What if the Moon had air?

On the Moon, the sky is black. It is black even when the Sun is up!

That is because the Moon has almost no air. What if it had air like ours?

On Earth, a sunny day means a bright blue sky. On the Moon, NASA says, the sky is pitch-black, no clouds ever form, snow never falls and thunder never rolls. The reason is that the Moon has almost no air.

So what if the Moon had air like Earth's? Would its sky turn blue? Would it get weather? And why doesn't it have air already? This is a thought experiment: choose a path and see what scientists know, and what they think might happen.

The Moon's barely-there envelope of gas is an exosphere: NASA notes that a cubic centimeter of Earth's atmosphere holds a hundred trillion times as many molecules as the same volume near the lunar surface, and the Moon Fact Sheet puts its total mass at about 25,000 kg.

This thought experiment gives the Moon an atmosphere and asks what would change: its sky, its temperatures, its weather. It then asks why it lacks one, using the Moon's low escape velocity and evidence that it once had an atmosphere for tens of millions of years. What would happen with Earth-like air is labeled as an idea.

The Moon gets very hot in the sunshine. It gets very cold in the dark.

No clouds form, and no snow falls. The sky is always black.

Without air to spread out the Sun's heat, the Moon's temperatures swing wildly. NASA says the ground near the Moon's equator can heat to over 121 degrees Celsius in daylight, then drop to minus 133 degrees after nightfall. In some deep craters near the poles that never see sunlight, it is colder than minus 246 degrees.

There is no weather as we know it. Instead, NASA says, the Moon's 'weather' comes from space: a stream of particles from the Sun, cosmic rays and space rocks.

NASA describes the lunar exosphere as too skimpy to trap or spread the Sun's energy, so temperature contrasts are extreme: over 121 °C (250 °F) near the equator by day, −133 °C (−208 °F) after nightfall, and below −246 °C (−410 °F) in permanently shadowed polar craters measured by the Lunar Reconnaissance Orbiter.

With no atmosphere to make weather, the Moon's environment is set by the solar wind, galactic cosmic rays, coronal mass ejections and impacts, and it has no global magnetic field to shield it.

Imagine the Moon with air like ours. Sunlight might bounce around in the air.

Then the Moon's sky might look blue in the daytime, like ours!

Imagine wrapping the Moon in air like Earth's. On Earth, NASA explains, the gases in the air scatter sunlight, and blue light is scattered the most, which is why our sky is blue. With similar air, the Moon's daytime sky might turn blue too.

Air would also spread the Sun's heat around. Today, NASA says, the Moon's huge temperature swings happen because its thin gas layer can't trap or spread that heat. With thick air, the swings might be smaller, and there might be clouds and wind. All of these are ideas, not things anyone has seen.

Two well-understood effects suggest what Earth-like air might do. Scattering by gas molecules, strongest for short wavelengths, makes Earth's sky blue; Space Place adds that sky color depends on what is in an atmosphere, which is why Mars's thin, dusty air looks orange by day. And an atmosphere that traps and redistributes heat might damp the extreme day-night contrast that NASA attributes to the exosphere's skimpiness.

There is also evidence. Needham and Kring estimated that the Moon's ancient volcanic atmosphere was thicker than Mars's today and was likely capable of weathering rocks and producing windstorms. A thick atmosphere today, though, is an idea, and the details would depend on its gases.

Air is made of tiny bits that zoom around very fast.

The Moon pulls only gently. So the zooming bits can slip away into space.

Air is made of tiny particles that are always moving fast. To escape a world for good, a particle has to move faster than its escape speed. Big worlds have strong gravity and a high escape speed, so they hold on to their air. Small worlds don't.

NASA's figures show the Moon's escape speed is only about 2.4 kilometers per second, compared with about 11.2 for Earth. As a University of Alaska Fairbanks scientist put it, small worlds like the Moon do not have enough gravity to hold on to fast-moving gas.

A molecule escapes when its speed exceeds the escape velocity, v = √(2GM/R). NASA's fact sheets give 2.38 km/s for the Moon and 11.186 km/s for Earth. The more massive the body, the higher this speed.

A University of Alaska Fairbanks Geophysical Institute article makes the comparison directly: giant planets' escape velocities are too high even for high-speed gas molecules, while the Moon and Mercury lack the gravity to hold them. Earth keeps its atmosphere because the mean speed of its gas particles is much lower than its escape velocity, and Mars has already lost most of its atmosphere.

Long, long ago, the Moon had volcanoes. They puffed out gas.

For a while, the Moon had a layer of air! Then it slowly went away.

In 2017, NASA scientist Debra Needham and planetary scientist David Kring studied Moon rocks brought back by the Apollo astronauts. They worked out that volcanoes on the young Moon released so much gas that the Moon had an atmosphere for a while, thickest about 3.5 billion years ago.

It may have lasted about 70 million years and was made mostly of carbon monoxide, sulfur and water. When the volcanoes quieted down, the gas was lost to space or became part of the Moon's surface. Some of that water might even be the ice found in the Moon's darkest craters today.

Needham (NASA Marshall) and Kring (Lunar and Planetary Institute) combined the timing and volume of mare basalt eruptions, 3.9 to 1 billion years ago, with lab analyses of Apollo basalts to estimate the volatiles released. Their result, published in Earth and Planetary Science Letters in 2017, is a short-lived atmosphere, about 70 million years, mostly carbon monoxide, sulfur and water, thickest about 3.5 billion years ago.

As volcanism declined, the atmosphere was lost to space or incorporated into the surface. The eruptions released so much water, they estimated, that if 0.1% reached the permanently shadowed regions it could account for all the water detected there: an idea about the origin of lunar ice.