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SYFY WIRE Bad Astronomy

Air de lune

By Phil Plait
Artwork depicting the formation of the ancient Moon's atmosphere. Credit: NASA/MSFC

Looking at the Moon right now, it’s pretty fair to say it’s airless. It does have an atmosphere, kinda ... if you call its whisper-thin layer of gas an atmosphere. Earth’s air at sea level is 100 trillion times denser. Essentially, standing on the Moon’s surface today is standing in a vacuum.

But that’s not always been the case. New research indicates the Moon once had a decently substantial atmosphere, and not only that, it was able to hold on to it for quite some time. The source of the air up there? Volcanic eruptions onto the lunar surface billions of years ago.

We think a lot of the Earth’s atmosphere came from the same sort of thing (what we call “outgassing”) — volcanic eruptions releasing tremendous amount of gas, enough to create an atmosphere — so it’s not silly to think the Moon may have undergone a similar process. And, in some ways, it’s easier to see this on the Moon. While the Earth’s ancient volcanoes were all rapidly erased due to plate tectonics and erosion long ago, the Moon’s surface is far more static. The evidence is still there.

You can actually see the results of these lunar eruptions with your naked eye. When you look at the Moon, you see the surface is patchy, with darker spots all over its face. Those are called maria (Latin for “seas”; when viewed during the day they take on the sky’s bluish hue and look like bodies of water). Billions of years ago, volcanic vents erupted molten material that then flowed and filled basins in the surface, which cooled and created the maria.

Using orbiting space probes like the Lunar Reconnaissance Orbiter, Clementine, and GRAIL (Gravity Recovery and Interior Laboratory), lunar scientists have been able to map out the ancient volcanic eruptions on the Moon and even get measurements of how thick the flows are (in general, a few kilometers). This, then, gives the total volume of the material that erupted over time.

Not only that, but by looking at the number of subsequent impact craters in these flows, the age of each the flows can be determined. That provides the volume of erupted material over time.

Timeline of the formation of the lunar maria. In each panel, red is the oldest material (so in the second panel, the purple overlays the red; in the third the blue overlays the purple, and so on). Credit: Needham and Kring

This is the part I love: This material had gasses in it, which then bubble out of the lava. But how much gas are we talking here? Well, you’d need to have a sample of that material, then look for trapped gases in it to measure their amounts. But hey, didn’t we send a bunch of people to the Moon to collect rocks … ?

Yup. Samples of volcanic glass from different places on the Moon were returned to Earth in Apollo 15 and 17. Inside these glasses are bubbles of gas, which can then be analyzed. The total amounts of the gas in the material can then be found, and then you only need to multiply it by the volume of material released in the eruptions to figure out how much gas the Moon would’ve had back then. I’m simplifying it here, of course, but that’s the general idea.

What the scientists found is that the creation of the mare was most vigorous between 3.8 and 3.1 billion years ago. During that time, gases like carbon monoxide, water vapor, hydrogen, hydroxide (OH), and sulfur were busily bubbling up out of the lava flowing over the lunar surface. So much gas was released so rapidly that the Moon formed an actual atmosphere!

At its peak, this lunar air would’ve been about 1% the thickness as Earth’s atmosphere. That may not sound like much, but it’s 1.5 times as thick as the current atmosphere on Mars, and that’s enough to cause erosion on the Martian surface and interact with the rocks there chemically. The same must have been true for the Moon billions of years ago, as well.

How long did this Moon air last? Planets and moons can leak away their atmosphere due to several processes. The principal one is just called thermal loss; when you heat a gas up, it expands. On a molecular level, what we think of as “temperature” is really just how fast the molecules are moving around. Each little molecule is zipping around at high speed, bumping into other molecules, and the hotter it is, the faster it moves. Molecules and atoms at the top of the atmosphere can be warm enough and move fast enough to escape into space! Over time, this process can remove a substantial amount of a body’s atmosphere.

Calculating this loss rate given the amount of material, the temperature, and the Moon’s gravity, the scientists figure the Moon may have had this atmosphere for 70 million years! That’s far longer than I would’ve thought. But, for a long time, the loss was balanced by outgassing, and was thick enough that thermal loss was slow compared to the amount of air there.

Artwork depicting the formation of the ancient Moon's atmosphere. Credit: NASA/MSFC

Imagine! If you could go back in time and stand on the Earth’s surface, the Moon would have looked very different (to be fair, so would the Earth; better have a spacesuit with you, since there was no atmospheric oxygen back then). The lunar maria we see now were still forming, and back then the Moon was much closer to Earth, about 150,000 km away versus 400,000 now. It would’ve looked three times bigger in the sky!

But most amazingly, it would’ve looked fuzzy, soft, with perhaps even a thin blue or silvery line around it — the thin air scattering sunlight and glowing.


One more thing: We know there is a lot of water frozen at the lunar poles, trapped in eternal darkness at the bottoms of craters there. Scientists have wondered where it came from: impacts from water-bearing comets or asteroids, or some other source? This new research indicates that if only 0.1% of the water vapor released by the lava flows migrated to the poles and froze there, it would account for all the water seen there.

It’s not clear how much of this water came from deep space or from inside the Moon itself, but it’s certainly possible a lot of it was locally sourced.

Think about that. Studying the Moon’s dry, lifeless surface now gives us clues about what it used to be like there, revealed the presence of an ancient atmosphere, and may even account for water ice that may someday be mined by astronauts to provide them life-sustaining resources to enable living there.

If I read this in a science fiction novel this would be pretty cool ... but it’s real. The next time you look up and see the Moon, take a moment to look at the dark maria, making up the familiar face of the Moon. Then take another moment to breathe in what they mean for the ancient Moon, and what they may mean for its future.

P.S. If you don't understand the title of this post, I suggest you waste no time familiarizing yourself with the works of Claude Debussy. Your life will be the better for it. 

Correction (Oct. 10, 2017 at 18:00 UTC): The original caption to the mare outflow sequence was incorrect, and I fixed it. My apologies for any confusion.

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