5: Building the Underworld

First, thank you all so much for your generous support during these first weeks. I’ve gotten a lot of great feedback, and even some fanmail! One listener said she could answer questions from students using the first three episodes- that’s exactly the spirit of Bedrock. Even if you’re not telling everyone on the street about the podcast (though I wouldn’t say no), I hope this information changes the way you see the world, and that you can pass it on to others, young and old. One final note- you can find transcripts and helpful pictures for each episode on our website: bedrockpodcast.com. Now, back to the show.

Last episode, we started Season 1 by introducing The Hadean, the oldest slice of Earth’s history from 4.6 to 4 billion years ago, January 1 – Feb 15 on our imaginary Earth Calendar. We learned that very little Hadean material remains on Earth, so we turn to outer space for evidence of the planet’s earliest days. Our sun formed when gravity pulled a cloud of atoms into itself- we see newborn stars forming the same way today. The leftover molecules around the sun assembled into minerals such as olivine which bumped into each other and grew into asteroids.

Today, we’ll cover the first week of the Earth calendar, 4.6 to 4.5 billion years ago. We will see how Earth was built from asteroids and started to separate into different layers. The cradle of stardust is about to be rocked.

Part 1: A Game of Survival

At the beginning of last episode, I painted a picture of two tiny mineral grains gently bumping into each other, the very first step to building a planet. So how do we get from these two specks of dust to a planet 12,000 kilometers wide? What clues can we gather?

To answer these questions in the real world would require a laboratory the size of the solar system. Instead, scientists who research early planet formation use computers to answer these questions. They create computer programs using real-world information (the size of dust particles, and whether they bounce off each other or stick). In some ways, it’s like creating a video game: For example, if you want to make a good tennis game, you need to know how fast the ball travels, how high it bounces, and what your court limits are. If the ball sticks to your racket, or instantly falls to the ground, the game won’t be true to life.

The same principle holds when creating a scientific computer model: we need as much real data as possible to get the closest answers to our question. And that question is:

If I surround the sun with billions of specks of dust, will they turn into planets?

The answer is yes… but it isn’t easy. Planet formation is a field where new discoveries are made every year, and the papers I’ve read fully acknowledge that there are still things we don’t know. But admitting that, not covering it up, helps scientists figure out what to research next. Finally, this is astrophysics, not geology, so if you notice a glaring mistake or omission I’ve made, please drop me a line.

With that out of the way, how does a planet form?

Let’s return and visit our two crystals from last episode, 4.6 billion years ago, just after they stuck to each other. These collisions will repeat for a while, and the infant Earth will slowly add more crystals and molecules over time- just like a tiny cosmic dust bunny. Some scientists even describe these clumps as “fluffy”, an adjective I wasn’t expecting to use this early.

From our perch atop the growing mini-Earth, only a few millimeters wide, we see a larger, more impressive dust bunny hurtle forward through the cloud, ten times our speed and size. Just as we think “Maybe that lump will become Jupiter or Saturn”, another, even larger particle comes careening out of nowhere. In a second, the two collide and shatter, leaving only pieces for others to pick up.

When your world is smaller than an ant, a dust cloud is a dangerous place to be.

*Bennu, an asteroid ~500 m wide, within the Hadean “danger zone”. It’s perfectly safe today, though.*

Even as Earth grows, the threat of annihilation will be present for the next week of the Earth Calendar, around 100 million years. We’re not talking about the asteroid that will kill the dinosaurs, but bodies that could obliterate our entire planet. The Earth survives the early gauntlet of collisions, slowly growing to look more like an asteroid. When the Earth is a meter wide, the size of a large dog, a completely different problem arises.

The early solar system is still full of gases like hydrogen and helium. When the Earth was tiny, this gas wasn’t a problem, but now that Earth is one meter wide, the gas acts more like water or molasses, slowing the speed of travel. Wading through molasses is never pleasant, but there’s a bigger problem. As Earth slows down in its orbit, it starts to drift closer and closer toward the sun. Throughout the early solar system, we see countless objects plummet and become incinerated in nuclear fire, and it looks like we’re next.

Let’s pause.

Clearly, our baby Earth wasn’t swallowed by the sun. But in most computer programs that follow the laws of physics, objects a meter in size slow down, fall into the sun and are destroyed. Smaller and larger objects don’t care about surrounding gas, but at some point in between, a growing planet has to reach one meter wide, the danger zone. One possible solution is that planets had rapid growth spurts, growing to the size of small countries in the cosmic blink of an eye. That way, a planet would only stay in the danger zone for a brief period of time.

