Now the Japanese space agency JAXA has approved its own mission to Phobos and Deimos, scheduled to launch from the Tanegashima Space Center in September 2024.
The Martian Moons eXploration (MMX) spacecraft will arrive at Mars in August 2025 and spend the next three years exploring the two moons and the environment around Mars. During this time, the spacecraft will drop to the surface of one of the moons and collect a sample to bring back to Earth. Probe and sample are scheduled to return to Earth in the summer of 2029.
Mars takes its name from the god of war in ancient Greek and Roman mythology. The Greek god Ares became Mars in the Roman adaptation of the deities. Mars’s two moons are named for Phobos and Deimos; in legend the twin sons of Ares who personified fear and panic.
Today, what the moons together personify is a compelling mystery, one regarding how in reality they came to be.
Both Martian moons are small, with Phobos’s average diameter measuring 22.2km, while the even smaller Deimos has an average size of just 13km. This makes even Phobos’s surface area only comparable to that of Tokyo. Their diminutive proportions means that the moons resemble asteroids, with irregular structures due to their gravity being too weak to pull them into spheres.
This leads to the question that has inspired a long-running debate: Were Phobos and Deimos formed during an impact with Mars, or are they asteroids that have been captured by Mars’s gravity?
Our own Moon is thought to have been created when a Mars-sized body slammed into the early Earth. Debris from the collision was thrown into the Earth’s orbit where is coalesced into our only natural satellite.
A similar scenario is possible for Phobos and Deimos. In the late stages of our Solar System’s formation, giant impacts such as the one that struck the Earth were relatively common.
Mars shows possible evidence for one such collision with a body the size of our Moon. The Martian surface has a dichotomy consistent with such an event, with the northern hemisphere sitting an average of 5.5km lower than the southern side. During such impacts, debris could have been thrown up from the Martian surface to birth the two moons.
An alternative scenario is that the resemblance of Phobos and Deimos to asteroids is not coincidental.
The two moons may have originally been part of the asteroid belt; a band of rocky left-overs from the planet formation process that circle the Sun between Mars and Jupiter. Scattered inwards towards the Sun during a chance collision, the asteroids may have been snagged into orbit by Mars’s gravity.
Observations of both moons suggest that their surface material is similar to that of other asteroids.
Disentangling these two possible births is the primary goal for the Martian Moons eXploration mission. If the moons were formed from the body of Mars itself, their rock type should resemble that of Mars. On the other hand, if the pair were captured then they would have formed in a different part of the Solar System with their own distinct composition.
Both options would reveal a great deal about the formation of our Solar System.
The young Mars is suspected to have been similar to the early Earth. If Phobos and Deimos formed during this time, the moons could be preserved time capsules of what conditions were like on the planets in this epoch. This would help us understand the formation of the Earth and maybe even the development of habitability on ancient Mars and Earth.
On the other hand, the Earth’s water is suspected to have been delivered to our planet after its formation by impacts from icy meteorites. This water delivery service may have originated in the asteroid belt (one rocky member of which is currently the destination of JAXA’s Hayabusa2).
If Phobos and Deimos are captured asteroids, they may be kin to the ice-packed rocks that hit the early Earth, revealing information about how volatiles were circulated about our Solar System.
The Martian Moons eXploration spacecraft is planning to study both moons and collect rock samples from Phobos. Phobos’s orbit takes it closer to Mars than Deimos, circling the planet at about 6,000 km above the surface. For comparison, the Moon is 385,000 km above the Earth.
This close proximity means that the surface of Phobos should have a loose layer of regolith or soil sprayed up from Mars during more recent impacts with meteorites. Samples taken from Phobos are therefore expected to contain Martian meteorites and the moon’s own material from deeper down.
This extra Martian regolith may be very different from the rocks that make it to Earth, such as the famous ALH84001 which was initially thought to contain a range of signs that life once existed in the rock. (The scientific consensus now is that the biosignatures can be explained as coming from processes other than life.)
The shorter journey to the close-by moon allows the transfer of lower density material that would never survive the trip to Earth. The regolith will also originate from all over the planet, rather than the small region of Mars that has been explored by landed rovers, providing a more wide spread sample than has previously been analyzed.
Excitement for the science a Mars moon mission could bring has led to strong international involvement. On April 10th this year, the president of the French space agency, CNES, visited the president of JAXA in Tokyo for a signing ceremony that formalized the agreement between the two agencies. CNES will be developing one of the key instruments for the mission as well as adding their expertise on flight dynamics for the tricky maneuvers around the Martian moons.
The planned French instrument is a Near-Infrared Spectrometer (NIRS), which combines a high-resolution infrared camera with the ability to analyze each pixel to determine the composition of the rock. Similar instruments have previously flown on ESA’s Mars Express and ExoMars, but with an image pixel size an order of magnitude larger than that now planned for the Martian Moons eXploration mission. The French space agency will also explore the possibility of providing a rover to explore the surface of Phobos on microscopic scales.
There are also plans for an instrument to be developed by NASA, which has put out an official “Announcement of Opportunity” inviting proposals for the instrument design. This would be a neutron and Gamma ray spectrometer, which probes the abundance of individual elements in the moons, rather than their combination within minerals that NIRS can see.
This will be JAXA’s third mission to sample material from a small body. Hayabusa visited the asteroid Itokawa, bringing back surface material to Earth from the asteroid in seven years ago this month. Its successor, Hayabusa2, is currently traveling to asteroid Ryugu and is expected to arrive in 2018.
With only 1/1000th of the gravity on Earth, landing on Phobos is a challenging task. But if samples can be collected for return to Earth, that will be a major scientific and engineering accomplishment.