Japan in planning to launch a mission to visit the two moons of Mars in 2024. The spacecraft will touchdown on the surface of Phobos, gathering a sample to bring back to Earth. But what is so important about a moon the size of a city?
Unlike the spherical shape of the Earth’s moon, the Martian moons resemble asteroids, with an asymmetric lumpy potato structure. This highlights one of the first mysteries about the pair: how did they form?
Light reflected from the moons’ surface gives clues to their composition, as different minerals absorb particular wavelengths of radiation. If an object reflects more light at longer wavelengths, it is said to have a spectra with a red slope. This is true of both Phobos and Deimos, which appear very dark in visible light but reflect more strongly in longer near-infrared wavelengths. It is also true of D-type asteroids, which orbit the sun in the outer edge of the asteroid belt that sits between Mars and Jupiter.
The similarities between both their lumpy shape and reflected light has led to speculation that the two moons are captured asteroids, snagged by Mars’s gravity after a collision in the asteroid belt scattered them towards the sun.
However, such a gravitational lasso would typically move the captured object onto an inclined or highly elliptical orbit. Neptune’s moon, Triton, is suspected to be captured as it orbits in the reverse direction to Neptune’s own spin and on a path tilted from the ice giant’s equator by 157 degrees.
Yet both Phobos and Deimos sit on near-circular orbits in the equatorial plane of the planet. This configuration suggests the moons may have been formed in a giant impact with Mars, which threw debris into orbit and this coalesced into the two moons.
This mystery will be one of the first tackled by Japan’s planned Martian Moons eXploration (MMX) mission, that is due to launch in the fiscal year of 2024. Onboard are multiple instruments designed to unpick the moons’ composition from close quarters, providing far more detailed information than that from distant reflected light.
If these moons are impact debris, their composition should be similar to Mars. Captured asteroids would show a more unique rocky formula.… Read more