The origin of Phobos and Deimos, the two Martian moons, remains 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 fast orbital period, taking only 7 hours, 39 minutes and 12 seconds (Phobos) and 30 hours, 18 minutes and 43 seconds (Deimos) to orbit Mars. Both are also irregularly shaped, leading many to speculate that they were once asteroids that were blown out of the main belt and 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 separate (aka the “separation hypothesis”). In a recent paper, an international team of scientists led by the Institute of Space and Astronautical Sciences (ISAS) revisited this assumption. They determined that a single moon in a synchronous orbit would not have produced two satellites as we see today. Instead, they claim, the two moons would have collided before long, producing a ring of debris that would have created an entirely new lunar system.
The article describing their findings recently appeared online and will be published in The Journal of Planetary Science. The research was led by Dr. Ryuki Hyodo, a researcher in the Department of Solar System Science at ISAS, 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 University of Paris, and the Orbital Dynamics and Planetary Group at Sao Paulo State University.
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As noted, the subject of the origin of the moons of Mars has become a hot topic for astronomers in recent years. Historically, astronomers have leaned toward the capture hypothesis, which states that Phobos and Deimos were once D-type asteroids. They are asteroids composed of organic-rich silicates, carbon, and silicates that do not contain d water (anhydrous) and which may contain water ice inside. This hypothesis is largely motivated by observations that have revealed similarities in the spectra between D-type asteroids and these moons.
Alternatively, the giant impact hypothesis states that an impactor hit Mars, creating a ring of debris around the planet that accumulated to form two rubble-pile objects. This is similar to the most widely accepted theory about the formation of the Earth-Moon system 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 have been primordial objects resulting from capture or impact, but the remains of a primordial moon that shattered.
This theory has been dubbed the “ring moon recycling hypothesis,” which was advanced in a 2021 paper by Amirhossein Bagheri et al. According to this hypothesis, this progenitor moon was torn apart 1 to 2.8 billion years ago, either by tidal forces or by 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, there are some problems with this model, including 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 were other issues:
“Studying the evolution of the tides of the moons over time, Bagheri et al. (2021) found a solution that Phobos and Deimos might once have intersecting orbits. Here is their proof: saying that Phobos and Deimos were once a single moon that was split to form Phobos and Deimos. Note that if you change the parameters that control the evolution of the tides, the orbits of Phobos and Deimos in the past do not intersect. The idea of Bagheri et al. is based on their parameters that have resulted in orbital crossings in the past.“
To test this hypothesis, Dr. Hyodo and his colleagues started by assuming that Phobos and Deimos were once one body. They then carried out numerical simulations combining geophysical and tidal evolution models of a Mars-satellite system. From this, Dr. Hyodo said, they determined that it was highly unlikely that Phobos and Deimos originated from a single object:
“We then calculated the successive orbital evolutions of the moons in the direct 3-body approach (Mars-Phobos-Deimos), which can accurately calculate close encounters, gravitational interactions and collisions between the moons. We found that the two moons would most likely collide (more than >90% by chance) within a very short time after separation (<10^4 years). This impact is very destructive (i.e. a high velocity impact), and therefore both moons (Phobos and Deimos) are catastrophically destroyed.
In short, if Phobos and Deimos were separated by a single progenitor Moon (1 to 2.7 billion years ago), they would have collided in 100,000 years. This would have left Mars with another ring of debris that would still be around today, rather than its two irregularly shaped satellites whose composition resembles that of an asteroid. These discoveries have renewed the debate over the origin of Mars’ moons and also suggest that it may not be resolved until sample return missions are sent to explore Mars’ satellites.
Several mission concepts are currently on the table. In 2008, NASA’s Glenn Research Center began investigating a possible sample-return mission known as the “Hall” concept. This New Frontiers class concept would perform a sample return of Phobos and Deimos. In January 2013, scientists from Stanford University, Massachusetts Institute of Technology (MIT), and NASA’s Jet Propulsion Laboratory began collaborating on a new Phobos Surveyor assignment. The mission is currently in the testing phase with a potential launch window of 2023 and 2033.
In March 2014, NASA proposed a Discovery-class mission called Phobos and Deimos and environment of Mars (PADME), which would place an orbiter in Mars orbit by 2021 to study Phobos and Deimos. A heritage concept called OSIRIS-REx 2 is envisaged to use parts of the first OSIRIS-REx (a sample return mission to 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 Martain satellites.
In 2015, Japanese Aerospace Exploration (JAXA) unveiled its concept for a sample return mission to Phobos known as Martian Moons Exploration (MMX). This mission would fly over Deimos before landing on Phobos several times to obtain samples – similar to what the Hayabusa2 mission carried out on the asteroid Ryugu. This mission is international in scope, with contributions from NASA and ESA, 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, ESA began evaluating a sample return mission to Phobos called Phootprint, which was scheduled to launch by 2024. This mission was originally conceived as a collaborative effort between ESA and Roscosmos, who have since ended their cooperation agreements in reason of Russia. invasion of Ukraine.
From all these proposals, it is clear that several 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 larger effort to explore and characterize Mars. In the meantime, the debate continues!
Further reading: arXiv
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