We may take the moon for granted, but
what a sight it is to behold in our sky.
It's big enough and close enough to us
that we can easily make out surface
details with the naked eye, like the
dark mare and bright craters.
Just looking at it through a telescope
is impressive enough. However, what I'm
about to show you in this video will
make the telescope view pale in
comparison.
You see, we are fortunate enough to have
not only visited the moon, but also have
an orbiter around it right now with a
powerful camera that has been scanning
the surface since 2009.
So, what has it seen?
I'm Alex Mccoan and you're watching
Astramm. Stick with me in this video and
I will show you some of the LRO's most
recent impressive and puzzling images of
the moon.
Let's start off with an innocuous little
unnamed crater. As you can see, there
are plenty of tiny craters within it.
And this crater is within another
crater. Again, maybe you can see where
I'm going with this. Zooming out, not
only are these craters in another
crater, but apparently they are
contained within two very nicely aligned
craters. Or is that really what this is?
Well, we aren't sure. Both of these
craters are named as one, the Bell E
crater. This peculiar type of crater is
known as a donut or concentric crater.
It is possible that they are the result
of two impactors aligning up nicely, but
further investigation suggests
otherwise. If they were the result of
chance collisions, then there should be
a random distribution of concentric
craters around the surface of the moon.
However, that is not the case. Have a
look at this. The population of
concentric craters actually clump up
around certain areas, especially around
the edge of this region of the moon here
called Oceanus Proellarum.
Another factor to consider is that most
of these craters are of similar ages.
Looking for clues in the crater itself
also reveals something interesting. This
outer crater should be around twice as
deep as it currently is when comparing
it to other similarsized craters around
the moon.
Now, while a few concentric craters on
the moon will certainly be the result of
double impacts, the location, age, and
depth of most craters means that
something else must be at play. One
theory is that some of these impacts
occurred during a time when the surface
of the moon in this region was in a
state in between solid and liquid with a
consistency similar to cool lava or
honey. As the impact happened, it caused
ripples which propagated outwards, but
then stopped and never smoothed off
until it was fully cooled and frozen in
place. Although this is seen as an
outside possibility,
the most likely theory is that when the
moon was more geologically active,
craters in the region were pushed up
from beneath by magma trying to escape
onto the surface. This would explain the
shallowess of the crater and why we see
concentric craters mainly around
specific points on the moon. However,
while this is the best theory we have at
the moment, we don't know for sure. What
do you think it could be?
Now, apart from the occasional meteor,
you probably think the surface of the
moon barely changes at all. And while
you are mostly right with that, we have
found evidence that material does move
on the moon occasionally.
See if you can spot what I'm talking
about in this image here as I pan
across.
This is the edge of a large 32 km wide
crater known as Kepler Crater. And what
you may notice along the crater wall is
evidence that landslides have occurred
here with the dark material apparently
having fallen down the slope.
Let's have a closer look at what's going
on by zooming in on the most prominent
of the landslides in this crater.
The material seems to originate from box
canyons towards the top of the crater
rim. The material coming down here is
clearly very fine, certainly less than a
meter across. as no individual rocks can
be resolved within the slide. However,
the largest rocks that got dislodged
seem to have all made it to the bottom
of the crater floor.
What's interesting is that the main mass
of the slide seems to actually be made
up of many smaller slide masses. Look at
these individual trails here.
So, it probably didn't all happen at
once, but is happening over time.
The slides were likely triggered by tiny
meteors striking the crater wall. These
tiny impacts and the subsequent
landslides round off the edges of the
crater, which is why the oldest types of
crater on the moon look so smooth
compared to the freshest craters.
Here's another puzzle to try and solve.
Here we have the remarkable messier
crater. Typically craters are round but
not messier crater. It is elongated with
a slit for a crater floor. What is going
on here?
The mystery continues if you zoom out a
bit.
Directly next to Messier crater are two
more craters. The one on the left seems
much older than the other as it seems to
have been weathered away compared to the
fresh impact crater on the right.
Did the newer crater just so happen to
cover an older one? But let's zoom out
again. What other clues can we see?
Actually, a big clue are these lines
coming away from the crater.
