4: The Cradle of Stardust
Our story begins in the twilight of space, in a dark cloud of dust. Newly-born stars glimmer within the cloud, a bustling nursery in the lonely void. In one corner, a spinning orb glows like a dull ember, not quite a star yet. Within its surrounding blanket, one mote of dust, an olive-green crystal barely visible to the eye, floats among countless others. From a distance, we see a dark speck of iron gently drift toward our green crystal. The collision is soft but holds, and is just enough for the two to slowly wheel around like a pair of dancers. In the dim light of the embryo sun, the particles cast the faintest of shadows as they make one full rotation, then start another. It is brief, it is cold, and it is dark, but this dance marks the first day of planet Earth.
Episode 4: The Cradle of Stardust
Part 1: The Hadean: The Invisible World
In this episode, we start Season 1 of Bedrock and finally begin to tell Earth’s story from the beginning. Today we’ll cover the birth of our solar system, from the Sun to the first building blocks of Earth. But first, let’s get to knaw some names and dates.
Season 1 covers a time period called the Hadean. The Hadean starts at the beginning of Earth history and ends 4 billion years ago. On our Earth Calendar, that’s Jan 1 to Feb 15. I’ll use the calendar frequently this season as a reference point, since all the dates will be 4-point-something billion years, and even I start to get cross-eyed trying to keep them all straight. For those keeping score, this episode will only cover January first.
The Hadean is named after Hades, the ancient Greek god of the underworld. Hades is also the name of his underworld realm- it’s like if I called my apartment “Dylan”. Basically, the name Hadean is geologists’ not-so-subtle way of saying that Earth’s beginning was hellish and intense. Most of the Hadean would fit perfectly on the cover of a metal album. That might not be apparent as we start, talking about interstellar gas, but trust me, things will heat up quickly.
While researching this episode, I found another, older meaning of Hades which is very relevant to this period of history: the word Hades could also be translated as “the invisible”, or “something which cannot be seen”. Why is this idea of invisibility so fitting for the Hadean?
In short, there simply isn’t a lot of Hadean material to study. Earth is a living and dynamic planet, constantly recycling itself over time. While this reinvention makes Earth a beautiful and exciting place to live, it comes at a cost for Precambrian rocks. The older a rock is, the more likely it will become recycled into something new. As we saw in Episode 2, a rock can be melted into magma, pulverized to sand, or transformed into new minerals through heat and pressure.
The amount of Hadean rocks still remaining on the Earth’s surface would only cover a few city blocks at best, and these rocks record the Hadean’s end. The early Hadean is mostly preserved by our tiny but tough mineral friends from last episode: zircons, Earth’s time capsules. Beyond zircons, almost every piece of the Hadean Earth has been scattered, buried, melted down or transformed by countless cycles of rock formation.
For me, studying Precambrian geology is like solving a cold case: you need to squeeze every piece of information you can from limited resources that have had billions of years to become altered or erased entirely. At the same time, you need to check your hopes and personal biases at the door and open your mind to whatever story the rocks tell you- even if it’s not the tale you expect, or even want to hear.
All these factors increase tenfold when studying the Hadean, which has so little material to work with. However, by using advanced chemical techniques and creative problem-solving, Hadean researchers have taken a time period that by all rights should be Invisible, beyond the realm of human sight, “Hadean” in the oldest sense of that word, and have pulled a cohesive story out of Earth’s forgotten past. For these feats, Hadean researchers have my deepest respect.
Fortunately for us, not every place in the solar system has been recycled like our planet has. Many Hadean time capsules exist beyond Earth, preserved in the vacuum of space beyond the destructive forces of magma or water. We can also peer even deeper into space to infant star systems light years away. The birthing pangs of these distant stars can help us piece together our own past. So, to start the tale of the Hadean, the story of Hell on Earth, we need to start looking in the heavens above.
Part 2: A Star Is Born
Space is usually described as void or nothingness- I’m also guilty of using those words in this episode. But in fact, the wide gulfs between stars are not completely empty. Even in the most remote corners of the universe, lonely atoms float in the darkness. On microscopic scales, each atom in interstellar space is like a single beach ball floating in the middle of the Pacific Ocean.
But space isn’t always this lonely. Some areas contain millions of particles in every centimeter. You may hear these particles described as “gas and dust”, but I personally think this term makes outer space sound like a condemned building. Astronomers also call this material “the interstellar medium”, which sounds like a much cooler space wizard. If you scooped handfuls of interstellar material from around the universe, you would find similar recipes of elements: 70% hydrogen, 25% helium, and pinches of heavier elements such as carbon and iron. More complex molecules and crystals can also form, but we’ll get to those later. Areas where interstellar particles are especially dense resemble dark clouds lit from outside and within by starlight. A single cloud is called a nebula, the plural being “nebulae”, from the Latin for mist or fog. Nebulae are much larger than our solar system, up to hundreds of light-years across, but are mere smudges compared to the Milky Way galaxy, which is half a million light-years wide.
