About a month ago, when NASA discovered 7 planets orbiting the star TRAPPIST-1, I took the opportunity to post on the Goldilocks Zone. This is the region around a star where liquid water can exist on a planet. Any colder and it would boil away. Any farther and it would freeze. Since liquid water is essential for life as we know it, these planets are considered prime candidates in searching for life.
But I ignored another important question. One I forgot to ask. Where did this water come from?
During the formation of the solar system 4.6 billion years ago, water was almost certainly present in the material that collected to form the Earth. Either in pure form or in the form of hydrated minerals.
But this was a violent time. The early Earth faced a barrage of space debri. During the Hadean epoch (4.1 billion years ago), these high energy impacts could have released enough heat to melt the surface of the Earth. Any liquid water would have boiled away. Any water on the surface would have had to come there afterwards.
So how did it get there?
The answer to this question has changed a lot in the last 5 years as new studies have come from study of moon rocks, asteroid minerals and hydrogen isotopes in comets. Let us look at a few of the ideas that have been proposed.
One of the most spectacular displays of water vapour comes from comet tails. Comets come from the outer reaches of the solar system from the Kuiper belt beyond Neptune. The Kuiper belt contain many chunks of rock and ice left over from the dawn of the solar system. When one of these bodies come close to the Sun, the ice vaporizes producing the long visible tail we see when a comet passes Earth.
Kuiper belt (let), comet tail (right)
Could comets have crashed and brought water to Earth. Probably not. The reason has to do with the type of water. A water molecule contains 2 atoms of hydrogen and 1 atom of oxygen (H2O). But hydrogen comes in 2 forms, the normal hydrogen (H) and a heavier isotopic form called Deuterium (D). Both forms occur naturally but Deuterium is far rarer than Hydrogen.
It turns out that the ratio of D/H in Earth’s water is very different from that in seen in comets (comets have a much higher ratio). Instead it is much closer to the value seen in asteroids between the orbits of Mars and Jupiter. So we can rule out comets from our list of water carriers.
A team at Woods Hole Oceanographic Intuition studied some of the oldest meteorites known which contain carbonaceous chondrite. These are believed to have formed at the time of the early solar system before the planets had fully taken shape. Next they compared these with asteroids originated from a large asteroid named Vesta which is believed to have formed in the same region as Earth. The meteorites had the same chemistry and a lot of water in them.
This means that water was part of the matter that formed the Earth and was also present in many of the asteroids that would have crashed into it billions of years ago.
Finally in 2015, University of Hawaii researchers who studied rocks that came from the deep mantle of the Earth also reported that they had found glass crystals containing droplets of water. The D/H ratio of the water was identical to that on our planet. This means that even from the start, water was present deep inside the earth and could have been brought to the surface in the form of water vapour which could condense to liquid once the temperature mellowed.