Mars is destined to get a spectacular new planetary ring system to rival that of Saturn, but don’t hold your breath because it will not be for another 30 million years or so, according to scientists.

The findings, published in the journal Nature Geoscience, are based on a new study to determine the fate destined to befall the diminutive Martian moon Phobos.

“We found that Phobos is too weak to withstand tidal stresses from Mars and we expect it to break apart in a few tens of millions of years and form a ring around Mars,” the study’s lead author Dr Benjamin Black of the University of California said.

Only the giant planets of the outer solar system have rings at the moment.

Phobos — the larger of the two moons circling Mars — orbits just 6,000 kilometres above the surface of the red planet, closer than any other moon in the solar system.

“Over time Phobos is creeping inwards towards Mars at a couple of centimetres per year,” Dr Black said.

“We wanted to figure out whether Phobos crashes into Mars or breaks apart to form a ring, so we needed to know how strong it was — is it going to be able to stand the increasing tidal stresses that are going to be pulling this little moon apart, or will it eventually succumb to these forces?”

Dr Black and co-author Tushar Mittal found that Phobos will be pulled apart by the red planet’s gravitational tidal forces in about 20 to 40 million years’ time.

“We concluded that Phobos would break apart between 2.4 and about 1.1 Mars radii, somewhere between 8,500 kilometres down to around 4,000 kilometres or so,” Dr Black said.

Early signs of doom

At its current 6,000-kilometre altitude Phobos is showing early signs of the structural failures that will ultimately spell its doom.

The surface of the 28-kilometre-wide moon features long, shallow grooves, which are interpreted as stretch marks caused by the powerful gravitational tidal forces of Mars which are slowly deforming and will eventually tear Phobos apart.

“Those grooves indicate the break up might be happening sooner than we expected,” Dr Black said.

When Phobos does start to break apart, the authors calculate the process may only take five orbits.

The debris created by the moon’s break-up will disperse very quickly, forming a ring around Mars in just 50 to 130 orbits, or between about 100 and 1,000 hours.

The authors found that some larger chunks could survive long enough to crash down onto the planet’s surface forming elongated craters.

According to Dr Black and Mr Mittal, the Martian ring will last between 1 million and 100 million years.

Mars vs Earth

The same gravitational tidal forces influencing Mars and Phobos also occur as our Moon orbits around Earth, causing the ocean tides to ebb and flow, and making both celestial bodies slightly egg-shaped rather than perfectly spherical.

However, both Earth’s Moon and the other Martian moon Demos are far enough away from their planets for these gravitational tidal forces to slowly push them away, rather than drag them down.

“Many people don’t realise it but Earth’s Moon is gradually moving away from the planet, but the Martian moon Phobos is doing the exact opposite — over time it’s creeping inwards towards Mars,” Dr Black said.

The authors reached their conclusions by combining geologic observations, visible and near-infrared spectral data and theoretical models to calculate the structure and strength of Phobos.

“We used evidence from meteorites, observations of Phobos how quickly its moving, we even used geotechnical models like engineers use to build tunnels and underground structures, to estimate how strong Phobos might be,” Dr Black said.

“And then once we had a better understanding of Phobos, we could make a prediction about what would happen to this little moon.”