These are called rays and they reveal
the direction the debris fell after the
impact. On rounded craters, debris can
go in all directions like the ones that
originate from Tao Crater.
However, here debris goes in three
distinct directions. North and southward
from this crater and only westward from
this one. So, what would cause that?
Well, the answer is an impactor striking
the surface at a very low angle, less
than 15°.
And in this particular case, it seems
like the impactor had already broken
apart into three parts before it even
hit the moon's surface. Yes, all three
of these craters likely hit the moon at
just about the same time, even the older
crater.
What actually happened here is that
ejector from this second crater likely
fell directly on top of the other crater
due to the low angle of the impact which
means that it has been artificially
aged.
There are some other really interesting
aspects of this image though like the
solidified pond of impact melt found at
the bottom of the crater
or this region here which appears to
have caved in a bit.
The impact melt in the first image also
appears to have flowed down towards the
left of the image. It really is a
fascinating set of craters.
Let's have a look at another
asymmetrical crater and try to figure
out why it has the shape it does. While
it could be that this crater is also the
result of two impacts or one impactor
breaking up into two just before it
collided with the moon, scientists think
that this is likely not the case here.
Notice the shadows in this image above
and below the crater. It is apparent
that this crater is right on the cusp of
a peak. Zooming out and looking at the
topographical map of the region reveals
that this is the case. In fact, this may
well have been the tallest peak in the
local area until by chance this impactor
came along and totally wiped it out.
Imagine Everest suddenly being taken out
by a meteor. The shape of this crater
was probably not only caused by the
angle the impactor approached from, but
also because it hit this steep slope. It
might not look that steep from the
oblique angled shot. However, over only
about 20 km, there's an 8 km difference
in elevation from the peak here to the
bottom of this nearby crater.
In this next image, there's not too much
to see. The only thing visible in this
wide expanse is this peak basking in the
light of the sun. Why is this
significant? Well, this peak is on the
rim of Apenus Crater, a crater found
near the north pole of the moon. Future
colonies on the moon will be located
somewhat near the north and south poles
because tucked away at the bottom of the
craters here, where the sun never
shines, are large pockets of water ice,
essential for any colony to subsist off
of. Water can be used for drinking,
washing, cooking, and farming. Plus,
breaking down the H2O into oxygen and
hydrogen provides breathable air and
rocket fuel. These poles also have the
added benefit that there are peaks here
that are almost always in the sun,
unlike other parts of the moon where the
day and night cycle is 28 days long. 14
Earth days in constant darkness is not
good for a solar powered power system. A
peak like this one, however, poking out
in the sun while the surrounding area
experiences nighttime would be an ideal
location for solar panels and powering a
colony there. It's not a perfect
solution, as peaks like this one will
eventually also become covered in
darkness depending on the time of year,
but 89% of the time is definitely better
than other regions on the moon where
you'd get sunlight for roughly 50% of
the time.
I'll just leave you with a couple more
islands in the darkness. This time from
the far side of the moon found in Barbar
Crater. These are the central peaks
found in the middle of this 80 km wide
complex crater. Do you believe the moon
to be barren, gray, and uninteresting?
Well, to me, that couldn't be any
further from the truth.
The moon is packed full of secrets about
its past and has clues dotted all over
its surface, which gives us information
about how it formed and evolved over
time.
From global events that created some of
the largest craters in the solar system
to tiny hills exposing layering in the
moon's crust, each helps us get a better
picture of our closest celestial
neighbor and as a result expands our
understanding about the formation of the
solar system as a whole.
It's no wonder then that NASA has a
spacecraft called the Lunar
Reconnaissance Orbiter which is in orbit
around the moon right now mapping out
its entire surface.
I'm Alex Mccoan and you're watching
Astramm. And in this episode of the LRO
series, we will investigate some of the
most unusual terrain found on the moon's
surface. And hopefully I'll be able to
make the gray, barren, and uninteresting
world into something fascinating and
wonderful for you.
Let's start with a place rich in
incredible contrast. This oblique angle
shot is of Jackson Crater. Sadly, not
visible to us on Earth as it is on the
far side of the moon.
A bit like Tao Crater on the near side
of the moon, when it formed, it created
a ray system stretching over 1,000 km.
