Starship Asterisk*

A close-up image of the planet Saturn. The rings are level with the viewer, and tilted slightly down. Credit: ESA/Hubble, NASA & A. Simon, A. Pagan (STScI)

---

The NASA/ESA Hubble Space Telescope has observation time devoted to Saturn each year, thanks to the Outer Planet Atmospheres Legacy (OPAL) program, and the dynamic gas giant planet always shows us something new. This latest image heralds the start of Saturn's "spoke season" with the appearance of two smudgy spokes in the B ring, on the left in the image.

The spokes are enigmatic features which appear across Saturn’s rings. Their presence and appearance varies with the seasons — like Earth, Saturn is tilted on its axis and therefore has four seasons. With Saturn's much larger orbit, each season lasts approximately seven Earth years. Equinox occurs when the rings are tilted edge-on to the Sun and marks the height of spokes’ visibility, while during a solstice when the Sun is at its highest or lowest latitude, the spokes disappear.

The shape and shading of spokes varies — they can appear light or dark, depending on the viewing angle, and sometimes appear more like blobs than classic radial spoke shapes, as seen here. The ephemeral features don't last long, but as the planet's autumnal equinox approaches on 6 May 2025, more will appear.

Scientists will be looking for clues to explain the cause and nature of the spokes. It's suspected they are caused by interaction between Saturn's magnetic field and the solar wind, which also causes aurorae to appear on the planet. The hypothesis is that spokes are the smallest, dust-sized, icy ring particles being temporarily electrically charged and levitated, but this has not been confirmed.

Saturn's last equinox occurred in 2009, while the NASA/ESA/ASI Cassini spacecraft was orbiting the gas giant planet for close-up reconnaissance. With Cassini's mission completed in 2017, Hubble is continuing the work of long-term monitoring of changes on Saturn and the other outer planets.

• Geophysical Research Letters 50(3):101904 (16 Feb 2023) DOI: 10.1029/2022GL101904