In 2013, scientists attempted to
recreate the evolution of the universe
from shortly after the Big Bang until
today.
This simulation, which took 19 million
CPU hours to produce, started with a
predicted amount of matter, dark matter,
and dark energy that should have existed
shortly after the Big Bang. The
simulation was allowed to run to see if
these parameters that were set at the
beginning can produce the galaxies and
the universe's structure we see today.
What you are looking at here is limited
to a 32 million lightyear cube of the
simulation.
The expansion of the universe is
considered with simply gas density and
temperature visible.
This is the intergalactic medium.
Even though space is a vacuum, there are
still a few particles in every cubic cm
of space. Blues are the coldest regions
here. Whites are the hottest. As gas
particles are drawn together by their
own gravity and that of dark matter, the
gas clumps and over the course of
millions of years coaleses into
galaxies, which increases the gas's
temperature drastically. What you'll
immediately notice is that there appear
to be explosions coming from the densest
clumps. But galaxies exploding, that
can't be right. I surely would have
heard of that before. But remember, this
is the intergalactic gas temperature we
are seeing. And each second passing in
the video is a few million realtime
years.
Quazars or extremely active black holes
are the brightest objects in the
universe, emitting more electromagnetic
radiation than entire galaxies combined.
When a quazar lights up, its rapid
increase in radiation blasts into space,
ionizing the intergalactic gas as it
expands out, heating it up to extreme
temperatures. This is known as quazar
mode or active galactic nuclei feedback.
Black holes don't remain as quazars for
lengthy periods of time. Rather,
quaazars are the result of a large
amount of mass falling into them,
lighting them up and causing them to
eject huge amounts of mass and energy.
They'll remain as quaazars as long as
there is matter being fed into them.
Although you may wonder, how can a black
hole emit anything? Don't they absorb it
all? And the answer is yes. It is in
fact the accretion disc around the
quazar that is so energetic and
luminous. A black holes accretion disc
is the result of matter passing by being
ripped apart and sucked into orbit.
These super massive black holes often
have billions of solar masses. The
gravity around them is immense. As the
material in the accretion disc orbits
and falls inwards, the friction from the
material in the disc rubbing together
creates energy so intense that a quazar
can be thousands of times brighter than
our Milky Way. In fact, a quazar's host
galaxy is often too dim to detect next
to the bright quazar. Although
techniques with the Hubble Space
Telescope have allowed a few of these
host galaxies to be seen, too.
quazars could light up from collisions
of galaxies when suddenly an abundance
of matter falls into the super massive
black hole. Although this doesn't always
happen.
You'll also notice these jets coming
from the quazar's poles.
These extend well beyond the galactic
disc and can be seen illuminating other
galaxies and dust clouds like a
spotlight.
Quaazars themselves are bright, but when
these jets are pointed towards us, they
are known as blazars.
The jets are believed to be powered by
the black hole's magnetic structure and
they can carry high energy plasma away
from the black hole at almost the speed
of light.
The days of quaazars and blazars are
thought to be over. The closest quaazar
to us is 600 million lighty years away
and thus was going on 600 million years
ago. However, you'll notice that the
explosions in the simulation don't let
up in this video as time passes. This
comes from the last in the active
galactic nuclei family of black holes,
radio galaxies. Typical quaazars and
blazars are so bright that they light up
in all frequencies of the
electromagnetic spectrum uniformly.
Radio galaxies originate from black
holes that unsurprisingly are brighter
in radio wavelengths. These explosions
now come from radio galaxies and this is
known as radio mode feedback.
The simulation stops at the present day.
Now, while impressive, the simulation
isn't perfect. For instance, it could
only simulate a trillion particles
compared to the countless number of
particles in the equivalent section of
space. Also, we don't have a perfect
knowledge of the perimeters of the
universe. And so, there were certain
mistakes evident in the model like the
overprediction of star formation. There
are plans to try this again at some
point with an updated understanding.
Being able to model how the universe
evolved can give us a confirmation about
how we believe it formed. And this is of
great interest to scientists. Let's see
what future results will bring. So, can
galaxies explode? Not in the
conventional sense, but if you are
talking about exploding with
electromagnetic radiation, then
absolutely.
In October 2022, a brilliant flash
pierced the cosmos, brighter and more
intense than anything human civilization
had ever seen. And that's not an
exaggeration. The strength of the blast
blinded our gammaray detectors the world
over and unleashed more energy in a
matter of seconds than our sun will emit
over its entire 9 billionyear lifespan.
Over the 18 months that followed, it
became the most widely studied gammaray
burst in history. Creatively dubbed the
boat for the brightest of all time. As
researchers began to decipher its cause,
their findings unraveled one mystery
after another. Scientists have been
cataloging gammaray burst for decades.
