In this illustration of the early Solar System, the dashed white line represents the snow line – the transition from the hotter inner Solar System, where water ice is not stable (brown) to the outer Solar System, where water ice is stable (blue). Image credit: Jack Cook / Woods Hole Oceanographic Institution.

One hypothesis was that the Earth and other terrestrial planets originally formed dry, due to the high-energy, high-impact process of planet formation, and that the water came later from sources such as ‘watery’ asteroids or comets.
Dr Monteleone, Dr Sarafian and their colleagues turned to another potential source of Earth’s water – carbonaceous chondrites, the most primitive meteorites.
Carbonaceous chondrites were formed in the same swirl of materials that gave rise to the Sun some 4.6 billion years ago, well before the planets were formed.
“These primitive meteorites resemble the bulk composition of our Solar System. They have quite a lot of water in them, and have been thought of before as candidates for the origin of Earth’s water,” said co-author Dr Sune Nielsen of the Woods Hole Oceanographic Institution.
In order to determine the source of water in planetary bodies, scientists measure the ratio between the two stable isotopes of hydrogen: deuterium and hydrogen.
Different regions of the Solar System are characterized by highly variable ratios of these isotopes.
The scientists knew the ratio for carbonaceous chondrites and reasoned that if they could compare that to an object that was known to crystallize while Earth was actively accreting then they could gauge when water appeared on Earth.
To test this hypothesis, they used meteorite samples provided by NASA from the asteroid 4-Vesta.
The asteroid, which formed in the same region of the Solar System as Earth, has a surface of basaltic rock – frozen lava.
These basaltic meteorites from 4-Vesta are known as eucrites and carry a unique signature of one of the oldest hydrogen reservoirs in the Solar System.
Their age – about 14 million years after the Solar System formed – makes them ideal for determining the source of water in the inner Solar System at a time when Earth was in its main building phase.
The team analyzed five different samples and found that 4-Vesta contains the same hydrogen isotopic composition as carbonaceous chondrites, which is also that of Earth. That, combined with nitrogen isotope data, points to carbonaceous chondrites as the most likely common source of water.
The study shows that Earth’s water most likely accreted at the same time as the rock. The planet formed as a wet planet with water on the surface.
While the findings don’t preclude a late addition of water on Earth, it shows that it wasn’t necessary since the right amount and composition of water was present at a very early stage.

Adam R. Sarafian et al. 2014. Early accretion of water in the inner solar system from a carbonaceous chondrite–like source. Science, vol. 346, no. 6209, pp. 623-626; doi: 10.1126/science.1256717

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