Did the Moon Once Have a Substantial Atmosphere?

Maybe it wasn’t always as dry and dead as it is today.

The Moon during an eclipse, with an orange cast.

In a new, eyebrow-raising study, Debra Needham and David Kring from the Lunar and Planetary Institute in Houston claim that the Moon had an atmosphere early in our solar system’s history.

The thin atmosphere, so the scientists reason, was created by gases derived from lava outpourings that engulfed huge areas of the Moon about 3.5 billion years ago, at a time when life already existed on Earth. So much gas was produced that the lunar atmosphere was 1.5 times thicker than the current atmosphere on Mars. And it lasted for millions of years. That’s a far different picture from how we normally envision the Moon—as a dead, dry rock in space.

The released gas was mostly carbon dioxide, but it also included water, meaning that some liquid water could have been temporarily stable on the lunar surface. By this time the Moon was already tidally locked to Earth (showing always the same face to our planet), and any liquid water would have existed only in the relatively narrow—and benign—area between the hot side of the Moon facing the Sun and the shaded cold side. So forget about lunar lakes or oceans: any water would have either been ice or gas.

Even on a tidally locked planet, enough heat can be distributed by atmospheric currents to create habitable temperatures on the surface. But for that to happen, the atmosphere would have to be much thicker than it was on the Moon 3.5 billion years ago. Might it have been thicker at some earlier time in the Moon’s history? Large asteroid impacts are known to release gas, and indeed, there was another peak release of gases, although a less extensive one, about four billion years ago, which coincides with a period known as the Late Heavy Bombardment. A thicker atmosphere could also have been present shortly after the formation of the Moon.

Surely, to imagine the early Moon even as borderline habitable is a stretch. But more evidence is accumulating for the presence of water on the Moon. And it’s the only other place where we might find trace fossils from the ancient Earth, dug up by asteroids impacting the Moon long ago. Any such evidence might even today lie buried in the mix of ice and lunar soil near the lunar south pole—together with entrapped gases, including water, from lava flows 3.5 billion years ago.

All of which is to say that the Moon may turn out to be a treasure trove for astrobiology.

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