Mysterious 'ghostly' shadows larger than planet Earth reappeared on Saturn's rings. NASA says it expects even more to appear soon.

• NASA's Hubble telescope observed the latest series of mysterious dark spots on Saturn's rings.
• Scientists have observed these "spokes" for decades but don't know their origin or cause.
• NASA said Saturn is approaching a key point in its orbit that may make them more common.

Saturn's rings hold a secret that has mystified scientists for decades.

Strange dark shadows, called spokes, occasionally appear and disappear on the planet's rings.

NASA's Voyager 2 spacecraft first observed Saturn's spokes in 1981, and other instruments, including Cassini and Hubble, have spotted them multiple times before.

The latest sighting was in October when Hubble spotted several dancing around the ringed planet.

Scientists have an idea of what they are and what causes them, but aren't certain.

Spokes have a "ghostly appearance" and rotate with Saturn's rings as they orbit the planet, NASA said in a statement.

And while they may look small compared to Saturna's rings, spokes can grow to lengths longer than our entire planet, NASA said.

What are Saturn's spokes?

Scientists suspect that spokes have something to do with interactions between Saturn and the sun.

Similar to Earth, Saturn has a magnetic field that surrounds the planet. And that magnetic field is susceptible to the charged particles in the solar wind that our sun regularly fires into space.

So when the solar wind smashes into Saturn's magnetic field at a million miles per hour, it's going to have some effects, like generating aurora borealis on Saturn's poles.

But scientists also suspect it may trigger the right conditions for those ethereal spokes on Saturn's rings to pop up.

"The leading theory is that spokes are tied to Saturn's powerful magnetic field, with some sort of solar interaction with the magnetic field that gives you the spokes," Amy Simon, a scientist with NASA's Goddard Space Flight Center, said in NASA's statement.

Electrostatic forces may cause dust or ice in Saturn's rings to levitate slightly higher than everything else around them, casting a dark shadow on the material below it, in the form of a spoke, per NASA. But this is just one possible explanation.

We're headed for max spoke activity soon

Astronomers must observe and analyze more spokes to determine what's triggering these mysterious marks.

Lucky for them, NASA predicts more spokes will be coming soon. Because even though scientists aren't sure what spokes are, or what causes them, they're fairly certain from observations over the decades that spokes vary with Saturn's seasons.

And Saturn's fall season, aka its autumnal equinox, is coming up.

When that happens, the planet's tilt allows solar winds to bombard the planet's magnetic field with more force, per Space.com.

"We are heading towards Saturn equinox, when we'd expect maximum spoke activity, with higher frequency and darker spokes appearing over the next few years," Simon said in the statement.

Saturn's northern hemisphere will reach autumnal equinox on May 6, 2025, per NASA. Perhaps this will be the decade scientists finally solve the spoke mystery, once and for all.