Seasonal variations on a planet without seasons

Among Jupiter’s many differences from Earth is that it has a small axial tilt. Combined with the gas giant’s distance from the Sun, the lack of tilt guarantees that Jupiter has negligible seasonal variability. So it came as a surprise when a new analysis by Glenn Orton of the NASA Jet Propulsion Laboratory and colleagues revealed cycles of temperature variations in the planet’s lowest atmospheric layer, the troposphere, that have periods close to that of Jupiter’s orbit. Those periodicities, along with other unexpected patterns and correlations, lead to questions that planetary scientists will want to address before applying climate models of Jupiter to gas giants and brown dwarfs beyond our solar system.

To probe the lower portion of Jupiter’s atmosphere, which hosts the famously colorful clouds that dart around the planet, Orton and colleagues analyzed four decades of IR measurements from ground- and space-based telescopes. They homed in on wavelengths around 20 µm, which are sensitive to the 120–130 K temperatures in that layer. They then looked for temporal and spatial patterns in the troposphere’s temperature changes. Most notable were temperature cycles of 10–14 years at various latitudes. Although that’s close to Jupiter’s 11.9-year orbital period, the timing of maximum and minimum temperatures did not coincide with that of the summer and winter solstices. The researchers also identified shorter-term temperature periodicities at low latitudes. Finally, Orton and colleagues discovered that temperature changes were strongly anticorrelated between the northern and southern hemispheres at multiple latitudes, including at 16° and 30°, as shown in the graphs below.

The researchers offer mechanisms that could explain some of the findings. For example, the shorter-term periodicities are similar to those reported in temperature analyses at higher altitudes, in Jupiter’s stratosphere, suggesting that some tropospheric heating may stem from the overlying layer. But the variety of cyclic changes at different time scales suggests that there are multiple, sometimes competing processes at play. If scientists want to use Jupiter as a model for understanding the climates of gaseous exoplanets, the researchers say, then they “must address the origins of these unexpected seasonal and non-seasonal periodicities on a virtually aseasonal gas giant.” (G. S. Orton et al., Nat. Astron., 2022, doi:10.1038/s41550-022-01839-0 .)