Our beautiful blue planet only has one
moon and it is a battered and sorryl
looking thing. It has obviously been
impacted many times, smashed into with
enough force to leave craters so big
that we can see them just by looking up
into the night sky. But this may not
have been a slow siege or a gradual
bombardment by the cosmos on the moon's
scarred surface.
Rather, it was a blitz creek. Evidence
from the moon's surface suggests that
these impacts may have all happened
within a very cosmologically short
period of time. A surge of falling
meteorites that swept across the moon in
an apocalyptic rain. Scientists have
named this event the late heavy
bombardment. And while it is a theory,
it would have been a truly devastating
period of history with some surprising
consequences.
It may be the reason we are here today
at all. But if it truly happened, what
caused it? And if the moon was so
scarred, how did the Earth avoid the
same fate?
I'm Alex Mccoan and you're watching
Astramm. Join me today as we journey
back in time, 4 billion years into the
past, to unravel a story of chaos around
our newly formed sun as its planet
settled down into the orbits that we are
now familiar with.
When the Apollo astronauts came home
from the moon, the rocks they brought
with them told a surprising story. The
astronauts had brought home samples
containing impact melt, rocks that had
superheated into magma due to being hit
by a meteorite, which had later
gradually cooled back into solid form.
But while the Apollo team had collected
these reconstituted rocks from different
areas and craters on the near side of
the moon, almost all of them were dated
to around 4.1 to 3.8 8 billion years ago
with very few showing older impacts. The
implication for some reason during this
period of roughly half a billion years
there was an abnormal spike in falling
meteorites that crashed into our moon.
This idea came to be known as the late
heavy bombardment.
This idea is controversial and not
everyone agrees that it happened that
way. Such is the nature of scientific
debate built on limited evidence. Uh we
don't get to go to the moon very often
to collect more samples. So it's hard to
tell if some process has been muddying
the waters. But if the late heavy
bombardment happened, what could have
caused it? And to answer that, we need
to consider conditions back at the dawn
of our solar system.
It is a well-known idea that our solar
system was formed from an accreting disc
of intergalactic dust and matter 4.6
billion years ago, circling around a
gravitational center that would later
house the sun. Gravitational
disturbances caused some of this dust to
bunch together. And once that happened,
each larger ball of matter found itself
snowballing in size. Larger clumps had
larger gravitational pull which pulled
in more mass to grow the clumps. At this
point in history, the vast spinning disc
of matter from which our whole solar
system was created had clumped together
into planet decimals. These were further
attracted to each other, crashing
together to form the planets we know
today. Within a mere 100 million years,
this process of planetary accretion had
gathered up almost all the material
nearer to the sun to form what we call
the terrestrial planets, Mercury, Venus,
Earth, and Mars. One of these impacts
formed our moon, flinging a huge lump of
molten rock into space, which formed
into a sphere and cooled and has
remained in orbit around our planet ever
since.
There would still be some isolated
leftovers between these planets. Some
are still out there today, like the 1.5
million floating asteroids orbiting the
sun between Mars and Jupiter. But this
process had pretty much cleared out
everything in the inner solar system.
But under this idea, you might expect to
have more impacts at the beginning, but
fewer and fewer impacts as time went on,
not a sudden spike.
So, why could there have been a surge in
impacts 4.1 billion years ago that gave
our moon such a beating? That has
created quite a puzzle for scientists,
although some now think they may have
solved it.
The finger of suspicion points towards
Jupiter, the largest planet in our solar
system with more than two and a half
times the mass of all the other planets
combined and the first to form along
with Saturn as our solar system took
shape. If you would like to learn more
about Jupiter and its influence on our
solar system, check out this video.
The suspected behavior of Jupiter is key
to explaining the observations that have
led to the proposal of the late heavy
bombardment theory. Jupiter is not
thought to have always orbited at a
distance of 768 million km from the sun.
Initially, Jupiter moved inward towards
the sun, reaching roughly where Mars
orbits now as Mars and the other planets
didn't exist there at that point, only
to then change direction and pull away
again. All of this possibly happened
within just hundreds of thousands to a
few million years.
What made Jupiter behave like this? The
short answer is its relationship with
Saturn. When Jupiter formed, there was
still a huge amount of gas swirling
around our developing sun. This gas
dragged Jupiter along towards the center
of the newly forming solar system. Once
Saturn had formed, it also joined this
death ride towards the sun. However, the
two new planets bonded as their
gravities combined, and the sudden tug
of Saturn behind it was enough to arrest
Jupiter's grand procession towards the
solar systems inner territory and to
gently pull it back to the orbit we see
it at today.
But this procession was not without
impact. Quite literally, our asteroid
belt with its 1 to two million asteroids
potentially only represents less than 1%
of the matter that used to be there. So
when Jupiter came sweeping through with
its massive farreaching gravitational
pull, a lot of orbits became disrupted
and many of those objects started to
fall inward.
Two types of asteroids were brought into
this process from both the inner and
outer solar system. This fact becomes
quite important later. Not all asteroids
are the same and you may be familiar
with the terms and cype asteroids.
Stype asteroids are the stony asteroids
from the inner solar system. Mainly made
up from silicut materials and nickel
iron. They are drier because they come
from closer to the sun. Sea type
asteroids. The condondrite or
carbonatous asteroids are the most
common and are among the most ancient
objects in our solar system. They come
from the outer asteroid belts and they
contain much larger amounts of something
that would later prove crucial to our
planet, water.
