and grey absorbers, respectively.

Results confirm the necessity of a haze and constrain its size and density. “Adapted” tholins provided the best spectral fit. Retrieved main cloud densities were significantly lower than ECCM and [1] predictions, with particle radii around 2-3 μm. Deep cloud densities aligned with the predicted ammonium hydrosulfide cloud top. These results may suggest that photochemistry may play a huge role in shaping the aerosol layers of Jupiter, combined with classic condensation. The observation of photochemistry in cold gaseous exoplanets such as GJ-1214b [12] makes Jupiter's troposphere a remarkably accessible laboratory for investigating these processes up close.

We formulate at least two options regarding the creation of these “adapted” tholins. The first is that they are the result of successful coagulation of some haze hydrocarbon small particles and the products of (gaseous) ammonia photolysis at pressures between 0.2 and 0.7 bar. However, we have to test if this process can efficiently create large particles around 3 microns. Alternatively, the tholins can be produced at the top of freshly produced ammonia ice clouds. Here, UV radiation can easily dissociate larger ice particles that successively can react with hydrocarbons and the gaseous ammonia photolysis products. This scenario is particularly intriguing as it should be consistent with the so-called “chromophore model” [13].  More accurate chemical models and laboratory measurements are, however, needed to verify our hypothesis.

References:

[1] Ackerman A. S., Marley M. S., 2001, ApJ, 556, 872

[2] Atreya S. K., Wong M. H., Owen T. C., Mahaffy P. R., Niemann H. B., dePater I., Drossart P., Encrenaz T., 1999, Planet. Space Sci., 47, 1243

[3] Baines K. H., Carlson R. W., Kamp L. W., 2002, Icarus, 159, 74

[4] Biagiotti, F., Grassi, D., Liuzzi, G., et al. 2025, Monthly Notices of the Royal Astronomical Society, staf38

[5] Adriani, A., Filacchione, G., Di Iorio, T., et al. 2017, Space Sci. Rev., 213, 393

[6] Villanueva G. L., Smith M. D., Protopapa S., Faggi S., Mandell A. M., 2018, Quant. Spec. Radiat. Transfer, 217, 86

[7] Dahl E. K. et al., 2021, Planet. Sci. J., 2, 16

[8] McKay, C. P., Pollack, J. B., & Courtin, R. (1989). Icarus, 80(1), 23–53.

[9] Rannou, P., McKay, C. P., & Lorenz, R. D. (2003). Planetary and Space Science, 51(14–15), 963–976.

[10] Moses, J. I., Fouchet, T., Bézard, B., et al. 2005, JGRE, 110, E08001

[11] Imanaka H., Cruikshank D. P., Khare B. N., McKay C. P., 2012, Icarus, 218, 247

[12] Kreidberg L. et al., 2014, Nature, 505, 69

[13] Baines K. H., Sromovsky L. A., Carlson R. W., Momary T. W., Fry P. M., 2019, Icarus, 330, 217