Four billion years ago, lava fell on the Moon’s crust, creating what we see in the Moon today. But volcanoes may also have left a much cooler legacy: ice.
A new study suggests that a two-billion-year volcanic eruption on the Moon may have created several short-lived atmospheres that contain water vapor. That vapor may have been carried through the atmosphere before settling as ice at the poles, researchers report in May. Planetary Science Journal.
Since the existence of lunar ice was confirmed in 2009, scientists have debated the possible origin of water on the Moon, including asteroids, comets or electrically charged atoms carried by the solar wind.SN: 11/13/09) or, possibly, water originated on the Moon, as steam emanated from volcanic eruptions between 4 billion and 2 billion years ago.
“It’s a really interesting question about how unstable they are [such as water] Been there,” says Andrew Wilkoski, a planetary scientist at the University of Colorado Boulder. “We still don’t have a really good handle on how many they are and where exactly they are.”
Wilkoski and his colleagues decided to begin by tackling the feasibility of volcanoes as a lunar ice source. During the rise of lunar volcanoes, eruptions occurred approximately once every 22,000 years. assuming that h2O constituted about a third of the volcano-spit gases – based on samples of ancient lunar magma – the researchers calculated that the eruption released a total of 20 quadrillion kilograms of water vapor, or about 25 over the volume of Lake Superior.
Some of this vapor may have been lost to space, as sunlight broke up the water molecules or the solar wind blew the molecules off the Moon. But at the colder poles, some could stick to the surface like ice.
For this to happen, however, the rate at which water vapor condenses into ice needs to exceed the rate at which vapor escapes from the Moon. The team used computer simulations to calculate and compare these rates. Simulations accounted for factors such as surface temperature, gas pressure and some vapor loss to mere frost.
About 40 percent of the total erupted water vapor may be deposited as ice, with most of the ice at the poles, the team found. Over billions of years, some of that ice converted back into vapor and escaped into space. The team’s simulation predicts the amount and distribution of ice that remains. And this is no small amount: the deposits can reach hundreds of meters at their thickest point, with the South Pole almost twice as icy as the North Pole.
The results are in line with the long-standing belief that ice dominates at the poles because it becomes trapped in cold traps that are so cold that ice will freeze for billions of years.
“There are few 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 probably depends on the presence of an atmosphere, say Landis, Wilkoski and their colleague Paul Heine, 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 difficult for them to escape into space. Each eruption started a new atmosphere, new calculations indicate, which lasted for about 2,500 years before disappearing until the next eruption some 20,000 years later.
This part of the story is most fascinating to Parvati Prem, a planetary scientist at the Johns Hopkins Applied Physics Laboratory in Laurel, MD, who was not involved in the research. “It’s really an interesting work of fiction. … How do you create environments from scratch? And why do they sometimes go away?” she says. “Polar ice is one way to detect that.”
If lunar ice is ejected from volcanoes as water vapor, the ice may retain that long-time memory. For example, the sulfur in polar ice would indicate that it came from a volcano, such as, an asteroid. Future Moon missions plan to drill for ice cores that could confirm the ice’s origins.
Looking for sulfur will be important when thinking about lunar resources. Researchers say that these water reserves may someday be collected by astronauts for water or rocket fuel. But if all lunar water is contaminated with sulfur, Landis says, “it’s a very important thing to know if you plan to bring a straw with you to the Moon.”
