What are the satellites of Mars?

Mars satellites, Phobos and Deimos, are an attributable part of its fascination with scientists: strange, tiny and unexplained in origin.

Since space missions have put Mars in the spotlight, the red planet has become a popular 'star' even outside the scientific community. Recent filmography bears witness to this. Part of the success, however, is also attributable to the two mysterious objects that gravitate in its orbit: the moons of Mars, in fact, are a 'funny mystery' that scientists have been trying for decades to interpret.

Let's talk about Phobos and Deimos, two words that in Greek mean "fear" and "terror" respectively. In fact, these Lilliputian celestial bodies are better suited to generate sympathy rather than panic. Let's find out why.

How many moons does Mars have and what do they look like?

As mentioned, the moons of Mars are Phobos, a tuber-like space body just over 27 kilometers in diameter, and Deimos about 14.5 kilometers wide. Why are they so intriguing? First, because they are still unexplored. What we know about them comes from observations made by Martian Rovers that took photos of the two moons, fortuitously finding themselves in the right place at the right time. These shots allowed us to take a mug shot, particularly of the larger Phobos: an irregularly shaped mass with a large crater and multiple scratches on its surface.

But nothing we know about their composition, which makes them super intriguing. In fact, scientists look with hope and attention to the space agency of Japan (JAXA) that in 2024 is ready for the mission "Martian Moons eXploration" (MMX), which will have as its objective the collection of rock samples on Phobos and their chemical analysis.

If the mission is successful we may be able to solve the most fascinating of the mysteries about the satellites of Mars: their origin. The two moons in fact seem to be asteroids alien to the red planet, yet they behave as part of the early history of Mars, that is, as real satellites.

The origin of Phobos

The most recent calculations would show that Phobos was once as much as 20 times larger in mass. The hypothesis in this regard suggests that it drifted spaceward toward Mars, breaking up into scattered material that would have largely rained down on Mars. The remnant of that material, left in the orbital ring, would then have clumped into the moon we know today just 200 million years ago (however much older the constituent matter is!). This cycle would have repeated itself more than once over billions of years, and each time Phobos would become smaller upon completion of the process.

The Mystery of Mars' Satellites

Going into more detail about the origin of Phobos and Deimos, a few things don't add up. Let's start by saying that remote observations can't reveal anything about their peculiar characteristics and mineral composition. Without such data, it is ultimately impossible to make claims about how they formed. Nevertheless, scientists, through the study of orbital behavior, have been able to hazard some hypotheses.

The most credited is that the moons of Mars are a cluster of debris formed by asteroids captured by the force of attraction and then remained trapped in the orbit of the planet. A patchwork of space debris from much larger bodies, then shattered by gravitational force, that have been circling Mars ever since.

But it is precisely the study of how Phobos and Deimos spin that undermines that hypothesis: the two moons orbit Mars' equator in a clean and orderly circular fashion. If they were asteroids, it would be very difficult to see them regularly wrapped in such a symmetrical, circular orbit. In addition, Mars has one-tenth the mass of Earth and exerts a gravitational pull that is not so strong that it would be considered highly likely to catch asteroids whizzing by in its vicinity.

The distance between the moons and the planet also does not fit the asteroid hypothesis: if they were formed after a colossal impact and were the result of residual debris, Deimos should orbit closer to Mars than it actually does. In conclusion, reconciling visual observation with the study of masses and orbits is currently a headache without a solution.