The Moon, for most intents and purposes, appears naked and bare, exposed to the vacuum of space. However, Earth’s satellite does indeed possess a thin, tenuous blanket of gases known as an exosphere. This ethereal atmosphere, while delicate, persists around the Moon, raising intriguing questions about its origin and maintenance.
Unlike Earth, which uses its magnetic field to confine its atmosphere, the Moon lacks such a protective shield. Consequently, scientists have long wondered how the lunar exosphere isn’t stripped away by relentless solar activity. Recent research from a team led by geochemist Nicole Nie at the Massachusetts Institute of Technology (MIT) offers a compelling answer: micrometeorites.
“We give a definitive answer that meteorite impact vaporization is the dominant process that creates the lunar atmosphere,” says Nie. Tiny micrometeorites, often no bigger than grains of dust, continually collide with the lunar surface, vaporizing lunar dust and releasing atoms into the exosphere. This steady bombardment ensures that the Moon’s atmosphere is constantly replenished, achieving a dynamic equilibrium over billions of years.
The challenge in studying the Moon’s exosphere lies in its diffuse nature. Although Apollo mission detectors confirmed the presence of various atomic components, pinpointing their exact origins remained elusive. Previous models implicated both micrometeorite impacts and a process known as ‘ion sputtering where solar wind particles dislodge atoms from the lunar surface.
Nie and her colleagues sought to clarify these processes by analyzing data from the Lunar Atmosphere and Dust Environment Explorer (LADEE), a lunar orbiter that operated between 2013 and 2014. LADEE’s observations indicated that both micrometeorite impacts and solar influences play roles in shaping the lunar exosphere. During meteor showers, an increase in atmospheric atoms was noted, while changes also occurred during solar eclipses when the Moon was shielded from the Sun.
To deepen their understanding, the researchers examined samples of lunar soil collected during the Apollo missions, focusing on the elements potassium and rubidium. These elements, known to exist on the Moon, are easily vaporized by impacts or solar wind. By crushing the Moon dirt into fine powder and analyzing it with a mass spectrometer, the team discerned the isotopic ratios resulting from each vaporization process.
Their findings revealed that micrometeorite impacts contribute significantly more to the lunar exosphere than ion sputtering, with a relative contribution of about 70:30. “With impact vaporization, most of the atoms would stay in the lunar atmosphere, whereas with ion sputtering, a lot of atoms would be ejected into space,” Nie explains.
This discovery not only enriches our understanding of the Moon but also has broader implications for other celestial bodies in the Solar System. Similar processes might occur on asteroids and moons, and analyzing samples from these bodies could reveal the effects of meteoroid bombardments and solar wind over geological timescales. Future missions, like the European Space Agency’s planned sample return mission to Martian moon Phobos, could apply these findings to better understand space weathering across the Solar System.
Thus, the humble micrometeorite emerges as a key player in maintaining the Moon’s fragile exosphere, offering a glimpse into the dynamic processes shaping celestial atmospheres.
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