198

Using supercomputer simulations, a team of NASA scientists recently modeled the destruction of an asteroid around Mars to investigate the possible origins of Mars’ two, mysterious moons — Phobos and Deimos. Since their discovery in 1877 and subsequent investigations by telescopes and Martian spacecraft, scientists have pondered how the two moons came to be, and how they gained their unique, rugged shapes.

Led by Jacob Kegerreis, the team’s new study revealed that a rogue asteroid may have passed too close to Mars. During its flyby, Mars’ strong gravitational pull would have disrupted, or ripped apart, the asteroid, leading to hundreds of thousands of small rocky fragments orbiting Mars.

While more than half of the fragments created from the disruption event are believed to have been ejected away from Mars, those trapped within the planet’s orbit would have continued to collide, creating more debris. After these collisions stopped and the fragments settled into a ring around Mars, the material within the rings likely began clumping together, creating the Phobos and Deimos we know today.

This theory was tested by Kegerreis et al.’s supercomputer models, which explored hundreds of different flyby simulations, each varying the asteroid’s size, spin, speed, and distance from Mars during the disruption event. The team employed two different computing codes for their simulations: a high-performance, open-source computing code called SWIFT (used to model the disruption event), and another computing code for modeling the orbits of the debris produced from the disruption event. Each computing code utilized Durham University’s advanced computing systems in the UK.

Kegerreis et al. report in their study that in many of their simulations, enough raw material is produced around Mars from the disruption event and subsequent collisions to aid in forming moons similar to Phobos and Deimos.

“It’s exciting to explore a new option for the making of Phobos and Deimos – the only moons in our solar system that orbit a rocky planet besides Earth’s. Furthermore, this new model makes different predictions about the moons’ properties that can be tested against the standard ideas for this key event in Mars’ history,” said Kegerreis, a postdoctoral research scientist at NASA’s Ames Research Center in California.

Interestingly, Kegerreis et al.’s models and findings don’t explicitly follow the two most popular theories about the origins of Phobos and Deimos. The first theory suggests that the moons were once asteroids that passed close enough to Mars to be wholly captured within Mars’ gravitational well and orbit. The second suggests that a giant impact on the Martian surface would have ejected enough material from the planet’s surface to create a disk and, finally, the moons. The latter theory closely aligns with scientists’ current theories behind the formation of Earth’s moon.

However, while the second theory accounts for the orbital trajectories of Phobos and Deimos more closely than the first theory, it fails to account for Deimos’ large orbital radius around Mars. If a giant impact did occur at the Martian surface, any material ejected from the surface would have settled into a disk that hugged Mars quite closely — much closer to Mars than where Deimos currently orbits. Deimos’ large orbital radius means it had to have formed at that distance.

Fortunately, Kegerreis et al.’s new models account for Deimos’ orbital distance, with the raw materials needed for the formation of the moons reaching out to Deimos’ orbit.

“Our idea allows for a more efficient distribution of moon-making material to the outer regions of the disk. That means a much smaller ‘parent’ asteroid could still deliver enough material to send the moons’ building blocks to the right place,” said co-author Jack Lissauer of NASA Ames.

While Kegerreis et al. primarily focus on Phobos, Deimos, and Mars in their simulations, Kegerreis explains that their simulations and models also allow for the exploration of the formation of a variety of moons around the solar system and encounters between objects like planets, asteroids, comets, and more. These violent events were extremely common in the very early days of our solar system, so Kegerreis et al.’s simulations could allow scientists to better model the early solar system, its environment, and its outcomes.

For Kegerreis et al. specifically, now that they’ve validated a formation theory with their model, the team will move to better modeling the formation of the disk that Phobos and Deimos formed out of.

“Next, we hope to build on this proof-of-concept project to simulate and study in greater detail the full timeline of formation. This will allow us to examine the structure of the disk itself and make more detailed predictions for what the MMX mission could find,” said co-author Vincent Eke, an associate professor at the Institute for Computational Cosmology at Durham University.

The Martian Moons Exploration (MMX) mission is a Martian sample return mission led by the Japanese Aerospace Exploration Agency (JAXA) that will travel to both Phobos and Deimos. While at the moons, MMX will extensively study the moons and their characteristics to learn more about their composition and origin. While at Phobos, the MMX spacecraft will collect samples of the moon’s surface to return to Earth, where it will be sent to a lab for in-depth study. The internal composition of the moons (i.e. what they’re made of) could be the major clue that helps scientists determine whether or not the moons were once asteroids or the results of an impact/disruption event.

MMX is currently set for launch in 2026 and features a variety of instruments and technology demonstrations, including NASA’s Mars-moon Exploration with Gamma Rays and Neutrons (MEGANE) instrument and a pneumatic sampler technology demonstration.

(Lead image: Mars, Phobos, and Deimos. Credit: NASA)