The MMX probe will visit the moons of Mars
JAXA
The mystery of how Mars acquired its moons Phobos and Deimos may begin to unravel in 2026 with the launch of a spacecraft that will eventually bring a piece of Phobos back to Earth.
“We are sure of the origin of Earth’s Moon, but we don’t know how Phobos and Deimos got there,” says Emilia Branagan-Harris at the Natural History Museum in London. “Understanding the origins of Phobos and Deimos and how they came to orbit Mars can hopefully tell us a little about the evolution of Mars in general and its history.”
There are two competing hypotheses on how these moons came to orbit Mars: The Red Planet could have captured them as a pair of asteroids that either connected and then separated, or orbited close together, or they could have been formed by an asteroid colliding with Mars itself, like Earth's moon was formed.
We have limited evidence for either scenario so far, but the Japan Aerospace Exploration Agency's Martian Moons eXploration (MMX) spacecraft, which will launch sometime after April 2026, should be able to definitively rule out either scenario, Branagan-Harris says. The spacecraft is equipped with an array of cameras and spectrometers it can use as it orbits the moons it plans to reach in 2027, as well as a rover it will send to the surface of Phobos to collect samples.
If observations reveal an abundance of carbon-rich molecules and water, it could indicate that the asteroid capture theory is correct. But if they are missing, then we may have to wait until the samples are returned to Earth for analysis, which is currently scheduled for 2031.
These samples will consist of rocks both from the surface of Phobos and several centimeters into the ground. Once we can test the material itself, we will be able to see if it shows signs of melting in the past and infer whether it resulted from an impact on the Martian surface.
Regardless of Phobos's origins, it orbits close enough to Mars that it may contain well-preserved samples from the planet earlier in its history. “There is a possibility that Phobos may contain parts of ancient Mars when it had liquid water, so we will also learn a lot about the history of Mars,” says Branagan-Harris.
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