Ray systems form when particularly fine
material is ejected far beyond the
crater rim, although their formation is
still being studied. Jackson crater
itself is about 70 km in diameter and
due to its size it is a complex crater
as can be seen by its terrace walls and
uplift in the central region.
This crater is actually tilted. The east
side of the crater is 6,000 m in
elevation and the west side is only
3,000 m high. The base of the crater has
an elevation of 1,000 m, and the peak
comprises of material that was pushed up
from another 1,000 m down.
Some of the dark patches you see along
the walls are shadows due to the sun's
angle in the sky, but there are also
sections of darker materials compared to
the predominantly lighter colored
ground. Although it's not as light as
this image would have you think, your
viewing angle and the angle of the sun
play a big role in how contrasts appear
on the lunar surface.
Focus here on the central peak in this
image.
We'll now switch to a top-down
perspective of this same peak taken at a
different time of the lunar day.
Suddenly, the crater basin and the tip
of the mountain appear much darker than
before.
But a sideby-side comparison does show
how the differences in contrast can be
seen in both pictures.
And that's not the only optical illusion
the moon can trick you with. Have a
close look at this image. What does it
appear like to you? Are these regions of
inverted bubbles or are these sections
actually rising higher than the wiggly
textured material surrounding them?
Well, for the longest time, I could only
see it's inverted. But maybe if you look
around the image, suddenly it will
switch perspectives for you. What type
of image did you see first? Are you like
me and need proof it's not actually
inverted? Well, have a look at the same
region, but from a different angle.
Seeing it like this makes me wonder how
I could have seen anything else. This is
a small region on the moon called inner.
It's only 2 to 3 km wide and 64 m deep.
And no one really knows how something
like this formed.
It's one of several similar regions on
the moon.
Although this one is the most prominent.
To a certain degree, a similar optical
illusion can happen with small craters.
Do you see domes here or craters?
Sometimes rotating the picture can help
get the right perspective.
That's why the LRO is so useful in my
opinion. Not only do we get top- down
views, but oblique perspectives, too.
Moving on to another unusual lunar
region. Let's have a look at Komarov
crater. This crater would be pretty
normal by lunar standards were it not
for the fact that it has huge fracture
lines running across the base. Komarov
crater itself is even bigger than
Jackson crater at 95 km in diameter.
Meaning these are up to 500 m deep and
2.5 km wide. It is believed that 2.6
billion years ago magma built up under
the crater causing large amounts of
pressure to fracture the crust. Although
it appears that the magma never made it
to the surface, meaning the fractures
were never filled in, and it remained
like that ever since.
But although it didn't happen in this
instance, there are examples on the moon
of magma breaking through and pooling on
the surface.
One such example can be found west of
Plato Crater, a large 100 km wide crater
seen towards the north of the moon,
visible with the telescope or binoculars
on Earth.
This image has a few rather spectacular
points of interest to see. The obvious
one being this channel which cuts
through the ground. This section here is
a lava vent back when the moon was a lot
more geologically active.
Running out of the vent in a
southwesterly direction is something
known as a rimma or a relay. These are
channels cut out by lava melting and
eroding its way down the slope. kind of
like a river on Earth.
To the east in this image, we see the
crater rim of Plato. Plato itself was
likely filled with lava at some point as
the base is darker and smoother than the
regions northwards. However, in this
image, we can see that a huge section of
crust has collapsed down from the crater
wall, creating a 24 km wide slump block.
In other words, this section was once
connected to the higher plane. However,
it has since collapsed under its own
weight, breaking away and falling
somewhat into the crater.
Let's have a look at one more crater.
This breathtaking view is from the
Apollo 15 mission overlooking Aristacus
crater. Aristicus is seen towards the
northwest of the moon. And although
Aristocus is only 40 km across, it's
bright enough to be seen with the naked
eye.
From Apollo's viewpoint, we can see a
really wide angled perspective of the
crater. Surrounding it are more relays
and lava events, and a small ray system
can be seen extending away from the
center. From this angle, with the shadow
extruding out from the rim, you get a
sense of how deep this crater is.
