{♫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!
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{♫Outro♫}
