Consider this the next time you pick up a "fresh" glass of water: some of the molecules in that water are billions of years old - much older than the solar system itself.
This
seems impossible at first: how could water on Earth predate the solar system in
which it exists? However, recent peer-reviewed research published in the
journal Science confirms this.
Astronomers
arrived at this conclusion by proving that water in our solar system had to
have been produced inside the thick cloud of gas and dust that preceded and was
required for the formation of the star we know as the Sun. This means that the
water that finally made its way to Earth through "wet rocks" such as
asteroids or comets existed before the Sun exploded into a star.
Ted
Bergin, an astronomy professor at the University of Michigan in Ann Arbor and
one of the study's authors, describes the discovery as
"extraordinary." “If you look back 4.6 billion years ago, there's an
incredible story to be told,” he says.
Earth
was formed from microscopic particles little larger than the width of a human
hair. Astronomers — who, Bergin says, are “very imaginative souls,” — call this
"dust."
These
dust particles would collect so much energy from the Sun at their distance from
it that they would become too hot for water to condense on them as ice. “This
means that when the Earth was born, it was dry,” Bergin says. “So that's an
interesting problem: Where did the water come from?”
If we consider the matter more generally, Bergin asserts, we must ask: Where did all the water in the cosmos originate from? “The universe isn't made of water, it's made of atoms," he explains. "So, at someplace, at some time, those atoms came together in the universe, via chemistry, to form water.”
Fortunately, astronomers can investigate that chemistry in Earth-based laboratories. They are capable of reproducing the circumstances that result in the formation of water. They do this using a process known as isotopic fingerprinting.
They do this using a process known as isotopic fingerprinting. The second kind is deuterium. These elements coexist in a more-or-less stable ratio throughout the solar system: for every deuterium atom, there are around 100,000 hydrogen atoms. Water has around this amount of hydrogen and deuterium.
“But
chemistry tells us that under very specific conditions there can be an excess
of deuterium," says Bergin. "That's what we call a ‘isotopic
fingerprint.’ Earth contains a surplus of deuterium, as do comets and asteroids.”
The
isotopic fingerprint appears only at very low
temperatures, between 10 and 20 degrees above absolute zero (-441 degrees
Fahrenheit). “So, because the Earth has this excess of deuterium,” Bergin
explains, “we know one thing already: that whatever the source of the water
was, it was really, really cold. So now we have to look at star and planet formation and ask, ‘Where is it that cold?’”
When a star starts to develop, temperatures may get so low in just two locations inside the enormous, violent system: within the cloud of gas and dust that surrounds the protostar, or within the accretion disc that begins to form around it. However, there is one more twist: water is also created by a chemical process called ionization. The researchers established that the disc is incapable of powering this chemical reaction by examining a comprehensive model of it.
“This
tells you that, of the two potential sources to make the water — the disc and
the cloud of gas and dust — the disc can't do it,” Bergin explains. Therefore,
the water with the isotopic fingerprint could only have emerged from
the gas and dust — about a million years before the formation of the sun.
Nonetheless,
this begs the issue of how this water reached Earth. According to Bergin,
planets are generated from the same cloud of gas and dust that collapses and
ignites to form a star. Within the cloud, rocks were thrown into space and
collided with the particles that eventually created Earth. Although some of
them lacked water, they collided with the Earth and amalgamated with it. Additional
boulders were flung our way from a greater distance, and these rocks were
chilly enough to contain water.
“So
as the Earth was being born, these rocks from greater distances supplied the
water,” Bergin says. “The water became part of the rocks, and it just
out-gassed via volcanoes, and that created the oceans and the atmosphere, and
this wonderful planet that we have today.”
0 Comments