This complex crater has prominent crater
walls. However, the uplift found in the
center seems pretty small. From LRO's
perspective, we have a much higher
resolution view of Ariscus again, and we
can have a close examination of the
walls and crater base.
The walls are similar in appearance to
Jackson crater.
However, looking at the peak towards the
crater center reveals some major
differences. Not only is the peak much
smaller, it also has a banded pattern
exposing layers in the crust that would
have otherwise been hidden hundreds of
meters down. The base of the crater was
also likely to have formed from molten
lava. Rock melted by the impactor.
Fracture lines from rapid cooling are
evident all over. And looking at where
the walls meet the crater base, you can
easily imagine how this base was once a
liquid.
Let's have a look at one last image.
This area is known as Mons Carpetus. And
what's immediately apparent here are the
variations in contrast. Again, generally
speaking, looking at darker regions on
the moon indicates older material, but
it also indicates what the material is
comprised of. The darkest regions in
this image are thought to have formed
from explosive volcanic activity over 3
billion years ago.
Lava would have also flown down through
valleys like these ones.
Also sprinkling the surface are white
dots. These are small impact craters and
appear white as they are a lot more
fresh than the surrounding regions and
space weathering hasn't had an
opportunity to darken them yet.
The moon,
Earth's natural satellite, orbiting
approximately 384,000
km away. A celestial object that
scientists have studied for thousands of
years, using its regular motions to mark
the passage of time in calendars. Its
dependable rhythms helped ancient
civilizations to track when to plant
crops. And its waxing and waning faces
cemented the moon's place deep in the
heart of symbol and tradition. It's a
wonder, a necessity, and a curse. While
the moon's desolate beauty has captured
the vision of poets, it also brings
desolation.
There is one lunar rhythm that is not
helpful to us. A 19-year cycle that
brings unexpected floods and ruin. And
NASA scientists are worried that in the
middle of the 2030s is about to hit its
hardest yet. For ecosystems that are
adapted to it in the right way, this
won't be a problem. But how adapted are
we?
I'm Alex Mccoan and you're watching
Astramm. Join with me today as we learn
about the innocently named lunar nodal
cycle and why we need to start
developing a much better understanding
of the fluctuating behavior of our moon
if we are to protect ourselves against
its dangers.
Much like the sun, the moon is an
inescapable part of life on Earth. The
moon has an immense impact on our
planet. You likely have already heard
how its cycles influence our wildlife,
affect our climate, and create tides. We
tend to imagine that the moon and the
earth's gravities cause them to circle
each other in a relatively stable,
synchronized harmony. But, as is so
often the case, nature is not as simple
as we imagine it. Instead, every 18.6 In
6 years, the moon's orbit undergoes a
subtle revolution. A shift in its
alignment between us and our sun that
causes high tides to grow even higher,
tipping us over the edge into dangerous
flood territory. But let's delve into
what this subtle revolution is. Its name
is the lunar nodal cycle or the
procession of lunar nodes. This complex
name refers to a specific feature of the
moon's orbit of the Earth. You likely
know that every 29.5 days the moon
orbits the Earth. However, this orbit is
not flat. Or to be more specific, there
is a 5° difference between the angle of
the moon's orbit and the ecliptic plane.
The 2D plane on which the Earth orbits
around the Sun. For half of the month,
the moon is slightly higher than the
plane of the ecliptic. For the other
half, it drops below it. Naturally, this
means that there are two crossover
points or two nodes, an ascending node
and a descending node that mark the
point where the moon goes from one side
over to the other. And it is these nodes
that move over the course of the 18.6
cycle, slowly rotating around the planet
in one complete revolution. The nodes
themselves are what causes the problem.
To understand why, let's recap what we
know about tides.
You may already be familiar with how the
moon's gravity pulls the Earth's water
towards it, causing a bulge in sea
levels on the side closest to it that we
call high tide. You likely also know
that this happens on the side of the
planet furthest away from the moon.
Rather than being caused by gravity,
this second bulge is caused by
centrifugal forces. As the Earth and the
Moon's gravitational pull on each other
causes them to behave like two dancers
holding each other by the arms and
spinning across the dance floor. While
it's mostly the moon moving due to the
Earth being much more massive, the Earth
is also swung around a little. The water
behind it is thus trying to fling off
into space through its rockous spinning
causing the second high tide.
