The Violent History of the Martian Moons

Mars is a funny little place, but it seems that Mars orbit may be even funnier. As we all know, Mars has two small and lumpy moons which would be no more than a pair of mundane asteroids if they weren’t orbiting Mars. Phobos and Deimos. But this seems to be only their latest incarnation.

No one’s entirely sure about the origin of the two potatoes in orbit around Mars. Sharing their planet’s orbital space with 14 satellites we’ve sent there, many people believe that they were captured main belt asteroids. They do seem to share a lot in common with C-type and D-type asteroids, but their orbits seem a little too perfect. They’re quite close to being circular, and both lie perfectly in Mars’ equatorial plane. Orbital drag and tidal forces can do this, but with Mars’ low gravity and thin atmosphere, it’s difficult to make this theory work. Additionally, infrared spectra of Phobos and Mars show very similar phyllosilicate minerals, suggesting a common origin.

A slightly more interesting possibility is that Mars may once have had rings, a little like Saturn, and Phobos and Deimos coalesced out of them. This may sound outlandish, but it’s a similar hypothesis to where we think Earth’s moon came from (with the big difference being that the Earth-Moon system is huge compared to the Mars system). And even asteroids can have rings, under the right circumstances!

There’s certainly evidence for a huge impact on Mars sometime in the past. The Martian Dichotomy is the name given to the interesting fact that Mars’ northern hemisphere is dramatically different to its southern hemisphere. The northern hemisphere is 1-3 km lower in elevation, and has a crust nearly half as thick as the crust in the south. At least one theory is that the entire northern half of Mars is, essentially, a huge impact basin.

An impact big enough to produce that kind of effect would be big enough to knock a lot of material into orbit. Some believe that this caused Mars’ orbit to have once been full of Phobos/Deimos-like objects. Others think it may have caused a ring system to form.

Similarly to Earth, a ring system like that is thought to have formed into a large moon. Unlike Earth, that wasn’t the end of the story.

In celestial mechanics, any object orbiting another has what’s known as a Roche limit. Any orbiting object which falls within this limit is destined to be torn apart by gravity. It’s described conveniently by this simple little equation;

where Rm is the radius of the secondary, Mm is the mass of the secondary, and MM is the mass of the primary. The equation gives d, the distance at which the secondary starts to be torn apart by gravity. In this case, the primary would be Mars, and the secondary would be its ill-fated moon.

So, one hypothesis goes that this former moon of Mars was ultimately shredded by the gravity of its parent planet. In fact, the process seems to still be occurring. After who-only-knows how many moons have been formed and crushed, Phobos and Deimos are all that remains, but Phobos is probably doomed to the same ultimate fate.

Deimos, on the other hand, is probably safe. It’s a lot further away. Any planetary orbit also has a radius called a synchronous orbit. Around Earth, we know this as a geostationary orbit – the point at which an orbiting satellite will always be staring at the same part of Earth’s surface. But there’s an interesting ramification of synchronous orbits. They’re the most stable kind. Inside this point, an orbit is destined to eventually decay, while objects outside will gradually orbit further away due to centrifugal force, and may ultimately be lost altogether.

Phobos sits, doomed, inside a synchronous orbit, while Deimos sits outside where it will eventually be free to leave. Similar situations in the past might explain why Phobos and Deimos are all that remains of the moon that once was.

Assuming, of course, that this hypothesis is true. It’s very difficult to say, until we find an errant asteroid which happens to match the composition of the current Martian moons. Additionally, while a ring of dust and small objects has been predicted to exist between Phobos and Deimos, no evidence has ever been found for such a ring.

Either way, the idea of a small, terrestrial planet like Mars having a ring system is rather pleasing!


Heard via Gizmodo.

Gif animation/video clip of Phobos eclipsing Deimos, as seen by NASA Curiosity.
Photomanipulations created by @InvaderXan/ using
• Comanche Outcrop observed by NASA Spirit,
• Phobos observed by NASA MRO,
Deimos observed by NASA MRO,
• Dione observed by NASA/ESA Cassini, and
• Mars observed by ISRO Mangalyaan.

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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1 Response to The Violent History of the Martian Moons

  1. Shantanu says:

    I loved the post!

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