Will Mars Really Lose One Of Its Moons?

Phobos is the larger and innermost of Mars' two moons, and it's a fascinating celestial body. It's small, irregularly shaped, and orbiting just about 6,000 kilometers above the surface of the Red Planet. That means Phobos is closer to its planet than any other moon in our solar system. It completes its orbit around the planet in only 7 hours and 39 minutes, and if looked at from Mars, Phobos rises in the west and sets in the east. But that's not all that's unusual about Phobos. Tidal forces from Mars are causing it to inch ever closer, approximately 1.8 centimeters every year. As a result, Phobos is on a slow but steady spiral toward its doom. Scientists estimate that it will fall into Mars' atmosphere within the next 50 million years.

Adding to its precarious state, Phobos shows signs of structural strain. Its surface is marred by long grooves and deep craters, which indicate that this moon is stretching, possibly due to tidal forces. It's been theorized that Phobos may be more like a loosely bound rubble pile than a solid rock. The shrinking orbit of Phobos, and its tidal deformations, make us wonder what its most likely destiny is. Will it collide with Mars, break apart into a ring, or meet another fate entirely? Let's explore these possibilities.

At just 22 kilometers across, Phobos is tiny compared to Earth's moon. It was discovered, together with Mars' second moon, Deimos, in 1877 by American astronomer Asaph Hall, who predicted the moons would have a close orbit to the Red Planet. What he couldn't even dream of is that the moon's irregular shape suggests a violent past. Spectral analysis indicates that Phobos is composed primarily of rock debris, dust, and ice, similar to type-C asteroids found in the outer asteroid belt. This has led many scientists to believe that Phobos is a captured asteroid, snagged by Mars' gravity long ago. 

However, not everyone believes this theory. If it were a captured asteroid, it would have a more erratic and tilted path. Its nearly circular orbit and alignment with Mars' equator suggest that Phobos, and its smaller brother, Deimos, formed from debris left over after an unknown celestial body impacted Mars. Whether born from Mars itself, or adopted from the asteroid belt, Phobos carries marks of instability. Its weak structure and spiraling orbit suggest it was never a fully settled moon. Phobos is Mars' temporary companion, destined for an early end.

The inward spiral of Phobos sets the stage for two possible outcomes: catastrophic breakup or a dramatic surface impact. According to 2015 research done by planetary scientist Benjamin Black from the University of California, Berkeley, Phobos is likely to be torn apart by Martian tidal forces before it hits the planet's surface. This breakup could occur in as little as 20 to 40 million years, and its result would be a ring of debris encircling Mars.

Once disrupted, the ring would form rather quickly, probably in only a few weeks, but it's predicted that this ring wouldn't be permanent. It would take anywhere from 1 to 100 million years for it to slowly fade, as its material would rain down on the surface of the Red Planet or erode over time. However, if Phobos maintains enough cohesion during its descent, denser fragments may survive the breakup and eventually crash into Mars. These impacts would be relatively slow and shallow but still capable of carving visible craters into the surface of Mars.

Alternative research offers a slightly faster timeline. An analysis suggests Phobos could reach a state of orbital disintegration in as little as 10 million years. In 2008, Dr. Bijay Kumar Sharma (via Cornell University) used a planetary-satellite dynamics model to calculate Phobos' orbital decay, concluding that this moon will meet its demise in approximately 10.4 million years. Once Phobos reaches an altitude of 3,400 miles from the surface of Mars, it will cross the threshold of the Roche limit. This is the point where the tidal forces of Mars will tear the moon apart, but the exact timing and the process will depend on Phobos' internal strength, density, and composition.