10: The Oldest Thing on Earth

Special shout-out to Tyler in Milwaukee, Wisconsin for solving last week’s hidden word search! The theme was tracks from Pink Floyd’s album The Dark Side of the Moon, and the track I didn’t mention was Brain Damage. It was kinda hard to fit that one in the script. Tyler also asked if solar eclipses would actually be shorter 4.5 billion years ago, since the Earth and Moon were spinning much faster. You might be right, Tyler- though I couldn’t find any papers on it. Either way, with the Moon ten times closer, a Hadean solar eclipse would be an incredible sight.

Speaking of which, we’ve spent the last two episodes covering the Moon’s first days, 4.5 to 4.4 billion years ago, the second week of the Earth Calendar. Our Moon was born after a violent planetary collision, collected into an angry ball of magma floating ten times closer than today, then cooled into a giant golf ball covered in the white mineral plagioclase. The dark spots that make the Man in the Moon won’t form for a long time yet.

Every time you look up at the moon, you’re looking at mountains and craters more than four billion years old. In contrast, Earth has almost nothing left from that time- a few square miles of twisted rock, a few grains of crystal you could hold in your hand.

This is the tradeoff between our two worlds. The Moon is a giant time capsule- without it, the Hadean would remain even more of a mystery. But that same stillness keeps the Moon from experiencing plate tectonics, oceans, a real atmosphere, and life. On Earth, all these beautiful things contribute to the destruction of our history. Nearly every Hadean rock has been recycled into the mantle, weathered by wind and rain, or broken into soil by plants.

In other words, the Moon and the Earth are like two toys. The Moon, a porcelain doll that stays on the shelf, and the Earth, a stuffed rabbit patched up over years of love. In the end, which do you remember more, which brought you more joy? For me, I would much rather be on a living world that forgot where it came from than a dead one which remembers everything.

Which brings us to today’s topic. With so much of our history lost to time, it makes us appreciate the handful of things that survived. For the past week of the Earth Calendar, Earth has given us nothing. We’ve relied on meteors, the moon, and models to tell our story, but no more! It’s finally time for me to show you the first survivor, the oldest thing on Earth.

Part 1: Jumbo SHRIMP

The oldest thing on Earth is a purple crystal, a tiny grain of sand- if you held it on your hand, it would be just barely visible. One good sneeze, and the oldest crystal would be lost to time. This tiny mineral is a zircon, a cousin of the cubic zirconia found in wedding rings. In Episode 3, we learned that zircons are the best way to date ancient rocks- they’re incredibly tough and capture uranium atoms which decay into lead. Like sand passing through an hourglass, we can compare the amount of uranium to lead and calculate a date. The more lead in a zircon, the older it is.

Measuring this lead is tricky, which brings us to the beginning of our story.

The year is 1973. Apollo 17 has brought back the final load of Moon rocks, ending the first era of human visits to the moon. In the Australian National University in Canberra, a new chapter in Earth research is just beginning. A group of scientists led by Prof. Bill Compston wanted to measure zircons more accurately than anyone before.

Let’s say you have a tiny grain of zircon, and you want to know how old it is. If you destroy the whole thing, there’s no way to double-check your results. And there’s another problem. When a zircon crystallizes out of magma like rock candy, it doesn’t form all at once. Instead, it slowly adds layers on the outside like a tree. In fact, the insides of zircons look like microscopic tree rings- so measuring the whole crystal would only give you an average date. Finally, while zircons are incredibly tough, they’re not perfect, and who can blame them after four billion years? They can be cracked, chipped or rounded down by time, which increases the error of measured dates.

The solution is to shoot a focused stream of atoms at these poor zircons, just like a pressure hose. The atom stream vaporizes a piece of the crystal, which is sucked into a machine that measures uranium, lead, and other elements. Like a video game, you can point this pressure hose wherever you want, avoiding cracks and aiming for the heart of the zircon. The best part: you can keep your crystal and repeat these experiments, since you’re only removing tiny pieces.