The sun also has a role to play in tide
formation, albeit to a lesser degree.
It's a bigger mass which would cause a
greater pull if it were closer, but its
further distance means that the sun's
effect is only 1/3 as big as the moon.
When the moon and the sun are aligned,
we get extra large tides called spring
tides. This happens six to eight times a
year.
When not aligned, they partially cancel
each other out causing smaller tidal
extremes known as neep tides.
So now consider the influence of lunar
nodes on this tidal tugofwar. During
spring tides, the pole of the sun and
the moon working in unison causes the
highest tides and the largest risk of
floods. However, the sun and the moon
are never more aligned than they are at
a node. During the rest of each 9.3year
phase, they are not quite tugging in the
same direction. So, tides are more
temperate. At a node, that's where
things get more serious and risk of
floods become highest.
The last time this alignment occurred in
September 2015, the UK and the US both
issued major flood warnings to its
citizens. In September itself, there
were floods, albeit minor ones. But it
was only when heavy rain combined with
the strength of the lunar nodes a couple
of months later that the real damage was
inflicted. In the US, in October, South
Carolina saw flash flooding that caused
property damage and people having to be
rescued by emergency services. At the
end of December 2015, the UK was hit by
some of the worst floods it had seen in
a century. Combined with the power of
Storm Desmond, flooding and storm damage
caused an estimated 1.3 billion pounds
in damages.
These floods can be highly damaging, but
that in and of itself doesn't completely
explain NASA's worry for the upcoming
alignment in mid 2030. There is an extra
element at play beyond the regular
rhythm of this rising flood risk we have
been seeing through the course of human
history.
Unfortunately, the next node's alignment
with the sun promises to be particularly
devastating. The danger is that this
phenomenon is combined with an already
strained system, even more strained than
it was in 2015. Climate change has
resulted in steadily rising sea levels.
When the next node aligns with the sun
in the mid 2030s, this will likely lead
to a dramatic increase of floods on
planet Earth.
Worryingly, a new study led by NASA's
sea level change science team predicts
that almost all US mainland coastlines,
Hawaii and Guam, will have a huge leap
in flood numbers when this happens. Some
predictions claim this node alignment
could cause four times the amount of
flooding from one decade to the next,
which will damage infrastructure and
change our coastlines around the world.
This means human life will inevitably be
affected by these floods impacting
shelter, clean water supplies,
electricity as well as the increased
risk of waterbornne disease outbreaks
like hepatitis A and cholera. Plus, the
receding flood water can create stagnant
pools of water where mosquitoes gather
which can spread other diseases like
malaria. This has a knock-on effect on
economic issues as these natural events
can make coastal life unaffordable with
increased cost of insurance on these
homes or an inability to find insurance
at all which could cause a reduction in
asset value in the community.
Consequently, this lunar nodal cycle
will damage the quality of life in
coastal communities where infrastructure
may not be rebuilt or adapted to this
force of nature.
It's not just bad for humans.
Ilia Rashlin, a visiting professor at
Rutkas University, analyzed that the
peak of the lunar wobble, where high
tides are higher, can drown salt
marshes. Salt marshes are a habitat for
a range of species such as
invertebrates. And these floods can
cause these creatures to drown, which
means that other species like fish,
seabirds, and others who rely on
invertebrates to survive also suffer.
And they aren't the only ones that rely
on salt marshes, as salt marshes hold a
multitude of marine life, which includes
75% of all fishery species. This means
that the lunar wobble impacts the food
chains of humans and animals, causing
disturbances to their natural habitat
and impacting their populations.
While this all does seem fairly doom and
gloom, it's interesting to note that not
all ecosystems on the planet are
negatively affected by flooding and high
tides. Ecologist Neil Saintan of Macari
University analyzed that the lunar nodal
cycle impacts heavily on the expansion
and contraction of mangrove canopy cover
over most of the Australian continent.
The analysis showed that the peaks of
the lunar nodal cycle coincided with the
cover of the mangrove canopy. It showed
that when the lunar wobble is at its
minimum phase, it causes the mangrove
ecosystems to become very dry, which
leads to thinner canopy cover. Yet, when
lunar wobble is at its maximum phase,
mangrove cover increases.