But this one was closer, brighter, and
unexplainably devoid of some key
signatures you'd expect to see. It also
raises some farreaching questions about
our standard model, the possibility of a
dark matter particle, and how heavy
elements like gold are made.
I'm Alex Mccoan, and you're watching
Astramm. Join me today as we dive into
the mystery of the biggest and brightest
gammaray burst of all time. What caused
such a colossal explosion? How is it
different from other gammaray bursts
before it? And what can it teach us
about our understanding of the universe
and the particles that constitute it?
Gammaray bursts are brief intense
flashes of high energy gamma radiation
lasting from milliseconds to several
minutes. The first of its kind was
detected in 1967 when American
satellites designed for detecting covert
Soviet nuclear testing picked up an
unusual pattern of electromagnetic
energy. Since then, gammaray bursts have
been of tremendous interest to the
cosmological community as they allow
scientists to study states of matter and
physics that are not reproducible on
Earth. Essentially, they provide
researchers with a glimpse of how stars
are formed and evolve across the whole
timeline of the universe.
There are two different kinds of
gammaray bursts. Short gammaray bursts
last less than 2 seconds and are
attributed to either the collision of
two neutron stars or the merger of a
neutron star and a black hole. They can
be followed by a kilon nova, an emission
of bright light resulting from the
radioactive decay of chemical elements.
This decay creates even heavier
elements, an important feature of nove
which we'll discuss more later. Anything
lasting longer than 2 seconds is classed
as a long gammaray burst. These are
thought to be caused by the explosive
deaths of massive stars and their
subsequent supernova. The collapsed core
may form either a neutron star or a
black hole. These typically occur close
to the edges of the observable universe
because they are characteristic of low
metalicity stars which formed when there
were less heavy elements around. When we
see one of these, we are witnessing
events from billions of years ago. In
the case of both long and short gammaray
bursts, the newly formed black hole
blasts out jets in opposing directions,
containing particles accelerated close
to the speed of light. When these
particles interact with surrounding
matter, they emit the gamma rays we
detect. So, what made the boat so
special? Let's start by analyzing some
of its key characteristics.
Firstly, the boat lasted 10 whole
minutes and was detectable for 10 hours
after the fact. It occurred in the
Sajitta constellation only 2 billion
lighty years away, which is much closer
than other gammaray bursts we've
detected until now. In fact, such a
bright explosion so close to Earth is
thought to be a 1 in 10,000year event.
Meaning the last time one happened,
humans had barely started farming. As is
the case with other long gammaray
bursts, we know a collapsing and
exploding star was behind it. But this
is where things start to get fuzzy.
A supernova alone isn't enough to
explain the magnitude of the gamma rays
emitted.
The boat was a whopping 70 times
stronger than any other gammaray burst
detected. Initially, the theory was that
this must be the supernova of a
ginormous star the likes of which we
rarely see. However, upon closer
inspection of the afterglow, scientists
found that the supernova behind the boat
was shockingly ordinary. To get a
clearer picture, astronomers pointed the
James Web Space Telescope in the boat's
direction. Web's near infrared
spectrograph revealed that the supernova
behind the boat was actually pretty
average. It wasn't nearly as bright as
you'd expect given the gammaray burst
that accompanied it. So what could have
caused such a flash?
One idea is that we simply perceive the
flash is bigger and brighter because of
Earth's relative position to the blast.
Imagine a flashlight shining in the
dark, diffused and soft. It lights the
path 1 to 2 m ahead of you. Now imagine
capturing all that light and focusing it
into a singular laser beam. It wouldn't
illuminate the path as widely, but it
would reach hundreds of meters into the
distance. And if Earth was in the direct
path of that laser, it would register a
super bright reading. That doesn't mean
the laser released more energy than the
flashlight. It just means the way it was
concentrated and then detected resulted
in a higher reading. The same concept
can be applied to these gamma ray
bursts. If a massive star is spinning
super fast when it collapses, then the
shape and structure of the near lighteed
jets it emits will be more narrow and
focused and therefore brighter. In fact,
the jets seen from the boat are some of
the narrowest we've ever seen.
But not only were these particle jets
brighter than expected, scientists also
detected way more of them going faster
than expected, they traveled with such
fervor that after 2 billion years
traversing the cosmos, they arrived here
and momentarily disrupted the Earth's
atmosphere.
