[Intro]

[Hank] Though the sun may shine on the far side of the moon, it is dark to us. Gravity keeps one side of the moon forever facing us, leaving the other side a source of perpetual mystery. And that side that forever faces away from us has sparked a ton of scientific questions over the years--but the first two questions were probably something along the lines of "what does it look like?" and "how can we see it?"; so here's Reid with the answers.

[Reid] The dark side of the moon: mysterious, intriguing. Why does it have so much lore associated with it? Maybe because it took three spacecrafts visiting before we had any idea what it looked like. So, here's how we finally got pictures of the dark side of the moon for the first time, and what it took to get those images back to Earth.

In October 1959, we got our first pictures of the dark side of the moon thanks to the former Soviet Union's Luna-3 spacecraft. It was the third spacecraft to go to the moon. So two missions were complete before we even tried to snap some pictures of its far side, and it wasn't an easy feat.

Luna-3 took pictures just like how we would take them on Earth back then, with a camera and film processing equipment. But the impressive part is that the spacecraft then sent those pictures to Earth using electrons.

First, a camera was mounted in the probe to take the original photos. The film had to be temperature and radiation resistant because of the unwelcoming environment of space. But this film, just like film on Earth, had to be developed, fixed, and dried. And all that processing happened automatically within the probe while it was still in space.

Then, six antennae mounted on the outside of the probe transmitted the images to Earth. They were similar to the antennae on old household tvs. 

A cathode ray, which is made of streams of electrons inside a vacuum tube, produced a bright light that shone through the developed film and onto a super sensitive detector called a photoelectric multiplier.

This light comes from the electrons crashing into gas inside the cathode ray tube, or a special material coating the ends. When that happens the coating glows. We can see it as light because the coating converts electrical energy into light energy. It's like when you eat and take the energy from your food to energize yourself. In Luna-3, the cathode ray scanned the whole image. Then, the intensity of that light was converted into an electric signal and scanned with a 1,000 line resolution. This is similar to how televisions used to work. Those same old ones with the antenna? Then, it passed the electric signals through a modifier and to a magnetic recording device on Earth.

When Luna-3 scanned the images, it produced light gradients to represent what was photographed, and it did this along slow, even, straight lines like a printer. That's why the images looked striped. Then, back on Earth, a light sensitive film was put in front of a screen to capture the image. But these images were far from a full story. We got a glimpse at just 70% of the dark side of the moon. And we didn't get 100% for a few reasons.

First, Luna-3 was on a trajectory to orbit the moon and take images while it traveled. It had photoelectric cells that use sunlight to align the probe to face the moon, but there were no rockets on the spacecraft so course correction was not an option. And it couldn't pause its orbit to get a better angle, so even if everything went according to plan, it was never supposed to image 100% of the dark side of the moon.

Another reason we didn't get a complete idea of what it looked like is that Luna-3 took 29 pictures, but only 17 full images were transmitted to Earth. A cathode ray isn't perfect. It's not super detailed in bright environments and can only use a limited number of gray shades to make an image. It's pretty impressive that we got any images at all. Now, the images could be sent to Earth at any point, but the signal was stronger when the probe was closer to Earth. Today, scientists think

  the partial images that were received on Earth were a result of Luna-3 being so far away. While we might not have gotten a complete picture of 100% of the dark side of the moon, those first images told us that the far side of the moon is full of craters of all sizes. This isn't how the near side of the moon looks at all. So it was an important step in learning more about our moon's history.

But the moon isn't the only mystery here. Four days after the last photo was sent, Luna-3 disconnected its communications and never came home. Its fate is still unknown. But, there are a few ideas about what happened to it. It might have burned up in an attempt to re-enter Earth's atmosphere. Or, maybe it stayed in orbit and flew in circles around Earth for another two years.
So, Luna-3 joins the list of moon-related mysteries.

Since the Luna-3 mission, humanity has revisited the dark side of the moon several times, and brought home higher resolution images. So today, that dark side of the moon is far less mysterious.

[Hank] We did it! And the images are awesome, despite being a little grainy. So we need to spend some more time going deeper into the details from those images, because humanity has been wondering about what they would look like for too long!
Here's Caitlin, with more of those exciting details.

