Ancient volcanic eruptions on the moon could provide an unexpected resource for future lunar explorers: water.
Between 2 billion and 4 billion years ago, the moon was a volcanic hotspot. Tens of thousands of volcanoes were erupting on the surface, releasing hundreds of thousands of square kilometres of lava across the lunar surface.
This activity created massive rivers and lakes of lava similar to features in modern-day Hawaii, but on a much grander scale.
"They dwarf almost all of the eruptions on Earth," said Paul Hayne, assistant professor in the department of astrophysical and planetary sciences and the Laboratory for Atmospheric and Space Physics at the University of Colorado Boulder, in a statement.
Hayne is a co-author on a new study published this month in The Planetary Science Journal about potential sources of water on the moon.
When these lunar volcanoes erupted, it's also likely they released giant clouds made of carbon monoxide and water vapour. These clouds moved around and could have created thin, temporary atmospheres.
But it's also possible that the water vapour settled on the lunar surface and formed layers of ice that may have existed in craters at the lunar poles today. These ice sheets could be between dozens and hundreds of feet thick.
"We envision it as a frost on the moon that built up over time," said lead author Andrew Wilcoski, a doctoral student in the department of astrophysical and planetary sciences and the Laboratory for Atmospheric and Space Physics at CU Boulder, in a statement.
Had humans been alive on Earth when this occurred, a shadow of frost may have been visible at the border of night and day on the lunar surface, the researchers said.
As the NASA Artemis mission prepares to return humans to the moon and land at the lunar south pole for the first time later this decade, that ice could provide drinking water and serve as a resource for rocket fuel, Hayne said.
"It's possible that 5 or 10 metres (16 to 33 feet) below the surface, you have big sheets of ice," he said.
Past research has lent support to the idea that the moon may contain more water than previously believed. Hayne and his colleagues estimated in a 2020 study that nearly 6,000 square miles (15,540 square kilometres) of the lunar north and south poles could retain ice.
The study was included in a NASA announcement that year about discovering water on the moon.
Scientists have been trying to figure out where the water originated, which led the researchers to the volcano theory. They imagined the water vapour clouds forming like frost on the lunar surface, similar to the way it forms on Earth after a cold night.
Wilcoski and Hayne teamed up with Margaret Landis, a research associate at CU Boulder's Laboratory for Atmospheric and Space Physics, to model what the moon was like a few billion years ago.
At the time, the moon was experiencing a volcanic eruption about every 22,000 years. The team calculated that 41% of the water vapour released during the eruptions could then form ice on the lunar surface.
That's about 18 quadrillion pounds (8.2 quadrillion kilograms) of volcanic water -- more water than the current level of Lake Michigan -- turning into lunar ice, according to the study. The thick polar ice caps may have even once been visible from Earth.
"The atmospheres escaped over about 1,000 years, so there was plenty of time for ice to form," Wilcoski said.
While most of that ice may still exist on the moon today, it is likely buried beneath several feet of lunar regolith, or dust.
"We really need to drill down and look for it," Wilcoski said.