Sitting just 50 to 1,000 km above the
surface of our planet, Earth's
ionosphere is rich in electrically
charged particles. When the boat struck,
it left a mark comparable to that of a
major solar flare, pushing the
ionosphere down into lower altitudes. If
photons from an explosion 2 billion
light-years away can have this kind of
effect on our planet, I don't really
want to think about what happens if
something like that in our neighborhood
explodes. The large high alitude air
shower observatory in Dawen County,
China managed to capture data on tens of
thousands of photons over the course of
the initial blast and into the
afterglow. This is a quantity unlike
anything seen before in gammaray
astronomy. In fact, it's so far out of
pocket that some astrophysicists think
that they might be pointing towards
something missing from our models.
According to our current understanding,
it's very unlikely these super high
energy photons are traveling for 2
billion years. Cosmic microwave
background radiation, interactions with
intergalactic dust, or red shifting
caused by the expansion of the universe
are all factors that can interfere with
a photon's trajectory. One hypothesis
put forward is that photons convert
themselves into a hypothetical particle
called an axion and then convert back
into gamma rays upon reaching our
galaxy's magnetic field.
Axons are thought to be an ultra light
particle responsible for dark matter.
Their existence is currently purely
hypothetical. We have no evidence for
them. And even if we did, they would lie
outside the standard model of particle
physics. We don't have time to delve
into detail in today's video, but let me
know if you'd enjoy a separate video on
this in the comments.
Okay, so far we've established that boat
was caused by a massive star collapsing
and turning into a black hole, which
incidentally is known as a collapsar.
Aside from generating a long gammaray
burst, collapsars are also known for
generating something else. Gold.
Wait, wait. How is gold connected to
gamma rays? Good question. To understand
that, let's take a minute to discuss how
elements are made.
The core of a star is a super high
pressure environment, some 200 billion
times higher than the atmospheric
pressure on Earth. In these conditions,
nuclear fusion reactions create heavier
elements out of lighter ones. For
example, one helium atom comes from
fusing four hydrogen atoms together.
Elements 2 through 26 on the periodic
table, that's helium to iron, are made
this way, process known as stellar
nucleioynthesis.
However, once you get to iron, it isn't
energetically favorable to continue
making bigger and bigger elements this
way. So, how do we account for the rest
of the periodic table? Where do these
heavier elements like gold come from? At
the moment, we know two different ways
these elements are formed. The first was
recently confirmed by the James Webb
Space Telescope. When two ultra dense
neutron stars collide, they emit an
immense amount of neutron particles.
Surrounding material captures these
neutrons, making their atoms temporarily
unstable. In order to stabilize, the
neutrons undergo radioactive decay into
protons, creating new, heavier elements.
This process is known as rapid neutron
capture or R processed nucleiosynthesis.
Some calculations suggest one neutron
star collision can produce up to three
Earth masses worth of heavy elements.
However, this explanation alone isn't
sufficient to account for all the heavy
elements in the universe. Neutron star
collisions are rare and take a long time
to happen in the order of billions of
years. On top of that, observations of
very old stars show that heavy elements
were already present in parts of the
universe well before most binary neutron
stars would have had a chance to
collide. So, how do you explain that?
There must be another source of heavy
elements in the cosmos. Which brings us
back to our boat.
There's another theory that collaps like
the boat could be another source of our
process nucleiosynthesis.
In their dying stages, massive stars
like the one that caused the boat are
surrounded by layers of exploding gas.
These explosions leave discs of matter
swirling around the resulting infant
black hole. As the black hole begins
devouring the surrounding material, it
can only ingest so much at a time. What
it cannot manage is swept away in a
neutron dense wind. Here the same R
process nucleiosynthesis occurs forming
heavier elements like gold, silver and
platinum. This seems promising but
unfortunately even factoring in these
kinds of supernova isn't enough to
account for the abundance of gold in the
universe.
To make matters worse, analysis of the
boat spectrum didn't show any traces of
heavy elements, raising questions about
the validity of this collapsar gold
making theory. Some scientists suggest
the boat's host galaxy might have
something to do with the lack of heavy
elements in the explosion. Upon modeling
the host galaxy spectrum, researchers
discovered it has the lowest metallicity
of all previous host galaxies where
gammaray bursts were detected. In other
words, maybe the environment didn't have
the right building blocks to make
heavier elements. How do we know how
much gold should be out there in the
first place? How does scientists predict
something like the relative abundance of
elements in the universe?