Mangrove canopies are beneficial to
Earth's environment as they are complex
ecosystems that fight against climate
change, protect wildlife, and shield
coastlines. They can also absorb four
times as much carbon dioxide than
rainforests of the same size. Their
growth is vital to the welfare of our
planet, so it's not all downside.
Still, it's clear that if we don't plan
ahead, coastal cities and environments
will face a serious crisis. The all
important question then is what can we
do about it?
One method is better protection. As
mentioned previously, the protection and
restoration of mangroves can act as a
shield against flooding as they can
mitigate the vulnerability of
communities on the coastlines. More
specifically, mangroves can avert damage
by decreasing the height and energy of
waves as they pass through mangrove
forests. The above ground roots and
branches diminish the height of the
waves and thus the waves lose energy,
ultimately stopping the waves emerging
onto the seabed and engulfing the
sediments.
The mangroves roots and branches also
reduce wind energy, which can stop the
formation of waves. According to
reports, densely packed mangroves can
half the height of a wave through just a
100 meter passage. For comparison, in an
open forest where roots and branches are
more sparse, it would take 500 m for a
wave to half its height. So,
preservation and reforestation of these
mangroves or plants with a similar
capability can become a great shield
against upcoming floods.
Another possible solution is to learn
how to live with these floodheavy
conditions. Working with nature rather
than against it. For example, let's take
a look at the flood defenses in the
Netherlands where onethird of the
country is below sea level and another
third is at risk of flooding. They've
built infrastructure that works with
water and manages the rising sea levels.
They do this by designing facilities
like boulders. Pders are bits of land
below sea level that have been
reasserted from a body of water. It's
always fully or partially surrounded by
an embankment to keep the water out that
comes from either the sea or a river.
These pders offer a network of drainage
canals and pumps to manage water levels
by disposing of excess water and running
water back to the sea or river to make
sure that the water doesn't run over
land.
PDERS can be used to protect houses,
farms, and factories and thus are used a
lot around the country. The Netherlands
also built dams and utilize sandjunes to
create ways to stay dry in their swampy
land. This shows that there are ways in
which we can observe nature and live
alongside it.
So, the bad news is behind its ethereal
beauty, our moon hides a power that if
just so combined, is set to overwhelm
humanity's coastal settlements. However,
there's always a bit of good news, too.
As knowledge is a power of his own. By
understanding our plight, we can look
for solutions both among already
existing ideas and ways forward that
have yet to be discovered. If we are to
endure what is coming, it's high time
for us to use our innate creativity and
drive to adapt and survive, to work with
our planet rather than against it.
With so many asteroids hurtling around
the solar system, it's a wonder that
Earth only has one moon. Because beyond
the inner solar system, the gas giants
have a lot more. Some of which are
regular moons, plus a lot of irregular
moons.
They are called irregular because their
orbits do not follow the orbital plane
of the planet. Instead following a
distant, inclined and eccentric orbit,
even sometimes orbiting retrograde to
the regular moons. The main reason for
this difference is that irregular moons
are mainly captured objects. Asteroids
that were orbiting the sun before they
approached too close to a planet and got
captured by the planet's gravity.
So, what about Earth? Well, I can
announce that Earth now has a second
moon called 2020 CD3.
But it probably isn't worthwhile trying
to spot this one in the sky. It's tiny,
only 2 to 4 m across and also very dark,
a type of asteroid known as a sea type
or a carbonatous asteroid.
These types of asteroids are as dark as
a wet asphalt.
This moon, like the irregular moons of
the outer planets, is a captured
asteroid that's been pulled into orbit
around the Earth. It was probably
captured as recently as 2017.
Although, before you warm to the idea of
Earth having two satellites, it seems
that this is only a temporary
relationship. Astronomers are expecting
it to escape Earth's gravity again by
April 2020. This is due to its chaotic
orbit. It never really got settled into
a stable orbit and so will be ejected
again in just a few months. A very
similar thing happened in 2006 with an
asteroid called 2006 RH120
where Earth had a second moon for just
short of a year before again it was
ejected.
