© olcano.oregonstate.edu/NASA
There are many volcanic features on the lunar surface. In fact, the dark lunar maria that you can see when there is a full Moon are massive, generally level deposits of basalt, a volcanic rock. The Gruithuisen Domes (shown by the black arrows in this Apollo 15 picture) are unusual silica-rich volcanoes.
New study finds that not once, but twice, volcanic eruptions left the Moon blanketed by gases for 70 million years.

Three to four billion years ago, the giant volcanic eruptions that created the Man in the Moon also gave the Moon an atmosphere, scientists say.

That's because volcanic eruptions release gases, and a new study, published online in Earth and Planetary Science Letters, calculates that these gases would have accumulated more quickly than they could escape into space, even under the Moon's low gravity.

The exact composition of that ancient atmosphere isn't completely known, but it would have included carbon dioxide, water, hydrogen, and sulfur, says David Kring of the Lunar and Planetary Institute in Houston, Texas. And there would have been enough of these gases to raise the surface pressure about 50% higher than on present-day Mars, about 1% of Earth's at sea level.

The research was based on prior studies that reanalysed volcanic rocks brought back by Apollo astronauts. These studies had concluded that the magma from which these rocks solidified contained substantially larger quantities of gases than had previously been believed. Kring's brainstorm was to wonder where those gases had gone.

With the help of Debra Needham, now at NASA Marshall Space Flight Centre in Huntsville, Alabama, he computed the volume of magma erupted in each of two known pulses - 3.8 and 3.5 billion years ago. The pair then determined the volume of gases these eruptions would release and calculated how long it would take for them to be lost to space.

The results, Needham says, were astounding.

Traditional views of the Moon have long made it dry and desolate. "Having a Mars-like atmosphere would be completely different," she says. "You might have wind. You might have processes which certainly don't exist today on the lunar surface."

It also means the Moon would have looked quite different from Earth than it does today. Not only would it then have been three times closer (and therefore three times bigger in the sky), but it would have been surrounded by a visible film of atmosphere.

Would the Moon's sky have been blue? Kring doesn't know. "We could calculate the abundances of some gases but not all," he says. "Until you know what the other gases are, you're guessing what the colour was."

Not that this lunar atmosphere would have persisted very long. Most of it would have been lost in about 70 million years, Kring and Needham calculate. But in that interval - which may have occurred twice, once after each pulse of volcanism - the Moon wasn't the airless expanse we know today. "It was, at least for a period of time, a small planet with an atmosphere," Kring says.

Matt Siegler, a Dallas, Texas, researcher with the Planetary Science Institute, sees the new study as a game-changer. In particular, he notes, weather-related processes on the early Moon might have helped convey water to the permanently shadowed "cold traps" at the lunar poles, where it is now known to persist.

Many scientists think that the Moon's polar ice comes from comet impacts in the fairly recent past - an idea consistent with the prevailing belief that Earth's own water was delivered to it late in our planet's formation by similar comet or asteroid impacts.

But the new study, Siegler says, provides a way that primordial water may have been retained by the Moon. And if that happened there, it's possible that the Earth's own water also dates from its earliest formation. "The big part of the story is how did the Earth get its water," Siegler says. "That's what we really want to know."