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This destruction was also found to occur at a distance from Mars greater than that estimated by previous models. These results, obtained through numerical simulations and analytical estimates, are based on the assumption that the physical properties of Phobos, in particular its low strength, are identical to those of recently visited asteroids.
The two moons of Mars, Phobos and Deimos, depicted in orbit around the red planet.
Credit: NASA
These properties will be measured by the Japanese space agency's (JAXA) MMX (Martian Moons eXploration) mission, scheduled for launch in 2026, of which these two researchers are members. These models and their results have implications for the origin, evolution, and fate of Phobos and can be applied to other small planetary moons.
A doomed moon: a revised scenario
Phobos, the more massive of Mars's two moons, orbits so close to the planet (approximately 5,600 miles or 9,000 km) that its orbit is decaying inexorably due to tidal forces. Until now, models predicted its destruction in the immediate vicinity of Mars, near the theoretical Roche limit (about 1.6 Martian radii). However, this new study reveals that Phobos will begin to disintegrate as early as 2.2 Martian radii (approximately 4,700 miles or 7,500 km from Mars' center), much earlier than expected, if its mechanical strength is as low as that of recently visited asteroids.
Why? Because if Phobos shares the same characteristics as small asteroids, its structure would be that of a rubble pile, a conglomerate of rocks bound together by their own gravity. The researchers have shown that in this case, as it gets closer to a planet, tidal forces first strip away surface material before causing total disruption. This mechanism, overlooked until now, explains why previous estimates underestimated the small moon's breakup distance.
These simulations start from the assumption that Phobos is much more fragile than previously assumed in earlier studies. This hypothesis is motivated by the observation that small visited asteroids have shown very low strength. The gradual, rather than sudden, destruction of Phobos opens the door to novel scenarios, such as collision erosion accelerated by the stripped debris.
From Figure 3 of the article: Representations of the "rubble pile" models of Phobos used in this study.
Top row: Top-down view from Phobos' rotational pole, with Mars oriented to the left.
Bottom row: Profile view, where the camera tracks Phobos' orbit and Mars remains on the left.
Implications far beyond Mars
This study is not just about Phobos. It offers a novel theoretical framework for understanding the fate of small, irregular moons in the Solar System, like those of Saturn or Jupiter. It also highlights the importance of space missions dedicated to small bodies, which allow for testing celestial mechanics and planetary formation models.
Phobos is a natural laboratory for studying satellite evolution processes and fate. Its coming years will teach us as much about its end as its origin.