Even though there is no air to breathe on the Earth's natural satellite, there is still an atmosphere, although it is very thin and rarefied. For a long time, scientists could not understand where it came from and why it is preserved. Unlike the Earth, the Moon does not have a magnetic field that repels the solar wind, and lunar gravity is too weak to hold gaseous substances near the surface for a long time. Therefore, according to all the laws of physics, the atmosphere should have flown away into space. From this we can conclude that there is some source on the satellite that constantly replenishes it.
The Moon has an atmosphere, but it is very thin. Photo source: thealphacentauri.net
Contents
- 1 What kind of atmosphere does the Moon have
- 2 How did the atmosphere on the Moon form
- 3 How meteorites provide stability to the exosphere
- 4 Could the young Moon have had a denser atmosphere?
What kind of atmosphere does the Moon have?
The Moon does not have an atmosphere in the usual sense of the word. However, scientists have found a so-called exosphere, which is located above the surface in a thin layer. The exosphere is an extremely rarefied gas shell. Its density is ten trillion times less than the density of the Earth's atmosphere.
However, the exosphere also creates a pressure at the surface, which is 10 nPa. It consists mainly of hydrogen, helium, neon and argon. This atmosphere has virtually no effect on the Moon itself, that is, it does not cause any processes on its surface.
Scientists have found a clue about the origin of the atmosphere in the lunar soil. Photo source: dzen.ru
How the atmosphere on the Moon originated
We have already told you that volcanoes once actively erupted on the Moon. As you know, volcanic eruptions are always accompanied by the release of gases. However, according to recent research, volcanic activity on the Moon ceased about two billion years ago. During this time, the “volcanic” atmosphere would have disappeared long ago if it had not had constant «recharge».
Clues about how the atmosphere actually formed and why it is stable were obtained in 2013 by NASA's LADEE spacecraft. The spacecraft studied the Moon's exosphere and its effect on lunar dust. The study found two processes by which the exosphere can be replenished with volatiles: impact evaporation and ion sputtering.
Impact evaporation occurs when the Moon is “bombarded” by meteorites. The high temperatures and pressures cause many materials on the Moon's surface and the meteorites themselves to evaporate. Some of this material escapes into space, but some remains above the Moon and replenishes the atmosphere.
The Moon's atmosphere is constantly replenished with volatile substances as a result of meteorite falls. Photo source: universemagazine.com
Ion sputtering occurs as a result of the interaction of the solar wind with the surface of the Moon. When high-energy charged particles hit the atoms of substances on the surface of the Moon, they literally “knock” them out into the exosphere. However, until recently it was unclear which of these processes was dominant.
How meteorites ensure the stability of the exosphere
In a recent study, the team used lunar soil samples in which they isolated two elements, potassium and rubidium. Both are volatile and can be vaporized by meteorite impacts and solar wind. The goal of the study was to determine the ratio of different isotopes of potassium and rubidium in the lunar soil.
Recall that an isotope is a variety of an element that has a different number of neutrons in its atomic nucleus. Isotopes with more neutrons are heavier than those with fewer. Light isotopes are more likely to remain in the Moon's atmosphere than heavy ones, which fall back to the surface.
The solar wind provides the Moon with only 30% of its atmosphere. Photo source: new.ras.ru
Impact evaporation and ion sputtering have different efficiencies in terms of isotope emissions into the lunar atmosphere. Based on modeling, scientists have found that ion sputtering releases light isotopes into the atmosphere. Thus, having determined the ratio of heavy and light isotopes in the lunar soil, scientists have found that impact evaporation is the dominant process. Ion evaporation accounts for only 30% of the gases entering the atmosphere. The authors of the work report this in the journal Science Advances.
According to researchers, impact evaporation has maintained the Moon's atmosphere for billions of years. The Moon has existed for about 4.5 billion years, and throughout this time it has been continuously “bombarded” by meteorites. Thanks to this, its atmosphere has always remained stable.
Could the young Moon have had a denser atmosphere?
More than 4 billion years ago, the solar system was much calmer than it is now. At one point, a planet the size of Mars crashed into Earth, resulting in the formation of Earth's natural satellite just a couple of hours after the catastrophe. At the same time, the young Moon itself was constantly subjected to massive asteroid impacts. Evidence of this are the numerous craters on the surface.
The numerous craters on the Moon are evidence of how “turbulent” the Solar System was billions of years ago. Photo source: www.vokrugsveta.ru
Accordingly, it can be assumed that the Moon at that time had a denser atmosphere, since more gases were released as a result of impacts. Over time, the Solar System became more “calm”. Although the Moon continued to be hit by meteorites, it was to a much lesser extent, that is, it happened less often, and the meteorites themselves were smaller.
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However, it should be taken into account that the Moon has very weak gravity, as a result of which it is not able to hold a full-fledged atmosphere. Therefore, it is unlikely that its exosphere was denser than it is now, even during the period when volcanoes erupted and meteorites constantly crashed into it.