New images of Saturn from NASA’s Hubble Space Telescope herald the start of the planet’s “spoke season” surrounding its equinox, when enigmatic features appear across its rings.
The cause of the spokes, as well as their seasonal variability, has yet to be fully explained by planetary scientists.
Like Earth, Saturn is tilted on its axis and therefore has four seasons, though because of Saturn’s much larger orbit, each season lasts approximately seven Earth years. Equinox occurs when the rings are tilted edge-on to the Sun. The spokes disappear when it is near summer or winter solstice on Saturn. (When the Sun appears to reach either its highest or lowest latitude in the northern or southern hemisphere of a planet.)
As the autumnal equinox of Saturn’s northern hemisphere on May 6, 2025, draws near, the spokes are expected to become increasingly prominent and observable.
Seven Hubble Space Telescope images, each taken about four minutes apart, are stitched together to show “spoke” features rotating around Saturn. The puzzling, transient features have defied easy characterization. Their rotation rate does not quite match up with the rotation of the rings or of the planet’s magnetic field. The spokes are known to appear during the period leading up to and following the planet’s equinox. With the northern hemisphere autumnal equinox approaching on May 6, 2025, scientists are hoping new observations by Hubble will help them to put the clues together and solve the spoke mystery—what are they, and why do they form? Hubble observations will be compared with those made by NASA’s Cassini spacecraft in the period surrounding Saturn’s last equinox, in 2009. With the Cassini mission completed, Hubble’s annual observations of Saturn as part of its Outer Planet Atmospheres Legacy (OPAL) program will be crucial to studying and better understanding this dynamic world. Credit: SCIENCE: NASA, ESA, Amy Simon (NASA-GSFC). ANIMATION: Alyssa Pagan (STScI)
The suspected culprit for the spokes is the planet’s variable magnetic field. Planetary magnetic fields interact with the solar wind, creating an electrically charged environment (on Earth, when those charged particles hit the atmosphere this is visible in the northern hemisphere as the aurora borealis, or northern lights). Scientists think that the smallest, dust-sized icy ring particles can become charged as well, which temporarily levitates those particles above the rest of the larger icy particles and boulders in the rings.
The ring spokes were first observed by NASA’s Voyager mission in the early 1980s. The transient, mysterious features can appear dark or light depending on the illumination and viewing angles.
“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 NASA senior planetary scientist Amy Simon, head of the Hubble Outer Planet Atmospheres Legacy (OPAL) program.
Since their discovery by NASA’s Voyager mission in the 1980s, temporary “spoke” features across Saturn’s rings have fascinated scientists, yet eluded explanation. Saturn’s upcoming autumnal equinox of the northern hemisphere on May 6, 2025, means that spoke season has come again. NASA’s Hubble Space Telescope will be on the job studying the spokes, thanks to time dedicated to Saturn in the mission’s ongoing Outer Planet Atmospheres Legacy (OPAL) program. Credit: NASA’s Goddard Space Flight Center; Lead Producer: Paul Morris
Saturn’s last equinox occurred in 2009, while NASA’s Cassini spacecraft was orbiting the gas giant planet for close-up reconnaissance. With Cassini’s mission completed in 2017, and the Voyager spacecrafts long gone, Hubble is continuing the work of long-term monitoring of 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 said.
While our solar system’s other three gas giant planets also have ring systems, nothing compares to Saturn’s prominent rings, making them a laboratory for studying spoke phenomena. Whether spokes could or do occur at other ringed planets is currently unknown. “It’s a fascinating magic trick of nature we only see on Saturn—for now at least,” Simon said.
Hubble’s OPAL program will add both visual and spectroscopic data, in wavelengths of light from ultraviolet to near-infrared, to the archive of Cassini observations. Scientists are anticipating putting these pieces together to get a more complete picture of the spoke phenomenon, and what it reveals about ring physics in general.
By Claire Andreoli, NASA’s Goddard Space Flight Center
The study is published in the journal Geophysical Research Letters.
Source: phys.org
