Phobos, moon of Mars

Today I'd like to tell you about the planet Mars' largest moon, Phobos.

The red planet has two small moons locked in orbit around it - Phobos and Deimos. Of the two, Phobos is the larger and lower altitude moon. This tiny moon resembles a moderate sized asteroid, with a corresponding low mass and low surface gravity.

As a possible target for future manned and unmanned missions to the Mars system, this little moon is of great interest to many researchers.

Close-up view of Phobos' scarred surface
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Orbit of Phobos

Unlike asteroids, Phobos is trapped in a permanent orbit around Mars. Like almost all solar system moons, Phobos occupies a mostly circular orbit (eccentricity ~ 0.01). It passes over the planet below just under 10,000 kilometers above the surface, which is only about one third of the distance between us and the geostationary satellites that provide us with satellite internet. This low altitude orbit means that if you were to stand on Phobos, you would see Mars take up a massive portion of the sky. It would be quite an incredible sight.

Relative size of Phobos' and Deimos' orbits
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Despite being quite tiny, both Phobos and Deimos can be easily seen from the surface of Mars. The Mars Curiosity rover spacecraft has sent back several images of the moon from Martian surface itself, including this one:

Image of both Phobos and Deimos, taken by the Curiosity Mars rover.
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Phobos is, perhaps, a little too close to Mars. Every year, Phobos inches ever closer to Mars due to tidal forces. Tidal forces are simply a small part of the gravitational force on an orbiting object that come from the fact that neither body is a perfect point mass. The end effect of tidal forces is, in this case, causing Phobos to slowly spiral into the planet. This change is totally insignificant on timescales we care about, but over longer periods of time, Phobos will draw nearer and nearer to the surface of Mars.

As Phobos gets closer to Mars, it will approach its Roche Limit. The Roche limit determines when an orbiting object, held together only by its own gravity, will be ripped apart by tidal forces. While a satellite flying over Earth is held together by a well-built structure, moons like Phobos are held together by their own gravity. For something as small as Phobos (only about 20 kilometers across), this gravitational attraction isn't terribly strong.

Once a moon in orbit around a planet drops below its Roche limit (given as an orbital altitude), the tidal gravitational forces from the central planet become strong enough that the moon can no longer be held together. The moon disintegrates, forming a planetary ring - much like the one Saturn has. If our own Moon was brought as close to Earth as Phobos is to Mars, it would be past its Roche limit, and the entire moon would be destroyed, forming one of the most spectacular rings in the solar system, and entirely decimating the grey world that hangs above our heads.

In Phobos' case, scientists predict that in several dozen million years, Phobos will pass its Roche limit and be destroyed by the tidal forces of Mars, reducing one of Mars' moons to an orbiting pile of rubble that would form a beautiful ring system. However, that's not going to happen for quite some time.

Rendition of what Mars could look like once Phobos is annihilated. Imagine looking at this beautiful sight from Earth!
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It's also possible that Phobos would simply crash into Mars, in which case it would leave behind a massive crater and little else. Either way, Phobos' low altitude orbit spells near-certain doom in the distant future.

Surface of Phobos

Phobos is quite tiny. Earth is over 12,000 kilometers across. Phobos is only about 20 kilometers apart (wider or thinner in some places since Phobos is not spherical). This tiny, lumpy world looks a lot like an asteroid. This small size means a small mass, and therefore a very small surface gravitational acceleration. On Earth, everything falls towards the ground accelerating at just under 10 meters per second, per second. On Phobos, things would only fall about 0.005 m/s^2. This means that a person standing on Phobos would feel almost 200 times lighter than on Earth. Something with a mass of 100 kilograms standing on Phobos would feel as if they had a mass of just 500 grams while standing on Earth (remember, only the weight changes, not the actual mass). Needless to say, there's effectively no significant gravitational acceleration here (excluding the still very significant acceleration due to Mars that you don't feel due to being in orbit). Stuff just wouldn't weigh much at all.

Map of Phobos produced by the United States Geological Survey
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The surface of Phobos is covered in impact craters of various sizes, indicating that a large number of meteoroids and small asteroids have met their final fate on the surface of Phobos. The largest crater by far is Stickney crater, a huge 9 kilometer wide crater almost half the width of the moon itself. This crater is absolutely enormous when considering how big it is compared to the size of Phobos itself, and the impact that produced likely almost took out the moon itself.

Stickney crater
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The surface of Phobos is likely a fine dusty substance, clearly visible in the pictures of the craters on the first Phobos image in this post. The moon itself isn't that dense, lending clues to its composition. Unfortunately, not a huge amount if known about the surface itself. The only mission to attempt to land on Phobos, the Russian Fobos-Grunt mission, failed to even reach Phobos itself. As such, nothing has ever landed here, and no from-the-surface images exist.

Several mysteries lie on Phobos. For one, the surface is scarred by long cracks or trenches that span almost the entire moon. Hypotheses for why these trenches exist range from leftovers from the Stickney impact to glancing blows with orbital rock debris to tidal forces from Mars.

There's a also a really weird looking rock on the surface of Mars, spotted by an orbiter. This rock resembles a small pillar sticking out of the Phobos dirt. Most sources call it a monolith, but I think this is way too suggestive that the rock is artificial in nature. Almost certainly, the rock is just that: A weird rock. Still, I would love to have a lander down there to get a closer look.

Phobos "monolith" rock on the surface
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Although numerous orbiters have gotten good views of the small surface of Phobos, the lack of lander spacecraft still makes this miniature world largely unexplored. I'd love to see what it looks like down there, and we likely will within the next few decades, as Mars is one of the few planets that's not ridiculously far away (although it still takes around 9 months to reach it!). But luckily, we've still got several million years to explore this place before it ceases to exist and is ripped apart by its home planet Mars.

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Thanks for reading!

Sources Utilized:
NASA: Phobos slowly falling apart
Berkely: Mars to lose its large moon, but gain a ring
Lawrence-Livermore/AstronomyNow: Stickney Crater Formation
BBC: Huge monolith of Phobos
Phobos Wikipedia Entry
ESA: Phobos