Constraining Exoplanetary Clouds with Jupiter Observations: Insights from Juno & JWST

Jupiter, the largest planet in our solar system, serves as a crucial model for understanding the giant exoplanets and their atmospheres. While its upper tropospheric chemical composition is well-known, the nature and structure of its clouds remain elusive.  To unveil them planetary scientists rely heavily on theoretical models and remote sensing data, as in the exoplanets field.

Traditional models, based on equilibrium cloud condensation (ECC) theory, are highly sensitive to input parameters such as the pressure-temperature profile and the chemical composition of the atmosphere. In the case of Jupiter, ECCMs predict the existence of distinct cloud layers, with the uppermost being composed of pure ammonia ice. More sophisticated models, like the Ackerman-Morley model (2001), incorporate turbulent diffusion and sedimentation, providing more realistic cloud densities and particle sizes. However, these models often neglect such crucial factors as the effects of atmospheric photochemistry and still rely on assumptions about the nature of condensed species. Remote sensing data can be used to retrieve cloud properties, but this process is highly complex and computationally expensive. For these reasons having a priori knowledge about some parameters and a large quantity of data acquired by different instruments is important to characterize the clouds and aerosols, both for planets and exoplanets.

Therefore, in this contribution, we briefly summarize the most important findings about Jovian clouds and aerosols obtained from an analysis of the data acquired by the JIRAM/Juno and NIRSpec/JWST instruments, as well as their implications for the study of giant exoplanets’ clouds.

Juno data suggest that theoretical cloud condensation models are not able to represent disk-averaged spectra of Jupiter, but they work well in the case of strong convective events and/or plumes; the presence of an extended layer of small reflecting particles (haze), not included in ECCMs, is also needed to obtain reasonable fits. Juno and JWST both suggest that the typical Jovian clouds are probably composed of materials that are the result of both photochemical processes in the upper troposphere and stratosphere, together with convection and condensation of volatile species in the lower troposphere. The optical properties of this unknown material can be approximated in the 2-3 micron range with similar refractive index spectra to those of Titan’s tholins, implying the presence of N-H stretch bonds within the aerosols of Jupiter’s clouds.

These findings lead to the following conclusions: (1) ECC and Ackerman-Marley models can (and should) be used as a first approximation to model clouds, bearing in mind that reality can be more complex because of phenomena like photochemistry; (2) modeling planetary clouds is extremely degenerate even if the most of the chemical and thermal structures are well known; therefore, it is essential to use iterative approaches and efficient radiative transfer suites; efficiency, however, should not sacrifice accuracy in the multiple scattering computations; (3) new laboratory measurements of ‘tholin-like materials’ optical constants are needed to improve atmospheric retrievals for both planets and exoplanets.