[Caitlin] The moon is our planet's constant, dependable companion. Whether it's waxing, waning, or shining in its full glory, it always presents the same face to Earth. A mottled, monochrome landscape of black and white rocks. Depending on where you are in the world, you might see different things in that great ink blot in the sky.
Some see the smiling face of the man in the moon. Others might see a rabbit stirring a pot of medicine. But whatever you make of it, that pattern is always the same. That's because the moon is tidally locked to the Earth. Gravitational forces between the two have slowed the moon's spin, so that it takes exactly the same time to complete a full rotation as it does to orbit the Earth.
That's given us plenty of time to examine this side facing us, but nobody knew what on the far side until relatively recently.

Turns out it's very different from the side we normally see, and astronomers are still working to pin down how that happened. The mysterious unseen side of the moon is sometimes called "the dark side", thanks Pink Floyd, but it isn't always dark. During a new moon, and when all we can see from earth are those toenail crescent slivers, this side is positioned behind the moon, lighting up the far side and leaving the side facing us dark instead. 

But it is dark from a knowledge point of view. Because of that rigid tidal locking, we'll never be able to see the far side from the surface of the earth. To do that, you have to physically fling something around the back of it. Fortunately, 60 years ago, flinging things around the moon was an integral part of the space race.

Although the US was the first to put an actual person on the moon, it was the Russians who led the space race at the beginning. And in 1959, their Luna 3 Spacecraft became the first to visit and take pictures of the far side. But it's not like you could just snap a digital picture back then. 

When Luna's cameras detected the sun-lit far side of the moon, they took photos using actual physical film. The negatives were then developed within the spacecraft and passed to a scanner, which shone a light through the image in a thousand horizontal lines and transmitted it back to earth as an analog signal, kinda like a fax.

The image was blurry and low-res, which isn't too surprising if you remember what faxes used to be like, but it was our first glimpse of a totally unknown lunar surface, and it was immediately obvious that the far side was completely different from the side we're used to seeing.

Where the near side is covered in dark patches, the far side was much brighter, with just two small black spots. Since then, many more spacecrafts have flung themselves around the moon to get a better look at its "dark side" and collected all kinds of data from high resolution images to altitude and density readings.

And with all that data, scientists can now clearly see the differences between the near and far sides in incredible detail. As that blurry Luna 3 image suggested, while the near side is covered with vast plains of dark lava, the far side has very few. Instead, it's a rugged cratered highland landscape of a type of light-colored rock that's rich in calcium.

Elevation maps also reveal something that's not easy to see from the pictures, one of the largest craters in the solar system. The South Pole-Aitken crater covers a huge area of the southern hemisphere on the far side. Around 2,500 kilometers in diameter and 13 kilometers deep. And even more surprisingly, measurements of the density of rocks beneath the lunar surface showed that the moon's crust is up to 35 kilometers thicker on the far side compared to the side that faces us. That was surprising, to say the least. With the Earth and other rotating planets as examples, we had reason to think that you'd get the same kinds of features on all sides of a planetary body, but it turns out that's not always a thing.

Today with detailed images of planets & moons all over the solar system we know that the idea of two-faced worlds isn't unique. Mars has radically different northern and southern hemispheres. Pluto is adorned with a huge heart shaped birthmark over one side, and Iapetus, a moon of Saturn, is truly two-toned with dark colored dust smeared over one hemisphere of its icy white surface. And yet the reasons for the differences between the near and far sides of the moon remains something of a mystery. It's hard to reconstruct a 4.5 billion year history, even of our closest neighbor. But like most mysteries, scientists do have some ideas.

The difference in the number of lava planes, for instance, is probably tied to that uneven crust. Some researchers think that in the early days of the solar system, while the earth and the moon were still forming, a massive asteroid struck the thick crust on the far side, creating that gigantic South Pole-Aitken crater. Later impacts left craters on the far side but were able to pierce the thinner crust on the near side, unleashing a flood of lava that filled the craters they left behind. The lava cooled and the Earth-facing side of the moon was left with a permanent scar, its mottled face.

But that doesn't explain why the crust was thicker on one side in the first place, or why the South Pole impact didn't send the moon shooting off into space. With government agencies and private companies starting to set their sights on the moon again, hopefully we'll get more data soon to figure that out.

Luna 3's photos of the far size of the moon were probably the first major lunar revelation of the space age, but nearly 60 years later we still have a lot more secrets to uncover.


[Hank] Now as time went on we have collected more data and found out more about that giant crater--here is what we know about it now.


