How To Date A Planet

The moon is old – at least, it looks old,
since its entire surface is strewn with the rubble of broken down rock and pockmarked
with scars from ancient asteroid impacts. We know that rocks weather here on Earth too,
and that meteorites have made some considerable dents over the years, but today Earth looks
reasonably fresh and new without widespread craters. Earth owes its youthful appearance to the
fact that its outermost layer is constantly renewing itself – hiding its age. New rock
spills onto the planet’s surface from the mouths of volcanoes; magma is released at
the seams of ocean floors and cools to become part of those floors; rocks metamorphose (and
mountains form) when continents collide; and the sea floor is recycled as continents ride
over it. The process of remaking rock doesn’t just
make it look young, it makes it young – at least, to us, because our best tool to date
rocks is to measure the amount of decay of certain radioactive elements in the rocks.
And when rocks melt and reform anew, all evidence of that decay is destroyed. Normally this is great, because it means rocks
carry around a record of their age – the time since they formed. For example, we can tell
that ocean floors are spreading because their rocks are youngest near mid-ocean ridges and
get progressively older as you head towards land, where they’re often recycled by sinking
under continents. However, because the earth remakes its rock
so much, and because remaking rocks resets their built-in clocks, it’s hard to look really
really far back into our planet’s history – most of the first solid rocks that formed
ages ago on Earth’s fiery surface have probably long since been remade. That’s where a resilient little mineral called
zircon comes in. Zircon is similar to quartz, both in its chemical structure and in its
durability, so it sticks around a long time after other minerals weather away. But zircon
also has two things quartz doesn’t: zirconium, which gives it its name, and small traces
of radioactive uranium. Uranium atoms are similar enough to zirconium that they can
occasionally slip into the mineral’s crystal lattice in zirconium’s place, but unlike zirconium,
the unstable uranium atoms eventually radioactively decay into lead atoms. Which is weird, because lead isn’t similar
to zirconium, and so would never have ended up in a zircon crystal on its own – so the
more lead you find in a zircon crystal, the older it is. And one of the oldest terrestrial minerals
ever found, quite possibly the oldest piece of the earth we know of, happens to be a very
lead-filled grain of zircon found embedded in sandstone in Western Australia. Somehow,
this crystal has managed to escape destruction by erosion and volcanic eruptions and asteroid
impacts over the millennia, and while we don’t know whether it formed in the very first rock
on Earth, if it didn’t then by definition the first rock must be even older. So despite earth’s best efforts to hide its
age from us, uranium-lead dating of little bits of zircon tells us our planet formed
at least 4.4 billion years ago. There’s also other evidence from meteorites that suggests
the earth is even older, but at bare minimum, this zircon lets us say with confidence that
the earth is literally older than dirt.


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