The oldest evidence of Earth's atmosphere may be hiding in rocks on the moon

The oldest evidence of Earth's ancient atmosphere may be lurking in rocks from the moon, a new study suggests.

That's the takeaway from a new analysis of moon rocks that Apollo astronauts brought back to Earth 50 years ago.

At the time, scientists detected traces of magnetism locked in the rocks — a sign that the moon had once had a magnetic field, much like Earth's. This was perplexing, because magnetic fields are driven by a planet or moon's core, and the moon's core is tiny, study co-author John Tarduno, a professor of Earth and environmental sciences at the University of Rochester, told Live Science.

But the new study shows that the moon has not had a magnetic field for at least 4.36 billion years.

Their new study, published Friday (Sept. 6) in the journal Communications Earth & Environment, finds that the moon could only have been protected by a magnetic field in the first 140 million years of its existence. That's exciting, Tarduno told Live Science, because it means the moon could hold a record of Earth's earliest existence that has long been wiped from our planet itself. Without a magnetic field to protect it, the moon could have picked up ions from Earth's atmosphere 4.36 billion years ago.

"One of the mysteries about Earth and Earth's evolution is really what was the earliest composition of Earth's atmosphere?" Tarduno said. "We have no way of really getting measurements of this on Earth."

Related: Scientists map 1,000 feet of hidden 'structures' deep below the dark side of the moon