The Moon is our pale guardian, fascinating us with eclipses, controlling our tides and even helping to generate our planet’s magnetic field. Today, it’s a dead sphere, one that just sits up there, tidally locked to Earth, unchanging. Its past, however, was far more dramatic.

Not only was it once an active volcanic object, but as a new paper published on arXiv has revealed, it also likely had a metallic atmosphere as thick as Mars’ is today. Unfortunately, the glowing embers of the early Earth ended up blasting it away into deep space, leaving us with the atmosphere-lacking Moon we see today.

Once upon a time – 4.5 billion years ago, in fact – the Earth was nothing like it is today. It had just begun to form, and its intense primordial heat ensured that it was a massive molten ball. Around this time, an object roughly the size of Mars impacted the magma-covered world and flung a decent chunk of this material out into space. Eventually, this material coalesced in an orbit around Earth and formed the Moon.

After cooling began to properly kick in, very primitive atmospheres would have formed over these two objects. A team of scientists at NASA’s Goddard Spaceflight Center wanted to know what the Moon’s might have been like, and how the intense radiation of the early Earth may have influenced its creation and destruction.

Back then, Earth’s surface was still around 2,000°C (3,632°F), and would have glowed like an ember at the bottom of a fire pit. The Moon then was around 15-20 times closer to Earth than it is today, which means that all this thermal radiation would have had quite the effect on it.

Using cutting-edge computer simulations, the study revealed that metallic elements in the Moon’s magmatic ocean would have been vaporized by the Earth’s intense heat, causing them to rise up and form an atmosphere. It would have been just 10 percent of the thickness (and pressure) of Earth’s, but for the Moon, that’s pretty thick.

Interestingly, the side of the Moon facing the Earth would have been far hotter (1,700°C/3,092°F) than the other side (-150°C/-238°F). This temperature differential would have caused winds with speeds of over a kilometer (0.62 miles) per second to rush from one side to the other, peppered with metallic elements – particularly sodium and silicon oxides.

When the winds approached the dividing line between the hot and cold hemisphere, all these metallic elements would have condensed out and fallen as snow.

Weirdly, as the Moon began to tidally lock itself to Earth – meaning that just one side faced the planet – the atmosphere would have become asymmetrical. Only the Earth-side would have had the chance to see this strange atmosphere accumulate, with the majority of the far side remaining atmospherically barren, so to speak.

After about 1,000 years post-formation, the Moon’s liquid magma ocean would have largely cooled and about 70 to 80 percent of it would have crusted over. The study explains that this would have led to the formation of massive "rockbergs" of volcanic minerals that would have floated atop the remaining pools like bizarre icebergs.

Without a source of vaporized metals, the atmosphere would have collapsed, and the embers of Earth would have blown away the remaining gaseous envelope into space.

The Moon is likely not unique in this sense. The authors of the study note that "such an atmosphere may also be a cousin to atmospheres expected on some of the most close-in and heated rocky exoplanets."

[H/T: Science News]