What could cause such rapid growth? Most hypotheses involve gravity- dense pockets of crystals and dust collapsing under their combined mass. If that sounds familiar, you’re right. Last episode, we talked about a similar process forming the sun, though on a much, much larger scale.

Scientists are still working out the details, and perhaps another solution will appear. Perhaps I’ll provide an update in a future episode, or I’ll interview an astrophysicist. The important takeaway is that each planet of our Solar System is a survivor, one in a million that escaped total obliteration by other bodies or the sun. And we’re not out of the woods yet.

After its growth spurt, Earth is about the size of Puerto Rico, 100 km wide. Earth is now large enough to have its own gravity, and can start gobbling up smaller asteroids and planetoids with no problem. As we move into January 2nd through 5th on the Earth Calendar, 4.5 billion years ago, our planet will grow closer to the size we know and love. At the same time, something special is happening deep below the surface. Let’s check it out- it’s time to make a journey to the center of the Earth.

Part 2: Interior Design

Last week, we talked about two types of minerals in meteorites- 1) olivine and 2) a large family that serve as timepieces for the early solar system- I won’t mention names, but if you want to learn more, check out “calcium-aluminum-rich inclusions” or CAIs.

Today we’ll introduce a new ingredient: metal, specifically a mix of iron and nickel. When we cut open an ancient chondrite like the Allende and Murchison meteorites from last episode, we’ll find olivine grains with bits of iron sprinkled in between. Olivine and iron metal make strange neighbors- they’re different in many ways, especially their densities. Iron is twice as dense as olivine- when you pick up a cube of each, the iron will feel much heavier.

Anyone who works in a kitchen will tell you that two ingredients with different densities will not stay mixed for long. The classic example is oil and vinegar- when you shake a bottle with both, they’ll mix for a while, but vinegar is denser, and will sink to the bottom. A chondritic asteroid, with scattered olivine and iron, is like taking mixed oil and vinegar, shaking it, then freezing it in place. As the asteroid grows in the early solar system, iron badly wants to sink to the center. All it needs is a little heat.

One way to heat up radioactive decay, which we discussed in episode 3. The transformation of unstable atoms into more stable forms releases heat- it’s like blowing off steam when you’re angry and need to settle down. In fact, this radioactive heat is one of the key factors in a nuclear reactor. Radioactive decay boils water into steam, which powers turbines and produces electricity.

We have two pieces of evidence that early asteroids were radioactive and hot. First, chemistry. When geologists analyze meteorites, they find high amounts of magnesium and nickel isotopes that are uncommon in nature. These isotopes are the products or “daughters” of unstable atoms- not the usual suspects like uranium or radium, but more mundane elements like aluminum and iron. Now don’t worry, you don’t need to put your aluminum soda can in nuclear waste- that’s the normal stuff.

Aluminum-26 and iron-60 are forged during supernovas, the explosive deaths of stars. These tell us that another star violently died just before our sun was born. It’s even possible that this supernova shook up our corner of the stellar nursery and triggered the birth of our solar system, but the details are still being worked out.

If you’re wondering about the safety of meteorite researchers, don’t worry- all the radioactive aluminum and iron decayed within a few million years, the first hours of the Earth Calendar. The remaining daughters are unusual, but perfectly safe.

But 4.6 billion years ago, radioactive elements began to heat up the early Earth from the inside, and now we return to our frozen oil and vinegar analogy. After we pull the tasty mixture out of the freezer, the liquids heat up and vinegar starts to sink and separate away from the oil. A similar process happened to the early Earth- heavy iron sank to the middle of the warming core.

That makes sense, but how do we know this?

Let me introduce you to iron meteorites. Until now, we’ve been talking about chondrites, grainy mixtures of minerals and metal. While chondrites are the most common meteorites, you are far more likely to see iron meteorites in museums. That’s because they’re easier to identify- big lumps of heavy iron where there shouldn’t be any, including the largest- the Hoba meteorite from Namibia weighing 60 tons, more than eight elephants.

As the name suggests, iron meteorites are almost entirely iron and nickel, no crystals of olivine or anything else. On the other hand, there are also stony meteorites that contain very little iron- these are much harder to spot because they look just like rocks. Finally, there is a strange and beautiful group called pallasites- I had never heard of these before this show, but now they’re my favorite meteorites. Pallasites contain large translucent olivine crystals floating in shiny, steely iron. They resemble both stained-glass windows in a cathedral and silver-mended pottery of the Japanese kintsugi style. Honestly, take a pause and check these out; I’ll wait.

