14: Waterworld
Before we begin, I have a very exciting personal announcement to make. I have a new paper published in the journal Geology, one of the top journals in the field. This is by far the most high-impact research of my career, and it took a long time to reach publication.
So what’s the topic? In short, Earth’s oxygen levels increased dramatically halfway through its history- this Great Oxidation Event is one of the landmarks of Earth’s past, around 2.4 billion years ago, late June in our imaginary Earth Calendar. The prelude to this Great Oxidation Event is highly debated: one idea suggests that bacteria in ancient lakes made local “oxygen oases”. Our team found direct evidence supporting this idea: fossilized pond scum from South Africa, 2.7 billion years old or the end of May on the Calendar. The fossils include former bubble cavities(!) with chemical signals of ancient reactions with oxygen, just like a rusting car.
The paper is free for anyone to access, and I’ll put links up on our website, www.bedrockpodcast.com. If there’s enough interest, I might do a side episode on the paper soon. Otherwise, it will take quite a while to reach this chapter of Earth history. Remember, we’re still around January 14th of the Earth Calendar, so we have nearly two billion years before we reach this important pond scum.
Click here to check out the article!
On that note, let’s pick up where we left off in the Hadean, 4.4 billion years ago. Our current plot arc involves the oldest material on Earth, the Jack Hills zircons of Western Australia, thousands of tiny purple crystals. These zircons are the final remnants of Earth’s earliest days, like fragments of pottery from a long-forgotten civilization. Just like those shards, each zircon tells a story- together, these stories paint a picture of the early Earth.
Scientists still argue about this picture, but there are a few basics that most folks agree on. 4.4 billion years ago, much of the Earth’s surface was black, gray, or green, similar to the dark basalts on the modern seafloor. There were also lighter-colored rocks peppered across the planet, granites and their volcanic cousins. Today, these lighter rocks are born around ocean trenches, as dark ocean crust melts back into magma.
Some scientists argue that the Hadean crust was remarkably similar to the modern Earth we saw in Episodes 11 and 12, with plate tectonics, trenches, and continents. Others say the crust was too thick or hot for modern systems to work, and continents didn’t show up until much later. This debate over the origins of plate tectonics will continue until the last season of this podcast. On our Earth Calendar, people have pinned the earliest continents everywhere from January 14th to November 6th. It’s truly one of the biggest mysteries in Earth Science, one we will return to repeatedly.
With that in mind, I think it’s high time for a palate cleanser, and there’s nothing better for that than a cool glass of water. Today, we start another tale from the Jack Hills zircons, the story of how our world became the blue planet.
Part 1: One Drop in a Bucket
Every rocky planet and moon from Mercury to Mars has a crust, but Earth is the only one we know that has a surface ocean. Saturn’s moon Titan has huge lakes, but these are liquid methane. Smaller moons like Europa and Enceladus probably have water oceans, but they are trapped under thick ice sheets.
In contrast, Earth is the only planet in our system where you can stand on a shoreline and gaze out over bodies of water that stretch beyond the horizon. Water is fundamental to life as we know it on Earth. One of the most common facts shared about the world is that 70% is covered with water.
All this is true, but it can create a misleading view of Earth, that the whole planet is sopping wet like a bathroom sponge. Instead, imagine a basketball, a bowling ball or a large melon. If you have one on hand, bring it out. Let’s say this orb represents the Earth. How much water would you need to make the oceans?
A gallon? Way too much. A basic glass of water? Still too high.
If the Earth was the size of a melon, the amount of water in all the world’s oceans, ice caps, and lakes would be smaller than a shot glass- about the size of a grape. There’s a really good visual of this on our website, I invite you to check it out. If we think back to Episode 6, this image makes sense. If you cut open our world like a melon, it’s full of iron and hot crystals, not water or melon juice.
