Did meteorites create the moon’s atmosphere?

• The moon has a thin atmosphere. It is so tenuous it is measured in individual atoms.
• Meteorite impacts are now thought to create most of the moon’s atmosphere, according to a new study of soil samples brought back by the Apollo missions in the late 1960s and ’70s.
• Meteorites hit the moon’s surface and kick up dust. A process called impact vaporization is thought to create the majority of the moon’s atmosphere.

Did meteorites create the moon’s atmosphere?

For most purposes, we say the moon has no atmosphere. The moon’s atmosphere is so thin that it’s almost negligible. It’s so thin that it’s measured in individual atoms, rather than in units of air pressure as on Earth. Still, it’s surprising those few atoms hang around the moon as long as they do. They tend to fall back to the moon’s surface. Or they escape into space. So the fact that they’re consistently present must mean they’re somehow being replenished. But how?

How has been the question, until now. A new study from researchers at the University of Chicago and Massachusetts Institute of Technology (MIT) suggests an answer. The researchers said on August 2, 2024, that the moon’s tenuous atmosphere is primarily the result of impacts from small meteorites.

The research team published their peer-reviewed findings in Science Advances on August 2, 2024.

Puzzle of the moon’s thin atmosphere

Scientists have had two main hypotheses about the moon’s atmosphere. Both involve lunar dust.

The first idea has been that meteorites hit the surface and kick up dust to create the moon’s atmosphere. This process is called impact vaporization.

The second hypothesis is similar, but involves the solar wind – the flow of charged particles from the sun – instead of meteorites.

Both processes are a form of space weathering. So which is correct? The answer is both. But – according to the new study – meteorite impacts are, by far, the biggest factor in creating the tenuous atmosphere on the moon.

Atmospheric reconnaissance with LADEE

NASA’s LADEE (Lunar Atmosphere and Dust Environment Explorer) orbiter studied the moon’s thin atmosphere in 2013. Those results suggested both meteorites and the solar wind might be responsible. But there was still some confusion. Nicole Nie, formerly at the University of Chicago and now an assistant professor at MIT, led the research team. Nie said:

Based on LADEE’s data, it seemed both processes are playing a role. For instance, it showed that during meteorite showers, you see more atoms in the atmosphere, meaning impacts have an effect. But it also showed that when the moon is shielded from the sun, such as during an eclipse, there are also changes in the atmosphere’s atoms, meaning the sun also has an impact. So, the results were not clear or quantitative.

Apollo soil samples provide an answer

The solution came from a new study of 10 soil samples brought back to Earth by the Apollo missions. Co-author and professor Nicolas Dauphas at the University of Chicago said:

It turns out the answer to this longstanding question was right in front of us, preserved in lunar soil brought back to Earth by the Apollo missions.

The researchers studied two elements in particular: potassium and rubidium. They examined the isotopes – the natural mass variance – of the elements. Both meteorite impacts and the solar wind can affect those variations. But they do so slightly differently. Meteorite impacts will disrupt atoms more that have heavier isotopes. But the solar wind will more greatly affect atoms with lighter isotopes. It can kick those atoms away from the moon and out into space. By comparing the abundances of heavier and lighter isotopes, scientists can determine whether meteorite impacts or the solar wind has a greater effect on the moon overall, and supplied more atoms to the atmosphere.

Nie explained:

It’s actually quite a clear difference, once you are able to count them. Lunar soils show distinct isotope patterns compared to other lunar rocks, due to meteorite impacts and solar wind bombardment, allowing us to determine which process supplied more atoms into the atmosphere.

Meteorites vs. solar wind

The study determined that meteorites play a much greater role in maintaining the moon’s thin atmosphere. The ratio, in fact, was 70% meteorites compared to only 30% solar wind. Nie said:

According to our analysis, at least 70% of the lunar atmosphere is created by these meteorite impacts. A much smaller percentage is created by the solar wind abrasion of the surface.

Nie added:

We give a definitive answer that meteorite impact vaporization is the dominant process that creates the lunar atmosphere. 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.

Human habitation and the solar system

The results also have implications for eventual human habitation on the moon or even elsewhere in the solar system. Nie said:

If humans want to move to different planetary bodies someday, we will have to understand what’s going on at the surface to be able to prepare. Each planetary body is different, and the more we understand about these processes, the more complete picture we’ll have.

The Apollo soil samples were essential to figuring out the mystery of the moon’s atmosphere, as Nie also noted:

Without these Apollo samples, we would not be able to get precise data and measure quantitatively to understand things in more detail. It’s important for us to bring samples back from the moon and other planetary bodies, so we can draw clearer pictures of the solar system’s formation and evolution.

Bottom line: How did the moon’s thin atmosphere form? In a new study of Apollo soil samples, researchers say by far most of it – about 70% – comes from meteorite impacts.

Source: Lunar soil record of atmosphere loss over eons

Via University of Chicago

Via MIT

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