For decades, scientists believed that comets were the primary source of Earth’s water.
These icy celestial bodies, hurtling through space, seemed like the perfect candidates to have delivered the life-sustaining liquid to our planet billions of years ago.
But new findings from the Rosetta mission are turning this theory on its head.
The European Space Agency’s Rosetta spacecraft, which has been orbiting and studying Comet 67P/Churyumov-Gerasimenko (Comet 67P for short), has revealed that the water on this comet is chemically very different from Earth’s water.
Specifically, the ratio of deuterium (a heavy hydrogen isotope) to regular hydrogen in Comet 67P’s water is about three times higher than that found in Earth’s oceans.
This discovery suggests that asteroids, not comets, may have been the primary source of Earth’s water.
As Kathrin Altwegg, the principal investigator on the Rosetta project, explains, “Our finding…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.”
But why does this matter?
Because understanding where Earth’s water came from could also shed light on how life began—and whether it could exist elsewhere in the Solar System.
Comets Aren’t the Cosmic Water Carriers We Thought They Were
At first glance, comets seem like the obvious source of Earth’s water.
They’re essentially giant, dirty snowballs, packed with ice and organic molecules.
But here’s the twist: comets may not be the cosmic water carriers we thought they were.
The Rosetta mission’s analysis of Comet 67P’s water has revealed a deuterium-to-hydrogen ratio that’s significantly higher than Earth’s.
This finding contradicts earlier data from Comet Hartley-2, a Jupiter-family comet like Comet 67P, which had a deuterium-to-hydrogen ratio similar to Earth’s.
This inconsistency suggests that the story of Earth’s water is far more complex than we imagined.
As Humberto Campins, a physics and astronomy professor at the University of Central Florida, notes, “Rosetta’s findings are interesting and important, but I wouldn’t necessarily say that this information makes asteroids a more likely source.”
How Rosetta Made the Discovery
Rosetta’s findings are based on mass spectrometry, a technique that measures the chemical composition of substances.
By analyzing the water vapor emitted by Comet 67P, Rosetta was able to determine the ratio of deuterium to hydrogen in its ice.
The results were striking: Comet 67P’s deuterium-to-hydrogen ratio is about three times higher than that of Earth’s oceans.
This makes it the most chemically distinct comet water ever analyzed.
The Case for Asteroids
While comets are no longer the leading candidates for Earth’s water, asteroids are emerging as strong contenders.
Previous research has shown that the deuterium-to-hydrogen ratio in water found on certain types of asteroids matches that of Earth’s oceans.
Asteroids, which are rocky rather than icy, are thought to have delivered water to Earth during the Late Heavy Bombardment period around 4 billion years ago.
This era, marked by intense asteroid and comet impacts, may have also brought the organic molecules that served as the building blocks for life.
What’s Next for Rosetta and Asteroid Research?
Rosetta’s mission isn’t over yet.
As Comet 67P approaches the Sun, scientists will continue to monitor changes in its water composition.
These observations could provide further insights into the comet’s chemistry and behavior.
Meanwhile, other missions are set to explore the asteroid connection.
Japan’s Hayabusa-2 mission, which aims to return an asteroid sample to Earth by 2020, and NASA’s OSIRIS-REx mission, launching in 2016, will analyze the water and organic molecules on asteroids.
These missions could bring us closer to understanding how Earth got its water—and perhaps even how life began.
What This Means for Life Beyond Earth
The question of where Earth’s water came from isn’t just an academic exercise.
It has profound implications for the search for life beyond our planet.
If water and organic molecules were delivered to Earth by asteroids or comets, the same process could have occurred elsewhere in the Solar System—or even in other star systems.
As Matt Taylor, the ESA’s Rosetta project scientist, puts it, “This outstanding observation certainly adds fuel to the debate about the origin of Earth’s water.”
A Cosmic Puzzle with Profound Implications
The Rosetta mission has challenged one of the most widely accepted theories about the origin of Earth’s water, opening up new questions and possibilities.
While the debate is far from settled, one thing is clear: the story of Earth’s water is more complex—and more fascinating—than we ever imagined.
As we continue to explore the cosmos, missions like Rosetta remind us of the interconnectedness of our Solar System—and the enduring mysteries that await us.
