Saturn‘s rings are one of the most magnificent spectacles in our solar system.
But every 15 years or so something odd happens to them — a phenomenon that has baffled scientists ever since its discovery four decades ago.
New Hubble Space Telescope images have now revealed mysterious smudges once again in Saturn’s rings, signalling that the planet’s ‘spoke season’ is in full swing.
It has earned this nickname because the dark and light marks resemble the spokes on a bicycle.
What exactly causes these spokes or smudges is unknown, but experts hope that the latest observations from Hubble will provide opportunities to study the phenomenon in greater detail.
Peculiar: NASA’s Hubble Space Telescope has revealed that peculiar smudges are once again appearing in Saturn’s rings, signalling that the planet’s ‘spoke season’ is in full swing
They appear only in the years preceding and following equinoxes on the sixth planet from the sun — so effectively spring and autumn in the Saturnian year
The smudges appear only in the years preceding and following equinoxes on the sixth planet from the sun — so effectively spring and autumn in the Saturnian year.
The autumnal equinox for Saturn’s northern hemisphere will arrive on May 6, 2025, but the marks tend to start appearing four years prior to this, which is why Hubble can already see them.
NASA wants to know why they only emerge seasonally and what causes them to disappear and reappear at certain times in Saturn’s year.
As the world is so much farther from the sun than us, its year is longer, too. It takes 29 years for Saturn to orbit our star, while each of its seasons is around seven years long.
‘Thanks to Hubble’s OPAL program, which is building an archive of data on the outer solar system planets, we will have longer dedicated time to study Saturn’s spokes this season than ever before,’ said planetary scientist Amy Simon of NASA.
Saturn’s last equinox occurred in 2009, when NASA’s Cassini spacecraft was still orbiting the planet as part of a close-up reconnaissance.
But Cassini’s mission ended in 2017, so Hubble is continuing the work of monitoring any changes on Saturn and the other outer planets.
‘Despite years of excellent observations by the Cassini mission, the precise beginning and duration of the spoke season is still unpredictable, rather like predicting the first storm during hurricane season,’ Simon added.
The ring spokes were first observed by NASA’s two Voyager probes, which flew past Saturn in 1980 and 1981.
But further analysis subsequently revealed yet more oddities — firstly that they’re not always there, but also that they usually appear dark from above and bright from below.
Cassini arrived in 2004 and relatively quickly revealed that the spokes weren’t caused by gravitational interactions with Saturn, its moons or the tiny moonlets that make up the planet’s rings.
A year later, the spacecraft confirmed that the spokes were likely related to Saturn’s magnetic field, sparking numerous theories on the process.
One is that charged dust particles, suspended above and below the rings, interact with the planet’s magnetic field, causing them to separate from the icy chunks in the rings and levitate separately.
What exactly causes these spokes or smudges is unknown, but experts hope that the latest observations from Hubble will provide opportunities to study the phenomenon in more detail
Scientists want to know why the smudges in the rings only emerge seasonally and what causes them to disappear and reappear at certain times in Saturn’s year
The Cassini spacecraft confirmed in 2004 that the spokes were likely related to Saturn’s magnetic field, sparking numerous theories in the process
One is that charged dust particles, suspended above and below the rings, interact with the planet’s magnetic field, causing them to separate from the icy chunks in the rings and levitate separately
It could also be that the solar wind interacts with Saturn’s magnetic field to create an electrically-charged environment that causes the dust particles to electrostatically stick together, therefore forming denser patches in the rings.
But it’s not clear if either of these theories are correct or why the phenomenon is seasonal, so scientists hope that Hubble data over the next few years will help to unravel the mystery.
The iconic telescope has been undertaking an observing campaign of the outer Solar System planets.
It was data from this Outer Planet Atmospheres Legacy (OPAL) programme that Simon and her team used to look for signs of the spokes appearing, and sure enough they did in 2021.
Then, in data from last September, the researchers tracked the spokes over an 11-hour period, confirming that ‘spoke season’ was truly under way.
The latest findings have been published in the journal Geophysical Research Letters.
WHAT DID CASSINI DISCOVER DURING ITS 20-YEAR MISSION TO SATURN?
Cassini launched from Cape Canaveral, Florida in 1997, then spent seven years in transit followed by 13 years orbiting Saturn.
An artist’s impression of the Cassini spacecraft studying Saturn
In 2000 it spent six months studying Jupiter before reaching Saturn in 2004.
In that time, it discovered six more moons around Saturn, three-dimensional structures towering above Saturn’s rings, and a giant storm that raged across the planet for nearly a year.
On 13 December 2004 it made its first flyby of Saturn’s moons Titan and Dione.
On 24 December it released the European Space Agency-built Huygens probe on Saturn’s moon Titan to study its atmosphere and surface composition.
There it discovered eerie hydrocarbon lakes made from ethane and methane.
In 2008, Cassini completed its primary mission to explore the Saturn system and began its mission extension (the Cassini Equinox Mission).
In 2010 it began its second mission (Cassini Solstice Mission) which lasted until it exploded in Saturn’s atmosphere.
In December 2011, Cassini obtained the highest resolution images of Saturn’s moon Enceladus.
In December of the following year it tracked the transit of Venus to test the feasibility of observing planets outside our solar system.
In March 2013 Cassini made the last flyby of Saturn’s moon Rhea and measured its internal structure and gravitational pull.
Cassini didn’t just study Saturn – it also captured incredible views of its many moons. In the image above, Saturn’s moon Enceladus can be seen drifting before the rings and the tiny moon Pandora. It was captured on Nov. 1, 2009, with the entire scene is backlit by the Sun
In July of that year Cassini captured a black-lit Saturn to examine the rings in fine detail and also captured an image of Earth.
In April of this year it completed its closest flyby of Titan and started its Grande Finale orbit which finished on September 15.
‘The mission has changed the way we think of where life may have developed beyond our Earth,’ said Andrew Coates, head of the Planetary Science Group at Mullard Space Science Laboratory at University College London.
‘As well as Mars, outer planet moons like Enceladus, Europa and even Titan are now top contenders for life elsewhere,’ he added. ‘We’ve completely rewritten the textbooks about Saturn.’
