Were Phobos and Deimos Once a Single Martian Moon That Split up? Not Likely, says New Study

Astronomers don't know the origin of the two Mars moons. The two bodies are less than the size of the Moon and less than 7 miles in diameter. They take just 7 hours, 39 minutes, and 12 seconds to complete a trip around Mars. Many people think that they were once asteroids that got kicked out of the Main Belt and were captured by Mars.

There is a theory that the moon split up after being hit by a large object. An international team of scientists led by the Institute of Space and Astronautical Science re-examined the hypothesis. The two satellites that we see today were not created by a single moon. They argue that if the two moons had collided, a debris ring would have formed, creating a new moon system.

The paper describing their findings will be published in a journal. A researcher with the Department of Solar System Sciences at ISAS led the research. He was joined by researchers from the Earth-Life Science Institute, the Tokyo Institute of Technology, the Paris Globe Institute of Physics, and the Orbital Dynamics and Planetary Group.

The subject of where Mars' moons came from has become a hot topic for astronomy. Astronomers believe that Phobos and Deimos were once D-type asteroids. These asteroids are made of organic-rich silicates, carbon, and silicates that do not have water in their interiors. Observations show similarities in the spectrum between asteroids and these moons.

The Giant Impact hypothesis states that an impactor struck Mars and created a debris ring around the planet. 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. The remains of a primordial moon may be what Phobos and Deimos are, according to a new proposal.

The progenitor moon was ripped apart by either tidal forces or an impact according to this theory. Phobos and Deimos were formed from the debris that formed a ring around Mars. The fact that Mars still has a ring system is one of the issues presented by this model. He and his team noted that there were other issues in the email.

“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: saying that Phobos and Deimos were once a single moon that was split to form Phobos and Deimos. 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.“

They assumed that Phobos and Deimos were once a single body. There are two models of a Mars–satellite system. They determined that it was not likely that Phobos and Deimos came 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 (<10^4>

If Phobos and Deimos were separated from a single progenitor Moon, they would collide in 100,000 years. This would have left Mars with another debris ring that would still be there today, instead of its two irregularly-shaped satellites that are similar to asteroids. It has been suggested that the debate about where Mars' moons came from may not be over until sample-return missions are sent to investigate the Martian satellites.

There are several mission ideas on the table. A sample-return mission known as the Hall concept was studied by NASA. A sample-return from Phobos and Deimos would be performed by this class. Scientists from Standford University, the Massachusetts Institute of Technology, and NASA's Jet Propulsion Laboratory began collaborating on a new mission in January of last year. There is a potential launch window of 2033 for the mission.

NASA proposed a mission in March of last year to study Phobos and Deimos on Mars. A concept called OSIRIS-REx 2 is being considered that would use parts from the first OSIRIS-REx mission and conduct a sample return mission. Other space agencies would like to send robotic spaceships to explore the satellites.

The concept for a sample-return mission to Phobos was unveiled in 2015. Similar to the Hayabusa2 mission, this mission would conduct a flyby before landing on Phombo to get samples. The goal of the mission is to return samples to Earth five years from now.

In the late 2020s, Russia will attempt to send a sample return mission to Phobos. The sample-return mission to Phobos, called Phootprint, was planned for launch by the end of the 20th century. The mission was originally conceived as a collaborative effort between the two organizations, but they have since ended their cooperation due to Russia's invasion of Ukranian.

Multiple space agencies plan to explore the Moons of Mars in the near future. The evolution of these moons is part of a larger effort to explore and understand Mars. The debate is going on.

ArXiv is further reading.