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Will Mars have a ring around it? Hank Green explains in this episode of Scishow Space News!

Hosted by: Hank Green
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Sources:
http://press.nature.com/wp-content/uploads/files/2015/11/ngeo2583-aop.pdf
http://www.space.com/20346-phobos-moon.html
http://redplanet.asu.edu/?p=2022
Mars is pretty cool already, I mean it's got liquid water and everything but wouldn't it be cooler if Mars had rings like Saturn. However, like all of the other rocky planets in our solar system, it is ring-less but not for long.

According to a new study by two American researchers published this week in Nature Geoscience, Mars is probably going to have rings in its future. Like 20 to 40 million years in its future but on an astronomical timescale that's practically tomorrow. These rings will be the remnants of Phobos, the larger of Mars's two moons. Ever since Phobos formed about 4 and a half billion years ago, it's been slowly spiralling in toward Mars at a rate of about 1.8 centimeters per year.  Eventually it will reach the Roche limit - the point in which the pull of Martian gravity will be stronger than the gravity holding Phobos together. That's when the moon will start to fall apart. 

The Roche limit for Mars is about 8500 kilometers from the planet's center, and Phobos is currently orbiting at about 9400 kilometers, meaning that the moon has less that a thousand kilometers to go before Martian gravity rips it apart. Then all of the chunks of rock and dust that used to be a moon will form a ring around the planet. The reason bodies like Phobos get torn apart rather than just crashing into the planet is that the planet's gravity doesn't act equally on the whole moon at once. Mars's gravity pulls harder on the near side of Phobos than the far side. That's how you get tides on both planets and moons and you can see it happening yourself if you live anywhere near an ocean. 

When our moon is close, its gravity pulls at the Earth hard enough for the water level to rise by several meters. It actually makes a bulge in the water than can be seen all the way around the planet. Earth exerts a similar force on the moon and Mars exerts the same kind of force on Phobos. Phobos doesn't have water so it doesn't have aquatic tides but it's still under that same kind of tidal strain, it's just that the strain acts only on the rock itself, and at the Roche limit that tidal bulge will start ripping chunks of rock off the moon.

So why won't those chunks of rock just crash into Mars instead of becoming rings? Well some of them probably will crash into Mars pretty quickly if they're big enough, but most of them won't do that for a while because they're still in orbit around a planet. Phobos orbits Mars three times in an Earth day. It's moving really fast so the dust and rocks that get torn off will also be moving really fast, and since those pieces will be at the Roche limit as well, they'll keep disintegrating into smaller and smaller fragments until they're just not heavy enough to fall inwards any more, at least not fast. The Martian rings will last bout a hundred million years before they are also pulled into the planet. Of course if Phobos were a denser, harder piece of rock it would be a lot more difficult to pull it apart, and it probably would crash into Mars in one catastrophic impact. But Phobos is small, only about 16 kilometers across, and it's porous. It's probably made of fairly weak carbon-rich rock. This new study in Nature Geoscience analyzed what we know about Phobos' composition, orbit, and the effect of Mars's gravity to give us the most complete model yet of how and when Mars will lose a moon and gain some rings. 

To assess Phobos' strength and by extension how well it will resist being pullet apart, the researchers examined the Stickney impact crater. This is a crater on Phobos that covers a sixth of the moon's surface, compared to the size of Phobos it is huge. If Phobos were made of rigid material, any impact strong enough to make a crater that big would have shattered it like glass, so the fact that Phobos held together tells us that it's made of a more porous material that wouldn't be as affected by a hard knock. The fact that it's a weaker kind of rock actually helps, the little pockets in the porous material acted like shock padding. We already suspected that Phobos' surface were full of this weak carbon-rich rock because it's dark, it reflects very little light which is why we didn't even know it was there until 1877. It's kind of like coal, which also has a lot of carbon. A big chunk of coal would be hard to see in space. 

But we now know that Phobos doesn't just have carbon-rich rock on the surface, it's likely made of the same stuff all the way through. The researchers analyzed Phobos with the same models that we use to gauge rock strength for building tunnels here on Earth, incorporating what they've learned about the moon's rock strength from examining the Stickney crater. Their conclusion? Phobos is made of weak stuff in the middle too, which means that it should break apart fairly easily. And thus, for around a hundred million years, Mars will be the most stylish rocky planet in the solar system. 

Thank you for watching this episode of SciShow Space News and thank you to the SciShow writing staff for not making a "if you like it then you should've put a ring on it" joke for this whole episode until now I guess. Thank you also to our patrons on Patreon who help make this show possible. If you'd like to help us make episodes like this you can go to Patreon.com/scishow, and don't forget to go to youtube.com/scishowspace and subscribe.