Origin of the Moon’s ephemeral atmosphere revealed

The Moon is covered in a barely-there layer of atoms known as an “exosphere.”

These atoms, bound tenuously to the Moon by gravity, bounce around on the surface before either returning permanently to it or escaping to space. It is nothing compared to Earth’s 600km-thick, life-sustaining atmosphere.

New analysis of lunar soil samples has finally confirmed the main process responsible for first forming and then sustaining the lunar exosphere – impact vaporisation.

“We give a definitive answer that meteorite impact vaporisation is the dominant process that creates the lunar atmosphere,” says Nicole Nie, assistant professor in the Department of Earth, Atmospheric, and Planetary Sciences at Massachusetts Institute of Technology in the US and lead author of the new paper in Science Advances.

“The moon is close to 4.5 billion years old and through that time the surface has been continuously bombarded by meteorites. We show that, eventually, a thin atmosphere reaches a steady state because it’s being continuously replenished by small impacts all over the moon.” 

Impact vaporisation is a process in which meteorite bombardment kicks up lunar soil, vaporising certain atoms on contact and lofting the particles into the exosphere. The Moon’s surface has been continuously bombarded, first by massive meteorites and then more recently by smaller, dust-sized micrometeoroids.

Nie and colleagues analysed 10 samples of lunar soil collected during NASA Apollo missions to determine the role of impact vaporisation and ion sputtering, another space weathering process, in creating the exosphere.

Ion sputtering is the process by which energetic charged particles from the Sun hit the Moon’s surface, transferring their energy to the atoms in the soil and ejecting them into the exosphere.

They isolated 2 elements, potassium and rubidium, which are easily vaporised by impacts and ion sputtering. These elements also exist as several isotopes, which have the same number of protons but different numbers of neutrons.

The idea was that lighter isotopes would be more easily lofted, while heavier isotopes would be more likely to settle back down in the soil. The researchers also predicted that the different space weathering processes would result in very different proportions of these isotopes in the soil.

The team found that the soils contained mostly heavy isotopes of both potassium and rubidium.

“With impact vaporisation, most of the atoms would stay in the lunar atmosphere, whereas with ion sputtering, a lot of atoms would be ejected into space,” Nie says.

“From our study, we now can quantify the role of both processes, to say that the relative contribution of impact vaporisation versus ion sputtering is about 70:30 or larger.”