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Caitlin: You wouldn't know it by looking at it, but it turns out that the moon has a tail. And it's not a small tail either.

This thing can grow to more than four hundred thousand kilometers (400,000 KM) long. And like some kind of inverse werewolf, we can only detect it right around the new moon.

The moon's tail was discovered by a team of astronomers in Texas during the nineteen ninety-eight (1998) Leonid meteor shower, which happened to coincide with the new moon. 

The scientists were observing the sky as usual when they noticed a big but faint yellow spot. That glowing spot was a tell-tale sign of excited sodium atoms in our atmosphere. Which emit yellow light when the go back to their normal state. 

By tracing the trail of the sodium the astronomers realized that it was coming from the Moon. And earth was passing through a stream of particles being whisked off it's surface.

That was exciting! And not just because they discovered that the Moon had a tail. These atoms were coming from something that astronomers had been wanting to know more about for a while. The Moon's atmosphere.

Because, it turns out, the moon has an atmosphere, too. It's a very thin atmosphere, to be sure, but still the surface of the moon is not the total vacuum that we might imagine. And astronomers have a special name for this incredibly thin atmosphere: an exosphere.

So where did it come from? Some of the particles in the exosphere come from a process called outgassing, where radioactive elements beneath the Moon's surface release gases as they decay. Potassium forty (40), for instance, releases argon.

But the gases could also be released by sputtering. Which is when solar wind or small meteorites hit the surface and dislodge some of the gas that's trapped underneath. That's where the sodium is coming from.

But the sodium doesn't hang around on the Moon for very long. Our faithful little rocky friend is simply too small to have enough gravity to hold onto all of these particles. 

And it doesn't have any way to protect them from being stripped away by solar wind. So the pressure from the Sun's radiation sweeps the sodium out behind the moon forming a long, teardrop-shaped tail. 

So now we know where the tail comes from, but there are still some open questions. Like, how exactly the sodium escapes from the surface, and whether or not a meteor shower makes the tail bigger with all those meteorites knocking out more gas.

In two-thousand six (2006) a team of astronomers began a two and a half year long study to answer these questions. They measured the brightness of the Moon's tail over thirty-one (31) new moons trying to figure out whether things like fluctuations in solar wind or meteor showers affected it. 

They thought that they'd be able to isolate one factor as the main influence on the tail. But they couldn't. What they found was that nothing changed all that much. The brightness stayed roughly the same for all thirty-one (31) months. 

The difficulty here is that we can only see the Moon's tail during the new moon, because that's the only time that the Moon is directly between us and the Sun. 

Since the tail is being pushed away from the Sun by solar wind, when the Moon is new it's basically blowing right in our face. Of course it hits the day side of the Earth first, but the planet's magnetic field pulls the tail in and extends it all the way around to the night side, where we can see the stream of sodium.

But in order to get a closer, clearer view of this thing in two-thousand thirteen (2013) NASA launched LADEE. Short for the Lunar Atmosphere and Dust Environment Explorer. The size of a small car, the probe orbited the moon for about one-hundred (140) days collecting data on the Moon's exosphere, including it's tail, before crashing into the lunar surface in April of twenty-fourteen (2014). 

Researchers aren't finished analyzing all of the data yet, but we're already starting to get some important insights. For instance, LADEE found the amount of sodium in the exosphere actually does change throughout the lunar cycle. Probably because of variations in solar wind. LADEE also found the first direct evidence that meteor showers do increase the density of the sodium in the Moon's tail.  

Even after four and a half billion (4,500,000,000) years palling around together, it turns out we still have a lot to learn about our little buddy up there.

So, Goodnight Moon... Tail.

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