Now that I’m done waxing poetic about pallasites, what was the point of that list?

Together, chondrites, pallasites, iron and stony meteorites tell a story of how asteroids and the early Earth evolved.

Earth probably started as a chondritic asteroid, a jumble of tiny minerals and metal grains. As the planet grew, radioactive elements heated the interior. Heavier metals like iron and nickel sank into the center, while lighter crystals like olivine remained on the outer shell. Other early planetoids were doing the same thing, but most of them did not survive. These unlucky worlds were broken into thousands of puzzle pieces, asteroids still falling on the Earth today. Their iron-rich cores became iron meteorites. The outer rocky shells became stony meteorites, and the areas in between, bridging the realms of iron below and stone above, became pallasites.

But not all these worlds were doomed to die. There are at least two protoplanets hiding in the asteroid belt, survivors from the earliest days of the solar system. They are named Vesta and Ceres. These two bodies are much smaller than our own moon, but they are still so interesting that NASA sent a probe named Dawn to orbit them between 2011 and 2018. Dawn helped confirm that Vesta and Ceres have iron-rich cores and olivine-rich mantles, just as scientists guessed from meteorite puzzle pieces. As I mentioned last episode, the Hadean does not leave us much to work with, so it’s always good to get some external confirmation.

Speaking of the Hadean- one final note.

Vesta and Ceres are named after Roman goddesses of the home and agriculture. These goddesses are sisters of the Roman god Pluto, AKA Hades in Greek. It’s only fitting then, that Vesta and Ceres provide a snapshot of the early Hadean. If you want to know what a baby Earth looked like, Vesta and Ceres are great places to start.

Summary:

Let’s review what we’ve learned about Earth’s earliest days. The Earth started to form when tiny pieces of crystal, metal, and dust assembled into asteroids, the dust bunnies of the solar system. These asteroids had to grow up fast to avoid being swallowed by the sun. The dozens that survived then had to watch out for their hungry siblings. The ultimate survivors were the eight planets and small hidden worlds like Vesta and Ceres. These bodies started to change inside, separating into iron-rich cores and rocky exteriors thanks to radioactive heat. The countless worlds that didn’t survive tell us these tale in the form of asteroids. So whenever you look at an iron meteorite in a museum, or catch a glimpse of Ceres with binoculars, you’re looking at a time that helped forge the earliest days of Earth.

Next time, we’ll continue to explore Earth’s interior as the core and mantle truly start to take shape, and keep an eye out for any rogue planets waiting to strike.

***

Thank you for listening to Bedrock, a part of Being Giants Media. As the show takes off, I would love to hear your input on style, topics, and people to interview- you can drop me a line at bedrock.mailbox@gmail.com. See you next time.

Images:

Asteroid game: Sergey Galyonkin

https://commons.wikimedia.org/wiki/File:Gamer_at_E3_2012_(7165443243).jpg

Oil and vinegar: Drazmoyde

https://commons.wikimedia.org/wiki/File:Balsamoil_Protean.jpg

Bennu: NASA/Goddard/U Arizona

https://commons.wikimedia.org/wiki/File:BennuAsteroid.jpg

Hoba: Compl33t
https://commons.wikimedia.org/wiki/File:Hoba_Meteorite_2007.jpg

Pallasite: Doug Bowman
https://en.wikipedia.org/wiki/Pallasite#/media/File:Esquel.jpg

Vesta: Pablo Budassi

https://commons.wikimedia.org/wiki/File:Vesta_black_background.png

Ceres: NASA/Justin Cowart

https://commons.wikimedia.org/wiki/File:Ceres_-_RC3_-_Haulani_Crater_(22381131691)_(cropped).jpg

Music:

Night on Bald Mountain by Modest Mussorgsky, Human Symphony Orchestra

Orpheus in the Underworld by Jacques Offenbach

https://commons.wikimedia.org/wiki/File:Offenbach_-_Orpheus_in_the_Underworld_-_Overture,_Can_Can_section.ogg

Tortues by Camille Saint-Saens

https://commons.wikimedia.org/wiki/File:Saint-Saens_-_The_Carnival_of_the_Animals_-_04_Tortues.ogg

Catacombs by Big Score Audio

Taking It In by Michael Brandon

4: The Cradle of Stardust

6: Working Out the Core