Book ReviewVOLUME 17 ISSUE # 27

Ice at the moon’s poles might have come from ancient volcanoes

Four billion years ago, lava spilled onto the moon’s crust, etching the man in the moon we see today. But the volcanoes may have also left a much colder legacy: ice.

Two billion years of volcanic eruptions on the moon may have led to the creation of many short-lived atmospheres, which contained water vapor, a new study suggests. That vapor could have been transported through the atmosphere before settling as ice at the poles, researchers report in the Planetary Science Journal.

Since the existence of lunar ice was confirmed in 2009, scientists have debated the possible origins of water on the moon, which include asteroids, comets or electrically charged atoms carried by the solar wind. Or, possibly, the water originated on the moon itself, as vapor belched by the rash of volcanic eruptions from 4 billion to 2 billion years ago.

“It’s a really interesting question how those volatiles (such as water) got there,” says Andrew Wilcoski, a planetary scientist at the University of Colorado Boulder. “We still don’t really have a good handle on how much are there and where exactly they are.” Wilcoski and his colleagues decided to start by tackling volcanism’s viability as a lunar ice source. During the heyday of lunar volcanism, eruptions happened about once every 22,000 years. Assuming that H2O constituted about a third of volcano-spit gasses — based on samples of ancient lunar magma — the researchers calculate that the eruptions released upward of 20 quadrillion kilograms of water vapor in total, or the volume of approximately 25 Lake Superiors.

Some of this vapor would have been lost to space, as sunlight broke down water molecules or the solar wind blew the molecules off the moon. But at the frigid poles, some could have stuck to the surface as ice.

“There are some places at the lunar poles that are as cold as Pluto,” says planetary scientist Margaret Landis of the University of Colorado Boulder. Volcanically sourced water vapor traveling to the poles, though, probably depends on the presence of an atmosphere, say Landis, Wilcoski and their colleague Paul Hayne, also a planetary scientist at the University of Colorado Boulder. An atmospheric transit system would have allowed water molecules to travel around the moon while also making it more difficult for them to flee into space. Each eruption triggered a new atmosphere, the new calculations indicate, which then lingered for about 2,500 years before disappearing until the next eruption some 20,000 years later.