Where did Earth’s water come from? – Zachary Metz


It has no taste, color or smell,
and we often look right through it. It covers over 70% of the Earth, cycling from the oceans and rivers
to the clouds and back again. It even makes up about 60% of our bodies. With all this water around and inside us, it’s easy to take
its presence for granted. But in the rest of the solar system,
liquid water is almost impossible to find. So how did our planet end up
with so much of this substance and where did it come from? As you probably know, a water molecule consists
of two basic parts. Hydrogen, the simplest of all elements, has been around since close
to the beginning of our universe. Oxygen entered the scene
several hundred million years later after stars began to form. The massive pressure at the center
of these fiery infernos was so great that hydrogen atoms
fused together to form helium. Helium, in turn, fused
to form heavier elements, like beryllium, carbon and oxygen
in a process known as nucleosynthesis. When stars eventually collapsed
and exploded into supernovas, these new elements
were spread across the universe and combined into new compounds,
like the now familiar H2O. These water molecules
were present in the dusty cloud that formed our solar system and more collided with our planet
after its formation. But there’s a big question
that we don’t have the answer to: how much water arrived on Earth, and when? If, as one theory goes, relatively small amounts of water were
present on Earth when the rock formed, the high temperatures
and lack of any surrounding atmosphere would have caused it
to evaporate back into space. Water would have been unable
to remain on the planet until hundreds of millions of years later when our first atmosphere formed
through a process called outgassing. This occurred when
molten rock in the Earth’s core released volcanic gasses to the surface, creating a layer
that could then trap escaping water. So how then did water
get back to the planet? Scientists have long suspected that much of it was brought
by ice-bearing comets, or more likely asteroids that bombarded
the Earth over millions of years. Recent research
has challenged this theory. In examining carbonaceous
chondrite meterorites that formed shorty after the birth
of our solar system, scientists have found
that not only did they contain water, but their mineral chemical composition
matched rocks on Earth and samples from an asteroid that
formed at the same time as our planet. This suggests that the Earth
may have accumulated a substantial amount of water early on
that was able to stay put, despite the lack of an atmosphere, though asteroids may
have brought more over the eons. If this turns out to be true, life may have formed much earlier
than previously thought. So we do not yet definitively know
whether the water on Earth came from its initial formation, later impacts, or some combination of the two. Regardless, the water that runs from our
showers, drinking fountains and faucets is something that didn’t just come from
a nearby lake or river, but first underwent a cosmic
and chaotic journey to get here.

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