Mars has two moons called Phobos and Deimos, the mythological personification of fear and dread, respectively. These objects are very different from our own Moon, which is large and mostly spherical, and for a long time, they were thought to be captured asteroids. Recent research has continued to find holes in this idea and a new study has a very intriguing claim. The two moons used to be one.

As reported in Nature Astronomy, researchers have used geophysical data and models about the orbital motion of the Moon, to wind back the clock. And it turns out that billions of years ago Phobos and Deimos were in the same place around Mars. The obvious suggestion is that they were once a single larger object.

Phobos is 22 kilometers (14 miles) across and Deimos a bit more than half of that. The work suggests that between 1 and 2.7 billion years ago, there was a single moon orbiting the Red Planet.

“Phobos and Deimos are the remainders of this lost moon,” lead author Amirhossein Bagheri, a doctoral student at the Institute of Geophysics at ETH Zurich, said in a statement. “The exact time depends on the physical properties of Phobos and Deimos, that is, how porous they are.”

Key to this work has been data collected by NASA’s InSight seismometer. The instruments measure marsquakes and all those vibrations the Red Planet experiences. This informs scientists of the internal properties of the planet and the effect of tidal forces from the moons on Mars and vice versa. These forces lead to energy dissipation, whose strength depends on the bodies; size, composition, and distance.

The complete picture of the Martian moon system is still lacking but we have long suggested that doom is on the cards for Phobos. Researchers expect that within 40 million years, the moon will be no more. Phobos is slowly but surely getting closer and closer to Mars. This might lead to an eventual collision with the planet. Or the moon might be ripped apart and provide Mars its very own ring system.

Another hypothesis sees this ring formation as part of a cycle where Phobos gets periodically ripped apart, which part of the material falling on Mars and part coalescing back into a moon.

More data is needed on the composition of Phobos. The Japanese Space Agency, JAXA, is planning for this purpose the Martian Moons Exploration mission which is expected to fly to Mars around 2025 and collect samples from Phobos before coming back to Earth.