Thanks to 80,000 Hours for supporting this SciShow video! 80,000 Hours is a nonprofit
that aims to help people have a positive impact with their career. 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]