The Martian moons Phobos and Deimos have always been a subject of fascination for astronomers. Unlike other moons in the Solar System that are round and regular, these two moons are irregularly shaped, resembling lumpy potatoes. Their odd shapes and unusual compositions have led scientists to question their origins.

There are two main hypotheses about how Phobos and Deimos came to be. One theory suggests that they are captured asteroids, while the other proposes that they are debris from an ancient impactor that collided with Mars. The latter theory is similar to how Earth’s moon is believed to have formed. However, the captured asteroid theory is also plausible, considering examples like Neptune’s moon Triton, which is thought to be a captured object.

Phobos and Deimos share similarities with carbonaceous C-type asteroids, the most abundant type of asteroid in the Solar System. Their compositions and albedos support the captured asteroid theory. However, their orbits are circular and close to Mars’ equator, which contradicts the eccentric orbits expected from captured objects.

The density of Phobos and Deimos is lower than that of silicate, the main component of Mars’ crust. This presents a challenge to the impact theory, as a powerful impact should have resulted in a disk of material around Mars containing more Martian silica. The conflicting evidence has left scientists searching for a definitive explanation for the moons’ origins.

Recently, researchers presented a new hypothesis at the 55th Lunar and Planetary Science Conference. They suggest that an ancient impactor, but one composed of ice, could be responsible for creating Phobos and Deimos. This icy impactor theory offers a fresh perspective on the moons’ formation.

Using simulations, the researchers showed that an impactor with a significant amount of water ice could have produced the debris disk from which Phobos and Deimos formed. Unlike previous studies with rocky impactors, the temperature in the debris disk with an icy impactor would have been lower, preserving carbonaceous materials and chondritic materials in the moons.

The researchers found that varying the ice content in the impactor influenced the size and composition of the debris disk around Mars. Higher ice content resulted in lower disk temperatures, allowing for the survival of chondritic material in Phobos and Deimos. The simulations provided insights into how an icy impactor could explain the unique features of the Martian moons.

While the idea of an icy impactor with 70% to 90% water ice mantles may seem unconventional, it offers a plausible explanation for the origins of Phobos and Deimos. Further exploration, such as Japan’s Martian Moons eXploration (MMX) mission, could provide more evidence to support or refute this theory. The delay in MMX’s launch may prolong the wait for samples from Phobos, but the potential discoveries about the Martian moons and the early Solar System will be well worth it.

Understanding the origins of Phobos and Deimos is a crucial piece in unraveling the mysteries of Mars and the formation of our Solar System. The ongoing research and upcoming missions hold the promise of shedding light on these enigmatic moons and their place in the cosmic story.

References:

• Universe Today Article – Did An Ancient Icy Impactor Create the Martian Moons?