After seven years, it’s time for a change of seasons on Saturn

Observing the full cycle of four seasons on Saturn is difficult when the orbit of the planet is 29 Earth years. The Cassini satellite—a project of NASA, the European Space Agency, and Italy’s space agency—began the work of understanding the details of Saturn’s weather, and now the James Webb Space Telescope (JWST) is building on it.

Cassini orbited Saturn from 2004 until 2017. Using its data, astronomers discovered that seasonal temperature change appeared to be driven by a transfer of air between hemispheres . In the Saturn summer, warm air rises upward in the stratosphere and cools via adiabatic expansion. Planetwide circulation patterns move the air to the opposite hemisphere, where it sinks and then is heated again. That process, known as Brewer–Dobson circulation, also occurs on Earth.

Since Saturnian seasons are seven years long, Cassini collected data for only two seasons—northern winter and spring. The wintertime observations showed air rising in the southern hemisphere and sinking in the northern hemisphere. Recent observations by Leigh Fletcher (University of Leicester) and his collaborators have used MIRI (the mid-IR instrument) on the JWST to support the previous work and see the predicted reversal of the atmospheric circulation pattern as the northern Saturnian summer comes to a close. The JWST saw air rising in northern summer, the opposite of Cassini.

The JWST‘s newer IR instruments provide higher-resolution spectroscopic observations than Cassini‘s and allow researchers to track the gases, aerosols, and temperatures as they change with time. The spectroscopic instruments also let the team use the location and density of aerosols in the atmosphere to better trace the shifting chemistry in the seasonal atmosphere. The data, however, are based on only two days of observations, a mere snapshot in the Saturnian year.

Although Saturn is a large target for the JWST, more than filling its field of view, astronomers studying the outer planets, like Uranus , are campaigning for annual guaranteed time on the JWST. That cadence could lead to a comprehensive understanding of Saturn’s seasons. Observations from the JWST time would be used with those from the Hubble Space Telescope’s Outer Planet Atmospheres Legacy program; together the instrumentation would cover wavelengths from UV to IR and give astronomers a 3D view of the weather of the gas giants. (L. N. Fletcher et al., J. Geophys. Res. Planets 128, e2023JE007924, 2023 .)