The Source of Water Ice on The Moon Could Be Traced to a Seemingly Unlikely Source

In comparison, there isn't much going on on the Moon. There is dust on the floor. There's rock to be found. There are basalt plains, which are the result of widespread volcanism throughout the Moon's history.

There's also water, as we've lately found. There's plenty of water. Embedded in the regolith of the moon. I'm encased in volcanic glass. Perhaps even in sheets of ice on or just beneath the surface, hidden in permanent shadow craters at the poles where it can't be melted by the Sun's heat.

It's still unclear where this water came from. However, recent study points to an intriguing source: volcanoes, a process we know has occurred often on the Moon in the past.

Planetary scientists have been wondering if the old Moon's volcanic outgassing had enough water molecules to fall down to the surface and create sheets of ice in permanent shadow. The answer now appears to be 'yes.'

"Our model suggests that [around] 41 percent of the total H2O mass erupted over this period could have condensed as ice in the polar regions, with thicknesses up to several hundreds of meters," wrote a team of researchers led by University of Colorado Boulder planetary scientist Andrew Wilcoski in their paper.

"Our work suggests that the volcanically active period of the early Moon would have been punctuated by short-lived, collisional atmospheres that enabled the efficient sequestration of large quantities of water ice at the poles and the temporary diurnal availability of water ice and vapor at all latitudes."

The Moon may appear to be tranquil these days, but it was once a boiling disaster. The vast plains of volcanic rock visible when looking at the full Moon are remnants of a period of large-scale volcanic activity that began as early as 4.2 billion years ago and lasted until about 1 billion years ago, with most of the activity occurring in the first two billion years or so of that time frame.

Thousands of volcanoes erupted on the Moon, filling the surface with volcanic landscapes (the most volcanic body in the Solar System today is Jupiter's moon Io, which has over 400 recognized volcanoes).

Furthermore, massive clouds of volcanic gases, mostly carbon monoxide and water vapor, would have resulted from those eruptions. These might have created flimsy, ephemeral atmospheres surrounding the Moon, which then dispersed into space. But, Wilcoski and his colleagues reasoned, what if part of the water vapor didn't disperse in the solar wind and instead settled like frost?

They did modeling based on a large eruption rate of once per 22,000 years on average. They next looked at how quickly the volcanic gases fled into space vs how much condensed, froze, and settled on the lunar surface.

They discovered that, over the course of 1,000 years, around 15% of the water in the atmosphere settles and creates a frost on the lunar nightside, amounting to around 8.2 quadrillion kilograms (18 quadrillion pounds). Some of the frost would sublimate in sunlight over time, but enough might have stayed over billions of years to make up a major amount of the ice that exists now, according to the researchers.

That does not imply that it will be simple to locate. It's possible that some of it is buried meters beneath the lunar surface. However, some water may have lingered on the surface at lower latitudes long enough to interact with minerals present there, or it may have been trapped in volcanic glass that gets re-melted in meteorite strikes.

On the Moon, such evidence for former water has already been discovered, providing a starting point for searching for evidence for old volcanic Moon frosts. Science is awe-inspiring.