When one looks up at the Moon, dark surfaces of volcanic basalt can be easily seen to fill large impact basins.
Those seas of basalt, known as maria, erupted while the interior of the Moon was still hot and generating magmatic plumes that sometimes breached the lunar surface and flowed for hundreds of kilometers.
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Analyses of lunar samples indicate those magmas carried gas components, such as carbon monoxide, the ingredients for water, sulfur, and other volatile species.
Researchers, from NASA and Lunar and Planetary Institute in the US, calculated the amounts of gases that rose from the erupting lavas as they flowed over the surface and showed that those gases accumulated around the Moon to form a transient atmosphere.
The atmosphere was thickest during the peak in volcanic activity about 3.5 billion years ago and, when created, would have persisted for about 70 million years before being lost to space.
The two largest pulses of gases were produced when lava seas filled the Serenitatis and Imbrium basins about 3.8 and 3.5 billion years ago, respectively.
The margins of those lava seas were explored by astronauts of the Apollo 15 and 17 missions, who collected samples that not only provided the ages of the eruptions, but also contained evidence of the gases produced from the erupting lunar lavas.
"The total amount of water released during the emplacement of the mare basalts is nearly twice the volume of water in Lake Tahoe," said Debra H Needham, Research Scientist of NASA Marshall Space Flight Center.
"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," said Needham.
"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 David A Kring, from LPI.
"When the Moon had that atmosphere, it was nearly three times closer to Earth than it is today and would have appeared nearly three times larger in the sky," Kring said.