One of the Cassini orbiter’s stand-out discoveries in the Saturn system was plumes of water-ice erupting from fissures in the moon Enceladus.
These ice particles make up the diffuse E ring surrounding Saturn.
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Credit: NASA/JPL-Caltech/University of Arizona/LPG/CNRS/University of Nantes/Space Science Institute
A team fronted by Niels Rubbrecht, at the Delft University of Technology in the Netherlands, has found something just as unique created by Enceladus’s icy eruptions.
This image captured by the Cassini spacecraft shows plumes of water ice erupting from the surface of Enceladus. Credit: NASA/JPL-Caltech/SSI
Finding rainbows in the rings
Rubbrecht and his colleagues were looking through old photos captured by Cassini during its 17th close fly-by of Enceladus on 27 March 2012.
They spotted distinct stripe-like features appearing around the moon in images taken by both the Visual and Infrared Mapping Spectrometer (VIMS) and the visible Narrow-Angle Camera (NAC) on the probe.
The team went back through other Cassini images taken under similar lighting conditions, including fly-bys of other moons such as Titan, Dione, Phoebe and Tethys.
Cassini’s view of Saturn’s moon Dione. Credit: NASA
But stripes were only apparent in two other Enceladus fly-bys, in December 2010 and May 2012.
The relative positions of these parallel stripes were consistent between different images, angled around 16° to the plane of the Solar System (or 43° out of the plane of Saturn’s rings).
So they’re definitely a real effect and not stray light in the camera lens.
And the fact that these observations span almost a year and a half suggests that this curious phenomenon persists for at least that time.
Multi-filter Cassini images show the plumes scattering light into faint stripes on Saturn’s rings. Credit: Niels Rubbrecht, Stephanie Cazauxa et al
What’s causing Saturn’s rainbows?
Tellingly, the separate bands making up the stripes appear to shift in position slightly when examined in different wavelengths of light.
This gave the team a big clue as to what’s behind this ‘light-dispersing phenomenon’.
Rainbows on Earth are created when sunlight reflects off the back of raindrops, with refraction occurring as the rays pass from the air to water and back out again, splitting the white light into a visible spectrum of colours.
The physics in the vacuum around Saturn’s rings is slightly different, but it’s creating a similar effect.
Sunlight reflecting straight off ice crystals produces the brightest band seen in the Cassini images, with a wavelength of around 5µm.
But because the ice particles are so tiny, they create what’s known as a reflection grating, diffracting the light and spreading it into its component wavelengths.
A view of Saturn’s rings captured on 30 June 2004. In this image, the varying hues are caused not by Enceladus’s plumes, but by of differences in their composition, as seen by the Cassini spacecraft’s Ultraviolet Imaging Spectrograph. Credit: NASA/JPL/University of Colorado
Are Enceladus plumes seeding the rainbows?
The fact that these stripes are only visible near Enceladus suggests the reflection grating is probably made up of the particles spewing out of the moon’s plumes.
The spectrum of the bright band produced by VIMS indicates that it’s caused by water-ice crystals with a smattering of carbon dioxide ice, which suggests the material has been freshly ejected into space.
However, the researchers aren’t yet entirely clear why these particles are larger than those in the plumes, and are inclined so far out of Saturn’s rings.
Saturn’s rings, as seen by the Cassini spacecraft. Credit: NASA/JPL-Caltech/Space Science Institute
They note that optical phenomena common in Earth’s atmosphere have also been discovered in other worlds.
In 2014, the Venus Express orbiter spotted a ring of refracted sunlight known as a halo on the Venusian upper cloud layer.
And the same effect has been observed by the Perseverance rover, created by hexagonal-shaped ice crystals in Martian clouds.
But this particular discovery is unprecedented, and has a wonderful poetic beauty to it: the fountains of Enceladus are creating space rainbows.
Lewis Dartnell was reading Peculiar Rainbows in Saturn’s E Ring: Uncovering Luminous Bands near Enceladus by Niels Rubbrecht, Stéphanie Cazaux et al. Read it online at: arxiv.org/abs/2502.18028.
This article appeared in the September 2025 issue of BBC Sky at Night Magazine