December 7, 2021

I, Science

The science magazine of Imperial College

It has long been hypothesised that Earth’s water originated from comets, but new data from the ESA’s Rosetta may indicate otherwise

rosetta imageThe European Space Agency’s Rosetta spacecraft recently discovered water vapor on Comet 67/Churyumov–Gerasimenko that greatly differs in composition from Earth’s water.

The origin of water on Earth is one of science’s most important unanswered questions. Scientists have hypothesised that Earth was so hot during its formation that any water originally existing on its surface would have evaporated into outer space. Once Earth cooled down, water was likely delivered to Earth via collisions with asteroids and comets from the Jupiter family, originating from the Kuiper belt.

In order to determine the original sources of Earth’s water, scientists observe the water composition of space objects. For projects like Rosetta, the proportion of deuterium (an isotope of hydrogen with an extra nucleus) to hydrogen is an important comparison.

Dr. Kathrin Altwegg of the University of Bern and colleagues from the Rosetta project found a deuterium to hydrogen ratio on comet 67P that is roughly three times the value of water on Earth.

Rosetta’s data by no means debunks the theory that Earth’s water was delivered via comets. In previous ESA missions, measurements of the water composition of other comets and meteorites matched that of Earth. Professor Edwin Bergin of the University of Michigan and his team, for example, previously found that comet Hartley 2 has the same chemical composition as our oceans.

For comet 67P specifically, scientists have speculated that deuterium to hydrogen ratios closer to Earth’s water could be found on different regions of the comet. This data could also provide new insight into the origin of these comets. As Dr. Altwegg notes, “This surprising finding could indicate a diverse origin for the Jupiter-family comets – perhaps they formed over a wider range of distances in the young Solar System than we previously thought.” She continues, “Our finding also rules out the idea that Jupiter-family comets contain solely Earth ocean-like water, and adds weight to models that place more emphasis on asteroids as the main delivery mechanism for Earth’s oceans.”

These data have sparked much scientific debate. Some scientists, including NASA Near Earth Object program manager Donald Yeomans, believe this new discovery likely rules out comets as a source of Earth’s water. Dr. Altwegg agrees, noting that “We have light water in some comets and very heavy water in other comets. We have to assume the mixture of all these comets is something that is heavier than what we have on Earth, so this probably rules out Kuiper Belt comets as the source of terrestrial water.” While this research yielded compelling results, scientists need more data from similar experiments of other space objects from the Jupiter family to further develop any speculation.

Image: Wikimedia commons