Hi, this is Kate from MinuteEarth, and that’s no moon, it’s a meteorite.
Although I’m starting to get what it must be like to live on a moonbase – like many
of you I’m stuck at home right now.
So we’re going to keep these compilations of our classic MinuteEarth videos coming for
you, your kids, my kids, and anyone else that is going a little loony!
So watch this space, because we’ve got four short videos about what lies beyond our Earth,
starting with where that meteorite probably came from.
Over the ages, the million or so rocks that make up the main asteroid belt between Mars
and Jupiter have sent chunks of space debris large and small crashing into Earth.
And after analyzing a lot of these meteorites, we've discovered something really weird: more
than a third of them have the same chemical signature, suggesting that they've broken
off from a single asteroid.
By using special telescopes to look at the mineral makeup of asteroids, we think we’ve
identified the culprit, known as Hebe.
Hebe is pretty big compared to other asteroids in the asteroid belt, but it still only makes
up a tiny fraction of the total mass.
And it’s been in lots of meteoroid-making collisions - including one violent enough
that it might have broken off a huge chunk of rock called Jebe - but lots of other asteroids
have also been in lots of wrecks.
Instead, Hebe owes its unique status to its location at the edge of an empty band in the
asteroid belt.
As asteroids on either side of this band orbit the sun, they pass Jupiter and get a little
extra gravitational pull, but that pull comes at a different place with each orbit, so it
kind of just averages out over time.
But any rock that finds itself inside the empty band orbits the sun exactly three times
faster than Jupiter does, which brings it closest to Jupiter at precisely the same two
places in its orbit, over and over and over and over and over and over again....this so-called
"orbital resonance" distorts the shape of the asteroid's orbit, and eventually it destabilizes
into a potentially Earth-crossing path.
Hebe feeds more space rocks into Jupiter’s reach than any other asteroid, thereby sending
more rocks rocketing toward us than anything else.
Fortunately, most of them miss us, like the one with 100,000 times more destructive power
than the Hiroshima bomb, which flew by us in 2012, just 20 moon distances away.
But we weren’t so lucky in 1976, when a boulder the size of a Toyota Camry crashed
into a field in northern China, or in 1868, when ten tons of pea-sized meteorites peppered
northeastern Poland.
Scientists are researching ways that we could divert a really big one if it were on a collision
course with Earth, but our anti-armageddon plan is still decades away from realization,
so there’s still time for a mega-meteorite to turn us into dinosaurs.
It’s enough to give you the Heebie-Jeebies.
Yeesh, that ending was dark.
And speaking of dark, do you ever wonder why we never see the “dark side” of the Moon,
other than... well?
Here’s why.
In 1959, the Soviet spaceship Luna 3 beamed back images of something Earthlings had never
seen before: the far side of the moon.
We always see the same old side of the moon because the moon rotates exactly once on its
axis each time it orbits Earth.
If it wasn’t spinning at all, or was spinning twice as fast, we’d get a full 360° view
with each lap.
But instead, our moon’s motions – like the spin and orbit of most other moons in
our solar system – are, remarkably, in perfect sync.
This wasn’t always the case: our best guess is that our own moon formed due to a massive
asteroid impact, and its initial spin and dizzying 10-hour orbit were almost certainly
not in sync with each other – though we don’t know which was faster.
At such close range, Earth’s gravity deformed the moon into a slight oval, with one of its
bulges facing Earth.
Those bulges quickly swung out of alignment, thanks to the moon's asynchronous spin and
orbit, but Earth’s gravity continually squeezed them back again.
What’s more, this gravitational tugging would have influenced the moon’s rotation
rate: if it was spinning more than once per orbit, earth would pull at a slight angle
against the moon’s direction of rotation, slowing our satellite’s spin; if the moon
was spinning less than once per orbit, Earth would have pulled the other way, speeding
its rotation.
Whatever the case, it took just 1000 years for the Earth’s pull to adjust the moon’s
spin enough that one rotation of the moon corresponded to one trip around the earth,
leaving one side forever locked facing Earth.
We do end up seeing slightly more than that one side, because the moon’s elliptical
orbit gives us peeks beyond its average eastern and western horizons, and its tilted axis
causes “moon-seasons” revealing more of the lunar poles.
But those glimpses only add up to an extra 9%, leaving 41% of the moon hidden from earth.
Satellites have allowed us to map the rest, but it’s safe to say that our relationship
with the moon is still pretty one-sided.
Fine, Moon, keep hiding your far side from us Earthbound stargazers, but we’ve still
got something you don't have - liquid water, and life, and cookies, AND, an atmosphere.
In a nutshell, our next video is about our relationship with our atmosphere, animated
by our friends at Kurzgesagt.
The earth and the moon are basically the same distance from the sun, yet temperatures on
the moon average an unlivable -18°C, and even deadlier, they range from -170°C during
lunar night to 100°C at lunar noon, regularly exceeding both the coldest and hottest temperatures
ever recorded on Earth.
And while the days and nights on the moon are about 14 times longer than those on Earth,
our planet’s relatively fast rotation isn’t what spares us from those loony temperatures.
