A new study challenges the long-held belief that Earth’s water was delivered by meteorites during the early history of the solar system. Researchers examining lunar samples say only a small fraction of Earth’s water likely came from space rocks that struck the planet billions of years ago.
The study, led by Dr. Tony Gargano of the Universities Space Research Association and the University of New Mexico, used high-precision triple oxygen isotope analysis on Apollo-era lunar samples.
The findings suggest that meteorites from the Late Heavy Bombardment, a period of intense impacts around 4 billion years ago, could not have contributed enough material to explain the amount of water found on Earth today.
Source of Earth’s water and meteorites’ role
Gargano’s team found that no more than 1 percent of the moon’s surface material comes from impact-related sources, likely carbon-rich meteorites that partially vaporized on collision. By setting an upper limit on the amount of water these meteorites could have delivered, researchers concluded that their contribution to Earth’s total water inventory was minimal.
While oceans cover over 70 percent of the planet’s surface, water makes up just 0.023 percent of Earth’s total mass. That equals around 1.46 sextillion kilograms (3.22 sextillion pounds). The study found that late meteorite impacts account for only a tiny share of this mass.
Co-author Dr. Justin Simon of NASA’s Astromaterials Research and Exploration Science Division said the research doesn’t rule out meteorites as a source entirely, but the moon’s preserved surface record makes it unlikely that they were the primary suppliers of Earth’s oceans.
Unlike Earth, the moon has no atmosphere and limited geological activity, allowing it to retain a clearer impact history.
The research team used oxygen isotopes to detect traces of impactor material, even in regolith that had been repeatedly melted and reshaped. This method also helps separate changes caused by impacts from the original composition of incoming materials.
Future lunar missions may rely on water ice for survival
Lunar soil is difficult to study due to repeated impacts that alter its structure and mix different elements. Gargano emphasized that despite these challenges, the moon offers a unique look into the solar system’s past. He described it as a place where the long-term impact record remains largely intact.
Today, most of the moon’s water is trapped in permanently shadowed craters at the poles. Future missions by NASA and other space agencies are targeting these regions for exploration. Easy access to water ice is key to sustaining human presence, supporting science, and enabling deeper space missions.
The study appears in the Proceedings of the National Academy of Sciences.
