On June 25, 2023, NASA’s James Webb Space Telescope turned to the known ringed world Saturn for the first near-infrared observation of the planet. The initial images from Webb’s NIRCam (Near-Infrared Camera) are already fascinating scientists.
Saturn itself appears extremely dark at this infrared wavelength seen through a telescope because the methane absorbs almost all of the sunlight hitting the atmosphere. However, the icy rings remain relatively bright, leading to Saturn’s unusual appearance in the Webb image.
This image was taken as part of the Webb Guaranteed Time Observation 1247 program. The program included several very deep exposures of Saturn designed to test the telescope’s ability to detect the faint moons around the planet and its bright rings. Any newly discovered moons could help scientists piece together a more complete picture of the current Saturn system, as well as its past.
This new image of Saturn clearly shows details in the planet’s ring system, along with several of the planet’s moons – Dione, Enceladus and Tethys. Additional deeper exposures (not shown here) will allow the team to examine some of the planet’s fainter rings that are not visible in this image, including the thin G ring and the diffuse E ring.
Saturn’s rings are made up of a variety of rocky and icy debris—particle sizes ranging from smaller than a grain of sand to several as large as mountains on Earth. Scientists recently used Webb to explore Enceladus and found a large plume spewing from the moon’s south pole that contains both particles and large amounts of water vapor—the plume that feeds Saturn’s E ring.
Saturn’s atmosphere also shows surprising and unexpected details. Although Cassini observed the atmosphere in greater brightness, this is the first time a planet’s atmosphere has been seen with this brightness at this particular wavelength (3.23 microns), which is unique to Webb. The large, dark, diffuse structures in the northern hemisphere do not follow the planet’s latitude lines, so this image lacks the familiar striped appearance typically seen from Saturn’s deeper atmospheric layers.
The irregularity is reminiscent of large-scale planetary waves in stratospheric aerosols high above the main clouds, potentially similar to those observed in early Webb NIRCam observations of Jupiter.
Comparing the planet’s north and south poles in this image, the differences in appearance are typical of Saturn’s known seasonal changes. For example, Saturn is currently experiencing northern summer as the southern hemisphere emerges from darkness at the end of winter. However, the North Pole is particularly dark, possibly due to an unknown seasonal process affecting polar aerosols in particular.
A small hint of brightening toward the edge of Saturn’s disk may be due to alpine methane fluorescence (the process of light emission after light absorption), trihydrogen ion (H3+) emission in the ionosphere, or both; spectroscopy by Webb could help confirm this.
Missions such as Pioneer 11, Voyagers 1 and 2, Cassini and the Hubble Space Telescope have been observing Saturn’s atmosphere and rings for many decades. These observations from Webb are just a hint of what the observatory will add to Saturn’s story in the coming years as the science team digs deep into the data to prepare the peer-reviewed results.
