Modern Earth is wet and temperate (last week's heat wave aside), but the early Earth was molten and hostile, meaning water and other volatile substances like hydrogen and nitrogen compounds must have been deposited after formation. The likely culprits are comets—full of water ice and organic compounds—and meteorites, which were likely more water-laden in the early days of the Solar System. Knowing exactly where Earth's water and organic molecules originated would reveal a great deal about our planet's history and help us understand the environment in which life arose.

In a Science article, Conel Alexander and colleagues compared the abundance of deuterium—the isotope of hydrogen with a proton and neutron instead of just a proton—and nitrogen isotopes across several types of delivery candidates. They found significant differences between comets and meteorites, indicating they likely formed in different regions of the Solar System. They also argued that comets were unlikely to be the source of water and other volatiles now present in the inner Solar System.

In particular, primitive meteorites known as CI chrondrites bear the closest resemblance to the primordial components of the terrestrial planets, meaning they may be responsible for the wet Earth we inhabit today.

CI chrondrites are a subclass of common rocky meteorites known as the carbonaceous chondrites (CC); CI stands for "carbonaceous Ivuna," named for the region in Africa where they were first identified. As the name suggests, CCs contain carbon compounds, while "chrondrite" refers to the internal structure of the meteorites. CCs—including CIs—are probably fragments of asteroids from the Asteroid Belt region between Mars and Jupiter.

Water is abundant in the Solar System, but the frequency with which deuterium replaces one of the ordinary hydrogen atoms in the H 2 O varies with location. Objects that formed close in to the Sun have less deuterium than those that formed farther out. Therefore, measuring the relative abundance of deuterium to regular hydrogen in an object provides some information about where it formed within the solar nebula—the cloud of gas and dust from which planets, meteorites, and comets formed. Deuterium isn't that common anywhere, so the abundance is generally reported in parts per thousand (‰) rather than the more familiar percentages, or parts per hundred (%).

The authors of the study examined 86 chondrite meteorite samples and compared them to a number of other Solar System objects, including comets and Saturn's moon Enceladus (using data obtained from the Cassini spacecraft). Since meteorites don't have a lot of water content, they couldn't do a direct comparison using water alone. Instead, they examined the deuterium fraction in general, and used other volatiles—carbon compounds and an isotope of nitrogen—as proxies for water content. They found a clear division between the comets and many of the CCs: comets were relatively high in deuterium, while most chondrites contained less than that found in seawater on Earth.

Enceladus turned out to be remarkably comet-like, while the Jupiter family comets (or JFCs, comets originating in the region between Jupiter and Saturn) resembled Earth in deuterium abundance. The researchers suggested this may be due to a complex relationship between deuterium content and distance from the Sun, since JFCs are not likely to be the source of Earth's water.

However, the CI chondrites had the most obvious correspondence to the chemistry of the Earth-Sun system in the early days of the Solar System. Based on their dissimilarity to comets, the authors argue they could not have formed in the outer Solar System, which has been suggested in other models. This has two immediate consequences: comets were unlikely to be the source of Earth's volatiles based on the isotope analysis, and Earth's water probably came from the Asteroid Belt, carried by CC meteorites.

Science, 2012. DOI: 10.1126/science.1223474 (About DOIs).