It's 'Spoke Season' On Saturn

The season on Earth might be spring, but out on Saturn, it’s spoke season. I talked with Dr. Philip Nicholson, a Professor of Astronomy at Cornell University, to learn more. Here’s our conversation, edited for brevity.

Can you tell us what Saturn's "spoke season" is, and how often it occurs?

Saturn has seasons similar to those of the Earth (or Mars). But due to the much greater length of the year on Saturn of 29.5 Earth-years, seasons on Saturn are each seven to eight Earth-years long. The “spokes” are rather enigmatic and highly variable features that appear in Saturn’s brightest and densest ring, known as the B ring. They are faint, roughly radial streaks, perhaps 10,000 kilometers (6,200 miles) in length, that seem to form just as the rings emerge from Saturn’s shadow into the sunlight and then dissipate (like a thunderstorm on Earth) over a period of five to 10 hours. Spokes are most prominent when the rings are almost edge-on to the direction to the sun, which occurs at Saturn’s spring and fall equinoxes, or every 15 Earth-years.

NASA's Hubble Space Telescope released an image showing that a new spoke season has begun. How long will the season last, and what will happen during it?

Based on previous observations of Saturn with Hubble and by the Cassini spacecraft, which orbited the planet from 2004 to 2017, spokes are only seen when the rings are inclined by less than about 20 degrees to the direction to the sun. This is a period of about eight Earth-years, centered on each equinox.

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The previous “spoke season” lasted from 2005 through 2013 or 2014. We expect the current one to last from 2021 through 2029, with the next equinox falling in May 2025. And Hubble has now spotted the first faint spokes in 2021 and brighter ones in 2022, right on schedule! During the eight-year “spoke season” the spokes should get steadily more numerous and prominent until the equinox, before gradually fading away again. The sequence is a bit like the Atlantic “hurricane season,” when hurricanes get both more frequent and stronger beginning in June and then fade away in November.

What causes these spokes?

For you as a scientist, what is fascinating about Saturn's spokes?

Very few things in Saturn’s rings change on a human timescale, let alone in 10 hours! The spokes are the most obvious exception, and the details of how they are formed have remained mysterious for 40 years. There are hints in the statistics of exactly where and when they appear that they are somehow controlled by unseen features in Saturn’s strong magnetic field, but again the details are elusive. We don’t know if the rings around other planets such as Jupiter or Uranus have spokes, but so far none have been seen.

What do you hope we'll understand over the next 50 years about Saturn that we don't know now?

Lots of things, but for me the really big questions center on how the rings were created, and when. Prior to Cassini, we assumed they were either formed along with the planet itself, probably about 4.55 billion years ago, or possibly by a later event involving the near-collision of an interplanetary object (a giant comet?) or a mid-sized moon with Saturn. But in the light of Cassini observations many scientists now believe the rings are relatively young, perhaps only 100 million years old.

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To put this in context, if the earliest dinosaurs had had telescopes they may have seen a ringless Saturn. And some recent theories suggest that several of Saturn’s 60-odd moons might also have formed quite recently, perhaps in the same catastrophic event that formed the rings.

But for many of my colleagues, the big question is whether or not Saturn’s small moon Enceladus, with its plumes of water vapor and ice, indeed harbors an ocean of salty water below its icy crust, and what might perhaps be swimming in that ocean?

Other scientists dream of the day when we will launch an unmanned boat (or submarine?) to explore the deep methane/ethane lakes of the largest moon, Titan, and the winding river canyons that empty into them.

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