The debate as to how Earth became a water-rich planet has been reignited following the publication of new research that points to a potential cosmic culprit: Jupiter-family comets.
NASA scientists have found that water on Comet 67P/Churyumov-Gerasimenko closely matches the molecular signature of Earth’s water, lending fresh support to the theory that comets played a critical role in delivering water to our planet.
The research revisits earlier findings from the European Space Agency’s (ESA) Rosetta mission, which had cast doubt on the idea that water had come from such comets.
But by analyzing thousands of water measurements collected from 67P and identifying key processes that may have skewed prior results, the team of scientists, led by Kathleen Mandt of NASA’s Goddard Space Flight Center, thrust Jupiter-family comets back to the forefront of Earth’s story.
ESA/Rosetta/NAVCAM
Comet 67P/Churyumov-Gerasimenko is no ordinary celestial object. It was the first comet to be orbited and landed on by robotic spacecraft from Earth.
ESA’s Rosetta spacecraft, carrying the Philae lander, rendezvoused with the comet in August 2014, accompanying it on its journey to the inner solar system and back out again.
Rosetta’s mission, which ended with a controlled impact on the comet’s surface on September 30, 2016, provided a treasure trove of data.
A Water Mystery Spanning Billions of Years
It’s not a given that water, the key ingredient for life on Earth, should be here at all. Its exact origins remain one of science’s enduring mysteries.
While some water likely existed in the primordial gas and dust that formed Earth about 4.6 billion years ago, much of it would have vaporized due to the planet’s proximity to the sun.
Over time, volcanic vapor condensed and contributed to Earth’s oceans. However, a significant portion of the water likely came from icy bodies like asteroids and comets that collided with Earth during a period of heavy bombardment around 4 billion years ago.
Asteroids—made mostly of rock—have long been considered a primary source of Earth’s water. Their deuterium-to-hydrogen (D/H) ratio—a molecular signature used to trace water’s origin—matches well with Earth’s.
Comets, on the other hand, which are made of ice, have yielded mixed results.
Jupiter-family comets, named for their gravitational ties to Jupiter, are icy relics of the Kuiper Belt, a region beyond Neptune.
These short-period comets, with orbits lasting less than 20 years, were thought to deliver water with higher deuterium levels due to their formation in colder regions of the solar system.
In 2014, Rosetta’s analysis of 67P surprised scientists by showing a deuterium level three times higher than Earth’s water—the highest level of any comet. This seemed to rule out comets like 67P as significant water sources.
“It was a big surprise, and it made us rethink everything,” Mandt said in a statement.
However, Mandt’s latest findings suggest that earlier measurements may have been distorted by the presence of dust particles in the comet’s coma—the gas-and-dust cloud surrounding its nucleus.
“So I was just curious if we could find evidence for that happening at 67P,” Mandt said. “And this is just one of those very rare cases where you propose a hypothesis and actually find it happening.”
By analyzing Rosetta’s full dataset, the team found that the true deuterium levels of 67P’s water align more closely with Earth’s.
The new findings not only bolster the hypothesis that comets contributed to Earth’s oceans, alongside asteroids, but also provide a window into the formation of the early solar system.
“This means there is a great opportunity to revisit our past observations and prepare for future ones so we can better account for the dust effects,” Mandt said.
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Reference
A nearly terrestrial D/H for comet 67P/Churyumov-Gerasimenko | Science Advances. (n.d.). Retrieved December 4, 2024, from
