The origin of Phobos and Deimos, the two Martian moons, has been a mystery to astronomers. These two bodies are a fraction of the size and mass of the Moon, measuring just 22.7 km (14 mi) and 12.6 km (7.83 mi) in diameter. Both have a rapid orbital period, taking just 7 hours, 39 minutes, and 12 seconds (Phobos) and 30 hours, 18 minutes, and 43 seconds (Deimos) to complete an orbit around Mars. Both are also irregular in shape, leading many to speculate that they were once asteroids that got kicked out of the Main Belt and were captured by Mars' gravity.
There's also the theory that Phobos and Deimos were once a single moon hit by a massive object, causing it to split up (aka the "splitting hypothesis"). In a recent paper, an international team of scientists led by the Institute of Space and Astronautical Science (ISAS) revisited this hypothesis. They determined that a single moon in a synchronous orbit would not have produced two satellites as we see there today. Instead, they argue, the two moons would have collided before long, producing a debris ring that would have created an entirely new moon system.
The paper that describes their findings recently appeared online and will be published in The Planetary Science Journal. The research was led by Dr. Ryuki Hyodo, a researcher with the Department of Solar System Sciences at ISAS, a part of the Japan Aerospace Exploration Agency (JAXA). He was joined by researchers from the Earth-Life Science Institute at the Tokyo Institute of Technology, the Paris Globe Institute of Physics at the Universite de Paris, and the Orbital Dynamics and Planetary Group at Sao Paulo State University.
As noted, the subject of where Mars' moons came from has become a hot topic of date for astronomers in recent years. Historically, astronomers have leaned toward the Capture Hypothesis, which states that Phobos and Deimos were once D-type asteroids. These are asteroids composed of organic-rich silicates, carbon, and silicates that contain no water (anhydrous) that may have water ice in their interiors. This hypothesis is largely motivated by observations that revealed similarities in spectra between D-type asteroids and these moons.
Alternately, the Giant Impact Hypothesis states that an impactor struck Mars, creating a debris ring around the planet that accreted to form two rubble-pile objects. This is similar to the most widely-accepted theory of how the Earth-moon system formed billions of years ago due to an impact with a Mars-sized object named Theia (also called the giant impact hypothesis). More recently, it has been proposed that Phobos and Deimos may not be primordial objects that resulted from capture or an impact but are the remains of a primordial moon that broke apart.
This theory has been dubbed the "ring-moon recycling hypothesis," which was put forth in a 2021 paper by Amirhossein Bagheri et al. According to this hypothesis, this progenitor moon was ripped apart 1 to 2.8 billion years ago, either by tidal forces or an impact. The resulting debris would have formed a ring around Mars that was eventually recycled to form Phobos and Deimos. As astronomers have noted, this model presents some issues, which include the fact that Mars would still have a ring system. As Dr. Hyodo explained to Universe Today via email, he and his team noted that there are other issues:
Phobos and Deimos were once a single moon that was split to form Phobos and Deimos said, studying the tidal evolution of the moons back in time, Bagheri et al. (2021) found a solution that Phobos and Deimos could once have orbits that would cross each other. This is their evidence. Note that if you change parameters that control the tidal evolution, the orbits of Phobos and Deimos in the past do not cross each other. The idea of Bagheri et al. is based on their parameters that have resulted in orbital crossing in the past.
To test this hypothesis, Dr. Hyodo and his colleagues began with the assumption that Phobos and Deimos were once a single body. They then conducted numerical simulations that combined geophysical and tidal-evolution models of a Mars–satellite system. From this, said Dr. Hyodo, they determined that it was highly unlikely that Phobos and Deimos originated from a single object:
We then calculated successive orbital evolutions of the moons in the direct 3-body approach (Mars-Phobos-Deimos), which can precisely calculate close encounters, gravitational interactions, and collisions between moons. We found that the two moons would most likely (more than >90% by chance) collide with each other within a very short timescale after the splitting (<104 years). This impact is very destructive (i.e., high-velocity impact), and thus the two moons (Phobos and Deimos) are catastrophically destroyed.
In short, if Phobos and Deimos were split from a single progenitor moon (1 to 2.7 billion years ago), they would have collided within 100,000 years. This would have left Mars with another debris ring that would still be there today, rather than its two irregularly-shaped satellites that happen to be asteroid-like in composition. These findings have renewed the debate about where Mars' moons came from and also suggest that it may not be resolved until sample-return missions are sent to explore the Martian satellites.
Several mission concepts are currently on the table. In 2008, NASA's Glenn Research Center began studying a possible sample-return mission known as the Hall concept. This New Frontiers-class concept would perform a sample return from Phobos and Deimos. In January 2013, scientists from Standford University, the Massachusetts Institute of Technology (MIT), and NASA's Jet Propulsion Laboratory began collaborating on a new Phobos Surveyor mission. The mission is currently in the testing phases with a potential launch window of 2023 and 2033.
In March 2014, NASA proposed a Discovery-class mission called Phobos And Deimos & Mars Environment (PADME), which would place an orbiter in Mars orbit by 2021 to study Phobos and Deimos. A heritage concept called OSIRIS-REx 2 is being considered that would use parts from the first OSIRIS-REx mission (a sample-return mission to the asteroid Bennu) and conduct a sample-return mission from Phobos and Deimos. Beyond NASA, other space agencies also hope to send robotic spacecraft to explore the Martian satellites.
In 2015, the Japanese Aerospace Exploration (JAXA) unveiled their concept for a sample-return mission to Phobos known as the Martian Moons Exploration (MMX). This mission would conduct a Deimos flyby before landing on Phobos multiple times to obtain samples similar to what the Hayabusa2 mission did on asteroid Ryugu. This mission is international in scope, with NASA and the ESA contributing, and is currently scheduled to launch in 2024 and return samples to Earth five years later.
Russia plans to repeat its previous attempt to send a sample-return mission to Phobos named Fobos-Grunt (Russian for "Phobos Ground") in the late 2020s. In 2015, the ESA began assessing a sample-return mission to Phobos called Phootprint, which was planned for launch by 2024. This mission was originally conceived as a collaborative effort between the ESA and Roscosmos, which have since terminated their cooperation agreements due to Russia's invasion of Ukraine.
From all these proposals, it is clear that multiple space agencies intend to explore the Moons of Mars in the near future. The origin and subsequent evolution of these moons are considered part of a wider effort to explore and characterize Mars. In the meantime, the debate continues.
Journal Information: Ryuki Hyodo et al, Challenges in forming Phobos and Deimos directly from a splitting of an ancestor single moon. arXiv:2208.04794v1 [astro-ph.EP], arxiv.org/abs/2208.04794