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Stefan: Somehow, the Earth has already celebrated its 4.5 billionth birthday, and when you're considering that big of a time scale, it makes you realize that humans have only been around for the proverbial blink of an eye.  There's no way we could know everything that's happened in our planet's history, because we weren't around for most of it, but thanks to geology, we don't need a time machine to explore how our planet was born.  Instead, we can study the oldest rocks and minerals, ones that have been around 14,000 times longer than our oldest human ancestor.  These samples have already shaken up what we thought we knew about the Earth's history, and the very oldest rocks on Earth can even teach us about the birth of the solar system itself.

We've found all kinds of old minerals and rocks, but in 2014, scientists confirmed that zircon crystals found in Western Australia took home the record.  Based on how certain atoms within the crystals had decayed, researchers pegged them at 4.4 billion years old.  That means they formed less than 200 million years after the Earth did.  These crystals grow in magma--molten or semi-molten rock that bubbles beneath the Earth's surface and made of silicon, oxygen, and zirconium.  The Australian zircon crystals were actually no bigger than a household dust mite, just 400 micrometers in length.  

At first glance, they don't seem like much, but these miniscule crystals are almost indestructible, are resistant to erosion, and are capable of outliving the rock in which they were formed.  They just kept washing in and out of sedimentary rocks over billions of years until they were found by intrepid geologists, but discovering them was about more than setting records.  Knowing when, where, and how they formed tells us how the Earth came to be the Earth.  They might also debunk some big ideas we had about our planet's baby days.  

See, we previously thought that during Earth's first 600 million years or so, our planet was all fire and brimstone.  We believed it was constantly bombarded with meteors and covered in a planet-wide ocean of lava.  Appropriately, that time is called the Hadean eon, like Hades, and it's when those oldest zircon crystals formed.  Unsurprisingly, constant meteor impacts and lava oceans aren't the best recipe for any Earth rocks or minerals to stick around, so finding these 4.4 billion year old zircons shook up our story a little bit.  It made scientists realize that the Hadean eon might have been less fiery than we originally thought, because like zircon crystals are really strong, but they still shouldn't have been able to survive those conditions, and that's not the only reason the zircons were significant.  

The researchers also examined what's known as the oxygen isotope composition of the crystals, or the ratio of heavier versus lighter oxygen atoms.  This can help them infer what the temperatures were like when the crystals were forming.  Surprisingly, the zircons' composition actually matched rocks from a later time period, called the archaen eon, when the Earth was cool enough to have oceans and continents.  Together, this suggests that the Earth had actually cooled down enough by 4.4 billion years ago, to form a solid crust.  That's 600 million years earlier than we used to think.  

The crystals are also a huge point in favor of what's known as the "cool early Earth" hypothesis.  This suggests that temperatures on Earth between 4.4 and 4 billion years ago were actually low enough to sustain liquid water oceans, so not really like Hades at all.  Still, these zircons are the only clues we have about this part of Earth's history, and we'll need to do more research to be certain.  Nothing's for sure yet. 

So far, the zircon crystals are the oldest pieces of Earth we've ever found, and they can teach us a lot, but funnily enough, there are even older rocks on our planet.  They just didn't start out here.  The oldest pieces of our planet are zircon crystals, but the oldest rocks on Earth were brought here from the moon or deposited by meteorites, and those can teach us about the whole solar system.  

In 1969, a two metric-ton meteor hit the atmosphere above Allende, Mexico.  Fragments of the meteorite are dated to 4.57 billion years ago, nearly 200 million years older than Australia's zircon crystals and older than almost anything else in our solar system.  Among other things, some scientists have used the fragments to try and confirm the order in which elements condensed when the solar system was just getting started. 

Conveniently around the same time, astronauts also began bringing back samples of lunar rocks.  In 1971, Apollo 14 brought back some of the oldest moon rocks, later determined to be 4.51 billion years old, and the age of these samples, much like Australia's zircon crystals, helps scientists infer that our moon formed within the first 60 million years after the birth of the solar system.  They also show that our moon had a magma ocean of its own, and that it cooled into a lunar crust before Earth's did.  

So far, we don't have that much insight into how the solar system or the Earth formed or into why we're lucky enough that our planet can support life.  Trying to piece together events that happened more than 4 billion years ago can, for obvious reasons, be tricky, but between the zircon crystals, meteorites, and moon rocks, we have plenty of time capsules to give us a good start, and as we keep discovering older samples, we'll just keep learning even more.

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