Back in 1973, this technology was possible, but Prof. Compston and his team wanted to improve it. They wanted to solve a huge problem in Earth history- the time gap between Moon rocks and Earth rocks. Moon rocks from the Apollo missions were 4.5 billion years old. In 1973, the oldest Earth rock was nearly a billion years younger. Geologists like Prof. Compston wanted to fill that gap, but finding the right material required the best technology possible.

Over 10 years, the Compston team in Canberra created a new machine that could shoot more accurate atom streams, thinner than a strand of hair. This machine would shape geochemistry for years, with copies built around the world. This titan of a machine, more than five meters long, is called… SHRIMP.

It's now 1980, and the SHRIMP is ready to be put to the test. The only task left is to bring it some old rocks to date. They were not looking for a needle in a haystack, they were looking for a grain of sand on an entire planet. The hunt is on.

Part 2: One Grain of Sand

So where do we start looking for the earliest zircons? Which rocks do we feed to the SHRIMP?

The most logical places are the oldest rocks on Earth. The first half of Earth history is only represented by 5% of the Earth’s surface, but that 5% is spread all over the world, including Western Australia. The Compston team already knew a location that was 3.6 billion years old, a hill called Mount Narryer. These weren’t the oldest rocks on the planet, but they were a good place to start.

Mount Narryer is deep in the Outback of Western Australia, in the lands of the Wajarri people among others. The closest major city is Perth, a nine-hour drive to the south. Imagine a land of low rolling hills covered in dry rusty soil, peppered with spiky grass and skinny green mulga trees. In the distance, you can see emus and red kangaroos lope across the hot landscape. In January, it’s up to 100 F, so it’s much better to work in July, when the high is only 70 F.

As we pick our way through the desert, what are we looking for exactly? Zircons are too small to see in the field, so we’re looking for two rocks: granites and sandstones. Let’s start with granites.

Zircons are born in cooling magma, like many other minerals we’ve already met. When we measure the age of a zircon, we are measuring when it crystallized from magma. In normal cases, this makes granite rock an excellent place to finds zircons.

But the Hadean is never a normal case. Even in 1980, geologists had dated enough of the world to understand that there was very little chance of finding a granite or any other rock more than 4 billion years old. Most Hadean rocks had been recycled back into magma, or ground down into sand. Which brings us to sandstone.

Let’s pick up a handful of sand in the Australian Outback, or your local beach. You have just picked up hundreds of rocks finely powdered over time. I would bet good money that there’s a few zircons in that handful. If we fed your beach zircons to the SHRIMP machine, it would give you a range of dates: this zircon is 10 million years old, that one is 100 million, another one a billion years old. Each grain came from a different rock that was much older than the sand you’re holding.

*Jack Hills zircons, Western Australia ~3.9 Bya*

Now we could sift modern beaches for Hadean zircons, but it makes even more sense to search in older beaches, closer to the source. As we discussed in Episode 2, the best place to find an ancient beach is in sandstone. Geologists take sandstones that are billions of years old, pluck out the zircons, and search for Hadean crystals, just like our imaginary measurements on beach sand. This is exactly the research that my guest Dr. Nadja Drabon performs at Harvard- if you want to hear more detail, check out my interview with her.

To recap: Dr. Compston and his Canberra crew are collecting ancient sandstones around Mount Narryer. The sandstones are only 3.5 billion years old, but we now know that we could find even older zircons in them, the pulverized remains of Earth’s oldest rocks.

We have the technology, we have the capability, we have the rocks. Let’s do some dating!

Part 3: Closing the Gap

In 1983, ten years of engineering and fieldwork paid off. Using the SHRIMP machine in Canberra, four zircons from Mount Narryer sandstones were dated between 4.1 and 4.2 billion years old, much closer to ages from the Moon and meteorites. The results were published in the journal Nature, but the story was only just beginning.

