Lunar particle flights: Where do the atoms in the thin, vacuum-like gas envelope of the moon originate? A research team has now addressed this question using samples from the Apollo missions. Their isotope analyses of the regolith indicate that less than a third of the atoms in the lunar exosphere were propelled into the atmosphere by the solar wind. The substantial remainder of the gas envelope results from impacts from micrometeorites, as reported in “Science Advances”.
By Earth’s standards, the moon has no atmosphere; technically, there is an ultra-high vacuum above its surface. Nevertheless, particles exist in the form of a thin exosphere composed of noble gases, trace elements, and hydrogen. Its density corresponds to an atmospheric pressure of merely one quadrillionth of Earth’s air pressure. Since this thin gas envelope continuously loses particles to space, there must also be processes that continually supply gas particles to the lunar exosphere today.
Two processes are particularly considered as possible sources for the lunar exosphere: the stirring up of ions by the solar wind (A) and evaporation due to micrometeorite impacts (B). The particles propelled into the gas envelope either fall back or get lost in space. © Nie et al./ Science Advances, CC-by 4.0
Data collected from lunar probes demonstrate that both the solar wind and interactions with Earth’s magnetotail, alongside the constant bombardment of micrometeorites, affect the lunar regolith, electrically charging and electrochemically changing it and even causing the moon dust to float. However, which of these processes is critical for replenishing the exosphere remains unclear, as analyses of the lunar gas envelope have yet to yield definitive conclusions.
Searching for clues in Apollo moon samples
To investigate this, Nicole Nie from the University of Chicago and her colleagues have closely examined the potential source of the exosphere particles: the lunar regolith. If this fine lunar dust loses atoms to the gas envelope, it would primarily be lighter isotopes that are thrust upward, as the team indicates. While some of these atoms return to the surface, others escape into space. Over time, this alters the isotope ratio of the elements within the regolith.
Crucially, the extent to which different isotopes are altered depends on the process impacting the regolith—micrometeorite impacts yield a different ratio compared to the solar wind’s influence. This is especially applicable to the isotopes of the alkali metals potassium and rubidium, as Nie and her colleagues explain. As a result, they analyzed nine regolith samples for these isotope values, which Apollo astronauts collected from various locations on the moon, along with a lunar basalt rock sample for reference.
Shifted isotope ratio
Indeed, clear differences emerged: “The moon dust is enriched with heavy isotopes, with proportions ranging from +1.2 to 12 per thousand for potassium and +0.02 to +2.2 for rubidium,” the team reports. The massive basalt, in contrast, maintained a standard isotope ratio, having not been altered by external influences. To identify which of the two processes—solar wind or micrometeorites—produces the relevant isotope ratios, the researchers reconstructed the effects using a computer model.
The measured isotope values align more closely with those expected from micrometeorite evaporation, as demonstrated by the modeling. © Nie et al./ Science Advances, CC-by 4.0
“In the computer model, we can easily vary the contributions of individual processes. We can calculate the ratio of potassium and rubidium isotopes that would be present if, for instance, the interaction with solar wind particles predominates or if micrometeorites have the greatest effect,” explains co-author Timo Hopp from the Max Planck Institute for Solar System Research in Göttingen.
Micrometeorites from solar wind
The conclusion: “Our study provides a clear answer: Evaporation due to micrometeorite impacts is the dominant process that forms the lunar atmosphere,” Nie reports. “We can now quantify the contributions of both processes, saying that micrometeorite impacts account for at least 70 percent of the lunar exosphere, while the solar wind contributes only about 30 percent.” This was evident from the isotope ratio of potassium and rubidium, which closely matched values typical for micrometeorite evaporation.
