The latest images to be returned from the Cassini space probe reveal a mystery in Saturn's rings. The unique close-up images show an unexplained clumping of material within the rings, which could help scientists understand how planets form.

Cassini entered into orbit around Saturn on 1 July, successfully negotiating a gap between the planet's rings before returning its fist images later the same day.

One of the new images from Cassini is of the Encke gap, a 325-kilometre void in Saturn's outermost main ring, known as the A ring. Scallops on the inner edge of the gap are thought to be caused by the gravity of the small moon Pan, which orbits inside the gap. The scallops leave a spiral wake behind them that spreads inwards from the edge.

"This is textbook ring physics," says Carolyn Porco, head of the imaging team. "I thought my team was playing tricks on me and showing me a simulation of the rings rather than the rings themselves."

Gravitational influence

In another image, more waves cross the A ring, driven by the gravity of three more moons: Janus, Pandora and Prometheus. These "density waves" are generated when the orbital period of ice particles in the rings match that of one of the orbiting moons, so that the moon's slight gravitational influence builds up.

But the graininess at the bottom left of the picture is harder to explain. "It almost looks like straw. I don't know what this is. I literally don't have a clue," says Porco, who is an expert on Saturn's rings. "It may be brand new, something nobody's predicted before. There may be processes going on that make the particles clump."

If so, it could be highly significant. Saturn's rings are thought to be a pocket-sized version of the disk of ice and dust that once surrounded the Sun, from which the planets formed.

"We may be seeing some of the processes that aided the development of the planets," says Porco.

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