Helix Nebula
Crab Nebula
Nebulae are among the most beautiful features in space. Typing “nebula” in Google Images will show stunning pictures with legendary names like the Horsehead, the Cat’s Eye, the Pillars of Creation, and the Lemon Slice. There’s even a nebula you can spot with your own eyes on a clear night. If you can see the constellation Orion, look for his sword, a tight line of three stars below his belt. The middle point isn’t a single star- it’s the Orion Nebula, the closest major nebula to Earth. It doesn’t look like much with your eye, but telescopes reveal bright sweeping arms of interstellar medium peppered with young stars.
The Orion Nebula has been extensively studied by astronomers, who have discovered multiple infant stars and even more excitingly, the earliest stages of planet formation around them. Looking at the Orion Nebula and others is like looking at the baby photos of our own Solar System. This fact leads astronomers to call nebulae “stellar nurseries”.
So how is a star born inside a stellar nursery? How did our sun come to be?
Inside of each nebula, there are two forces fighting each other for dominance. One force is gravity, which pulls inward on the cloud toward the center of mass, just like Earth is constantly trying to pull you into its core. The opposing outward force is gas pressure. The best visual example of gas pressure is when you blow up a balloon. The air you blow is made of thousands of independent gas molecules, providing a constant outward force on the balloon. A small nebula keeps gravity and gas pressure balanced. But if the cloud becomes too massive, gravity takes over and pulls the interstellar medium into a focused core. The attraction soon becomes irreversible- as more molecules are stolen from the nebula, the core becomes heavier, which attracts more molecules, and so on.
This core releases energy as it grows, nearly 2000 C, twice as hot as a magma chamber. The orb is now literally glowing red-hot, but we cannot call it the sun just yet. First we need to get nuclear.
As a proto-star grows and gathers material, atoms trapped within are now much closer and moving much faster than in cold outer space, and strange things start to happen. For example, when two hydrogen molecules meet in the interstellar medium, they pass like ships in the night. Within a star’s core, they can physically collide and fuse into one new particle: a single atom of helium. This nuclear fusion releases energy: either heat, light, or other forms of radiation that leaves the star. This energy also balances the inward pull of gravity and, for a while, keeps stars from completely collapsing into a black hole or neutron star.
When a star’s nuclear spark ignites, it has eaten up nearly all of the hydrogen, helium, and other elements in its area. For example, our newborn sun how contains more than 99% of our solar system’s mass. The star also clears out the solar system by blowing out a constant stream of electrons and protons called the solar wind. Let’s hitch a ride on this wind, leave our infant sun behind and search for those crystals we met at the beginning of the episode.
When astronomers look at the Orion Nebula using powerful telescopes, they see pancake-shaped clouds of material surrounding new stars. These molecular pancakes are called proto-planetary discs- they form before planets, and they’re relatively flat. Unlike a pancake, these discs are not stationary, but gently spin around their stars. This spin speeds up as the star pulls more atoms into its core. You can see the same principle when a ballet dancer or figure skater performs a spinning pirouette. When the dancer wants to spin faster, they bring their arms closer to their body, just like a star pulling in hydrogen. In our solar system, the echoes of those first pirouettes can still be seen in the orbits of planets around the sun.
Protoplanetary discs in the Orion Nebula
The Orion Nebula with the locations of several protoplanetary discs
If you’ve looked at a map of the solar system and wondered why all the small, rocky planets are close to the sun, and all the gas giants are farther away, one answer is the solar wind we’re currently riding. This wind of electrons, protons and helium atoms are small, but through sheer numbers and electrostatic charge, they push lighter molecules such as hydrogen and methane farther from the sun. But heavier elements such as calcium and iron are not so easily pushed around, and stubbornly remain closer to the sun.
Our world and other inner planets are largely built by atoms heavier than hydrogen or helium. To us, even lonely worlds like Mars or Mercury seem more familiar than gas giants like Jupiter – they are also made of stone and metal, built by silicon, aluminum, and iron. Never forget that these elements make up less than 1% of the entire solar system. The atoms that shape our planet, our buildings, and even our bodies are incredibly rare commodities in the universe.
As with the concepts of deep time and deep space, this idea could easily make you feel lonely or insignificant in a sea of cold, uncaring hydrogen. I personally like to view it the other way- the physics of the universe have shaped our corner of the galaxy into a beautiful array of rocky planets, gems around the neck of the sun, precious and worth studying and preserving.
And speaking of gems, let’s meet the first minerals in the solar system.