“The moon is nearly 4.5 billion years old, and throughout this time, its surface has been continuously bombarded by meteorites. We demonstrate that a thin atmosphere eventually stabilizes because it is perpetually replenished by small impacts occurring all over the moon,” states Nie. The findings not only clarify the origin of the lunar exosphere but may also shed light on the formation of similar ultra-thin gas envelopes around other planets, such as Mercury. (Science Advances, 2024; doi: 10.1126/sciadv.adm7074)
Source: Massachusetts Institute of Technology, Max Planck Institute for Solar System Research
August 6, 2024 – Nadja Podbregar
Lunar Particle Flights: Unveiling the Origins of the Moon’s Exosphere
Lunar particle flights: Where do the atoms in the thin, vacuum-like gas envelope of the moon come from? A research team has now answered this question using Apollo moon samples. Their isotope analyses of the regolith reveal that less than a third of the atoms in the lunar exosphere were catapulted into the air by the solar wind. The large remainder of the gas envelope comes from the impacts of micrometeorites, as the team reports in “Science Advances”.
The Nature of the Lunar Exosphere
By Earth’s standards, the moon has no atmosphere—technically speaking, there is an ultra-high vacuum above its surface. Nevertheless, there are particles present in the form of a wafer-thin exosphere made of noble gases, trace elements, and hydrogen. Its density corresponds to an atmospheric pressure of just one quadrillionth of the Earth’s air pressure. Because this thin gas envelope is constantly losing particles to space, there must also be processes that continue to supply gas particles for the lunar exosphere today.
Understanding the Supply of the Lunar Exosphere
Two processes in particular come into consideration as possible suppliers for the lunar exosphere: the stirring up of ions by the solar wind and evaporation by micrometeorite impacts. The particles that are hurled into the gas envelope can either fall back to the surface or be lost into space. Data from lunar probes show that both the solar wind and the Earth’s magnetotail, as well as the constant bombardment of micrometeorites, are affecting the lunar regolith, electrically charging it, altering it, and even making the moon dust float.
Investigating Apollo Moon Samples
To determine the primary source of the exosphere particles, a team led by Nicole Nie from the University of Chicago has turned to lunar regolith samples. They posited that lighter isotopes in the lunar dust would be more likely to be hurled into the gas envelope, allowing for the potential of isotope ratios to vary depending on the dominant process influencing the dust. The focus was on potassium and rubidium isotopes, as they would exhibit distinct variances under different conditions.
Results of Isotope Analysis
The team analyzed nine regolith samples collected by Apollo astronauts in various lunar locations, plus a comparison with a lunar basalt rock sample. The analysis revealed significant differences:
| Element | Isotope Range (per thousand) | Sample Type |
|---|---|---|
| Potassium | +1.2 to +12 | Lunar Regolith |
| Rubidium | +0.02 to +2.2 | Lunar Regolith |
| Basalt | Normal Isotope Ratio | Lunar Basalt |
The findings indicated that the lunar dust is enriched with heavy isotopes, while the basalt remained unaltered. The relationship between the isotope ratios provided critical insight into the effects of solar wind versus micrometeorite impacts on the lunar surface.
Modeling the Processes
The research team utilized computer modeling to simulate the effects of both solar wind and meteoritic impacts on the isotopic ratios. This allowed them to determine which process had the more substantial impact on the observed values. Co-author Timo Hopp from the Max Planck Institute for Solar System Research contributed to developing a model where they could adjust the variables to reflect each process’s contribution to the observed isotope ratios.
Key Findings: Micrometeorites Dominate
The results from the simulations provided clear evidence that evaporation caused by micrometeorite impacts is the primary process that sustains the lunar atmosphere. Specifically, the analysis indicated that micrometeorites contribute at least 70 percent to the lunar exosphere, while the solar wind contributes around 30 percent. This conclusion was drawn from the closeness of the measured isotope ratios to those typical of meteorite evaporation.
Long-Term Stability of the Lunar Exosphere
The moon is nearly 4.5 billion years old, and during this extensive timeframe, its surface has been consistently bombarded by micrometeorites. The continuous replenishment from these impacts not only sustains a thin atmosphere but also suggests it reaches a stable state over eons. These findings not only clarify the origin of the lunar exosphere but could also shed light on atmospheric formations around other celestial bodies, such as Mercury.
Implications for Future Research
This research fuels a deeper understanding of the dynamics behind thin atmospheres on planetary bodies. The methodologies developed through studying the lunar exosphere can be applied to other solar system objects, enhancing our comprehension of the conditions necessary for atmospheric retention and evolution. Additionally, the isotopic technique used here can lead to further investigations into the geological and atmospheric processes occurring across different planetary surfaces.
