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 (SN: 11/13/09). 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 volatile [such as water] got there,” says Andrew Wilcox, 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.”
Wilcox 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.
For that to happen, though, the rate at which the water vapor condensed into ice would have needed to surpass the rate at which the vapor escaped the moon. The team used a computer simulation to calculate and compare these rates. The simulation accounted for factors such as surface temperature, gas pressure and the loss of some vapor to mere frost.
About 40 percent of the total erupted water vapor could have accumulated as ice, with most of that ice at the poles, the team found. Over billions of years, some of that ice would have converted back to vapor and escaped to space. The team’s simulation predicts the amount and distribution of ice that remains. And it’s no small amount: Deposits could reach hundreds of meters at their thickest point, with the South Pole being about twice as icy as the North Pole.
The results align with a long-standing assumption that ice dominates at the poles because it gets stuck in cold traps that are so cold that ice will stay frozen for billions of years.
“There are some places at the lunar poles that are as cold as Pluto,” says planetary scientist Margaret Lands of the University of Colorado Boulder.
This part of the story is most captivating to Party Poem, a planetary scientist at Johns Hopkins Applied Physics Laboratory in Laurel, Md., who wasn’t involved in the research. “It’s a really interesting act of imagination.… How do you create atmospheres from scratch? And why do they sometimes go away?” she says. “The polar ices are one way to find out.”
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.
Volcanically sourced water vapor traveling to the poles, though, probably depends on the presence of an atmosphere, say Lands, Wilcox and their colleague Paul Haynes, 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
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