Scientists have discovered that water from a rare comet is almost identical to the water in Earth’s oceans, offering strong new evidence that comets may have played a key role in making our planet habitable.
The finding adds weight to the idea that comets could have delivered much of Earth’s water—and possibly some of the ingredients needed for life—billions of years ago.
The international research team, led by Martin Cordiner of NASA’s Goddard Space Flight Center, studied the Halley-type comet 12P/Pons-Brooks during its journey toward the Sun.
Using the powerful Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, the scientists mapped both ordinary water (H₂O) and “heavy” water (HDO), which contains a heavier form of hydrogen known as deuterium, in the comet’s coma—the cloud of gas surrounding its solid nucleus.
This is the first time researchers have been able to create such a detailed map showing where these two forms of water are located in a comet.
ALMA’s high sensitivity allowed the team to detect the faint signal of heavy water coming from the innermost regions of the coma—an achievement never before accomplished in comet research.
To get an even clearer picture, the ALMA data was combined with observations from NASA’s Infrared Telescope Facility (IRTF) in Hawaii, which measured water and other gases in the comet.
Together, the two sets of data gave scientists an accurate reading of the ratio of deuterium to hydrogen (D/H) in the comet’s water. This chemical “fingerprint” helps trace the origin and history of water in the solar system.
The results were remarkable: the D/H ratio in 12P/Pons-Brooks’ water, measured at (1.71±0.44)×10⁻⁴, is virtually identical to the ratio found in Earth’s oceans. It is also the lowest D/H ratio ever recorded in a Halley-type comet and sits at the low end of values seen in other comets.
“Comets like this are frozen relics from the birth of our solar system 4.5 billion years ago,” Cordiner said. “Since the early Earth is thought to have formed without water, impacts from comets have long been proposed as a way our planet got its water.
Our results provide the strongest evidence yet that at least some Halley-type comets carried water with the same isotopic signature as Earth’s.”
Halley-type comets, which take between 20 and 200 years to complete an orbit, rarely visit the inner solar system. Earlier studies often found their water to have a D/H ratio different from Earth’s, casting doubt on the idea that comets supplied our water. This new study changes that picture, showing that some comets—like 12P/Pons-Brooks—could indeed have been major contributors.
The research also confirmed that the water detected was coming directly from the comet’s icy nucleus, rather than forming in the coma itself. According to co-author Stefanie Milam of NASA, this means the measurements reflect the comet’s true, original composition.
These findings not only bring us closer to understanding where Earth’s water came from, but also shed light on how conditions for life might have emerged in the early solar system.
Source: National Radio Astronomy Observatory.
