Artist's conception of the moon about three and a half billion years ago with volcanic activity feeding an atmosphere. NASA/MSFC

From about 4.1 to 3.8 billion years ago, just a few hundred million years after the Earth and moon formed, both celestial bodies were being hammered by asteroids and other space rocks. It's a period astronomers call the Late Heavy Bombardment. Shortly thereafter, the moon was an active world of tectonic movements and volcanic eruptions—some of which spewed lava that flowed for hundreds of miles, which scientists can tell from collections of volcanic basalt in lunar craters known as mare basalt.

That's not all. According to a new study, this period of volcanic activity released enough gases into the air that the moon would have had a substantial atmosphere.

Research scientist Debra H. Needham of NASA Marshall Space Flight Center and senior staff scientist David A. Kring of the Lunar and Planetary Institute (LPI) studied lunar samples collected by the Apollo program. They say that two heavy periods of volcanic activity in particular, 3.8 and 3.5 billion years ago, are thought to have fed a relatively thick atmosphere compared to the infinitesimal molecules that make up the moon's hardly detectable atmosphere today. The findings were recently published in theEarth and Planetary Science Letters.

"This work dramatically changes our view of the moon from an airless rocky body to one that used to be surrounded by an atmosphere more prevalent than that surrounding Mars today," said Kring in a press release.

The Earth and moon were drastically different places three and a half billion years ago. The moon was three times closer that it is today, so if you could have stood on proto-Earth and looked up, the moon would have appeared three times bigger in the sky. The strong tidal forces that existed when the two bodies were so close together helped to maintain the volcanic and geologic activity on the lunar surface.

Artist's impression of the moon during the Late Heavy Bombardment (above) and today (below). Timwether/Wikimedia

To measure the exact amount of gas that was released during these heavy periods of volcanism, Needham and Kring analyzed the composition of samples taken from mare basalt deposits in the Serenitatis and Imbrium basins on the moon, which were collected by the astronauts of Apollo 15 and 17. The same samples allowed scientists to calculate the ages of major lava flows within the basins: 3.8 billion years ago for Serenitatis and 3.5 billion for Imbrium.

The mare basalt also contains clues about the composition of material spewed out in volcanic plumes on the moon. The eruptions carried carbon monoxide, oxygen, hydrogen, sulfur, and other volatile gases. The plumes released enough material to blanket the moon in an atmosphere 1.5 times the thickness of Mars, or about 1.5 percent that of Earth.

"The total amount of H2O released during the emplacement of the mare basalts is nearly twice the volume of water in Lake Tahoe," said Needham. "Although much of this vapor would have been lost to space, a significant fraction may have made its way to the lunar poles. This means some of the lunar polar volatiles we see at the lunar poles may have originated inside the moon."

The findings have significant implications for Earth as well, as our home planet and its only natural satellite shared a significant amount of material during their formation. The moon itself is thought to be a chunk of the Earth that was smashed off in an enormous collision and then accreted from the debris. Geologic material on the moon has been largely undisturbed for billions of years, and it can tell us what the Earth was like in the distant past, perhaps even providing clues as to the origins of life.

Map of basaltic lavas that emitted gases on the lunar nearside. Debra Needham

The high abundance of volatiles—chemical elements and molecules that have relatively low boiling points, such as water, carbon dioxide and hydrogen—in the mare basalt craters also has significant implications for future exploration and construction on the moon. The volcanic material is rich with resources that could be used to make water and fuel to support a lunar outpost. Locations near mare basalt deposits will likely be studied as possible moon base sites.

Above all else, the new study shows us just how much we have to learn. The closest heavenly body to our reach, the only extraterrestrial world the species has walked, had a secret atmosphere for years and years without anyone being any the wiser.

Source: Lunar and Planetary Institute

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