Part 3: The First Building Blocks of Earth
The date is February 8, 1969. The Beatles have given their last public performance, Richard Nixon has been sworn in as the US President, and in five months, humanity will make that giant leap for the moon.
It’s 1 AM in the small village of Pueblito de Allende, in northern Mexico. The town is settling in for another quiet night, when suddenly the silence is shattered. A fireball is screaming down the sky from the southwest. The object is the size of a car, weighs 5 tons, and travelling at 16 km a second until it violently explodes into a thousand pieces in the air. The chunks scatter over 200 square kilometers, but fortunately, no major damage is reported. More than two tons of the meteorite are recovered, and scientists quickly realize this bountiful rock- named Allende after the town- was special in many ways. In November of the same year, a similar event happens over the town of Murchison in southeastern Australia. The same year that humanity picked up the first rocks on another world, the Allende and Murchison meteorites would change our perception of space forever.
One point to clear up trivia debates: a rock is an asteroid as it’s floating in space, a meteor as it’s falling through Earth’s atmosphere, and a meteorite once it’s on the ground.
A fragment of the Allende meteorite, 8 cm wide
Chondrule grains within an Allende fragment, scale = 1 cm
The most common meteorites are called chondrites, from the ancient Greek word for grain. That’s because when you cut open a chondrite, it is filled with rounded sand-sized mineral grains. The Allende and Murchison meteorites are a more rare variety called “carbonaceous chondrites” because they have more carbon than other meteorites- while 85% of meteorites are chondrites, only 5% are carbon-rich.
Chondrites are a complex mixture of materials. I’m going to briefly cover two before we finish. There are two other components we will cover in much greater detail in later episodes: water and organic carbon. Today, let’s stick to minerals.
First, olivine. Olivine is a very common ingredient of chondrites. As you can guess, olivine is the olive-green crystal I described in the cold open- gem fanciers or fans of the show Steven Universe might know this green mineral by another name- peridot (pear-eh-dot or pear-eh-doh). Olivine has a fairly simple chemistry, a mix of iron, magnesium, and silicon, and is common in the solar system. Olivine has been discovered on the moon, Mars, and in comet tails. Remember those protoplanetary discs we discussed, the dusty pancakes light years away? Yep, scientists have found signatures of olivine there, too.
You can also find olivine right here on Earth- it is a major ingredient in the mantle, and is commonly found in the volcanic rock basalt. When water breaks down basalt, olivine sand can sometimes form stunning green beaches, the most famous of which is Papakolea Beach on Hawaii. Olivine quickly breaks down on Earth’s surface, which is why green sand beaches are very rare. But this instability has recently been a source of great interest to climate scientists. The transformation of olivine to other minerals removes carbon dioxide from surrounding air or water. Since humans are still producing carbon dioxide on massive scales, scientists and engineers have proposed using olivine like a sponge to soak up extra CO2 and help slow down climate change. This sounds like science fiction, but several scientists I know are seriously proposing this little green mineral to help save the world.
Green olivine sand weathered from basalt, Papakolea Beach, Hawaii
Olivine sand, Papakolea Beach, Hawaii
Olivine is a very important mineral on planet Earth, and it will show up time and again on our show- we can shelve it next to zircon in our mental mineral collection. But olivine is not the reason why the Allende and Murchison meteorites from 1969 are so special.
In the last three episodes I hammered home the point that Earth is 4.6 billion years old- you’re probably sick of hearing me say that by now. But I’ve kept a little secret: this date was not measured from an Earth rock. As I mentioned earlier, almost all of Earth’s minerals have been recycled since the Hadean, but meteorites provide time capsules that preserve the solar system’s earliest moments.
You might throw your hands in the air and say “What good is this date, then? How can we trust meteorites to tell us the age of the Earth? How do we know that Earth wasn’t formed earlier or later?”
I’m not trying to set up a strawman here- these are important questions to ask when presented the date of Earth’s birthday. I’ll present three quick arguments to support the Allende case- each answering a question. (If you want to learn more about how to date a rock, check out Episode 3: The Dating Game.
1: Could Earth have formed after 4.6 billion years?
If so, it’s not by a lot. The oldest minerals on Earth are 4.4 billion years old- that’s January 14 on the Earth Calendar, not too far off. No surprise, these old fogeys are zircon crystals. Their presence implies a dynamic rock cycle and probably the presence of liquid water. What I’m saying is, by the time these zircons formed, Earth had already been around for a while, so 4.6 billion is not unreasonable using Earth material alone.
2: Why should we trust the Allende meteorite date?
Unlike Earth, chondrites do not have zircons, and the minerals that do form don’t hold uranium atoms well. Instead, chondrites contain three lead isotopes. Scientists can compare the ratios of these isotopes and calculate a starting date. To use our hourglass analogy from last episode, this method is like comparing two different hourglasses to determine when both were flipped- it’s complicated, but doable.