Yes, liquid water is a fundamental part of our planet. Yes, it is crucial to life as we know it. And yes, it is far more plentiful here than elsewhere in our solar system. But that doesn’t mean we should take it for granted. Our neighbors Mars and probably Venus once had liquid water, too. As the Sun grew more powerful, Venus became too hot, and Mars’ atmosphere was stripped away by the solar winds. In both cases, water slowly vaporized away into space. Farther away from the Sun, water became frozen solid inside icy moons or lonely comets.
In Episode 5, we saw the Earth and the other planets jockey for positions to avoid being swallowed by the Sun. Earth’s orbit brought it to a zone where water could remain stable, while its’ larger size prevented a slow dry death like Mars. Some scientists call this area “The Goldilocks Zone”, after the fairy tale character who enjoyed breaking and entering, looking for the spot that was just right to live in. If the Earth was little closer, a little farther, a little bigger or smaller, this podcast would be very different. Perhaps there are other worlds out there where those tales are being told right now.
These are some basic facts about water on the modern world, but as with the Moon and the crust, we now arrive at a basic question: where did all this water come from?
Part 2: The Snow Line
We began our journey into Earth history in Episode 4, in the Cradle of Stardust. There, we met a special type of meteorite called a chondrite. If you cut open a chondrite, you would see thousands of tiny rounded crystal grains, dust bunnies from the beginning of the solar system. Until now, we’ve followed the path of these minerals as they forged Earth’s core, mantle, and crust. But if we picked up a chondrite in the cold vacuum of space, we would notice another mineral- ice.
You heard me correctly. Ice is technically a mineral. Last episode, we said a mineral is a natural material with a well-defined chemical recipe and a regular crystal structure. Ice fits all these requirements: Is it natural? Check. Does it have a chemical definition? Yes: H2O. And does it have a crystalline texture? Double-check, just look at any snowflakes or frozen ponds. The big difference between ice and quartz is that ice melts at much lower temperatures. Using this mineral definition, water is technically ice lava. So, tell your friends that you’re going to drink some lava, then pull out your water bottle, because technically correct is the best kind of correct.
As I mentioned last section, ice is pretty common in the solar system, especially the outer planets. That makes sense: the closer to the sun you are, the more ice will melt- anyone who has sat around a campfire knows the feeling. In fact, there is an invisible line in the solar system that separates the cooler outer planets from the warmer inner ones- it’s called the snow line. Just like a snowline on a mountain, the farther you go, the more likely you’ll see snow.
Solar System, showing the snow lines for water and CO2, Marocchi & Piani 2019
The Solar System’s snow line is around the asteroid belt between Mars and Jupiter, and it’s here that we come back to our chondrite asteroid friends. Just like the planets, the snow line separates chondrites into two families. We talked about one family way back in Episode 4, kudos to you if you still remember it. For us mortals, these are carbonaceous chondrites. I wouldn’t throw such a mouthful at you if it wasn’t important- we will see them throughout Earth’s early history.
Carbonaceous chondrites are the cool asteroids, they form beyond the snow line between Jupiter and Neptune and are full of ice crystals. In contrast, non-carbonaceous chondrites are closer to the sun, between Mercury and Mars. The classic view of these sun-worshipers is that they are baked totally dry, no ice, no water.
Two houses, both alike in dignity, in the fair Solar System, where we lay our scene. If we compare the Earth to these two asteroid families, our planet aligns more closely with non-carbonaceous chondrites closer to the sun. That makes perfect sense- our planet was built from asteroids that surrounded it. However, there is one big problem here: these are the dry asteroids, so where did Earth’s water come from? Remember, water is locked up as ice crystals out beyond the snow line.
The answer comes from an unexpected planetary ally.
Part 3: Jupiter Ascending
I can’t emphasize just how big Jupiter is. If you hollowed out the entire planet, you could fit 1,000 Earths inside it. Jupiter is more than 2.5 times heavier than all the other planets combined. Jupiter’s size and mass have an incredible effect on the modern solar system, moving groups of asteroids like chess pieces, and even aligning planetary orbits- but not in the astrology sense.
