The Hubble Space Telescope has confirmed that a new "spoke season" is kicking off at Saturn.

These are the times of the Saturnian year, centered around the equinoxes, when mysterious radial smudge marks, like the spokes of a wheel, appear across the planet's rings.

What exactly causes the spokes is unknown, but their re-emergence, combined with a Hubble planetary observing program, will provide opportunities to study them in greater detail.

Scientists hope to get to the bottom of not just what the spokes are but why they only emerge seasonally, disappearing and reappearing at certain times in Saturn's year.

We first discovered Saturn's spokes in images from the two Voyager probes, which flew past Saturn in 1980 and 1981, respectively: temporary streaks and smudges that usually appear as radial features, moving with the rings as they orbit Saturn.

Further observation and analysis revealed more oddities. The spokes usually appear dark from above, while from below, they typically appear bright, and they're not always there.

Typically, the spokes appear only in Saturn's spring and autumn, for the eight-year period centered around the equinox, and disappear during summer and winter, for the period centered around the solstice.