Mars might be smaller than our own planet Earth, but it hosts two moons as opposed to one. These are Phobos and Deimos and are the only other moons in the inner Solar System, yet remarkedly little is known about them. The existence of Phobos and Deimos remained hypothetical until August 1877 when Asaph Hall and American Astronomer finally found them within five days of one another. Upon discovery it became obvious why they had not been seen previously. They were both extremely small, with diameters of just ~12.4 km (Deimos) and ~22.5 km (Phobos), making Phobos the same size as the distance between Milton Keynes and Bedford. Furthermore, their surfaces were very dark and Phobos, in particular, orbits extremely close to Mars, making these objects very hard to spot.
In fact, Phobos orbits so close to Mars that it lies within the synchronous orbit, meaning that it orbits faster than the red planet spins, resulting in it rising in the west three times every day. Phobos is also in a degrading orbit, which takes it closer to Mars every year, and closer to the Roche Limit ̶ the distance in which a moon would disintegrate under a planet’s gravity. Within 50 million years Phobos could be completely destroyed!
Phobos’ surface is quite ordinary compared to other moons in the Solar System. It is irregularly shaped, unlike our own round moon, and has one of the darkest surfaces in the Solar System. These features led scientists to suggest that Phobos was an asteroid that had been captured by the gravity of Mars. More recent spectral investigation by spacecraft such as Mars Global Surveyor, Mars Express and Mars Reconnaissance Orbiter have shown that Phobos’ surface corresponds well with some asteroid types, namely D- or T-type asteroids, as well as carbonaceous chondrite meteorites that originate from asteroids.
However, Phobos resides in a near-circular equatorial orbit, one which is directly above Mars’ Equator. This is inconsistent with the orbit of a captured asteroid, which would more likely occupy an elliptical and near-heliocentric orbit, one which is on the same plane as the Solar System rather than tilted ~25 degrees to match the axial tilt of Mars. It would be a rather complex and unlikely orbital dynamics scenario for a captured asteroid to have transitioned from this capture orbit to the one we observe today.
This led scientists to suggest alternative hypothesises, such as the formation of the moons in-situ. Firstly, Phobos and Deimos could represent the last two fragments of an earlier parent moon that was broken apart on entering the Roche limit. But this doesn’t seem to be consistent with Deimos orbiting much further away.
Scientists wonder whether life once existed on Mars. If it did then they hope that it left behind so-called ‘biomarkers’ of its existence on the surface.
Secondly, Phobos and Deimos could have formed from a debris disk that remained after the formation of Mars, but this is inconsistent with the difference in composition between the two moons and Mars.
Lastly, they could represent re-accretion products from a debris disk made of rocky material that was ejected from the surface of Mars following a giant impact. This scenario has recently been investigated and it has been suggested that a giant impact by a large asteroid similar in size to Vesta, one of the largest objects in the asteroid belt, could have provided sufficient material to create Phobos and Deimos.
As we can see, there are several possible formation mechanisms for Phobos and Deimos, each with their caveats. To date, remote sensing observations, which remain the only source of evidence to inform us on the likely origin mechanism (since we haven’t sent landers or humans to Mars’ moons yet) have only made the story more unclear. This highlights the need for a future in-situ mission to analyse material on the surface to shed light on this conundrum.
The Japanese Space Agency (JAXA), along with the space agencies of France and Germany, are looking to do just that! Their Martian Moons eXploration (MMX) mission, set for launch in 2024, is looking to put a microwave-sized rover on the surface of Phobos to investigate its composition. It will be followed by a lander that will scoop up a sample from the surface, as well as take a core of the subsurface, returning these samples back to Earth for further analysis in around 2029. This mission is likely to revolutionise our understanding of inner Solar System dynamics and might finally put to bed the argument about the origin of Phobos.
As a bonus, these samples might even contain evidence of past Martian life! It is thought that large impacts on Mars could throw up material into space that eventually ends up raining down on Phobos. Scientists wonder whether life once existed on Mars. If it did then they hope that it left behind so-called ‘biomarkers’ of its existence on the surface – preserved biological evidence of life. If one of these large impacts occurred in a region where life had developed on Mars, then such biomarkers could be transported and deposited on Phobos, waiting to be found by future missions.
For such a small body, Phobos has much more to offer than first meets the eye.