Imagine what Jupiter’s influence must have been like back in the earliest days of the solar system, when there were hundreds of smaller planetoids careening around the neighborhood. Some of these little worlds were captured by Jupiter, becoming moons like Ganymede, Io, and Europa. But others would get pushed around by Jupiter’s gravity like a slingshot. Space agencies use the same trick today to get a speed boost for probes like Voyager and New Horizons, hurling them deep into the outer solar system.
Those slingshots are carefully prepared by humans for years using complicated equations. Back in the Hadean, Jupiter was hurling out curveballs in every direction. Since Jupiter is out beyond the snow line, it’s more likely to be visited by icy bodies like our old colds friends the carbonaceous chondrites. These cool characters would be flung unceremoniously into the warmer neighborhoods of the inner solar system, where they could crash into the face of Earth.
Everything about the early Solar System, even the delivery of life-giving water, was done on a violently Promethean level. Things were truly more epic back then.
This all makes for a cool story, but can we back it up? Broadly, yes.
Water is made of two hydrogen atoms and one oxygen atom, H2O. Those hydrogen atoms come in two different flavors: light and heavy. It turns out that water from different spots in the Solar System has very different ratios of heavy and light hydrogen. If Earth water came from cold carbonaceous chondrites in the outer solar system, the water should look similar. And overall, it does!
But the story isn’t fully resolved yet. This tale is the most accepted for now, but there are a few wrinkles. Some argue that a glass of water from the modern Earth would be very different from the ancient Earth, so the similarities with carbonaceous chondrites are just a coincidence. Other recent studies have found intriguing evidence that more local asteroids, closer to the sun, are not as dry as we originally thought. If so, there might not be any reason to include Jupiter in the story after all.
If this cycle all sounds familiar, you’re right. On this podcast, we’ve covered the formation of magma oceans, the Moon, the first crust, and now the first water on Earth. Each time, I’ve given you the most well-accepted hypotheses as of 2022, but there are always counterpoints, usually with decent evidence. Going into each and every alternative here would drive all of us mad, but ignoring them outright does a disservice to the scientists who spend years to get their work published. Believe me, I know the feeling.
For now, here’s what we can comfortably say.
The asteroids that built the Earth not only brought olivine crystals and iron metal, they delivered the first water. Some of these asteroids were from our local neighborhood, while others were hurled from the outer reaches of the Solar System. As these hunks of rock screamed down the sky, ice crystals began to melt in the atmosphere. One violent impact at a time, the meteor shower brought drops of water that would grow into ponds, lakes, and eventually oceans, the thin sheen of water that covers our planet’s surface today.
Next time, we’ll see how this new ingredient interacts with Earth’s crust, and how this dance is recorded within the Jack Hills zircons
***
Thank you for listening to Bedrock, a part of Be Giants Media.
If you like what you’ve heard today, please take a second to rate our show wherever you tune in- just a simple click of the stars, no words needed unless you feel like it. If just one person rates the show every week or tells a friend, that makes us more visible to other curious folks. It always makes my day, and that one person could be you. You can drop me a line at bedrock.mailbox@gmail.com. See you next time!
Images:
South African stromatolites: Wilmeth et al., 2022
Water Pearl: https://www.usgs.gov/media/images/all-earths-water-a-single-sphere
Snow Lines: Marocchi & Piani, 2019
Jupiter: https://commons.wikimedia.org/wiki/File:Portrait_of_Jupiter_from_Cassini.jpg
Music:
Hebrides Overture, Fingal’s Cave: https://commons.wikimedia.org/wiki/File:Mendelssohn_-_Hebrides_Overture_Fingal%27s_Cave.ogg
TV Mambo by Daniel Belardinelli
Catacombs by Big Score Audio
Voliere (Aviary) from Carnival of the Animals, Camille Saint-Saens: https://commons.wikimedia.org/wiki/File:Saint-Saens_-_The_Carnival_of_the_Animals_-_10_Volière.ogg
The Planets: Jupiter by Gustav Holst, performed by the United States Air Force Heritage of America Band:
https://commons.wikimedia.org/wiki/File:Holst_The_Planets_Jupiter.ogg
Seven Days of Flying by Remember the Future