A brownish and gray image of the ringed planet Saturn appears against a black background, with portions of its northern hemisphere and rings overlain with colorful new spacecraft images.
The James Webb Space Telescope captured new images of Saturn’s northern hemisphere and rings (seen here against a background image of Saturn from the Hubble Space Telescope), revealing seasonal atmospheric changes. Credit: NASA/ESA/University of Leicester/L. N. Fletcher/O. King
Source: Journal of Geophysical Research: Planets

Winter is coming—and not just for Earth’s Northern Hemisphere. Northern summer on Saturn, which completes its orbit around the Sun about once every 30 years, is coming to a close after about 7.5 years, with its fall equinox coming up in 2025.

Just like on Earth, Saturn’s changing seasons are accompanied by changes in its weather. Now, as reported by Fletcher et al., images captured by the Mid-Infrared Instrument (MIRI) aboard the James Webb Space Telescope (JWST) show how Saturn’s atmospheric dynamics have evolved since the Cassini-Huygens spacecraft ended its 13-year investigation of the planet in 2017.

Launched in December 2021, JWST set its sights on Saturn in November 2022, with the goal of putting MIRI’s small fields of view to the test against the planet’s large size, rapid rotation, iconic rings, and unusually high infrared brightness compared with MIRI’s other targets. Researchers used MIRI to capture infrared images of Saturn bit by bit and create a mosaic map of Saturn’s northern hemisphere in summertime.

MIRI appears to have passed the test. The images captured the structure of Saturn’s clouds and allowed researchers to measure the spatial distribution of different temperatures and chemicals in the atmosphere, revealing a number of notable seasonal changes.

For instance, the images show that the planet’s north polar stratospheric vortex—a high-atmosphere circulation pattern of gases first detected by Cassini during Saturn’s spring—warmed during the summer; it should cool and dissipate as winter approaches.

The images also highlight a complete reversal of an airflow pattern in Saturn’s stratosphere that Cassini observed during the northern winter. At that point, large quantities of air rose to higher altitudes in the southern hemisphere, crossed the equator, and sank to lower altitudes in the northern hemisphere, enriching the air in gases like hydrocarbons. Now, the MIRI data suggest that air is rising in the north and flowing south, creating a scarcity of hydrocarbons at northern latitudes. This seasonal circulation pattern may continue to change as fall approaches.

Because of MIRI’s exceptional sensitivity and its ability to capture wavelengths of light that Cassini could not, the new images also map the distribution of several gases for the first time, including water in the troposphere and ethylene, benzene, methyl, and carbon dioxide in the stratosphere. The new images also reveal high levels of ammonia at the equator, suggesting that Saturn’s equator may feature processes similar to Jupiter’s, which is also rich in ammonia.

Together these findings provide the first real glimpse into late summertime in Saturn’s northern hemisphere and demonstrate the advanced capabilities of JWST and MIRI. (Journal of Geophysical Research: Planets, https://doi.org/10.1029/2023JE007924, 2023)

—Sarah Stanley, Science Writer

Citation: Stanley, S. (2023), James Webb Space Telescope captures Saturn’s changing seasons, Eos, 104, https://doi.org/10.1029/2023EO230371. Published on 28 September 2023.
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