Can you imagine Mars with rings? That could be the case within 30–50 million years, as planetary scientists say the planet's moon Phobos is spiralling towards the Red Planet.
Phobos is one of Mars's two moons – the other being Deimos – and orbits about 3,700 miles (6,000 kilometres) above the surface of the Red Planet.
It completes an orbit of Mars once every hours and 39 minutes, according to the European Space Agency, but that is slowly changing.
Phobos's orbit is 'decaying', meaning the moon is slowly spiralling inwards and will eventually either be ripped apart by gravitational forces, or impact Mars and be destroyed entirely.
This will likely happen some time in the next 30–50 million years.
If Phobos is ripped apart, the leftover debris could form a ring system around the planet, much like Saturn's rings.
This is in contrast to Earth's Moon, which, say scientists, is actually moving away from Earth at a distance of about 3.8cm per year.
Phobos forming a ring system
Because Phobos is fractured and made up of porous rubble, rather than crashing into the Red Planet, the moon could be torn apart in its atmosphere, potentially creating rings around Mars, like those around Saturn, that could last from one to one hundred million years.
In 2015, two scientists at the University of California, Benjamin Black and then-graduate student Tushar Mittal, published a study that said the largest chunks of Phobos would crash into the Red Planet and create craters on the Martian surface.
Much of the debris would remain in Mars’s orbit and circle the planet for millions of years before dropping to the surface in moon showers.
Mars’s other moon, Deimos, would remain.
"While our Moon is moving away from Earth at a few centimetres per year, Phobos is moving toward Mars at a few centimetres per year, so it is almost inevitable that it will either crash into Mars or break apart," Black said.
"One of our motivations for studying Phobos was as a test case to develop ideas of what processes a moon might undergo as it moves inward toward a planet."
About the study
Black and Mittal examined data of fractured rocks on Earth and meteorites that have struck Earth that have a density and composition similar to Phobos.
They also looked at results from simulations of the Stickney impact crater on Phobos, which formed when a rock smashed into the Martian moon.
The crater is huge, spanning about one sixth of the lunar circumference.
The duo then modelled the predicted evolution of the ring that might form around Mars following Phobos’s demise.
"If Phobos broke apart at 1.2 Mars radii, about 680 kilometres above the surface, it would form a really narrow ring comparable in density to that of one of Saturn’s most massive rings," Mittal said.
"Over time it would spread out and get wider, reaching the top of the Martian atmosphere in a few million years, when it would start losing material because stuff would keep raining down on Mars."
It is, however, not clear whether the rings would be visible from Earth, but they might reflect enough light to make Mars appear brighter.
"Standing on the surface of Mars a few tens of millions of years from now, it would be pretty spectacular to watch," Black said.