When Jared Landry wanted to understand what Earth's air was like billions of years ago, he looked not at Earth — but at the Moon.
Landry, an astrobiology Ph.D. student at the Earth Life Science Institute in Tokyo, has been analyzing Apollo lunar samples in a clever twist: using Moon rocks as a mirror of early Earth. His research, shared at the Origins 2026 conference in Paris, suggests Earth's ancient atmosphere was far richer in carbon dioxide and sulfur than it is today — potentially explaining how our planet stayed warm enough for liquid oceans despite a younger, dimmer Sun.
"The Archean is one of the biggest unknowns, especially in atmospheric chemistry," Landry said in Paris.
The Archean Eon stretched from 2.5 to 4 billion years ago, a time when our planet was still finding its footing. Conventional ways of studying Earth's deep past have been lost to the planet's active geology and weathering. But Landry spotted an opportunity in an unlikely place.
For billions of years, gases from Earth's upper atmosphere have been drifting outward and landing on the Moon's surface. The Moon passes through an outflowing channel of charged particles from Earth, picking up these chemical fingerprints like dust on a windowsill. Landry's model accounts for how much material would have landed during the Archean, around 3.5 billion years ago.
The findings paint a striking picture: Earth's ancient atmosphere likely held roughly 100 times more carbon dioxide than today, along with high levels of sulfur. This thick, chemical-rich air could have helped Earth avoid a deep freeze. With the Sun putting out less energy back then, scientists had estimated that keeping oceans liquid would have required about a tenth of an atmosphere of carbon dioxide. Landry's model suggests Earth had nearly that much.
"The samples suggest high carbon dioxide and methane abundances, strong enough to overcome the faint young sun and sustain a liquid ocean," Landry said.
The sulfur-rich environment may have also been a cradle for life's origins. Complex organic molecules can form more easily in sulfur-rich, aqueous settings — and the Archean oceans likely received plenty of sulfur from the air.
The Apollo samples all came from the Moon's near side, the same side that faces Earth and absorbs this atmospheric outflow. Landry noted that as long as a sample's age is known, its exact landing spot doesn't matter — the near side history is the same.
For Landry, the open question is what kept so much sulfur floating in the air instead of dissolving into the oceans. "You either need to have low hydrological activity or a cool environment, and we don't have that today," he said. Solving that puzzle could deepen our understanding not just of Earth's past, but of what makes a planet habitable at all.