Observed by advanced observatories in Chile adn Hawaii, the comet, approximately the size of Mount Everest, made its closest approach to Earth in April 2025 during a total solar eclipse. This was its first near pass as 1954.

Comet 12P/pons-Brooks belongs to the Halley-type family, ancient objects that orbit the Sun roughly every 71 years. Scientists believe these comets are relics from the early solar system.

Water Signature Matches Earth’s Oceans

The key finding centers on the water composition of the comet. Scientists measured both normal water and heavy water (deuterium) within the comet. The ratio of deuterium to hydrogen in the comet’s water vapor was found to be nearly identical to that of Earth’s seas.

“The Pons-Brooks data gave one of the strongest similarities yet,” said NASA astrophysicist Martin Cordiner.

researchers mapped both forms of water and confirmed that the vapor originated directly from the comet’s nucleus, eliminating the possibility of chemical reactions in the surrounding coma producing the water.

According to NASA scientist Stefanie Milam, the study revealed gases “emerging from frozen ices locked deep inside the comet’s core.”

Revisiting the Origin of Earth’s Water

For many years, the prevailing theory has been that Earth formed too hot to retain water, suggesting that water arrived later through impacts from icy comets or asteroids. Though, previous comet studies frequently enough showed mismatched water signatures.

This new data from Comet 12P/pons-Brooks presents a compelling case, prompting a re-evaluation of how Earth acquired its oceans. If comets like this one delivered water to the early Earth, they may have also influenced the conditions necessary for life to emerge.