Featuring image: Erruption of the Raikoke Volcano on June 22, 2019. Volcanos can exhaust a large amount of gases and dust during eruptions. Is this enough to create an atmosphere on the Moon? NASA’s Earth Observatory, public domain (CC0).
Authors: A. X. Wilcoski, P. O. Hayne and M. E. Landis
The Moon is a silent and dry, yet beautiful desert. Where it comes from and how much ice exits is still a mystery. It can be found in the darkness of its pole regions as ice. Surprisingly, the eruptions of volcanos might have helped the Moon to keep its water.
The gas that is set free during a volcano eruption contains different volatile molecules, including water. On small celestial objects without an atmosphere like the moon, most of the gases are released to space. A new study suggests that not all water vapour from such eruptions escaped from the Moon during its history. Instead, local and short-lived atmospheres might have formed during eruptions, allowing a part of the water vapour to cool down and deposit as snow and ice.
The atmosphere of the Moon is so thin that it is basically non-existent. It is ten trillion times less dense then that of the Earth. Nevertheless, we observed the existence of water ice on the Moon. Most of it can be found inside the shadows of old craters and in the polar regions, where temperatures are low enough to prevent the ice from melting. Due the Moon’s low atmosphere, liquid water is not stable there. Thus, melting ice turns to water vapour and immediately escapes to space. However, we know of one process that is able to increase the atmosphere, at least temporally: volcanic eruptions. Today, we don’t know of any active volcanos on the Moon. But before it cooled down completely, the Moon had a very active phase from 4 to 2 billion years before present. Andrew Wilcoski and co-workers investigated how long such a small local eruption-atmosphere would have lasted and if it was long enough to allow water to deposit as ice on the Moon’s surface.
Where does the water in volcanic gases come from? Rocks and magma always contain some water. When a volcano erupts and the magma is transported to the surface, the pressure decreases and water, along with other volatiles, degases from the magma. From the solidified lava flows that we can find on the Moon, Wilcoski could estimate the amount of magma and volatiles that were exhausted by lunar eruptions. And there are a lot of these lava flows. All the dark spots on the Moon are the remnants of big volcanic eruptions. While it is difficult to reconstruct these ancient volcanos, the researchers were able to show that an average eruption produced a dense atmosphere that lasted for about 2500 years. Considering the average time between eruptions of ~22000 years, this is not enough to sustain a stable lunar atmosphere, but long enough to allow a part of the water to cool down and deposit as snow and ice. The fraction of water that remains on the Moon depends on the size of the eruption. Bigger eruptions produce denser atmospheres and less water can escape to space.
While most of the ice will thaw and escape from the Moon during the lunar day, the temperatures at the polar regions are low enough to keep most of the ice. Moreover, as long as the atmosphere remains, water can travel from one ice field to another by atmospheric exchange. This sounds strange, but smaller, warmer ice fields lose water which is then trapped by neighbouring cooler ice fields. In this way, ice might have been concentrated in shadow-rich, cooler places. Interestingly, the south pole of the Moon has a geography which is especially rich in these ice traps. Based on their theory, the researchers propose a higher abundance of ice on the southern hemisphere.
This exciting study suggests that volcanism contributed significantly to the abundance of ice on the Moon. Moreover, the soil in the Moon’s polar regions could be mixed with a high amount of ice, which was buried over the last aeons. Now, the Moon looks like an eternal, never changing desert. But in ancient times it might have been an spectacular place of fire and snow.