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The idea that the Moon is a blown-off chunk of the Earth is known as the giant impact hypothesis - but the presence of carbon on the Moon throws this hypothesis into question.

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[♪ INTRO].

Scientists have long thought that Earth got its Moon when a Mars-sized object blasted into our developing planet. But for decades, different clues have called this hypothesis into question.

And last week, a paper published in the journal Science Advances is revealing a new twist: The Moon is emitting carbon. Carbon should have boiled off the Moon a long time ago if it formed from a single violent collision. But it's there—and it looks like it's been there for billions of years… which hints that our lunar origin story is not as straightforward as we thought.

The idea that the Moon is a blown-off chunk of the Earth is called the giant impact hypothesis, and early evidence for it came from the Apollo missions. Apollo astronauts brought back rocks from the Moon that looked so much like Earth rocks that scientists reasoned they had to have come from here. The Moon rocks also appeared to be missing volatiles, which are elements and compounds with low boiling points, like carbon and hydrogen.

And that seemed to check out, because the intense heat from a giant impact should have vaporized any volatiles on the surface of the newborn Moon. But in the study published last week, a team of physicists and planetary scientists from Japan used data from the Kaguya satellite to take a better look at the lunar surface. They looked through a year and a half of data from the late 2000s, when the satellite was operational—and that's where they saw evidence of charged carbon atoms escaping from the surface.

These charged atoms are called ions, and they form on the Moon when micrometeorites, sunlight, or charged solar particles hit the surface. Any of those can knock electrons off atoms and kick the resulting ions out into space. Previous satellites weren't sensitive enough to detect those ions, but Kaguya was—and it detected more than anyone expected.

In fact, the researchers were able to map carbon concentrations across the entire Moon, and they found that the amount of carbon in the ground varied by location. Older surfaces emitted less carbon than younger surfaces, implying that the Moon started out with carbon and is gradually losing it to space, leaving older regions more depleted than younger ones. The idea that carbon has just… been there all along raises a big challenge for the giant impact hypothesis.

Before we have a clear picture of how the Moon formed, scientists will have to explain how volatiles like carbon survived the event that created the Moon. Some sort of collision is still the leading hypothesis for the Moon's formation, but exactly what the circumstances were like is still up for debate. While some scientists are working to better understand our own solar system, others are thinking about how to explore worlds way beyond it.

Last week, a paper in the journal Acta Astronautica laid out what it would take to create a laser-powered light sail, a type of probe that could one day be our best bet at visiting far-off worlds on human timescales. The idea behind a light sail is that light carries momentum, so you can physically push something through space just by shining a light on it. After enough time, it could get going at a good fraction of the speed of light, meaning it could potentially reach neighboring star systems in a matter of decades—as opposed to the tens of thousands of years it would take a conventional probe.

We've launched a few successful light sails already as proofs of concept, but those use sunlight, which just isn't powerful enough to get a sail going at any significant speed. But the authors of this recent paper investigated what it would take to make a light sail that could reach another star. First of all, they calculated that it would take nearly the entire output of the Hoover Dam to power a laser that could push even a small light sail anywhere close to the speed of light.

That's… no laser pointer. Like, don't get in the way of this thing. One reason that number is so high is because researchers had to account for one important detail: special relativity.

Special relativity is Einstein's theory of how space and time get distorted at high speeds, and because of this distortion, it will be harder to accelerate the sail the faster it goes. As the sail goes faster, the laser will also have to shine at a higher frequency, because the light will appear stretched out from the ship's perspective—delivering less force than it would if the sail were moving much slower. The authors also sorted out some details about what a future light sail would look like.

It would need to be huge, to catch as much light as possible, but also light enough to be moved by a laser—so we're talking just nanometers thick. Except, it also has to be strong…. Oh, and it has to be reflective so that light bounces off, transferring as much momentum to the sail as possible.

So there are some hurdles to get past, which is why we're not going to be making interstellar voyages anytime soon. But just identifying those hurdles is a really important and also fascinating step. And last month, a separate team of engineers published another paper in Acta Astronautica revealing a model for a super-light sail made of graphene.

Now, it was only three millimeters wide, which is likely millions of times smaller than an interstellar probe would have to be, and scaling it up will come with its own problems. But! It demonstrated one way of making an efficient sail that could pick up momentum from light.

The researchers even tested it in a freefall chamber to see how it would fare in microgravity—and the sail sailed! It'll take some time and serious research before these things get past the proof-of-concept stage. But we're shooting for the stars here, literally, and these are some of the first few steps toward getting us there.

Thanks for watching this episode of SciShow Space News! And if you want to learn more about how we might one day use sunlight to propel spaceships, we have an episode just for that! [♪ OUTRO].