On World Earth Day 2026, scientists are revising theories about the origin of Earth’s water after new data from the Rosetta spacecraft challenged the long-held belief that comets delivered most of the planet’s oceans.
The Rosetta mission, operated by the European Space Agency, found that the deuterium-to-hydrogen ratio in water vapor from comet 67P/Churyumov-Gerasimenko differs significantly from that in Earth’s oceans, according to analysis published in April 2026.
This discovery contradicts earlier assumptions that comets like 67P were a primary source of Earth’s water, prompting researchers to consider alternative origins such as asteroids.
Comets are considered ancient building blocks of the solar system, and scientists had previously thought they delivered both water and complex organic molecules necessary for life to early Earth.
Rosetta made history in August 2014 as the first spacecraft to orbit a comet, and its lander Philae touched down on the comet’s surface in November 2014.
The mission, launched in March 2004, concluded in September 2016 after over twelve years of studying the comet’s composition and behavior.
Scientists say the new data helps refine understanding of how icy bodies contributed to solar system evolution, even if they did not supply most of Earth’s water.
What does the Rosetta data suggest about Earth’s water source?
The data suggests that asteroids, rather than comets, may have been the main carriers of water to Earth, based on the mismatch in isotopic ratios between comet 67P’s water and ocean water.
Why is the deuterium-to-hydrogen ratio important in this research?
This ratio acts as a chemical fingerprint; if comet water matched Earth’s ocean water, it would support a cometary origin, but the difference rules out comets as a major source.