Another Potential Source of Moon Ice
By Iva Fedorka
A new study suggests that billions of years of volcanic eruptions on the Moon may have created multiple transient atmospheres, including at least one that contained water vapor. Researchers reported in the Planetary Science Journal that the water vapor may be responsible for ice at the Moon’s north and south poles.
The History
The existence of lunar ice was confirmed in 2009 when an impactor released by NASA's Lunar Crater Observation and Sensing Satellite (LCROSS) found water in Cabeus Crater, near the Moon's south pole.
Since then, scientists have suggested that asteroids, comets, or electrically charged atoms from solar wind could be the source of the moisture. Now, some are speculating that the water came from the Moon itself — an aqueous vapor released by volcanic eruptions that occurred between two and four billion years ago.
“It’s a really interesting question how those volatiles [such as water] got there,” Andrew Wilcoski, a planetary scientist at the University of Colorado Boulder, told Science News. “We still don’t really have a good handle on how much are there and where exactly they are.”
The Simulation
Wilcoski and his colleagues first decided to assess volcanic eruptions for the presence of water. During the active lunar volcanism period, eruptions occurred roughly every 22,000 years. If water constituted about a third of the gasses emitted (a figure based on lunar magma samples), about 20 quadrillion kilograms of water vapor would have been released. That’s the same amount of water currently found in the U.S. Great Lakes.
Of course, some of the water vapor was likely degraded by sunlight or solar wind. But, depending on the rate of condensation, much would have remained on the surface as ice. The team of researchers used a computer simulation to account for surface temperature, gas pressure, loss of vapor as frost, and other factors. Based on their calculations, about 40 percent of the projected total water vapor may have accumulated as ice.
Over billions of years, some of that ice would have become vapor again and escaped to space. The simulation also predicted that remaining deposits could be hundreds of meters thick, with twice as much at the south pole as at the north. Like on Earth, the Moon’s poles are typically colder.
The Results
The team’s results align with a long-standing assumption that ice dominates at the poles because it gets stuck in cold traps that keep the ice frozen for billions of years. The simulation outcomes show the potential present-day distribution and thickness of ice at the lunar poles following those ancient volcanic eruptions. “There are some places at the lunar poles that are as cold as Pluto,” says planetary scientist Margaret Landis, also from the University of Colorado Boulder.
The travel of volcanic water vapor to the poles also likely depends on the presence of an atmosphere, say Landis, Wilcoski, and their colleague Paul Hayne, another planetary scientist at the University of Colorado Boulder. Atmospheric transit would let the water molecules travel around the Moon and make it more difficult for them to escape into space.
If lunar ice came from volcanoes as water vapor, the ice may retain evidence of its origin. For example, the presence of sulfur in lunar ice could indicate its source as volcanic rather than asteroidal.
Future Moon missions plan to drill for ice cores to confirm the ice’s origin. But the presence of sulfur may affect the availability of lunar resources. If astronauts in the future wanted to harvest the ice for water or fuel, sulfur contamination would be important. “That’s a pretty critical thing to know if you plan on bringing a straw with you to the Moon,” says Landis.