And just like back then, once this one
leaves Earth's orbit, we'll be back to
just the one natural satellite.
Or will we?
There is another asteroid orbiting Earth
at a distance of about 9 million km,
much further out than the moon, called
2016 HO3.
Except although it's circling around
Earth, it's not really orbiting the
Earth at all.
Instead, it's orbiting the Sun, and its
orbit is very similar to Earth's, just a
little more eccentric.
At some points of this orbit, it is
ahead of the Earth and 6 months later,
it falls behind the Earth. This is
something known as a quasi satellite.
And Earth has five of them that we know
of. Moons which circle the Earth and are
even influenced by its gravity, but
without truly being in orbit. And so,
unfortunately, they aren't and can't be
classified as true moons. We aren't the
only planet to have quasi satellites. In
fact, they all might considering these
objects are very difficult to spot.
But why do gas giants have so many
irregular moons and Earth only has the
one right now anyway?
Well, being big and massive is only part
of the answer. Yes, the more massive you
are, the bigger your gravity, but
Earth's gravity is hardly a slouch. It's
easily comparable to Saturn, Neptune,
and Uranus. So, what's the difference?
Well, simply put, there's another force
of gravity in the solar system called
the sun. Moons in orbit around planets
are constantly being tugged and pulled
at by the sun. However, they are within
something called the planet's hillphere.
Within the hillphere of a planet, the
planet's gravity has more of an
influence on the object than the sun's
gravity. And this influence will be
different depending on how close you are
to the sun. Because the closer you are,
the stronger you are under the influence
of the sun's gravity. This means that
the hillphere around Earth isn't very
big, only 1.5 million km.
If Earth was close to the sun, its
hillphere would be smaller. If it was
further away, it would be bigger.
And now you can see why these gas giants
have so many captured moons. Their hill
spheres are simply much bigger all the
way out there.
In fact, Neptune, while not being
anywhere near as massive as Jupiter, has
a much bigger hillphere, meaning it can
capture stray asteroids which are tens
of millions of kilome away.
In order for Earth to permanently
capture an object, it would have to
happen in just the right way, likely
needing some helpful gravitational tugs
from our moon along the way to tame its
orbit. And as far as we know, that
hasn't happened yet. Meaning anything
that does come too close only sticks
around for a few orbits before it leaves
again.
So enjoy 2020 CD3 while you still can.
Can a moon have a moon? Well, yes. In
theory, it can. And yet, we don't see
any examples of this in our solar
system. So why is that? The first
important thing to know is that all
celestial objects have gravitational
pull. That includes tiny little
asteroids all the way to giant stars or
even black holes. The mass of an object
dictates how strong its gravitational
pull is.
A region around an object where its
gravitational influence is greater than
any other celestial object near them is
called its hill sphere. Due to the mass
of our Earth, it has a hillphere which
has a radius of 1.5 million km. Meaning
that if you were within this hillphere,
you would be pulled more towards Earth
than towards the sun.
If the Earth was closer to the sun, its
hillphere would be smaller. And if it
was further away, it would be larger.
The sun also has its own hillphere,
which contains the entire solar system.
Its hill sphere is massive almost two
light years radius as the nearest
celestial object its gravity is
competing against are other stars
which means in a way everything orbiting
our sun is the sun's moon or rather its
satellite. The term moon is really
reserved for satellites of planets.
So why don't moons also have natural
satellites?
Well, lots of moons are extremely close
to their parent planets, meaning their
hillpheres are very small.
Let's take our moon as an example.
Against the earth, the hillphere of the
moon is only 60,000 km radius or only
16th of the distance from the moon to
the earth.
Moons like Io have an even smaller hill
sphere as it is competing against the
gravity of Jupiter.
This makes it quite hard for moons to
capture an object, but not technically
impossible.
So surely there must be a moon that has
a natural satellite somewhere, right?
Well, most moons also have one other
major problem. They tend to be tidily
locked to their parent planet.
Because of this, any satellite that
orbits a tidly locked object will have
its orbit decay from tidal forces until
it eventually crashes into the moon.