Three years later, the team hit a jackpot in the nearby Jack Hills of Western Australia. Most researchers are lucky to find a few dozen Hadean zircons. The Jack Hills have given us thousands. Some Jack Hills sandstones are up to 12% Hadean zircons, which is mind-blowing considering their usual rarity. The Jack Hills quickly became a hotbed of international geology research. In 2001, a joint team from Western Australia, Wisconsin, and Scotland found the oldest zircon grain yet, a date that stands to this day.

You have been very patient with me as I’ve rambled through this story, but finally, it’s time to reveal that date.

The oldest thing on planet Earth is a deep purple zircon grain from the Jack Hills of Western Australia, in the lands of the Wajarri people. The grain is 200 microns wide, barely visible to the human eye, and is labeled W74/2-36. This zircon is 4 billion 404 million years old, give or take eight million years. In short, 4.4 billion. On our Earth Calendar, that translates to Jan 14, two weeks away from the beginning of the Earth.

*The zircon W74/2-36, the oldest thing on Earth, 4.404 billion years. Jack Hills, WA.*

As of 2022, more than 100 papers have been published on the Jack Hills. It’s not uncommon to see eight or ten Jack Hills papers come out in a single year- nearly all of them are talking about Hadean zircons. There are other locations around the world, including my guest Dr. Nadja Drabon’s work in South Africa. But the Jack Hills remains the oldest and widest window onto Earth’s earliest past.

That window was opened in the 1980s, flung wide in the 2000s, and as technology has improved, we’ve been able to open this window a little wider every year. You see, zircons trap more elements than just uranium- they can capture oxygen, titanium, and many more. Each of these atoms was a part of the Hadean world, each has a story to tell. Some atoms will tell us how the earliest crust formed. Others will give us a peek into the first ocean.

Next episode, we’ll keep looking at the Jack Hills and start asking these big questions, looking for clues like a message in a 4.4 billion-year-old bottle.

Some housekeeping before we finish. First, I want to thank everyone who has listened and shared this story. It’s been incredible seeing the fan base grow over these 10 episodes. We’ve had listeners on all continents except Antarctica. You might not be aware that we also have a website- bedrockpodcast.com. Here, you can find transcripts of each episode with helpful pictures if you want more information, plus a hand-made version of the Earth Calendar.

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.

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Thank you for listening to Bedrock, a part of Be Giants Media. You can drop me a line at bedrock.mailbox@gmail.com. See you next time!

Images:

Zircon from Pakistan by Robert Lavinsky: https://commons.wikimedia.org/wiki/File:Zircon-dtn1a.jpg
Prof. Bill Compston: https://archives.anu.edu.au/exhibitions/building-australias-national-university-75-years-australian-national-university-39

Compston photo taken by Gap Carpay

SHRIMP: https://science.anu.edu.au/research/facilities/sensitive-high-resolution-ion-microprobe-shrimp

Zircon cross-section: Valley et al., 2014

Mulga Scrubland by Mark Marathon: https://commons.wikimedia.org/wiki/File:Acacia_aneura_habit.jpg

Jack Hills Zircons: Cavosie et al., 2005

W74/2-36 picture taken by John Valley

Music:

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

The Goldberg Variations: No. 3 by Johann Sebastian Bach, performed by Kimiko Ishizaka https://commons.wikimedia.org/wiki/File:Goldberg_Variations_04_Variatio_3_a_1_Clav._Canone_all%27Unisuono.ogg

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
The Carnival of the Animals: Hémiones by Camille Saint-Saens, performed by Neil and Nancy O’Doan https://commons.wikimedia.org/wiki/File:Saint-Saens_-_The_Carnival_of_the_Animals_-_03_Hémiones_(animaux_véloces).ogg

Prelude in C Minor by J.S. Bach, performed by Jason M.C. Han. https://commons.wikimedia.org/wiki/File:C_minor_Prelude_Bach_(BWV847),_Performer_JMC,_Han.wav

Night Cruiser by Arenas

Space 80s by Ilegot

Seven Days of Flying by Remember the Future

9: The Great Gig in the Sky

11: Heretics and Heroes