Investigating Thermal Contrasts Between Jupiter's Belts, Zones, and Polar Vortices with VLT/VISIR

<p><br />Ground- and space-based remote sensing, from Voyager, to Galileo, Cassini and Juno, has revealed the existence of circulation cells in the troposphere of Jupiter. These circulation cells, which may be similar to terrestrial Ferrel cells [1], show properties that vary significantly as a function of depth, showing circulations of opposing directions above and below the expected level of the water condensation cloud near 4-6 bar [2]. Moreover, the location of each vertical branch of the Ferrel-like cells are correlated to the Jupiter's temperature belt/zone contrast, suggesting a dynamical and thermal link between winds, temperatures, aerosols, and composition. </p> <p>To provide infrared support for Juno spacecraft observations, we have been observing Jupiter with the VISIR mid-infrared instrument on the Very Large Telescope (VLT) since 2016.  We analyse images at multiple wavelengths between 5 and 20 µm  to study the thermal, chemical and aerosol structure of Jupiter's belts, zones, and polar domains. In particular, an observing run in May 2018 (conciding with Juno's 13 perijove) provided global coverage of Jupiter in  thirteen narrow-band filters.  These data sense stratospheric temperature (7.9 µm), tropospheric temperature via the collision-induced hydrogen-helium continuum (13, 17.6, 18.6, 19.5 µm), aerosol opacity (8.6 and 8.9 µm), and the distribution of ammonia gas (10.5, 10.7 and 12.3 µm).  These wavelengths primarily sound the upper troposphere at p<0.7 bar, above the cloud tops, so are sensitive to the upper cell of the belt/zone Ferrel-like circulations.  By stacking the data in all 13 filters, we invert the data using the optimal-estimation retrieval algorithm NEMESIS [3] to derive temperature, aerosol and chemical structure over the whole planet. Meridional gradients of temperature, wind shear (derived from thermal balance equation) and chemical species will be examined to understand the upper-tropospheric circulation cells.  </p> <p>We confirm that the pattern of cool anticyclonic zones and warm cyclonic belts persists throughout the mid-latitudes, up to the boundary of the polar domains.  This implies, via thermal wind balance, the decay of the zonal jets as a function of altitude throughout the upper troposphere. Aerosol opacity is often (but not always) highest in the anticyclonic zones, suggesting condensation of saturated vapours, but we caution that aerosol opacity is not a good proxy for atmospheric circulation on any giant planet.  The thermal and compositional gradients derived from the VISIR maps are consistent with those from Voyager and Cassini, but opposite to what would be inferred for the Ferrel-like circulations of the deeper cell of [1], which was suggested by [2] to exist only below the water-cloud layer based on Juno microwave observations.</p> <p>Concerning the Jovian polar regions, the analysis of VISIR imaging shows a large region of mid-infrared cooling poleward ~67˚S, co-located with the reflective aerosols observed in methane-band imaging by JunoCam, suggesting that they play a key role in the radiative cooling at the poles, and that this cooling extends from the upper troposphere into the stratosphere.  These VISIR observations also reveal thermal contrasts across polar features, such as numerous cyclonic and anticyclonic vortices, as well as evidence of high-altitude heating by auroral precipitation. Comparison of zonal mean thermal properties and high-resolution visible imaging from Juno allows us to study the variability of atmospheric properties as a function of altitude and jet boundaries, particularly in the cold southern polar vortex.  To investigate the radiative processes and influence of auroral precipitation on the southern cold vortex, a radiative-convective model tailored for Jupiter's atmosphere [4], with an updated polar aerosol distribution from Juno mission results, will be used to determine the aerosol distribution needed to reproduce the thermal structure of the cold polar vortex of Jupiter.   </p> <p> </p> <p>[1] Duer, K., Gavriel, N., Galanti, E., Kaspi, Y., Fletcher, L. N., Guillot, T., Bolton, S. J., Levin, S. M., Atreya, S. K., Grassi, D., Ingersoll, A. P., Li, C., Li, L., Lunine, J. I., Orton, G. S., Oyafuso, F. A., Waite, J. H.: Evidence for Multiple Ferrel-Like Cells on Jupiter, Geophysical Research Letters, 2021.</p> <p><br />[2] Fletcher, L. N., Oyafuso, F. A., Allison, M., Ingersoll, A., Li, L., Kaspi, Y., Galanti, E., Wong, M. H., Orton, G. S., Duer, K., Zhang, Z., Li, C., Guillot, T., Levin, S. M., Bolton, S: Jupiter's Temperate Belt/Zone Contrasts Revealed at Depth by Juno Microwave Observations, Journal of Geophysical Research: Planets, 2021</p> <p>[3] Irwin, P. G. J., Teanby, N. A., de Kok, R., Fletcher, L. N., Howett, C. J. A., Tsang, C. C. C., Wilson, C. F., Calcutt, S. B., Nixon, C. A., Parrish, P. D.: The NEMESIS planetary atmosphere radiative transfer and retrieval tool, Journal of Quantitative Spectroscopy & Radiative Transfer, 2008</p> <p><br />[4] Guerlet, S., Spiga, A., Delattre, H., Fouchet, T.: Radiative-equilibrium model of Jupiter's atmosphere and application to estimating stratospheric circulations, Icarus, 2020</p>