And all of these asteroids began to fall
through the patch of space our planet
was orbiting. So to answer my initial
question of how the Earth avoided all
these asteroids when the moon was
constantly apocalyptically bombarded, it
didn't. The timing of this, as far as
our planet is concerned, was crucial. By
then, the surface of the Earth had
cooled to form a more stable crust, what
we know as the lithosphere today.
This insulated anything above from the
enormous heat of the magma below. The
magma near the surface was much hotter
than it is today, around 1,600°
C, and it was much more liquid. Heavier
elements, especially iron, had sunk
towards the central core, leaving the
lighter elements and minerals at the
surface. As the asteroids rained down
onto the surface, they brought with them
a whole range of elements and minerals
that would remain as part of this
surface layer. It is now thought that
many of the important deposits we find
at the surface today, ores and metals
such as lead, nickel, copper, and gold
were brought here in this way.
More important than valuable metals,
however, was the arrival of water. Vast
amounts of water. Had all this water
arrived earlier when the surface of our
planet was hotter and less able to
maintain an atmosphere, much or all of
this water would have escaped back into
space, possibly leaving the planet an
arid desert like Mars and the moon.
Surrounding Earth during the bombarding
chaos 4 billion years ago was a toxic
atmosphere of methane and ammonia
covering an unstable but solidifying
surface of bassalt type rock. Dark gray,
almost black in color and with brightly
glowing lava erupting through faults
where it was shattered by the incoming
hail stom of asteroids.
Steaming oceans of water were forming,
helping to further cool the developing
surface. a true vision of hellfire and
brimstone.
It is partly because of this churning
nature of the surface floating on liquid
magma that today we see no trace of the
craters left by these impacts. The
forming lithosphere was fractured into
many small tectonic plates that moved
around and recycled themselves much
faster than they do today. And no trace
of these early rocks remains. The oldest
surviving rock formations on our planet
roughly marked the end of the hadian
period. And by this time, this intense
bombardment of our planet had likely
come to an end.
The end of the hadian era also marks the
beginning of a new develop in our
planet's fascinating history. The story
of life. It has long been debated how
the very beginnings of life may have
evolved on Earth. Key building blocks
would have been needed like hydrocarbons
and other complex molecules. What
conditions could have led to the origins
of this organic molecular soup?
Interestingly, while there is debate
over the reality of the late heavy
bombardment, this theory does help to
answer this vital question.
It is now known that all five of the
essential building blocks of DNA and RNA
exist in space and have been found on
meteorites.
But while the meteorites we see today
burning up in the night sky as they
enter our atmosphere could never bring
anything as delicate as these molecules
to the surface of our planet, they would
burn away to nothing before even
reaching the stratosphere.
But when much larger objects from space
were raining water and ice onto our
newly formed surface, it is certainly
possible that they seeded the Earth with
some of the right ingredients that made
life viable. There is compelling
evidence that life began at the very
time when this bombardment was ending
and new oceans had formed.
Finding the answers to many of these
questions is almost impossible on Earth
because so little remains from this
time. This is why scientists are so keen
to study clues that may still exist on
the moon. The reason why this evidence
is so well preserved on the moon is that
unlike on Earth, the surface of our moon
was already quite stable without the
churning and recycling of the surface
that occurred here on Earth at that
time. So the craters remain intact.
Also, the surface of the moon is not
being constantly eroded by weathering.
There are no seas to grind it down. No
wind and acid rain to wear away the
rock. The dust that covers the surface
is pulverized rock that remains
undisturbed following those dramatic
impacts from billions of years ago.
There are craters on our planet's
surface such as the Behringer Crater in
Arizona, USA, but that dates only from
50,000 years ago. Of the late heavy
bombardment era craters themselves,
there is nothing visible at the present
time. If such craters do survive
somewhere on our planet that have not
been completely eroded away, they could
be buried under later sediments or lava
flows and we may never find them.
We've been able to make progress in our
understanding of this era thanks to the
astonishing 382 kg of moon surface rock
brought back to Earth from the Apollo
missions. Radiometric dating of these
rock samples indicated that they were
last molten during a narrow period of
time around 3.9 billion years ago,
consistent with when we believe that
Earth and other inner planets were being
bombarded by asteroids.
Those large craters you see on the moon
are where asteroids have impacted the
surface and when lava has flowed out
forming those characteristic smooth
craters that we can see on clear nights.
Not everybody agrees, however, and as
more refined analytic techniques have
been developed over time, questions have
been raised about the time scale and
duration of this period of asteroid
activity. Some dating inconsistencies
suggest that the craters may have formed
over a much longer period of time,
tailing off from the planetary accretion
that formed the terrestrial planets
rather than a more concentrated period
of activity.
The sample materials that scientists
have available from the Apollo missions
are still limited as they stem from just
a few carefully selected locations. More
samples from future missions will
hopefully resolve some of those
controversies. And the NASA Arteimus
missions could see another landing on
the surface of the moon as early as
2026.
I can't wait to see what new discoveries
this could bring. The late heavy
bombardment theory may not be a perfect
explanation, but it tells a convincing
story that fits our observations of the
moon and of the parts of our planet
surface that we have explored so far.
And it provides some possible answers to
the question of how such a vast amount
of water came to exist on the surface of
what had once been a molten ball of
rock.
Whether or not this turns out to be the
correct story, next time you fill the
kettle or paddle in the sea, you might
remember where that water came from and
how lucky the timing of it all must have
been for it to actually stay here and
create a planet so conducive to life. I
hope you will join me for the next video
in this series when we will explore
another chapter of the incredible
history of our beautiful blue marble,
planet Earth.
[Music]
the link to this playlist for more
Astramm content. I'll see you next