Webb provides fresh insights into Saturn’s changing seasons

Saturn's seasons are significantly longer because it orbits the Sun in nearly 29.5 Earth years.
Mrigakshi Dixit
Montage of JWST MIRI/MRS observations of Saturn in November 2022, requiring four tiles to study Saturn’s northern hemisphere and rings.
Montage of JWST MIRI/MRS observations of Saturn in November 2022, requiring four tiles to study Saturn’s northern hemisphere and rings.

NASA, ESA, and Amy Simon (NASA-GSFC); Image Processing: Alyssa Pagan (STScI)). 

The advanced James Webb Space Telescope (JWST) has provided the latest weather report of the gas giant Saturn

This huge outer planet goes through seasonal cycles, much like Earth, due to its axial tilt. However, its seasons are significantly longer because Saturn orbits the Sun in nearly 29.5 Earth years. As a result, one season lasts approximately 7.5 Earth years.

The University of Leicester scientists reviewed Webb's data and discovered new insights regarding Saturn's changing seasons. 

The atmospheric observations by Webb's advanced instrument

Webb's MIRI instrument acquired infrared data and images of the ringed planet's atmosphere. This led to the measurement of various atmospheric parameters, including temperatures, gaseous abundances, and the existence of clouds at multiple altitudes. 

As per the official release, this instrument can split infrared light into its constituent wavelengths, enabling scientists to detect the distinct signatures of the diverse range of chemicals present in a planet's atmosphere.

“The quality of the new data from JWST is simply breath-taking – in one short set of observations, we’ve been able to continue the legacy of the Cassini mission into a completely new Saturnian season, watching how the weather patterns and atmospheric circulation respond to the changing sunlight,” said Professor Leigh Fletcher, from the University of Leicester School of Physics and Astronomy, in a press release. 

Fletcher added: “JWST can see in wavelengths of light that were inaccessible to any previous spacecraft, producing an exquisite dataset that whets the appetite for the years to come.”

The observations indicate that Saturn is approaching its northern autumn equinox in 2025, suggesting that the northern pole is entering prolonged periods of polar winter.

Saturn is about to experience autumn

The main image showcases this north pole in blue due to strong thermal emission. The image also exhibits a large 1500km-wide storm called the North Polar Cyclone (NPC), first spotted by the Cassini probe. The different color hues depict temperature variations at various stratospheric levels.

The authors describe around this cyclone a wider area of warm gases known as the north-polar stratospheric vortex (NPSV), formed during Saturn's long summer season (spring).

This north polar vortex will progressively cool and dissipate with the arrival of the autumn equinox in 2025. The recent observations offer a final look at Saturn's north pole for the next few years. Soon, this pole will start moving into the darkness of the polar winter.

The scientists also modeled Saturn's spectrum data obtained by Webb in 2022 and discovered a considerable variation from what the Cassini mission saw. 

The new measurements indicated that the distributions of stratospheric temperatures and gases during this summer phase of Saturn's seasonal cycle differed significantly from those detected by the probe during northern winter and spring. 

The stratospheric air circulation on Saturn has most likely now reversed. Cooler air temperatures and lower quantities of hydrocarbon gases were detected in the north, between 10°N and 40°N. 

Simply put, the new observations reveal that the planet’s northern summer is experiencing a “cooling trend” due to the reverse flow of the air as autumn approaches.

“No spacecraft has ever been present to explore Saturn’s late northern summer and autumn before, so we hope that this is just the starting point, and that JWST can continue the legacy of Cassini into the coming decade,” said Fletcher. 

The team has outlined plans for more such observations of Saturn and other gas giants using the Webb in the coming months. 

Fletcher concluded: “This work on Saturn is just the first of a programme of observations of all four giant planets, and JWST is providing a capability beyond anything we’ve had in the past – if we can get so many new findings from a single observation of a single world, imagine what discoveries await?”

The results have been published in the Journal of Geophysical Research Planets. 

Study abstract:

Saturn's northern summertime hemisphere was mapped by JWST/Mid-Infrared Instrument (4.9–27.9 µm) in November 2022, tracing the seasonal evolution of temperatures, aerosols, and chemical species in the 5 years since the end of the Cassini mission. The spectral region between reflected sunlight and thermal emission (5.1–6.8 µm) is mapped for the first time, enabling retrievals of phosphine, ammonia, and water, alongside a system of two aerosol layers (an upper tropospheric haze p < 0.3 bars, and a deeper cloud layer at 1–2 bars). Ammonia displays substantial equatorial enrichment, suggesting similar dynamical processes to those found in Jupiter's equatorial zone. Saturn's North Polar Stratospheric Vortex has warmed since 2017, entrained by westward winds at p < 10 mbar, and exhibits localized enhancements in several hydrocarbons. The strongest latitudinal temperature gradients are co-located with the peaks of the zonal winds, implying wind decay with altitude. Reflectivity contrasts at 5–6 µm compare favorably with albedo contrasts observed by Hubble, and several discrete vortices are observed. A warm equatorial stratospheric band in 2022 is not consistent with a 15-year repeatability for the equatorial oscillation.

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