So How Did Mars Get Its Moons?

It’s slightly different behind the scenes this week as I have had a request to do a write up on a specific paper, which saves me a job because it can take me as long to decide what to write on as it does to do the actual writing. Having said that this paper is a bit more technical than what I would normally do so let’s see how it goes.

The paper can be found here (open access) and was presented at the 47th Lunar and Planetary Science Conference in Houston, Texas just before easter. It concerns the creation of the two moons of Mars, Phobos and Deimos, the potential demise of which I wrote about back in November. Surprisingly little is known about how these two little moons came to be where they are. Phobos, the larger and nearer to Mars of the two, is only about the size of London, isn’t especially spherical and looks like it’s been caved in on one side. Deimos is about half as small again but orbits five times further out.

Phobos. Image courtesy of NASA.

Given these unusual characteristics it has been suggested that both were captured asteroids that strayed too close to Mars, but given that they have nearly perfectly circular orbits this seems unlikely. If they had been captured we would expect them to be far more elliptical. Satellites with circular orbits would normally form from an accretion disc orbiting the planet. This is how we got our moon, at least that’s what the leading theory says. An object about the size of Mars smashed into us around 4 billion years ago and nearly blew us to smithereens. The debris kicked up by the impact is what went on to form the moon. Further evidence that this may have been the case on Mars is that a day on Mars is very short, 25 hours. Mars and us here on Earth have relatively short days compared to our celestial brethren Venus and Mercury. On Mercury a day lasts for 58 Earth days and a day on Venus is so long that it has less than 2 per Venusian year. Being smashed into by a giant rock would give just the right amount of kick to boost the spin of a planet and give it shorter days. It seems likely, then, that Mars suffered such an impact in the past and that an accretion disc would have been present from which to form a couple of moons.

There are problems with this hypothesis, however; moons formed from such a cataclysmic event would be far larger than Phobos is, orders of magnitude larger. Plus, Phobos has a crazy fast orbit of only about 7 hours meaning that if you were stood on the surface of Mars you would see it rise in the west and set in the east twice each day. This isn’t congruent with Phobos having formed from a disc of material orbiting Mars.

To get through the rest of this paper we’re going to need to understand what the Roche Limit is. Named after Edouard Roche who first proposed it in the 1840s, the Roche Limit is the distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body’s tidal forces exceeding the first body’s gravitational self-attraction. Put another way, it is how close a moon can get to a planet before it is torn to pieces by the gravity of the planet. Something very cool happens as a body starts to move within the Roche Limit: because it is now at the point where the pull from the planet is starting to exceed the pull from the moon, the regolith on the surface would start to float away from the moon. All the dust, gravel and other debris on the surface, and you along with it, would literally just start to levitate away.

If none of that makes sense look at the illustration below. As the moon approaches the Roche Limit it starts to get stretched; as it passes the limit it is literally torn apart and the debris begins to form a ring system just like you get on the outer planets, most notably, Saturn.


It is theorised that the larger moon(s) created by the accretion disc were subject to just such a fate though if that were the case where is the ring system? The authors do not address this point. The main meat of the paper is that the authors ran simulations of an accretion disc that spread beyond the Roche Limit to see if it’s likely that this could have led to the survival of two little moons.

They first ran simulations with impacts of various speed and differing angles of impact to see which would give them a 25 hour day; it turns out quite a few different combinations will yield realistic results. They were also able to use some fancy pants maths far beyond my understanding to find out that a portion of the ring material (10-20%) would be far enough away from Mars to be beyond the Roche Limit.

Within a million years of the impact between 5 and 10 moons have formed outside of the Roche Limit but their orbits are not stable. Two moons tend to fall into synch with each other which pushes their orbits out further from Mars and clears all the debris as they go, thus hoovering up the material that would be needed to form Phobos and Deimos. Not good.

10 million years into the simulation and things have gotten worse. The remaining moons have collided and ultimately formed 2 large moons about 100 times larger than Phobos is, but these have strayed within the Roche Limit, evolve inwards and are lost. Essentially the results of the simulation mean that it is impossible (given the parameters of their simulation) for Phobos and Deimos to have formed in such a way from an accretion disc. The authors have a couple of ideas for tweaks they might make to the simulation that could increase the likelihood of an endpoint nearer to real life and are actively investigating them, but sadly their best shot failed.

This doesn’t completely rule out the possibility that Mars’ moons coalesced as opposed to being captured, but it does seem to imply that neither scenario is very satisfactory. In a nutshell: we just don’t know. Which is where we were in the first place.

For those of you who have read this far I salute you. You really are the hardcore. If you are part of that brave band then you can comment/tweet/message/shout me the secret code word to let me know you made it to the end and as a reward my esteem for you will be raised. The code word is: regolith.


One thought on “So How Did Mars Get Its Moons?

  1. Fascinating subject; astronomy was my first love, and I’ve seen much of this information in the past; I had not known of these latest simulations. As you noted, it seems to indicate there is still a lot to be learned in our first science, which, when one thinks about it, can be said of every scientific field humanity has engaged in exploring… there is still more we don’t know than there is we do… which means lots more to learn…

    I’d say we should begin with learning to understand, accept, and learn to handle our own nature…. then, finding out the secrets of a regolith will naturally become more accessible to our explorations…

    gigoid, the dubious

    Liked by 1 person

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