“A journey is a person in itself; no two are alike. And all plans, safeguards, policing, and coercion are fruitless. We find that after years of struggle that we do not take a trip; a trip takes us.” -John Steinbeck
Here on Earth, we all get to enjoy the delight of being located in an extremely fortuitous place in our Solar System. Not just today, mind you, but billions of years ago, when the Solar System’s planets were first forming!
Located close enough to our Sun, when the Earth first formed, like our neighbors Mercury, Venus and Mars, we were chock full of heavy elements. Not just the carbons, nitrogens and oxygens that abound on all the known worlds, but important ones much higher up on the periodic table, including silicon, sulphur, iron, nickel, tin, lead, and even the radioactive uranium!
This might not sound so special to you, but our world would be a lot more boring if we had formed farther from the Sun. See for yourself:
Jupiter and Saturn aren’t simply less dense than Earth is because they retained all that excess hydrogen that our wimpy gravitational field couldn’t hold. Although that’s true, they’re also made out of elements that are intrinsically less dense!
We can identify where an object found anywhere in the Solar System — including meteorites that fall to Earth — simply by examining what they’re made out of.
One of the most spectacular applications of this knowledge came when we first journeyed to the Moon. For the first time, we’d be able to analyze rocks from the Moon and their chemical composition! What we found was simultaneously profound and the most boring thing imaginable.
Because the Moon is made out of the same stuff that the Earth is!
Before you start saying, “Duhhhhh,” as some of you are wont to do, let’s remember that it didn’t have to be this way. We need only look at our nearest moon-ed neighbor.
Because Mars’ two moons, Phobos and Deimos, are not made out of the same stuff Mars is! They formed significantly farther out in the Solar System, originating in the asteroid belt. Only a chance encounter with another body (probably Jupiter) flung them in to the inner Solar System, where they were gravitationally captured by the red planet!
If it happens for Mars from the asteroid belt, you may be wondering about that other, even bigger belt in our Solar System, and what the possibilities might be from there.
I refer, of course, to the Kuiper Belt, the band of leftover proto-planetesimals from the formation of our Solar System. These small, icy objects are only about a third the density of Earth, but are more dense than the gas giants that lie interior to them.
If the asteroids could be flung towards the inner, rocky worlds due to the gravitational influence from Jupiter, it stands to reason that these Kuiper Belt objects could similarly be flung inwards thanks to Neptune.
There are some dead giveaways that your object isn’t from the same part of the Solar System as its initial planet is. There’s the elemental composition / density argument, of course, but simply via random chance, 50% of these objects that get captured will wind up revolving around their planet the wrong way. One obvious guy like this is Neptune’s largest moon, Triton.
Triton is maybe the easiest one; he’s so large that if he were still in the Kuiper belt, he’d be the largest object there, dwarfing (burn!) both Pluto and Eris!
But there are others, elsewhere, that don’t quite look like they belong. And a Saturnian mystery may, in fact, be on the cusp of being solved thanks to this idea.
This is Iapetus, one of Saturn’s moons, looking like it always does: like it came out on the wrong side of a trip through the mud. Iapetus does all the moon-like things correctly: it revolves the right way around Saturn, it’s got the right density for its spot in the Solar System, its surface is even made of the same elements — as far as we can tell — as it ought to be.
Except for that muddy mess that discolors one of its face. What’s going on here? It turns out Iapetus isn’t alone.
A giant, diffuse outer ring, well beyond the Saturnian rings you’re used to, pollutes Iapetus’ orbit. As the tidally-locked moon speeds around Saturn, these grains from the ring smack into Iapetus, discoloring it like billions of bugs on a windshield.
The question, of course, is where did this ring come from? Because it isn’t Saturn. The answer is much more fun than that!
Say hello to Phoebe, Saturn’s very suspicious moon, located in the same vicinity as both this outermost ring and Iapetus. Phoebe is full of craters, a different color and elemental composition than the other moons, and — this is a big and — it revolves around Saturn the wrong way!
In other words, this outsider came all the way from the Kuiper Belt to become a moon of Saturn! And the journey was no picnic for Phoebe, either.
Those craters on its surface are from a lifetime of bombardment! The once-spherical Phoebe has lost a lot of its mass, and that’s almost certainly where the material that makes up both the outermost ring and the diffuse discoloring of Iapetus comes from!
Let this be a lesson to all of you: if you want to be adopted by another planet, make sure you orbit the same direction as everything else! In the meantime, know that objects from the asteroid belt or Kuiper Belt could come in at any time, and could even become our planet’s next additional moon!