The Star-Crossed Fates of Phobos and Deimos

The planet Mars was named after Mars, who to the Romans was the God of War. Its two moons, Phobos and Deimos, carry the names of the Greek Gods of fear (from which we get phobia) and terror, respectively.

However, despite the naming scheme that seems to be inspired by Death Metal, Phobos and Deimos are not very intimidating. The look much less like something menacing and much more like two misshapen, cosmic potatoes of especially low quality.

But we really shouldn’t be making too much fun of Phobos and Deimos. After all, they are doomed. “How can a moon be doomed?” viewers at home may be wondering. Here is how:

The Death of Phobos

From what we have observed about Phobos, it appears to be formed of many segments of rock weakly held together by gravity, coated by a thin crust. This loose conglomeration of rock does not fare well when tidal forces are applied. And unfortunately for Phobos, it is being subjected quite strong tidal forces, as it is much closer to Mars than Deimos. Moreover, it is being pulled in closer to Mars at a rate of 2 meters every hundreds years. At this rate, scientists expect that Phobos will either collide directly with Mars, or break up into a fancy planetary ring.

Neither of these options are very appealing for Phobos.

The Rejection of Deimos

Deimos is suffering (or will be suffering, at any rate) from the opposite problem that is afflicting Phobos. Tidal acceleration is slowly but surely increasing Deimos’ orbit, and eventually Mars will lose gravitational hold on Deimos. Deimos will then be sent to drift about the Solar System indefinitely, or until it crashes into something. It is thought (though there is not a consensus) that Deimos and Phobos are asteroids that were captured by Mars at some point in the past, so perhaps it would be fitting for Deimos to rejoin its friends in the asteroid belt. Still, one can’t help but feel for Deimos’ impending loss of glory, falling from the status of moon to lowly space rock.


4 thoughts on “The Star-Crossed Fates of Phobos and Deimos

  1. The first time I saw these two “potatoes,” I could hardly relate them with moons due to their shapes. Do they look so irregular because they do not have enough mass to form spherical shapes?
    Also, at the rate of 2 meters every hundreds years, it would take hundreds of millions of years for the Phobos to collide with Mars (if it does not unfortunately break up)! Maybe that’s a relief since the death of Phobos is still so far away.


  2. Wow, this is a fascinating post! I also wrote about Mars, but I talked about something entirely different (life on Mars). I didn’t realize that Mars’ moons were so potato-like, and I certainly didn’t know that they were doomed! Your post taught me something new.
    One question I have is how long it will take for Deimos to be sent out of Mars’ orbit. After all, our Moon is also gradually moving away from Earth, but the rate of separation is so slow that it will still be in orbit when the Sun engulfs both!


  3. One of the neat things about Phobos and Deimos is that they can be seen by the Mars rovers. In fact, Curiosity captured them eclipsing one other:

    I imagine they’re less exciting to see than our own Moon is, but I am fascinated that we go (or for now, send machines) to other planets and experience their moons. It really drives home the immensity of the feat of interplanetary travel!


  4. This was super interesting! I knew of these two moons, but I had no idea they were doomed. Your post has inspired me to learn more about the moons of other planets. I sure hope that Deimos can find some potato friends in the asteroid belt when it returns.


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