Good morning, Interweb…let's worldbuild!
Here's a fun idea: let's re-imagine the history of Earth except…with rings!
Hold on, Earth?
Rings?
Is that even possible.
Well, Saturn has rings.
Jupiter has rings.
Uranus has rings.
Neptune has rings.
Hell, even found two centaurs with rings: (10199) Chariklo and (2060) Chiron.
So, if big things can have rings and little things can have rings, why not intermediate
things too?
I mean, none of the leading ring formation theories exclusively rule out the possibility
of terrestrial rings.
And we're not talking man-made rings of space junk either.
We got those.
We're talking 100% organic, naturally occurring, geologically stable, terrestrial rings systems.
However improbable, terrestrial rings are possible!
As for earth in particular, well, it's complicated.
PHYSICAL
In my opinion, the best “earth-ring” formation scenario goes like this: back in the early
days of the solar system, after the earth and the moon formed, a wandering asteroid
got a little too close to earth and was captured.
Over time the asteroid's orbit decayed to such an extent that the earth's tidal forces
ripped it apart and the resulting debris formed the rings of earth.
So, we're gonna need an asteroid.
Meet 48 Doris – an actual, real life, main belt asteroid I'll use to model our hypothetical
asteroid off.
Now, as Doris approaches the Earth, she will eventually cross whats known as the Earth's
Roche Limit.
The Roche Limit is the theoretical boundary around any astronomical object, outside of
which satellites can exist but inside of which they will be torn apart and form rings.
As such the outer boundary of any ring system will always be at the roche limit!
Unless otherwise acted upon, rings will exist entirely within this region.
The Roche limit is given by: 2.44 x R x 3√(Pp/Ps) - in this case, Earth is our primary and Doris
is our secondary.
So, it's -- 2.44 times the radius of the earth, 1, 'cause, as always, we're working in earth
units.
Times the cubed root of the density of earth, again 1.
Divided by the density of Doris, which wikipedia tells me is roughly 0.38 times as dense as
earth.
Run the numbers and we'll find that the outer limit lies 3.37 earth radii out from the centre
of earth.
Which, to scale, is about here!
It's worth noting that the Roche Limit depends on the ratio of the densities.
An asteroid made of solid iron, say, could get much closer to earth without being torn
apart.
The size of any ring system will depends primarily on the density of the moon, comet or asteroid
from which they formed: high density objects yield tight in rings, low density objects
yield more expansive rings.
Now for the inner boundary.
This will likely coincide with the outer limit of the earth's atmosphere.
Why?
Because the rings need to be totally free of atmospheric drag lest their orbits decay.
So, that's beyond the troposphere, beyond the stratosphere, beyond the mesosphere, beyond
the thermosphere and even beyond the exosphere.
Earth's atmosphere, at it's fullest extent, is deep.
Really deep! 10,000 km deep.
So our inner boundary goes about here – 2.57 earth radii from the centre of earth!
And, hey, presto we got ourselves some “Earth-rings”! 10,000km above the earth's surface we'll have
a 5161 km wide “earth-ring”.
Now, the ring's mass will be roughly equivalent to Doris' mass – minus a bit to allow for
any material ejected during formation.
So, again thanks to wikipedia, our “earth ring's” will be about about half (0.56)
the mass of Saturn's rings.
However, unlike saturn's icy rings, our “earth-rings” will be made of silicates – after all Doris,
a rocky asteroid, was made of silicates.
This is very important.
In the inner solar system, so close to the sun, plain old sunlight would sublimate any
icy ring particles from a solid to a gas and then the UV rays would break apart the water
molecules, i.e., bye bye ring system.
In short, close in planets should have rocky rings.
Distant planets; icy rings.
Now, like Saturn our “earth-rings” will be very thin.
I ran some crude volumetric equations to very roughly gauge the average thickness of our
“earth-rings”.
Turns out they're about 9.5 meters thick, which sounds about right, given that the average
thickness of Saturn's rings is about 10 meters.
So, that's distance, mass, composition and thickness covered...what about gaps?
The Cassini Division is probably the most famous gap in Saturn's rings.