Now, this still takes a lot of time over
astronomical standards, but it means if
any of the moons in our solar system did
have a satellite at one point, chances
are that it has since crashed into it.
This leaves one question, I think. Why
don't moons of planets also eventually
crash? The difference is that none of
our planets are tidly locked to the sun
as they are far enough away from the
parent star which means their moons have
a stable orbit.
This is one of the reasons we believe
that none of the Trappist system planets
have moons as they are so close to their
parent star that we assume the planets
are all tidily locked.
I hope this didn't disappoint you too
much. So, I'm going to leave you with
this. There are some other curious
objects in the solar system far away
from any other object so that they
aren't tidy locked to anything. This
particular asteroid called IDA looks
like a standard 30 km wide asteroid as
seen by the Galileo spacecraft. But you
might notice this little blob here. This
is actually Ida's moon, Dactyl. It's
only 1.5 km across and orbits only 60 km
away from IDA. We don't know especially
how stable this orbit is, but a very
cool thing to observe nonetheless.
We are used to tectonic activity on
Earth. It being a daily occurrence
somewhere on the planet.
But what about our closest neighbor, the
moon?
During the Apollo missions, seismometers
were left on the moon to detect the
presence of moon quakes.
From these instruments, we now know that
the moon does have quakes and relatively
frequently too. But unlike Earth, most
can be attributed to meteors striking
the moon's surface.
Although there is also evidence to
suggest that a small percentage of these
quakes could be due to the moon
shrinking and contracting.
Early in the solar system's life, most
of the celestial objects were hot, a
result of the process of how they were
formed,
and followed up by a period of heavy
bombardment.
Since then, impacts have become much
less frequent, and these objects,
including the moon, have cooled down.
As something cools, it contracts.
Photos taken by the Apollo missions
imaging the moon's surface and photos
from the current Luna mission, the Lunar
Reconnaissance Orbiter,
show that the moon is covered with these
kilome long ridges or lobate scarves
which can rise up to 100 m high.
Scientists believe they are connected to
the moon cooling.
They predict that these scarves are
caused by thrust faults. Faults caused
by the lunar crust being pushed together
until it gives, causing a distinctive
scarab on the surface, which is steep on
one side and shallow on the other.
It could be that some of these moon
quakes detected by the old Apollo
missions are caused by these thrust
faults even today.
The images that we have of loates scarps
and the moon show that they must be
pretty young. Certainly formed within
the last 100 million years.
We know this because they can cut right
through older craters. Craters which are
big enough that they would have likely
formed billions of years ago, not
recently.
Tidal forces from the Earth could also
influence the location of these scarps
as the Earth's gravity pulls on the
moon's crust. Here
you can see the location of the scarps
across the moon and the arrows represent
the direction of pull caused by the
earth's gravity. Even with all these
influences, however, it is thought that
the moon has only shrunk a total of 250
m since it was formed.
Mercury, on the other hand, has had a
much more tumultuous past.
It is thought that early in its life it
was actually a molten world and has
since cooled and solidified.
Mercury is also special in that it has a
massive neighbor exerting an incredible
amount of tidal forces on it. The sun
this means its contractions were much
bigger than our moons. Here you can see
one such lobate scarp which is about 2
km high and hundreds of kilome long. In
comparison to the moon shrinking 300 m,
Mercury is thought to have shrunk by up
to 4 km.
So yes, the moon is shrinking, but we
don't need to worry about it
disappearing anytime soon.
I bet you didn't know the moon did this.
This isn't the camera moving. It's a
real time lapse of the moon. While the
same face of the moon roughly faces the
Earth at all times, there's much more
wiggle room to that than you might
think. By taking a time lapse of the
moon as it circles the Earth over the
course of one month, we discover the
truth of its wobbling path. Go home,
moon, you're drunk.
A massive thank you to our astronauts on
Patreon. This video had no sponsors, but
it was still made possible thanks to the
hundreds of members we have there. Link
is in the description to join our
growing community. Patreon is where
Astramm truly take shape. A place for
people who love space, who want to see
these videos keep improving and reaching
more curious minds. Every new member
keeps the channel focused on what really
matters, making the complexity of space
available to everyone. If you enjoy what
we do, come join the Astramm community
today.