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You can start   planning a career that helps solve the  most pressing problems in the world at   80000hours.org/scishow. [♪ INTRO]  So, you may have heard lots of hullabaloo lately  about humans going back to the Moon. And there are   a bunch of reasons to head up there, again, from  scratching humanity’s itch to explore the unknown,   to more practical things like mining.

But from the  surface of our dynamic and ever-changing planet,   it might seem like those journeys will be a little  bit boring. It’s easy to dismiss the Moon as a   lifeless, frozen rock. Because we can’t currently  go inside the Moon to check on how geologically   active it is, we have to rely on clues from its  surface to tell us what’s going on.

And some of   those clues tell us that the Moon could very  much be alive. Before we talk about the Moon’s   reputation for being geologically dead, we need  to talk about how it was born. We think the Moon   formed about four and a half billion years ago,  back when the young Earth hadn’t quite grown to   its full size.

Our proto-Earth and a Mars-sized  planet called Theia had a bit of an altercation.   They smashed right into one another and most of  their guts ended up merging together. But this   galactic car crash also threw tons of material  into space, which eventually coalesced to form   our Moon. The baby Moon would have loomed large  in Earth’s sky.

Their mutual gravitational tugging   may have caused massive tides of lava to spill  over the surface. The Moon would have glowed red   hot, from the surface down into the core. And  it's been slowly cooling down ever since.

But   cooling isn’t the same as frozen. And if there are  still some liquidy bits churning around beneath   the surface, we might see signs on the surface  as recently solidified lava. When we look at   the Moon, we see a bunch of gray splotches called  maria, which is Latin for seas.

They’re darker in   color from the rest of the surface because they’re  made of a different kind of rock, called basalt,.   That’s a type of frozen lava, so planetary  scientists are pretty sure maria are evidence   that the Moon was once geologically active. But is  it still? And if not, how long ago did that end?   Well, it turns out, dating the Moon can be tricky.  And scientists have two main strategies.

The first   is simply looking at craters. If a smaller crater  is on top of another, we know that that crater has   to be younger. Otherwise, it would have been  destroyed when the larger meteorite hit.

We   can also look at the number of craters in an  area. Generally the greater the crater count,   the older a patch of rock is because space rocks  have had more time to smash themselves into it.   These are both forms of relative dating. Craters  can’t tell us the absolute age of a particular   part of the Moon.

But we can use them to tell  how old a feature is compared to another one.   Most of the Moon’s surface appears to be billions  of years old. But by counting craters, scientists   have identified lava that may be a mere 18 million  years old! If the Moon were my age, that’s, like,   less than two months ago.

But to confirm it’s  actually that young, we’d need to get our hands   on actual rock samples to get a precise age. And  that’s where the second dating method comes in. We   can cross reference crater counts with radiometric  dating to get a baseline for age estimates.   Radiometric dating looks at the composition  of the rocks themselves.

Some isotopes,   or versions of atoms, are radioactive and break  down over time. And they do that breaking down at   a very predictable rate. So to estimate how old  a given moon rock is, scientists will compare   two measurements: how much of the original isotope  is left, versus how much there is of the stuff it   decayed into.

Unfortunately, real lunar samples  are a little bit hard to come by. You can’t   exactly just pop up there whenever you like to  date whatever part of the Moon you want. But the   Apollo missions of the 1960s and 70s did bring  back hundreds of kilos of rocks.

And in 2020,   China’s Chang’E-5 lander performed its own sample  return. The Apollo rock collection is chock full   of samples that formed over four billion years  ago. Meanwhile, Chang’E-5’s basalts were dated   to be as young as two billion years old!

So just  going by these limited samples, it looks like   lunar volcanism went quiet a long time ago. Which  actually could be good news for future explorers.   After all, the Moon’s surface is dangerous enough  without having to worry about new pools of lava   erupting from the ground or a volcanic eruption  ruining your day. But some features call this into   question.

Take Ina for example. It’s a small  but bright crater in a spot called the Lake   of Happiness. Ina isn’t like other craters.

It  wasn’t caused by an impact. If it were, we would   expect to see its floor lower than the surrounding  surface. Instead, it looks more like a volcanic   feature, and its rock looks more shiny and less  worn down than the surrounding material, making it   appear super young.

Now, we’ve only photographed  Ina from space. No mission has ever landed in it,   so one team started counting its craters. And in  2006, they came back with the incredibly young   estimate of two million years, if not younger!

To  explain this age, they proposed what might seem   like a pretty quirky hypothesis: The Moon let out  a giant burp of subsurface lunar gas. When the gas   escaped, the rock collapsed with the sudden lack  of support. But it is worth noting that not every   astronomer agrees Ina is this young.

There’s  another gassy hypothesis out there that Ina’s   rocks were made out of a super foamy lava, kinda  like pumice. Their texture could be hiding a lot   of craters, making Ina look literally billions  of years younger than it really is. Either way,   two million years is still a long time for us  humans.

So Ina might not be enough evidence to   say the Moon isn’t dead now, but we’re not done  yet. Even if humans have never seen an eruption,   we do know that underneath its surface, the  Moon is rumbling. Lunar scientists have analyzed   nearly a decade of moonquake recordings, thanks to  seismometers left up there by Apollo astronauts.   But what could be causing them?

There shouldn’t  be much heat left over from the Moon’s formation,   but radioactive elements deep underground could  be releasing enough energy to power tectonic   activity. And while the Moon doesn’t have giant  plates of crust sliding around like Earth does,   there might be enough of a difference in heat  between rocks deeper underground and those near   the surface to allow rocks to shift positions.  As the rock slowly cools, it becomes denser and   sinks, while warm, less dense rock pushes its  way up. And the ongoing mutual gravitational   tugging between the Earth and Moon, and even the  occasional space rock impact, could add to the   total heat budget, as well.

Collectively,  maybe it’s warm enough for some liquid   magma to be moving around down there. Upcoming  missions may be able to learn more. While there   probably won’t be any explosive Mt.

St Helen’s  style eruptions for our explorers to witness,   the Moon still has many secrets beneath its  surface. We have only just begun to get a glimpse.  And if you’re searching for a glimpse at a new  job that really makes a difference in the world,   you can start with 80,000 hours. 80,000  Hours is a nonprofit created to help you   find a fulfilling job. They aim to help people  have a positive impact with their career and   really do some good in the world.

All of their  curated high impact career postings are free to   access through their job board. But whether  you’re ready to look at job postings or not,   you can always start with their blog posts  and podcast that explore different global   problems and careers that help solve them. Just  like the Moon, the concept of a rewarding job   search isn’t as dead as you think.

For a free  copy of the 80,000 hours in-depth career guide,   click the link in the description down below or  go to 80000hours.org/scishow. While you’re there,   you can sign up for their newsletter full of  updates on their research and job opportunities.   And just to be clear, everything they provide  is always free; their only aim is to help you   find a fulfilling, impactful career. Thanks  for watching this SciShow video and thanks to   80,000 hours for supporting it! [♪ OUTRO]