Japanese space agency’s mission to discover how the moons of Mars were formed
Phobos and Deimos, unlike our own Moon, are small. Phobos measures 22.2km in diameter, while Deimos is even smaller at 13km. Deimos is slowly moving away, while Phobos will reach the surface of Mars in about 20 million years.
It is impossible to make the moon’s spheres because the gravity of the two small satellites is too weak. The pair instead have the irregular, lumpy shape of asteroids. This has raised a question: do asteroids capture these moons, or did Mars form them?
Impact or capture?
It is thought to have been formed by a Mars-sized asteroid hitting the early Earth. The collision hurled material into Earth’s orbit, where it coalesced into our Moon.
A similar event could have produced Phobos and Deimos. During the final stages of Solar System formation, the terrestrial planets experienced a torrent of impacts.
Mars may show evidence of such a major impact. The northern hemisphere of the planet is approximately 5.5km below the southern terrain. The moons could have been created by debris from the crash or another.
Jupiter’s gravitational pull could have pushed asteroids outwards, causing them to scatter. Mars could have stolen the moons by snagging them with its gravity. This is the mechanism by which Neptune acquired Triton. Triton is believed to have been once a Kuiper Belt object like Pluto.
There are compelling arguments for both the #TeamImpact and #TeamCapture scenarios.
Our own Moon is thought to have formed when a Mars-sized object hit the early Earth. REUTERS/Eliseo Fernandez/Reuters
The orbits of the two moons are circular and in the plane of Mars’s rotation. While the chance of this happening during a capture event is extremely low, observations of the moons suggest they may have a composition similar to that of other asteroids.
Definite determination of the moons’ composition would act as a fingerprint to distinguish the two models. A collision event should result in moons made from the same rock as Mars. But if the moons were captured, they would have formed in a different part of the Solar System with distinct minerals.
This is where the new mission comes in. JAXA’s Martian Moon eXploration Mission (MMX) is due to launch in September 2024 and arrive at Mars in August 2025. The spacecraft will then spend the next three years exploring the two moons and the environment around the red planet.
During this time, MMX will drop to the surface of Phobos and collect a sample to return to Earth in the summer of 2029.
Due to their weak gravity, collecting a sample from small rocky bodies is a difficult challenge. But this is JAXA’s specialty. The space agency has previously returned samples from asteroid Itokawa in 2010. The sequel to that mission, Hayabusa2, is due to arrive at asteroid Ryugu next year.
The excitement for a Mars moon mission has led to strong international involvement in MMX. On April 10, JAXA president Naoki Okumura met his counterpart from France’s Centre National d’Etudes Spatiales (CNES), Jean-Yves Le Gall.
The meeting cemented a collaboration between the two space agencies. CNES will provide an instrument for MMX as well as combining expertise on flight dynamics for the tricky encounter with the Martian moons.
The French instrument will combine a high-resolution infrared camera and spectrometer, a technique that analyses the composition of each image pixel. This will allow the rocks of the Martian moons to be investigated down to a few tenths of a meter.
With a pixel size an order of magnitude smaller than that of similar instruments on missions such as NASA’s Mars Reconnaissance Orbiter and ESA’s Mars Express, the spectrometer will also be able to help MMX select the best landing site on Phobos and take the sample.
Phobos and Deimos could be asteroids that were scattered inwards from the asteroid belt by the looming gravitational influence of Jupiter. NASA/JPL-Caltech/Handout/Reuters
CNES will also explore the possibility of building a rover to explore the surface of Phobos. A decision will be taken in November this year.
In addition to the collaboration with France, MMX will carry an instrument from NASA. While the CNES spectrometer will examine the type of minerals on the moons, the NASA instrument will pick out individual chemical elements. This is done by analyzing the high-energy Gamma rays and neutrons that are produced during the bombardment of cosmic rays from the Sun or more distant sources.
Together, these instruments will reveal a more thorough composition of Mars’s mysterious satellites.
Both #TeamCapture and #TeamImpact offer fantastic science. Moons formed from collisional debris would be preserved time capsules of conditions on the young Mars. In this early epoch, Mars and the Earth are expected to have been far more similar than now. A sample from this time could reveal how a planet becomes habitable.