It appears the process of coming apart at the seams has already begun. Images of Phobos taken by the Viking orbiters and other spacecraft show a network of grooves in the tiny moon’s surface. At first they appeared to radiate from near Stickney Crater, and geologists assumed that the grooves were cracks caused by the ancient impact. Some certainly are just that, but not all. In 2015, Terry Hurford of NASA’s Goddard Space Flight Center and his colleagues reported a new analysis of the grooves. Most of them actually radiate from the side of Phobos that constantly faces Mars; tidal forces caused by Mars’ gravitational pull are deforming Phobos. Hurford believes the grooves are stretch marks, a visible sign of the inexorable grip of tidal forces on the moon.

The future martian ring will not be the only one in the solar system, of course. Nor will it be the only ring whose existence depends on a moon. There are rings across the giant planets: the four dusty rings of Jupiter; Uranus’ 13 dark, thin rings; and Neptune’s five faint rings and four enigmatic ring arcs are — like the future ring around Mars — all intimately linked to moons and moonlets. And of course, the most familiar ringed planet is Saturn, whose icy surrounding matter can be seen even through a small telescope.

Galileo Galilei saw what turned out to be Saturn’s spectacular ring system in 1610. (He said the features looked like ears or handles.) But it wasn’t until 1655 that Christiaan Huygens identified them as an entire system of icy rings. In 1856, famed physicist James Clerk Maxwell showed that the rings must be composed of a huge number of tiny particles (he called them “brick-bats”), each independently orbiting Saturn.

Since then, debate has raged over the origin, age, and composition of Saturn’s rings. Are they leftovers from the formation of Saturn, or the remains of a shattered moon? As old as Saturn itself, or a relatively new addition? And why so much ice? Planetary scientist Robin M. Canup of the Southwest Research Institute in Boulder, Colorado, recently published a proposal that answers these questions. Canup suggests that Saturn’s rings are the very ancient remains of a Titan-sized moon.

“Saturn originally had multiple massive moons like Jupiter,” says Canup. These moons were large enough for their interiors to differentiate into layers of ices and a rocky core — less like a large comet and more like the four larger moons of Jupiter. When the large moons spiraled inward as Saturn finished its accretion, Canup says, the outer icy layers of at least one were stripped away. The core plunged into Saturn, and the icy remains eventually formed the planet’s main rings. This, she says, explains why the particles making up the rings are 99.9 percent pure water ice. It also explains the striking difference between Jupiter’s and Saturn’s rings and satellites.

“The existence of Saturn’s much more massive ring system is linked to Saturn having lost its large primordial inner moons,” Canup explains. “Jupiter retained its large inner moons, [while] its dusty ring system is vastly less massive than the ring system at Saturn.”