{♫Intro♫}.

Around the end of July 2019, China's Yutu-2 rover was exploring the far side of the Moon when it noticed something strange. It was green.

And in the shadowy light of the crater, it almost seemed to glisten. It wasn't aliens. It's never aliens.

But there was a mysterious substance on the Moon! And in a paper published online last month in Earth and Planetary Science Letters, a group of astronomers in China announced that they're pretty sure they've figured out what it is. For their analysis, they used data from three types of cameras on the rover: a pair of panoramic cameras, an imager that could collect data in both visible and near-infrared light, and a navigation camera that happened to get good images of the nearby terrain.

The goal was to compare the so-called “unusual substance” with the surrounding regolith — or loose, rocky dirt, basically — and learn more about its properties to figure out where it came from. Now, when you see dark, shiny stuff on the. Moon, that generally means one thing: glass.

And that glass usually comes from one of two sources: either impact melt, which is formed when something crashes into the lunar surface, or volcanic eruptions. According to the team, this green stuff seems much more like impact melt than volcanic glass. More specifically, they noted that it looks like two impact melt samples returned to Earth during the Apollo missions.

One consists of rock fragments held together by black glass, and the other is more like a bunch of small soil particles coated in glass. So those are the two main options:. This substance could be mostly impact melt, like the first Apollo sample, or just a bunch of regolith coated in a layer of glass, like the second one.

We don't really have enough data to know which. But there are a few things we can extrapolate. Like, if this stuff is mostly impact melt, it probably didn't come from the same impact that made the crater we found it in.

When the team ran the numbers, they found that the density and speed of the object that would have made the crater, which is about two meters wide, would only have made impact melt about six centimeters wide. And this shiny green stuff is 52 by 16 centimeters. So if this glass is mostly impact melt, the team thinks it's much more likely that it was made by an impact somewhere else and bounced into this other crater.

On the other hand, if the green stuff is more like a coating on regular, unmelted rock, it was probably caused by the impact of a small, two-centimeter-wide meteorite. So, we're still working out the fine details, but overall, the conclusion here is that this “unusual substance” is probably just some dark, greenish glass made by an impact. Which is great progress... and definitely rules out aliens.

Like everything else on the far side of the Moon, this green stuff wasn't something you could see from Earth — we have to actually go out there to take a look. But there's another rare sight you can see from Earth these days: a comet so bright it's visible to the unaided eye. The last bright comets were in 2011 and 2007, but both of those were mostly visible from the Southern Hemisphere.

The last time those of us on the northern half of the planet got to see a really awesome comet was when Hale-Bopp flew by in 1997. This new object is called Comet C/2020 F3, but its unofficial name is Comet NEOWISE, after the space telescope that spotted it in March. It makes sense that we wouldn't have seen it before: Comets have famously lopsided orbits, and this one takes more than 6000 years to finish one loop around the Sun.

But at long last, it's back. On July 3, it survived its closest approach to the Sun, passing within about 43 million kilometers of it without being destroyed by the heat. And now, its orbit is taking it closer to Earth.

The comet has been visible for the last couple of weeks, but it will be closest to us — within about 103 million kilometers — on July 22. After that, we're not sure exactly how long it'll be visible, but we should have at least another week or two. Seeing the comet might be tricky, though.

In terms of raw brightness, it's technically on par with some of the brighter stars in the night sky, but because that light is spread over a larger object instead of a single speck, it's harder to see. Still, people are reporting that with clear skies away from light pollution, they've been able to see it as a fuzzball even with the unaided eye. And if you have access to binoculars, you can get an even clearer view, including its split tail.

Until last week, the best view was in the northern hemisphere, about an hour before dawn. But these days, you should be able to see it just after dusk, looking toward the northwest horizon. In the southern hemisphere, the timing is a little trickier, but we'll link to a website in the description where you can get more info for your location.

If you do get to see Comet NEOWISE, let us know in the comments below! And thanks for watching this episode of SciShow Space News! We couldn't make episodes like this every week without our patrons on Patreon — so to everyone who supports the show, thank you.

If you want to keep up with the latest in space news, you can subscribe to our channel. And if you want to help us make more of these, you can learn more at patreon.com/scishow. {♫Outro♫}.