The James Webb Space Telescope observes the changing seasons on Saturn (video)

By | September 14, 2023

Astronomers have observed in detail the summer giving way to autumn on the planet Saturn.

Using observations of Saturn by the James Webb Space Telescope (JWST) made in November 2022, a team led by scientists at the University of Leicester observed a cooling trend on the gas giant caused by planet-sized airflows reversing direction as Saturn’s long seasons change.

“No spacecraft has ever been around to explore Saturn’s late northern summer and autumn before, so we hope this is just the starting point,” said Leigh Fletcher, a professor in the School of Physics and astronomy at the University of Leicester. said in a statement. “The quality of the new JWST data is simply breathtaking: with a short series of observations, we were able to carry forward the legacy of the Cassini mission into a completely new Saturnian season, observing how weather patterns and atmospheric circulation respond to the changing light of the sun.”

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four colored inserts of different color bands on the top of a ringed planet

four colored inserts of different color bands on the top of a ringed planet

The astronomers made their observations of Saturn using JWST’s Mid-Infrared Instrument (MIRI), which allowed them to observe the planet’s atmosphere in wavelengths of infrared light just out of reach of our eyes. MIRI allowed the team to measure the temperature of Saturn’s atmosphere and see signatures of chemicals within it, measuring their abundance and 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 ice rings, experiences seasons just like Earth because, like our planet, it is tilted away from the sun. Although 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 that seasons on the gas giant are much longer than those on Earth.

Saturn takes 29.4 Earth years to orbit the sun, leading to seasons that last about 7.5 Earth years. These new JWST observations give scientists new insights into those extended seasons and the phenomena at play when they begin to change. The JWST observations complement data collected by NASA’s Cassini-Huygens (Cassini) space probe, which observed the gas giant planet for 13 years during its winter and spring seasons.

Saturn, its rings and three moons stand out against the darkness of space in this photo from the James Webb Space Telescope

Saturn, its rings and three moons stand out against the darkness of space in this photo from the James Webb Space Telescope

The new JWST observations have been compiled into an animation showing how the gas giant’s appearance changes across different wavelengths of light, just as the transition from summer to autumn can be seen by the yellowing of leaves on the trees. trees here on Earth.

In the movie, thermal emissions from the planet’s north pole are represented by bright blue, while the yellow color represents the bright, hot parts of Saturn’s atmosphere. The contrast in temperatures can be seen by the purple-colored areas, which represent the coldest and darkest parts of the planet’s atmosphere. The gas giant planet’s infamous banded appearance is clearly visible when observing the wavelengths of light in Saturn’s troposphere, the lowest layer of its atmosphere.

The animation also shows visible light observations 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-wide (1,500-kilometer) polar cyclone (NPC) at Saturn’s north pole. This is surrounded by a large region of hot gases that have been observed gathering during the spring in Saturn’s northern hemisphere, called the north polar stratospheric vortex (NPSV).

These hot vortices swirl through the gas giant planet’s upper stratosphere, where they were heated during Saturn’s summer season. In 2025, the planet will experience the autumnal equinox, the point at which the sun is directly above the planet’s equator (Earth has its autumnal equinox in September). As the autumnal equinox approaches, the NPSV will begin to cool and disappear as the Northern Hemisphere fully falls into autumn and darkens.

These vortices in Saturn’s atmosphere had previously been observed up close and personal by Cassini. Infrared data from JWST observations of the gas giant show different temperature distributions in Saturn’s stratosphere than those collected by the spacecraft, which however crashed into the gas giant in 2017. This is because Cassini made its observations in the winter and in northern spring of the planet.

a swirling gray vortex

a swirling gray vortex

The distribution of gas through Saturn’s atmosphere also varied between the JWST and Cassini observations. This is the result of air rising from the Southern Hemisphere and crossing Saturn’s equator during the northern winter and southern summer as NASA’s spacecraft made its observations. This process was reversed during the northern summer/southern winter, during which the JWST observed the planet, causing the powerful space telescope to see hydrocarbon-poor gas flowing from Saturn’s north to south.


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Saturn was chosen as one of the first solar system targets for the JWST to test the capabilities of the $10 billion telescope because the bright rotating planet poses a challenge to the MIRI instrument’s small fields of view. MIRI can only observe a small slice of Saturn at a time due to the planet’s brightness compared to some of its other targets, which are often billions of light-years away.

The challenging nature of this investigation means that planning for these Saturn observations had been underway for approximately 8 years leading up to 2022. Even with nearly a decade of planning, Fletcher said he and the team were amazed at the quality of the data provided from JWST, with the University of Leicester professor describing this work as a career highlight.

“JWST can see into wavelengths of light that were inaccessible to any previous spacecraft, producing an exquisite data set that whets the appetite for years to come,” Fletcher said. “This work on Saturn is just the first in 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 discoveries from a single observation of one planet, one world, imagine what discoveries await you?”

The team’s research is published in Journal of Geophysical Research Planets.

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