But it's hardly alone: Saturn's rings have thousands of gaps all varying in size!
Why?
Because moons.
Lots of moons...60 odd moons in fact - they all interact gravitationally with Saturn's
rings giving rise to the complex structures we see.
Earth, with it's one, abnormally massive, moon will have very different looking rings.
The orbital resonances between our moon and the rings will make for a ring system with
fewer, more regularly spaced gaps.
Think pin-stripe rings…
Kinda like this.
Ye, they're not as visually pleasing…but luckily the moon has another trick up it's
sleeve: tides.
Lunar tides might pull the “earth-rings” into a spoked configuration akin to those
seen on Saturn's rings.
I say luckily, but truth be told, the lunar tides are so strong they'd probably tear apart
the rings.
Hell, the Earth's so close to the sun, the solar tides would likely do the same.
And this is the “it's complicated” bit.
Whilst terrestrial planets do have the potential to host rings, Earth doesn't: it's setup is
just all wrong.
Ideally, for Earth to have rings, it would need a smaller moon and would need to orbit
further from the sun.
But, hey, we're balls deep at this stage, so, on with the rampant speculation!
Let's talk culture.
CULTURE
The “earth-rings” would certainly play a huge role in shaping the evolution of human
culture and society.
Standing at the equator they would appear directly over head as a thin, bright line
running east to west.
As you move away from the equator, the rings would appear wider and wider and ever closer
to the horizon - to the south in the northern hemisphere and north in the southern hemisphere.
Kinda like this, albeit more terrestrial and less saturnian.
They would never set and would, to an extent, be constantly visible - day and night.
They would be so apparent that they would change the apparent color of the moon.
The moon is a very dark object: we only perceive it as being so incredibly bright because,
relative to the blackness of the night sky, it is really bright.
Relative to a our “earth-rings” the moon may seem darker: a dark grey, perhaps.
Imagine the rich mythological tradition that would arise centered on this great celestial
arc with it's grey companion.
That is, if there even were humans around to create those mythologies...See, the rings
would also cast a shadow on the earth's surface, resulting in colder winters in both hemispheres
- a lot colder.
Possibly inhospitably cold.
Plants and animals, if they even evolved, would have to develop methods to deal with
the colder temperatures and decreased amounts of light.
It's not all bad though.
Joseph Shoer of Quantum Rocketry, speculates that if the earth were ringed, and humans
were around, the progress of science would be rapidly accelerated.
Using nothing but the naked eye, and some basic geometry, it would be possible for ancient
cultures to look at the rings and figure out (a) the Earth's axial tilt, (b) that the Earth
and Moon are spherical, (c) the distances to the moon, (d) that the solar system is
heliocentric, (e) the relative brightness of distant stars AND (f) the distances to
those stars.
All of this just from studying how the shadows of the Earth and Moon interact with the rings,
and how much of a star's light the rings block as the star passes behind them.
The ancient Greeks or Chinese, using nothing but the sacks of fluid embedded in their faces,
would have been able to tell you the rough distance to THE STARS!
From just looking up at the rings.
Madness.
Hell, the Romans, they might have been able to tell you want a parsec is..
And I, I might have lived on Mars...who knows.
Well, actually I do.
See spaceflight is a tricky undertaking at the best of times.
Imagine the complications a 5,000 km wide ring of rocky debris, aka space shrapnel,
would cause.
At best space flight would be really complicated, at worst, impossible.
Any launch trajectories would need to be carefully plotted so as to go “over” or “under”
the rings.
Satellites would also have to avoid the rings, which is difficult considering directly above
the equator is the best place to put them.
Many of earth's current satellites regular plough through the region the rings would
occupy.
And even if humans found safe orbits for their satellites, the ring particles may scatter
any incoming radiowaves thus rendering weather, communications, military, GPS and spy satellites
useless.
All in all, terrestrial rings are not that crazy a concept to think about.
They'd be awesome to look at, great for the advancement of pre-scientific societies, but
they'd ultimately hinder any spacefaring civilization.
Unless
of course, Stargates!