Saturn’s rings outshine Saturn in new JWST image

At long last, JWST has released its views of Saturn.

The full image of Saturn taken by JWST reveals the planet, its main rings and a variety of features therein, and the inner moons Dione (top), Enceladus (middle), and Tethys (bottom).

Joining previously-imaged Jupiter,

This animation showcases JWST’s unique near-infrared views of Jupiter. In addition to the bands, the great red spot, and the “atmospheric haze” visible at the day/night boundary of Jupiter, a number of moon, ring, and auroral features are seen and labeled. A single NIRCam or MIRI frame is just barely large enough to hold all of Jupiter’s disk within it, enabling spectacular views of this world with JWST. With an expected lifetime to last until the mid-2040s, JWST will observe multiple Jovian solstices and equinoxes, but won’t last until Uranus reaches its equinox phase.

Neptune,

This image, a portion of a wide-field view of Neptune taken with JWST’s NIRCam imager, showcases Neptune, its giant moon Triton, faint features on and around Neptune, including its rings and smaller moons, and a smattering of background galaxies and stars from within the Milky Way.

and Uranus,

This wide-field view of Uranus, taken with JWST, reveals the planet, cloud-like features on it, the inner rings that surround it, as well as the 6 brightest (annotated) of Uranus’s 27 known moons. Background objects, like galaxies, are also visible owing to JWST’s incredible capabilities.

it’s now seen all four of our gas giants.

Now that Saturn has been imaged by JWST, the first “family portrait” of the gas giant worlds as seen by JWST’s eyes can be composed. Here, each planet is shown with an angular size that’s calibrated to how they would appear relative to one another as seen by JWST. Planets can be as large as about twice Jupiter’s size, but may be as small as 1000 km or even less.

Saturn, in particular, appears astoundingly different at various wavelengths.

Saturn has a substantial axial tilt just like Earth does: of 26.7 degrees, leading to its seasons. Whereas Earth’s seasons last roughly 3 months apiece, seasons on Saturn last ~7 years each. The change in the rings, as shown here, represent Hubble observations at the same time of year from 1996, 1997, 1998, 1999, and 2000. The rings were perfectly edge-on in 1995, and then again in 2010, and will be once again in 2025.

Its hydrogen and helium atmosphere contains traces of ammonia, phosphine, water vapor, and hydrocarbons.

Although Saturn normally appears yellowish-brown from Earth, this unique perspective from Cassini show’s Saturn’s upper atmosphere, with ring shadows on them, at an angle far above the Saturnian clouds. With only hydrogen and helium at these high altitudes, Saturn’s cloud-free skies are shown to be blue: just like Earth, Uranus, and Neptune.

In optical light, Saturn appears a yellowish-brown color.

Saturn, as photographed here by Cassini during the 2008 equinox, isn’t quite round (as it’s more of an oblate spheroid), but is in hydrostatic equilibrium. With its low density and rapid rotation, Saturn is the most flattened planet in the Solar System, with an equatorial diameter that’s more than 10% larger than its polar diameter. Its colors and “bands” are largely due to different atmospheric layers appearing dominant in visible light at different latitudes.

Clouds — composed of ammonia crystals, ammonium hydrosulfide, and water — preferentially reflect those colors.

Each year, Hubble takes a new image of the prominent gas giant worlds in our Solar System. This 2023 image of Saturn showcases its approaching autumnal equinox (which will arrive in May of 2025), with Saturn’s classic yellow-and-brown features on full display in optical wavelengths of light.

But in infrared light, Saturn shows a different side of itself.

This annotated JWST image of Saturn shows its three imaged moons, the main disk of the planet, and many features in the main rings of Saturn, including the Cassini division and the Encke gap.

Only one-third of Jupiter’s mass, similar-sized Saturn generates little internal heat, appearing far fainter.

Shown here as calibrated to the same relative “actual size” as one another, the appearance of Saturn is faint and dark compared to Jupiter. This is due to a mix of absorptive features in Saturn’s upper atmosphere that aren’t as dominant on Jupiter but also to the fact that Jupiter is intrinsically much brighter than Saturn in infrared light, as Jupiter produces much of its own heat internally.

Even in infrared wavelengths, Saturn’s appearance is dominated by reflected sunlight.

This animation switches between a visible light (Hubble) view of Saturn and its main rings and the infrared (JWST) view of the same sight, albeit a little bit closer to Saturnian equinox. Both images are dominated by reflected light, where the different wavelengths highlight different features in Saturn’s atmosphere and rings. Although Newton knew the size of and distance to Saturn, he had no way of knowing its reflectivity.

It’s northern hemisphere summer on Saturn, but Saturn’s north pole currently appears particularly dark.

Saturn appears dark at its north and south poles in infrared light likely due to some new process that affects the formation and distribution of aerosols in its upper cloud layers. The planet itself shows darker spots, likely caused by planet-wide gravity waves.

This suggests that stratospheric aerosols, made of absorptive hydrocarbons, play a major role.

From its unique vantage point in the shadow of Saturn, the atmosphere, the main rings, and even the outer E-ring are all visible, along with the visible ring gaps of the Saturnian system in eclipse. Saturn’s aerosol-rich atmosphere has some latitude and seasonally dependent features that can only, thus far, be detected in infrared wavelengths of light.

Those aerosols are likely shaped by a planetary-scale atmospheric phenomenon called gravity waves.

Gravity waves are an atmospheric phenomenon that can appear on any planet with an atmosphere, creating pockets of compressed and rarefied air that can be imprinted on features such as clouds. They can persist in just a region of a planet’s atmosphere or can be planet-wide, coming in a wide variety of sizes and scales.

But Saturn’s rings, by contrast, appear bright and brilliant.

This version of the JWST view of Saturn has been artificially darkened to reveal deep features within Saturn’s rings. The inner structure visible within the main rings, in particular, appears spectacular.

Composed almost entirely of water-ice, they’re too cool to radiate thermally.

The Cassini orbiter took this image of Saturn’s rings in 2004, with the rings assigned colors based on their temperature: red is ~110 K, green is ~90 K, and blue is ~70 K. All of these temperatures are too cool to register directly in JWST’s NIRCam imager.

But water-ice is incredibly reflective, even in infrared light.

In this enhanced contrast image from JWST’s NIRCam imager, many of Saturn’s main rings, including the gaps between them, are individually visible, shining far brighter than the planet itself. Whereas Saturn’s atmosphere and clouds are largely absorptive in the infrared, its water-ice rings are almost perfectly reflective.

JWST sees reflected sunlight from Saturn’s rings and in water-ice plumes from Enceladus.

Enceladus is a moon of Saturn made primarily of water-ice, which ejects plumes made of water vapor, ice particles, and organic chemical compounds from it. About 30% of those emissions feed Saturn’s E-ring, while the remaining 70% goes elsewhere into the Saturnian system.

Future JWST views will reveal fainter, thinner, and even more diffuse Saturnian rings.

Saturn’s extremely reflective moon, Enceladus, is covered in a thick crust of water-ice with cracks across it and geysers emanating from the south pole. Enceladus is the source of Saturn’s E-ring, visible here in reflected sunlight from Cassini. JWST, even from afar, has the potential to see this diffuse E-ring, the thin G-ring, and perhaps even the enormous but ultra-diffuse Phoebe ring.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.