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Whenever we look for extraterrestrial life,
we look for water—because we know that
it’s all thanks to water that life formed on Earth.
So water is important for our continued existence,
and this is a watery planet…
but somehow, we’re not entirely sure
where that water came from.
Astronomers have thought for years
that Earth was dry in the beginning
and it got its water most likely delivered
on comets or asteroids
from the outer solar system that pummeled
Earth as it was forming.
But a paper published last week
in the journal Science suggests
that Earth got at least some of its water
much closer to home—
and that it might have actually started out wet.
The evidence comes from a type of meteorite
called an enstatite chondrite or EC.
ECs originated in the inner solar system—
they’re basically the leftovers of the material
that glommed together to form Earth.
In the past, scientists have thought that
objects that formed so close to the Sun
wouldn’t have been able to hold onto water,
which would just vaporize in such high temperatures.
But some models have suggested
that these objects could have retained some water
through adsorption, the process of physically
or chemically locking water to a surface.
It turns out ECs actually have
a lot more water than we'd assumed.
The authors of the paper figured this out
by measuring the amount of hydrogen in 13 ECs,
assuming that all this hydrogen was
either currently or previously part of a water molecule.
It’s not a perfect method,
and hydrogen can take many other forms,
but at least it provides
an upper limit for how much water
is locked up in the rock.
And it tells us that if Earth formed
mostly out of ECs, the water inside these rocks
could account for at least three times the amount
of water currently on the Earth’s surface.
Which isn’t out of the question:
Overall, the composition of these rocks is
really similar to Earth’s,
so the authors believe it’s likely
that ECs were the main building blocks for Earth.
But… even if Earth formed
with a lot of water,
that doesn’t necessarily mean
it kept all the water it formed with.
Remember, this was a really hot process,
like, magma-ocean hot.
So, the authors acknowledge that
while Earth may have kept that initial water,
it may have also lost a lot of it to evaporation.
But even if it did, ECs could still be
our water source!
Now that we know how much water
there is in these rocks,
we don’t necessarily need comets
and asteroids from the outer solar system
to explain how all this water ended up here.
At least some of that water could have
come from nearby ECs, which were also colliding
with Earth.
And not only does that tell us something about
where our water comes from,
it also tells us something about
how watery planets can form in general!
In other meteorite news,
the authors of a study published last week
in the journal Science Advances
took a look at the special kinds of minerals
that form during meteorite impacts.
These events produce such high temperature
and pressure that they rearrange the molecules
in minerals to form new crystal structures
called polymorphs.
These days, the only other place on Earth
with conditions like these is
deep in the mantle,
and we can’t drill down that far.
So impact sites are a good way
to get an idea of what Earth’s interior is like
without actually going there.
Scientists are especially interested
in what happens to quartz in these conditions.
Quartz is important because
it’s one of the main components of Earth’s crust,
and it gets recycled over time
as tectonic activity churns things up.
But we don’t actually know
what exactly happens to material from the crust
as it gets pulled into the depths of the Earth.
We can get an idea, though,
by looking at what happens to quartz at impact sites.
In the early 1960s,
scientists started doing research
at a crater in Arizona and found
a polymorph called stishovite.
Like quartz, it had an ordered,
crystal lattice structure made up of silica,
a combination of silicon and oxygen atoms.
As a result, scientists assumed that the stishovite
was once quartz that had been transformed
in the impact.
But the authors of last week’s study realized that was a mistake:
They’d been trying to figure out how quartz transforms into stishovite by giving it a
shock of high temperature and pressure in the lab.
Except, instead of getting stishovite, they produced a totally new polymorph!
Unlike both quartz and stishovite, which have a nice crystalline structure, its molecules
were mostly disordered.
It was also not very stable.
It would revert back to a mix of crystalline quartz and glassy silica after a little bit
of time—which explains why no one has found this polymorph in any impact craters.
But what it doesn’t explain is where stishovite comes from, since that definitely has been
found in impact craters.
The key is that silica can take many different forms in nature.
It can have ordered crystal structure, like both quartz and stishovite, or it can have
no structure at all, like opal and glass.
And researchers found that stishovite just comes from a different form of silica instead
of quartz: It comes from glass!
Even though this study didn’t turn out as expected, all of these findings help us better
model what’s going on inside the Earth, and how rocks composed of crystals react to
the conditions in the mantle.
And that’s important—because not only is this happening deep within Earth today,
when Earth was young, transformations like these were happening all the time.
So, like the search for the origin of Earth’s water, the study of impact craters can help
us understand Earth’s earliest days.
Thanks for watching this episode of SciShow Space News, which was sponsored in part by
HBOmax’s Raised by Wolves, now streaming.
These days, we’re always learning something new about the other worlds out there, and
sometimes, you can’t help but wonder about what life would be like on a different planet.
If you’ve ever thought about that, you might like this new sci-fi series Raised by Wolves
from Ridley Scott, the director of The Martian.
It explores a world where Earth has been destroyed and it’s up to two androids to raise children
on a mysterious, uninhabited planet.
As the new colony of humans starts breaking apart over religious differences, the androids
learn how difficult and dangerous it is to try to control the beliefs of humans.
If you’re interested, you can click the link in the description to stream it now!
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