Ancient rings around Mars? Clues from its moon

Every crater tells a story, every tilt a tale. Until recently, the 2° orbital tilt of Mars’s oldest moon, Deimos, hasn’t been considered particularly fascinating. Now, researchers claim that it points towards the existence of an ancient Martian moon-ring cycle.

The slight orbital tilt of Mars’s oldest and farthest moon, Deimos, gives us clues that a moon-ring cycle of creation and destruction has already happened multiple times throughout history.

This new research will be published in Astrophysical Journal Letters, and was presented today at the 236th meeting of the American Astronomical Society conference.

In 2017, Hesselbrock and Minton published a paper outlining the cyclic Martian moon theory. They proposed that, over billions of years, generations of Martian moons have been destroyed, forming Martian rings of dust and moonlets. Each time, this planetary ring system gives rise to a new, smaller moon.

Simulations have shown that Phobos, the largest and youngest of Mars’s two moons, will again succumb to this fate 22 to 40 million years from now.

Unlike our own moon, which is moving gradually away from earth, Phobos is moving closer to the surface of Mars, at a rate of about 2 centimetres per year. This decaying orbit is a result of tidal deceleration. Once this tidal stress becomes too much, Phobos’s orbit will drop below what is known as the “Roche limit”, and Mars’s gravity will tear it apart to make a ring around the planet. This exterior Martian ring will remain for anything between 1 and 100 million years.

The ring-moon cycle

Moons form on the outer fringes of planetary ring systems. The new moon is pushed outward, away from the planet. The rings then fall onto the planet, leaving only the moon. “Once the ring is gone, the moon will start falling inwards towards the planet again, because the ring is the only thing pushing it out,” says lead author Matija Ćuk, a research scientist at the SETI Institute. “The moon then gets very close to Mars and is torn apart by tidal forces, creating a new ring.”

Secrets spilled by Deimos’s tilt

Deimos’s orbit is close enough to the planet’s centre for us to suppose that it formed at the same time as Mars. However, it is not precisely in plane with Mars’s equator. “The present inclination of Deimos, which is about 2 degrees, is best matched with a resonance with a prior moon that was about 20 times the mass of Phobos,” says Dr Ćuk.

With each ring-moon cycle, both the ring and moons become smaller, so the ancestors of Phobos would have been much larger than Phobos itself. The formation and outward movement of a large proto-Phobos moon away from an ancient ring system is, according to Ćuk and his colleagues, compatible with the skewed inclination of Deimos.

The 2° difference would only have been seen if this proto-Phobos was moving outward – the only thing that can move it outward are rings. This would have happened around 2.5 billion years ago.