Geochronologists have another trick up their sleeve to test chondrites. Just like in magma chambers, certain minerals should have formed earlier than others as the solar system grew. Chondrites have a wide variety of minerals, from tough customers that should have formed very early, to late bloomers that take more time to get settled. Comparing lead hourglasses in each mineral confirms this expectation- the tough ones formed earlier, the late bloomers later. These clocks are accurate within 1-2 million years, mere hours on our calendar- that’s a 99% precision.
Even better, many other meteorites show similar dates around 4.6 billion years- so we’re not betting everything on just one rock. In fact, Allende wasn’t the first meteorite to be dated. In 1956, when the Beatles were the Quarrymen, and Nixon was Vice President, a scientist named Clair Patterson calculated an age of 4.6 billion years from five meteorites. You can thank Dr. Patterson for having that number drilled into your head.
3: Could Earth have formed earlier?
Here’s something I learned while researching this episode: there’s material in these meteorites that is even older than 4.6 billion years. A 2020 study on the Murchison chondrite found that some of these grains are up to 7 billion years old, half the age of the Universe. On our Earth calendar, that’s May of the previous year. So why don’t we go around saying that the Earth is 7 billion years old?
Once again, the answer is chemistry. Our sun has a specific chemical fingerprint. All the crystals that are 4.6 billion years have our sun’s fingerprints all over them. However, the 7 billion-year old material has traces from a completely different star- we even know what type- a weird variety of red giant that I don’t have time to get into. For now, 4.6 billion remains the oldest date- the January 1st of our solar system.
Summary
Speaking of time, let’s review what we’ve learned about the first day of the Earth Calendar.
Our sun formed in the corner of a nebula, a cloud of interstellar material including hydrogen, helium, and fractions of heavier elements like iron and magnesium. The heavier elements began to clump together in the inner solar system, forming many different minerals including abundant olivine. Other minerals trapped different types of lead, which can be used to calculate the age of the Solar System, and Earth by proxy. The different minerals assembled into chondritic asteroids, which are still falling on Earth today. If you find a chondrite while meteorite hunting, or if you simply look up at the Orion Nebula, you are looking at snapshots from the earliest days of stars and their solar systems.
Next episode, we will start building the Earth out of chondrites and move beyond January 1. These will be days that better reflect the Hadean’s name, days of magma oceans and meteor impacts. It’s time to go to Hell on Earth.
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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:
Orion Nebula: NASA, M. Roberto
https://commons.wikimedia.org/wiki/File:Orion_Nebula_-_Hubble_2006_mosaic_18000.jpg
Earth Calendar, Hadean: Dylan Wilmeth
Helix Nebula: NASA/JPL
https://commons.wikimedia.org/wiki/File:Comets_Kick_up_Dust_in_Helix_Nebula_(PIA09178).jpg
Crab Nebula: NASA, J. Hester and A. Loll
https://commons.wikimedia.org/wiki/File:Crab_Nebula.jpg
Protoplanetary discs: NASA/C.R. O’Dell (single image), NASA, M. Robberto & L. Ricci (composite)
https://commons.wikimedia.org/wiki/File:M42proplyds.jpg
https://commons.wikimedia.org/wiki/File:Orion_Nebula_with_proplyd_highlights_(captured_by_the_Hubble_Space_Telescope).jpg
Allende meteorite: H. Raab (outer surface), Matteo Chinellato (inside cut)
https://commons.wikimedia.org/wiki/File:AllendeMeteorite.jpg
https://commons.wikimedia.org/wiki/File:Allende_Matteo_Chinellato.jpg
Olivine sand: Wilson44691 (closeup), Aren Elliott
https://commons.wikimedia.org/wiki/File:Papakolea_Beach_sand_low_mag_052915.JPG
https://commons.wikimedia.org/w/index.php?search=papakolea&title=Special:MediaSearch&go=Go&type=image
Music:
In the Steppes of Central Asia by Alexander Borodin, performed by the Czech National Symphony Orchestra
https://commons.wikimedia.org/wiki/File:Alexander_Borodin_-_In_The_Steppes_Of_Central_Asia.ogg
Guten Abend, Gute Nacht by Johannes Brahms, performed by Stephan
https://commons.wikimedia.org/wiki/File:Lullaby_wound_up_clock_guten_abend_gute_nacht.ogg
Cold String by Tiny Music
The Goldberg Variations: No. 5 by Johann Sebastian Bach, performed by Kimiko Ishizaka
https://commons.wikimedia.org/wiki/File:Goldberg_Variations_06_Variatio_5_a_1_ovvero_2_Clav.ogg
Their Arrival by Emmett Cooke