Astronomers have observed summer giving way to autumn on the ringed planet Saturn in detail.
Using James Webb Space Telescope (JWST) observations of Saturn made during November 2022, a team led by the University of Leicester scientists observed a cooling trend on the gas giant caused by planet-sized air flows reversing direction as Saturn's long seasons switch.
"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," University of Leicester School of Physics and Astronomy professor Leigh Fletcher said in a statement. "The quality of the new data from JWST is simply breathtaking — 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."
The astronomers made their observations of Saturn using the JWST's Mid-Infrared Instrument (MIRI), which allowed them to observe the planet's atmosphere in infrared wavelengths of light just outside the capability of our eyes. MIRI let the team measure the temperature of Saturn's atmosphere and see the fingerprints of the chemicals within it, measuring their abundances while also observing the churning clouds that exist at the top of Saturn's atmosphere — the stratosphere.
The gas giant planet, which is the sixth planet from the sun and is known for its striking icy rings, experiences seasons just as Earth does because, like our planet, it is tilted with regard to the sun. Though Saturn's axial tilt of 26.7 degrees is similar to that of our planet, which is 23.5 degrees, its much wider orbit means the seasons on the gas giant are much longer than those of Earth.
Saturn takes 29.4 Earth years to orbit the sun, which leads to seasons lasting around 7.5 Earth years. These new JWST observations give scientists new insight into those drawn-out seasons and the phenomena at play when they start to shift. The JWST observations complement data collected by NASA's Cassini–Huygens (Cassini) spacecraft, which observed the gas giant planet for 13 years during its winter and spring seasons.
The new JWST observations have been compiled in an animation that shows how the appearance of the gas giant changes over different wavelengths of light, just as the shift of summer to autumn can be seen by the yellowing of the leaves on trees here on Earth.
In the footage, the thermal emissions of the planet's north pole are represented by bright blue, while the color yellow represents bright and warm parts of Saturn's atmosphere. The contrast in temperatures can be seen by purple-colored areas, which represent cooler and darker parts of the planet's atmosphere. The infamous banded appearance of the gas giant planet is clearly seen in the observation of wavelengths of light in Saturn's troposphere, the lowest layer in its atmosphere.
The animation also shows observations in visible light collected by the Hubble Space Telescope in September 2022. These are shown in the background to act as a contrast to the JWST images.
Also seen in the JWST data is a 932-mile (1,500-kilometer) wide polar cyclone (NPC) at Saturn's north pole. This is ringed by a broad region of warm gasses that were observed as they gathered during spring in the northern hemisphere of Saturn called the north-polar stratospheric vortex (NPSV).
These warm vortices swirl through the high stratosphere of the gas giant planet, where they have been heated during Saturn's summer season. In 2025, the planet will experience the autumn equinox — the point at which the sun is directly over the equator of the planet (Earth has its own autumn equinox in September). As the autumn equinox approaches, the NPSV will begin to cool down and will disappear as the northern hemisphere falls fully into autumn and darkens.
These vortices in the atmosphere of Saturn were previously observed up close and personal by Cassini. The infrared data of the JWST observations of the gas giant show different temperature distributions in the stratosphere of Saturn than those collected by that spacecraft, which crashed into the gas giant in 2017, however. This is because Cassini made its observations in the planet's northern winter and spring.
Also varying between the JWST and Cassini observations was the distribution of gas through the atmosphere of Saturn. This is the result of air rising from the southern hemisphere and crossing the equator of Saturn during its northern winter and southern summer as the NASA spacecraft made its observations. This process was reversed during the northern summer/southern winter, during which the JWST observed the planet, resulting in the powerful space telescope seeing gasses low in hydrocarbons flowing from the north to the south of Saturn.
Saturn was chosen as an early solar system target for the JWST to test the capabilities of the $10 billion telescope because the bright rotating planet provides a challenge for the small fields of view for the MIRI instrument. MIRI can only look at a tiny slice of Saturn at a single time because of how bright the planet is compared to some of its other targets, which are often located billions of light-years away.
The challenging nature of this investigation means that planning for these Saturn observations had been underway for around 8 years prior to 2022. Even with almost a decade of planning, Fletcher said he and the team were astounded by the quality of data provided by the JWST, with the University of Leicester professor describing this work as a career highlight.
"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," Fletcher said. "This work on Saturn is just the first of a program 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 team's research is published in the Journal of Geophysical Research Planets.