What protects us is our atmosphere.
By day, it serves as a shield, blocking out the most harmful and energetic of the sun’s
rays and about one-third of the less-intense visible light.
At the same time, it traps the infrared radiation – aka heat – radiating out from Earth’s
sun-warmed surface, keeping us from freezing solid at night.
In order for our atmosphere to absorb any kind of radiation, it needs to have some electrically
charged particles for passing electromagnetic waves to push around.
And most of our atmosphere is made up of gas molecules that don’t have an electric charge
– they all have a balanced number of positive and negative particles.
But some hold most of their negatively-charged electrons closer to one side, lending them
a lopsidedness that can jiggle back and forth to absorb the energy of incoming infrared
rays.
For example, water, ozone, and nitrous oxide are all electrically lopsided, so they all
absorb infrared radiation.
Then there are gases like carbon dioxide and methane.
On paper, neither molecule looks lopsided, so it doesn’t seem like they should be able
to absorb any radiating heat.
But in reality, gas molecules aren't motionless – they crash into each other billions of
times per second, knocking each other in different directions, and also into different modes
of rotation and vibration.
And it turns out that both carbon dioxide and methane spend most of their time “shaking
it” in electrically-lopsided ways, allowing them to absorb infrared rays and help insulate
the earth.
Even though many different kinds of molecules can absorb infrared radiation, the vast majority
of our atmosphere can’t, because it’s made of nitrogen and oxygen, which don't get
lopsided even when they are vibrating - they’re too symmetric.
Nevertheless, the lopsided 1% are such good infrared absorbers that they manage to intercept
about 90% of Earth's outgoing heat.
Each captured ray gets pinged around the atmosphere, and most end up returning to the surface at
least once before escaping to space.
We don’t need to visit the moon during frigid lunar night to know just how important the
game of radiation-pinball is for Earth – ice records from our own coldest climate show
that small, natural variations in atmospheric carbon dioxide produce relatively big changes
in temperature.
They also show that, compared to the last 800,000 years, the game today is much, much
harder.
Gotta admit, I wouldn’t mind having a game of pinball to keep me - and these kids - occupied.
But instead, here’s a story of what things might have been like back when the Earth,
and the Sun, were just kids ...
In 1972, Carl Sagan and a colleague discovered something that’s come to be known as the
faint young sun paradox: according to stellar physics, our sun has been growing brighter
over time, thanks to increasing hydrogen fusion in the star’s core.
This means that the sun that shined on early Earth was roughly 25% dimmer than today’s
sun, which should have kept our baby planet cool enough for ice at the poles to grow and
reflect more sunlight and cool the planet further - producing a literal snowball effect
and turning Earth into a big ice cube.
BUT: according to rock and fossil evidence, ancient Earth was actually a melty, warm,
watery haven for life, where simple single-celled organisms developed and thrived.
Hence the paradox – how could the sun be dim but the earth warm?
Scientists have proposed a range of possible explanations, but the most likely one is that
Earth’s early atmosphere included one or more ultra-insulating gases that kept its
surface unseasonably toasty.
We still don’t know for sure what those gases were or where they came from, but scientists
have been toying with an intriguing possibility: that whatever created Earth’s mega-greenhouse
effect also supplied key ingredients for life.
One hypothesis is that a constant barrage of rocky debris left over from the creation
of the solar system melted sizable chunks of earth, releasing greenhouse gases like
carbon dioxide and methane, and drawing sulfur – an essential component of some amino acids
– up to the surface.
Another out-of-this-world hypothesis points to the sun itself.
Magnetic storms on the sun’s surface unleash streams of high-energy particles into space.
Today, these so-called solar winds can disrupt Earth’s magnetic shield enough to penetrate
the atmosphere and interact with gases, giving rise to the Auroras.
But back when our sun was a baby, it threw much wilder tantrums, hurling violent streams
of high-energy particles that interacted with earth’s primordial atmosphere much more
frequently to create large amounts of two gases: nitrous oxide, a greenhouse gas 300
times as powerful as carbon dioxide, and hydrogen cyanide, a poison that can, ironically, also
help produce some basic building blocks of life.
Whatever the real story, it’s safe to say that our early Earth somehow managed to create
a perfect home for life under the faint young sun.
It’s also safe to say that, as our sun continues to burn ever brighter into the future, Earth
will snowball in another, hotter, direction, and eventually water and life will boil away
under the bright old sun.
I'm off to catch some rays from our middle-age sun.
But first, here’s one more thing to keep you sane while stuck at home – The Great
Courses Plus.
It’s like Netflix for learning, with a library of more than 11,000 lectures, like “Life
in Our Universe,” which is all about how life came to be on our Earth, and how we may
discover it on another planet.
Once you’re done gorging on science content, you can check out courses on cooking, playing
guitar, and training dogs...which I definitely need.
To get a free trial subscription to The Great Courses Plus and show your support for MinuteEarth,
go to TheGreatCoursesPlus.com/minuteearth, or click the link in the description below.
Thanks Great Courses Plus!
And we’ll see you next time.