Scientists have discovered that the oldest evidence of Earth’s atmosphere may be hidden in rocks on the Moon, a conclusion based on a new analysis of lunar rocks brought back to Earth by Apollo astronauts 50 years ago.

Scientists have previously found traces of magnetism in rocks, suggesting that the Moon once had a magnetic field much like Earth’s. But a new study suggests that the Moon has been devoid of a magnetic field for at least 4.36 billion years.

According to John Tarduno, professor of earth and environmental sciences at the University of Rochester, “One of the mysteries of Earth and its evolution is what the earliest composition of Earth’s atmosphere was.” Scientists can’t measure it on Earth because there are very few rocks older than 3.5 billion years, and what remains has been largely altered by Earth’s moving tectonic plates. The moon, on the other hand, is geologically “quiet,” with layers of lunar soil that may have remained untouched for billions of years.

“If we can find a place on the Moon where this ancient regolith is preserved, we could use direct measurements to understand what the early Earth atmosphere was like,” Tarduno said.

Magnetic fields are generated by the motion of magnetic materials in the core of a planet or moon, which protect the surface of the planetary body from the solar wind. Some iron-containing rocks can retain their magnetic field state as they cool and solidify.

In 2021, Tarduno and his team discovered that the Moon had no magnetic field 3.9 billion years ago. In the new study, Tarduno looked at older lunar samples and pushed back the date of possible magnetism on the Moon by 400 million years. This updated timeframe represents Earth’s first eon, the Hadean. There are no Hadean rocks left on Earth, and the planet’s early atmosphere remains a mystery.

“This is very interesting for people who think about the evolution and settlement of planets,” Tarduno said. “If we don’t fully understand Earth, how can we say anything about the evolution of other planets?