Moon Monday: Mars’ Mysterious Moons Phobos & Deimos

Phobos and Deimos NASA

Mysterious moons: Martian moons Phobos (left) and Deimos (right).

Every reason why these moons should spark your curiosity

By Mélissa M Azombo

The red planet has been the subject of our curiosity for decades, now.  It has sparked the imagination of writers with sci-fi alien invasion film Mars Attacks and H.G. Wells’ War Of The Worlds novel.


Pictured: A view of the red planet, Mars.

In the real world, multiple efforts have been made to discover the origins, composition and potential for life of Mars’ surface. With 26 missions to its name (plus 30 failed missions), these investigations continue. Launched in November 2011, the Mars Curiosity rover was sent to determine if the planet had ever been able to support microbial life. We learnt more about the red surface, as the rover mission graced us with images of its polar ice caps and evidence of an ancient oasis. With plans for a future human settlement on our neighbouring planet, these studies will continue to teach us more about Mars. Meanwhile, its 2 moons Phobos and Deimos remain a mystery. So, what do we know and what has yet to be discovered?

Phobos and Deimos in orbi around Mars

Revolving Moons: A diagram of Martian moons Phobos and Deimos in their respective orbits around Mars. Credit:

Mars, named after the Greek god of war has 2 moons named Phobos & Deimos, meaning panic and flight. They were discovered by American astronomer Asaph Hall in August 1877 at the Naval Observatory in Washington D.C.. Phobos orbits Mars every 7.39 hours at a distance of 5989 km from Mars and Deimos orbits the planet every 30 hours 4 times as far away at a distance of 23460 km. For comparison, the Moon orbits the Earth every 28 days located 384400 km away. So, both of Mars’ moons are much closer to their host planet. In fact, Phobos’ orbit is closer than any moon in the solar system. As a result, for an observer at the polar region of Mars, the curvature of the planet would obscure the view of Phobos.


A Martian Companion: Don’t panic! It’s just Phobos. Credit: ESA

Phobos, the larger of the two moons,is thought to have dimensions of 13 x 11.39 x 9.07 km. The non-spherical object has a heavily cratered surface. Scientists believe this could be as a result of impacts with other meteors or ejecta (debris) from the Martian surface below.


Taking flight: Deimos has a smoother appearance and is the smallest and furthest away Moon. Credit: ESA

Deimos, the smaller of the 2 moons has a 6 km radius. While it is too light for gravity to squash it into a sphere, it has a more uniform shape and is nearer to spherical than Phobos. The asteroid of mass 1.8 x 1015 kg and dimensions also boasts a heavily cratered surface, with some very visible deep circular red rings and white rings at the polar regions. Its largest crater is about 2.3 km in diameter.

Despite being heavily cratered like Phobos, Deimos manages to look smoother partly due to its craters being filled with dust. Due to its mass, Deimos is too light to hold on to the dust produced by impacts. So, it is the gravity from the Martian surface below that keeps a ring of the dust around itself in about the same region as Deimos’ orbit. As the moon revolves, this dust is deposited over its craters. This means that using craters to calculate its age accurately would prove very difficult.


However, both moons have been estimated to be 4.5 billion years old. Their formation history and how they came to be in Mars’ orbit is still up for debate, but scientists have some ideas.

Phobos may have originated from elsewhere in the solar system, before passing near enough to Mars to be captured by its gravitational pull, unable to escape its orbit. One theory is that it is an object from the asteroid belt, that Jupiter’s gravitational force catapulted into Mars’ orbit. The Phobos 2 mission supported this idea.

However, missions since suggest Phobos formed around Mars. Mars Express revealed the density of Phobos does not fit the capture scenario, because it is likely to have lots of interior space. Instead, Phobos will have formed from Mars’ ejecta, or from a collision between a previous Martian moon and another body. Scientists are not yet ruling out that Phobos was captured, but its near-circular orbit and composition from primitive materials, do point towards formation at Mars.

Deimos could be a captured asteroid.

Due to how comparatively small Mars’ moons are, they have very little effects on each other and Mars. In fact, Phobos creates a tiny tidal bulge on Mars of only a fraction of a millimetre.

Phobos is actually moving closer to the red planet by around 1.8 cm a year. Within 100 million years, it will crash onto Martian surface, creating a shower of debris, as it is ripped apart by the gravitational pull of the red planet. Some of this debris might also come together to form a ring around Mars.

Deimos is doing the opposite and is moving away from Mars. It will continue to do so until it is too far away for Mars’ gravitational pull to retain it in its orbit, leaving Mars moonless.

Before then, we have a lot more to discover about these moons. Scientists think Deimos is carbon-rich, with a mixture of ice and captured asteroids but we still need to know more about its interior. As both moons are too light to hang onto an atmosphere to protect them from the Sun’s UV radiation and impacts, we don’t expect life to be possible on these moons. However, analysing their surfaces more accurately may allow us to confirm how they formed. Deimos’ orbital path and formation history have yet to be uncovered.

The Phobos-Soil mission was set to collect information about the rotation, orbit and composition of Phobos but failed to launch in 2011.

Future missions like Japan’s MMX in 2024 that will collect samples from Phobos and observe both moons, will shed more light on the Martian satellites. It might even uncover just by how much Deimos is moving away from Mars per year, and exactly when we should expect it to disappear from Mars’ orbit.