[Reid] We've been looking up at the moon for as long as there have been people to look. And because the natural satellite is tidally locked to earth, it only ever shows the same face to us. And for a long time we knew absolutely nothing about its so called "dark side", but its secrets have gradually been revealed over the last 50 years. And scientists probing the far side of the moon have recently discovered something new and unexpected. There's a chunk of metal buried deep beneath an even bigger crater, and they hope that studying it will help solve some of the outstanding mysteries around our solar system's formation.

We first got a good look at the moon's far side about 60 years ago when various U.S. and Soviet probes returned the first images. These pictures revealed a huge crater near the South Pole, which is now known as the South Pole-Aitken Basin. At about 2,500 kilometers across and 12 kilometers deep, it's the biggest and oldest crater we've found in the entire solar system. And scientists think that around 4 billion years ago, an asteroid about 170 kilometers wide impacted the moon, creating the crater. That impact would have been about 1,000 times more explosive than the asteroid that brought about the end of the dinosaurs. But, its incredible size isn't the only remarkable thing about the South Pole-Aitken crater: there seems to be something surprising hidden beneath the surface. Modern surveys with new instruments and technologies have revealed the structure of the moon in unprecedented detail. For instance, the lunar orbiter laser altimeter, or LOLA, onboard the lunar reconnaissance orbiter, has given us precise measurements of the moon's topography. And NASA's gravity recovery and interior laboratory, or GRAIL, mission has measured the strength of gravity all over the moon.

Combining these two datasets revealed an incredibly high gravity reading in the floor of the South Pole-Aitken basin. And since extra gravity must come from extra mass, that means there’s something huge and very dense hidden there under the surface. 

This mass concentration seems to extend several hundred kilometers beneath the lunar surface, and its density is closer to that of metal than rock. And there’s a lot of it. Scientists estimate that it’s about five times the size of the big island of Hawai’i, amounting to about 3.5 million cubic kilometers of metal.

Whats more, the extra gravity is enough to pull down the middle of the South Pole-Aitken crater by another 800 meters.  Other than the dip there’s nothing else on the crater bottom to suggest what it is, but the lunar scientists from Baylor University in Texas have a couple of ideas.

They believe that the signal could be coming from the asteroid’s core that collided with the Moon to make the South Pole-Aitken crater.

The theory goes that a big enough asteroid would have been able to differentiate--which is when things like planets, or in this case an asteroid, melt and the stuff in them separates from one another, forming layers--just like what happened to Earth back in the day. 

So when the asteroid impacted the moon it shattered into bits of rock, but the metal core would have plowed through and into the moons mantle, leaving a metallic core of iron and nickel surrounded by a rocky shell. And because the mantle was already partly solidified, it just kinda stuck there and stayed there to this day. 

There’s another interpretation for this extra mass which doesn’t call for interplanetary ballistics.

It’s possible that the extra density comes from a concentration of heavy metal oxides which formed near the end of the mantle solidification.

But these kinds of processes aren’t very well understood. And the fact that the metal is right beneath the biggest known impact crater in the solar system is a big clue that the two are probably connected.

Ultimatly impact sights like the South Pole-Aitken basin are important natural laboratories for understanding planetary formation processes. By studying this crater and the metal underneath, scientists hope to learn more about the cooling and solidification of the moon, the differentiation of asteroids and the timing of collisions in the early solar system. 

For example, we still don't know exactly when the South Pole-Aitken basin formed. Getting a handle on that would help us date other events in the solar system, for which we currently only have relative ages from things like overlapping craters. So scientists are currently making a case for sample return missions to the far side of the moon.

They won't be able to access the massive chunk of metal buried beneath the surface, but physical samples of the surface material would tell us a lot, like the state of the asteroid and the moon during the South Pole-Aitken impact.

Scientists can also pinpoint which radioactive elements got trapped within the melted rocks to potentially determine the precise age of the impact. Combining these and other data will help researchers to figure out the true nature of the extra buried mass and decide whether it really is asteroid shrapnel or something else. 

So even as we look further afield to uncover the secrets of the cosmos, there are still plenty of mysteries to be solved in our own planetary neighborhood. 

[Hank] Even after all that exploration, there is still plenty we do not know about the dark side of the moon, so future generations can keep on gazing up in the night sky full of questions just like their ancestors. 

Now if you find yourself full of questions and in the position to support SciShow videos that bring you some of the answers, you can check out patreon.com/scishowspace and even submit a question that might be answered in future episodes. Thank you to everybody that supports us through patreon, and also everybody who supports us just by watching our videos. 

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