S u m m a r y :
The young moon might have had a thick, metallic atmosphere together with supersonic winds, suggests a new study published online at arXiv.org.
The research is based on a simulation designed to understand how heat from the surfaces of the Sun, a young Earth, and the moon vaporised lunar metals to form a thick atmosphere around our satellite. The aim behind this new model is to test theories of moon formation. The findings are also thought to be helpful to scientists who want to study exoplanets—they would allow them to do so without leaving the Earth.
Formation of the Moon
The most common theory explaining the formation of the moon mentions a massive collision that happened between a Mars-size planet and the Earth around 4.5 billion years ago—this impact would have scraped off our planet’s surface, hurling hot molten material from it into space, in Earth’s orbit. This mass of materials would have eventually cooled down into the moon.
This primordial moon is believed to have had a deep ocean consisting of hot liquid rock (magma) all around it. At that time, the Earth was exceptionally hot (2000° Celsius), making it glow like a red dwarf star. The star-Earth would have sent out radiation to the baby moon. The latter would have also received the same treatment from its own magma ocean, and from the sun. Study author Prabal Saxena from NASA’s Goddard Spaceflight Center in Greenbelt considered a combination of these conditions using their model. According to the researchers, their model is the first to incorporate all of these interactions.
Young Moon’s Metal Atmosphere
The radiation combo hitting the moon would have changed volatile atoms from the magma ocean rich in metals into a gaseous state which would have constituted a lunar metallic atmosphere. The primordial moon atmosphere would be similar to Mars’ in terms of thickness, at around one-tenth the that of Earth’s. This heavy metal atmosphere was apparently dominated by supersonic winds, indicates the model. The lunar winds would have influenced the moon’s magma ocean, lifting the waves up.
The Supersonic Winds
The molten ocean would constantly send sodium atoms into the atmosphere, a process that would continue for as long as it remained in the liquid state. The atmosphere, upon receiving the sodium, would whip them through the metallic air, and because the moon experienced an extreme temperature difference between the side facing the Earth and its farside (the former, heated to over 1700°C, while the latter would be as cold as ‒150°), the resulting winds would be of great speeds, over a kilometer per second. These winds might have stirred waves in the moon’s magma ocean.
These winds would travel to the twilight zone separating the hot and cold regions, at a place where the air would have condensed, resulting in a band of sodium snow.
Ocean Turned to Rock
The magma ocean did not remain an ocean for long, though. Around a thousand years after the formation of the moon, its temperature would have decreased, ultimately causing it to have a solid, rocky crust. As the liquid was no more, the whole of the atmosphere would have ceased to exist.
“The moon’s atmosphere was like a hard-partying rock star,” says Saxena. “It had a really violent, heavy metal existence, but it rapidly just fell apart.”
Is the Simulation Reliable?
Testing this model should be easy. One way would be to search for a ring of an excess of sodium in lunar rocks at the transition zone; the presence of the metal would indicate the past existence of an extremely hot atmosphere with high winds. This extra sodium would also answer yet another pressing question: how did the moon really form, was it a single major impact as described above, or did it happen in several, but smaller, impacts?
Learning About Exoplanets
Furthermore, the young moon having lived near to a star-glowing Earth might serve as a model for rocky exoplanets orbiting red dwarfs.
“If we can characterize what the early moon looked like, it can tell us about the physical mechanisms that are operating on these close-in extreme exoplanets,” Saxena says.