The Japan Aerospace Exploration Agency (JAXA) has announced a mission to visit the two moons of Mars and return a rock sample to Earth. It's a plan to uncover both the mystery of the moons' creation and, perhaps, how life began in our Solar System.
The Solar System's planets take their names from ancient Greek and Roman mythology. Mars is the god of war, while the red planet's two moons are named for the deity's twin sons: Deimos (meaning panic) and Phobos (fear).
Unlike our own Moon, Phobos and Deimos are tiny. Phobos has an average diameter of 22.2km, while Deimos measures an even smaller 13km. Neither moon is on a stable orbit, with Deimos slowly moving away from Mars while Phobos will hit the Martian surface in around 20 million years.
The small size of the two satellites makes their gravity too weak to pull the moons in spheres. Instead, the pair have the irregular, lumpy structure of asteroids. This has led to a major question about their formation: were these moons formed from Mars or are they actually captured asteroids?
Impact or capture?
Our own Moon is thought to have formed when a Mars-sized object hit the early Earth. Material from the collision was flung into the Earth's orbit to coalesce into our Moon.
A similar event could have produced Phobos and Deimos. The terrestrial planets were subjected to a rain of impacts during the final throes of Solar System formation.
Mars shows possible evidence of one such major impact, as the planet's northern hemisphere is sunk an average of 5.5km lower than the southern terrain. Debris from this or other impacts could have given birth to the moons.
Alternatively, Phobos and Deimos could be asteroids that were scattered inwards from the asteroid belt by the looming gravitational influence of Jupiter. Snagged by Mars's gravity, the planet could have stolen its two moons. This mechanism is how Neptune acquired its moon, Triton, which is thought to have once been a Kuiper belt object, like Pluto.
There are compelling arguments for both the #TeamImpact and #TeamCapture scenario.
The orbits of the two moons are circular and in the plane of Mars's own rotation. While the chance of this happening during a capture event are 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 speciality. 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 metre.
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.
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 analysing 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.
But moons captured from the asteroid belt would be kin to the meteorites that rained down on the young Earth. These are thought to be the deliverers of our oceans and even our first organic molecules. A sample from Phobos would be a taste of the package that was flung around the early Solar System.
Phobos's ever decreasing distance to Mars also means that the top layer of the moon's soil should be littered with meteorites scattered from the planet. Such a short journey would allow much lower-density material to survive the trip, unlike the Martian meteorites that manage to reach Earth.
This transferred material will also be from all over Mars, rather than the small patch that rovers have examined. And it might result in a more complete picture of Mars, as well as of its moons.
MMX is an exciting mission, bringing information about moon formation, Mars and water delivery around our Sun. As we wait for 2024, are you voting for #TeamImpact or #TeamCapture?