Mars is pretty cool already. I mean, it has liquid water and everything!
But wouldn’t it be cooler if Mars had rings, like Saturn?
However, like all the other rocky planets in our solar system, though, Mars is ringless.
But not for long!
According to a study by two American researchers published this week in Nature Geoscience,
Mars is probably going to have rings in the future!
Like, 20-40 million years in the future, but on an astronomical timescale, that’s practically
tomorrow.
Those rings will come from Phobos, the larger of Mars’s two moons.
Ever since Phobos formed, about four 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 [roash] limit: the point at which the pull of Martian
gravity will be stronger than the gravity holding Phobos together. That’s when the
moon will start to come 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 than a thousand kilometers to go before Martian gravity rips
it to pieces. Then, 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 because 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 is 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 that can be seen all
the way around the planet.
Earth exerts a similar force on the Moon, and Mars exerts that 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 that 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 forming rings?
Well, some of them will probably 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 the
planet.
Phobos orbits Mars three times in an Earth day. It’s moving really fast, so the dust
and rocks that get torn off of it will also be moving really fast.
And since they’ll be at the Roche limit, 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 for about a hundred million years, before
they’re 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 analyzed what we know about Phobos’ composition, orbit, and the effect
of Mars’ gravity, to give us the most complete model yet of how and when Mars will lose a
moon, and gain its rings.
To assess Phobos’s strength -- and by extension, how well it will resist being pulled apart
-- the researchers examined the Stickney impact crater. This is a crater on Phobos that covers
about a sixth of the moon’s surface. Compared to the size of Phobos, it’s HUGE.
If Phobos were made of a 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 was 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 lots of carbon. A big chunk of coal would be really
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’d 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 for around a hundred million years, Mars will be the most stylish rocky planet in the
Solar System.
Thanks for watching this episode of SciShow Space News, and thanks to our writers for
not making a "If you want it then put a ring on it" joke, . . . until now I guess. Thanks
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