We have comets and asteroids to thank for Earth's water, according to the most widely-held theory among scientists. But it's not that cut-and-dried. It's still a bit of a mystery, and a new study suggests that not all of Earth's water was delivered to our planet that way.
We have comets and asteroids to thank for Earth's water, according to the most widely-held theory among scientists. But it's not that cut-and-dried. It's still a bit of a mystery, and a new study suggests that not all of Earth's water was delivered to our planet that way.
Hydrogen is the most abundant element in the Universe, and it's at the center of the question surrounding Earth's water.
This new study was co-led by Peter Buseck, Regents' Professor in the School of Earth and Space Exploration and School of Molecular Sciences at Arizona State University.
In it, the authors suggest that the hydrogen came, at least partially, from the solar nebula, a cloud of gas and dust left over after the Sun formed.
Before we dig in to the details in this new study, it's helpful to look at the long-held theory that it may replace.
Earth's Water: The Widely-Held Theory
For a long time, most scientists believed the water-from-comets-and-asteroids version of water's origin here on Earth. It all starts with the formation of the Sun.
When the Sun formed out of a molecular cloud, it swept up most of the material in the cloud, leaving a little left over for everything else: planets, asteroids, and comets.
Once the Sun burst into life with fusion, a powerful solar wind sent a lot of hydrogen from its outer layers out beyond where the inner rocky planets - Mercury, Venus, Earth, and Mars - are today.
This is the realm of the gas giants, and more importantly, comets and asteroids.
Comets are icy, rocky bodies, thought to contain significant amounts of the hydrogen blown out there by the early Sun, and asteroids too, although to a lesser extent. They became a significant reservoir for hydrogen.
When Earth formed, it was a molten ball, its surface kept in that state by repeated collision with asteroids. So far, so good, since the early Solar System was a much more chaotic place than it is now.
As asteroids and comets struck this hot Earth, the water and the hydrogen in it were boiled off into space. As Earth cooled over time, water from comet and asteroid collisions was allowed to condense on Earth, and not be boiled off into space. The water stuck around.
The evidence for this lies in isotope ratios. The ratio of the heavy hydrogen isotope deuterium to normal hydrogen is a chemical signature.
Two bodies of water with the same ratio must have the same origin, the thinking goes. And Earth's oceans have the same ratio as water on asteroids. That's a very simplified version of the widely-held theory of how water got to Earth.
Earth's Water: A Leaky Theory With Holes In It
But scientists are malcontents, always trying to have a better, more thorough understanding of things. They were questioning the "water from comets" theory before this newest study came out.
Back in 2014, some scientists studied the issue by looking at meteorites of different ages. (Meteorites are just asteroids that have struck Earth.) First they looked at what are known as 'carbonaceous chondrite meteorites'.
They're the oldest ones we know of, and they formed about the same time as the Sun did. They're the primary building blocks of Earth.
Next, they studied meteorites that we think originated from the large asteroid Vesta. Vesta formed in the same region as Earth, about 14 million years after the solar system was born.
According to this 2014 study, the ancient meteorites resembled the bulk Solar System composition and have a lot of water in them, so they've been widely considered to be the source of Earth's water.
The measurements in this 2014 study showed that these meteorites have the same chemistry as the carbonaceous chondrites and rocks found on Earth. They concluded that carbonaceous chondrites are the most likely common source of water.
Hydrogen is the most abundant element in the Universe, and it's at the center of the question surrounding Earth's water.
This new study was co-led by Peter Buseck, Regents' Professor in the School of Earth and Space Exploration and School of Molecular Sciences at Arizona State University.
In it, the authors suggest that the hydrogen came, at least partially, from the solar nebula, a cloud of gas and dust left over after the Sun formed.
Before we dig in to the details in this new study, it's helpful to look at the long-held theory that it may replace.
Earth's Water: The Widely-Held Theory
For a long time, most scientists believed the water-from-comets-and-asteroids version of water's origin here on Earth. It all starts with the formation of the Sun.
When the Sun formed out of a molecular cloud, it swept up most of the material in the cloud, leaving a little left over for everything else: planets, asteroids, and comets.
Once the Sun burst into life with fusion, a powerful solar wind sent a lot of hydrogen from its outer layers out beyond where the inner rocky planets - Mercury, Venus, Earth, and Mars - are today.
This is the realm of the gas giants, and more importantly, comets and asteroids.
Comets are icy, rocky bodies, thought to contain significant amounts of the hydrogen blown out there by the early Sun, and asteroids too, although to a lesser extent. They became a significant reservoir for hydrogen.
When Earth formed, it was a molten ball, its surface kept in that state by repeated collision with asteroids. So far, so good, since the early Solar System was a much more chaotic place than it is now.
As asteroids and comets struck this hot Earth, the water and the hydrogen in it were boiled off into space. As Earth cooled over time, water from comet and asteroid collisions was allowed to condense on Earth, and not be boiled off into space. The water stuck around.
The evidence for this lies in isotope ratios. The ratio of the heavy hydrogen isotope deuterium to normal hydrogen is a chemical signature.
Two bodies of water with the same ratio must have the same origin, the thinking goes. And Earth's oceans have the same ratio as water on asteroids. That's a very simplified version of the widely-held theory of how water got to Earth.
Earth's Water: A Leaky Theory With Holes In It
But scientists are malcontents, always trying to have a better, more thorough understanding of things. They were questioning the "water from comets" theory before this newest study came out.
Back in 2014, some scientists studied the issue by looking at meteorites of different ages. (Meteorites are just asteroids that have struck Earth.) First they looked at what are known as 'carbonaceous chondrite meteorites'.
They're the oldest ones we know of, and they formed about the same time as the Sun did. They're the primary building blocks of Earth.
Next, they studied meteorites that we think originated from the large asteroid Vesta. Vesta formed in the same region as Earth, about 14 million years after the solar system was born.
According to this 2014 study, the ancient meteorites resembled the bulk Solar System composition and have a lot of water in them, so they've been widely considered to be the source of Earth's water.
The measurements in this 2014 study showed that these meteorites have the same chemistry as the carbonaceous chondrites and rocks found on Earth. They concluded that carbonaceous chondrites are the most likely common source of water.