There are two main methods of
calculating this. The spectroscopy of
stellar photospheres and meteorite
analysis. By analyzing the absorption
lines in the spectra of stars,
astronomers can determine the relative
abundances of elements in the
photosphere of those stars. The
composition of meteorites, remnants of
an early solar system, are analyzed in
parallel to determine the relative
abundances of elements. Meteorites are
especially useful for measuring the
abundances of volatile elements like
hydrogen, helium, and noble gases that
are unrepresented in stellar
photospheres. The results of both of
these methods are usually congruent,
indicating we're probably doing
something right. But this is physics, so
of course nothing is so cut and dry. One
famous exception to this rule is
lithium. According to the standard big
bang nucleiosynthesis theory, the early
universe should have produced about
three times more lithium 7 than is
currently observed. The plot thickens
when we consider its isotope, lithium 6,
where we observe 1,000 times more than
our predictions can account for. This
discrepancy is known as the lithium
problem and remains unsolved, presenting
a significant challenge to the standard
cosmological model. It highlights the
importance of understanding the
processes that shape the relative
abundances of elements in the universe
and suggests that our current
understanding of nucleiosynthesis might
be incomplete.
Just because boat didn't yield gold as
expected doesn't mean we should discard
these kinds of extreme gammaray bursts
as places where heavy elements could be
made. Observations of nearby stars have
provided strong evidence for an early R
process that enriched the universe with
heavy elements. But the boat findings
cannot be ignored as they suggest there
may be alternative currently unknown
processes responsible for this elemental
enrichment of our cosmos. The results
may call into question our entire model
of understanding regarding collab stars
and their role in creating heavy
elements. This discovery is much bigger
than just the boat or gamma rays. It's
about the literal building blocks of our
universe as we know it. Where do our
different atoms come from? And why do
they exist in the proportions they do?
How much of our model is accurate? And
how much is missing? What role does dark
matter play in all of this? We need more
time and research before we know for
sure. But the boat is a great example of
how new findings keep our understanding
of physics ever evolving, just like the
universe itself. Imagine standing on a
quiet hill watching the sky burst into a
symphony of colors as the sun peaks
above the horizon. It's a daily
spectacle that many of us take for
granted. The warm hues of orange, red,
and pink splashing across the sky. Yet,
this beautiful sunrise is only the final
chapter in the light's long journey. If
we trace the path of these photons from
their end point in the retina at the
back of your eye through the Earth's
atmosphere across the 150 million km
void separating our planet from its host
star and finally through the varied and
tumultuous layers of that star. We
discover the source of the warmth on
your face and the colors painted across
the sky. The sun's core, a nuclear
engine that has been raging for 4 and a
half billion years. Born in a cosmic
nursery from the remnants of ancient
stars, the glowing sphere of searing
plasma that dominates our sky is the
product of complex forces that have
played out over millennia.
So, let's take a journey back in time,
tracing the origins of the sun,
exploring its intricate layers, and
mapping the elements that make up its
very structure. I'm Alex Mccoan, and
you're watching Astramm. Join me as we
dive into the depths of our sun and
learn what secrets lie behind the
blinding curtain of light and time.
To understand how the sun formed, we
must go back to before our solar system
even existed about 4.6 billion years
ago. At that time, in a relatively quiet
region of the Milky Way galaxy, a
massive cloud of gas and dust known as a
giant molecular cloud drifted silently
through space. This was no ordinary
cloud. It was immense, possibly spanning
hundreds of light years across with a
mass equivalent to millions of suns.
Within this cloud lay the scattered
remnants of ancient stars that had long
since exhausted their nuclear fuel and
exploded as supernova, casting their
enriched contents into the cosmos. This
stellar debris included a mix of
elements such as hydrogen and helium,
the building blocks of stars along with
heavier elements like carbon, oxygen,
nitrogen, and iron.
The presence of these elements made this
molecular cloud fertile ground for
forming new stars and planetary systems.
However, such a massive cloud requires a
trigger to initiate the cascading
collapse that results in stellar birth.
Scientists believe that this trigger
could have been a shock wave from a
nearby supernova explosion that
compressed parts of the cloud, causing
it to fragment and collapse into regions
of higher density. This collapse marked
the beginning of the pre-olar nebula,
the dense region within the molecular
cloud that would eventually give birth
to our solar system. Some scientists
have even proposed a name for this
hypothetical exploding star Kati Quay.
Named after the Aztec goddess of Earth
and fertility, Kati Quay symbolizes the
death and rebirth cycle where in this
case the death of one star conceded the
formation of new ones.
As the gravitational collapse of the
giant molecular cloud progressed, it did
not result in a single massive object,
but rather fragmented into multiple
dense clumps. This fragmentation occurs
due to a phenomenon known as genes
instability, named after the British
physicist Sir James Jeans. The genes
length is a critical distance within a
cloud at which the forces of gravity are
balanced with the natural internal gas
pressure that keeps the cloud propped
up. When a defined section of the cloud
exceeds the gene's length, it will
collapse in on itself. Different regions
within the molecular cloud exceed this
gene's length at different times due to
variations in density and temperature.
As these regions become gravitationally
unstable, they collapse independently to
form multiple protostellar cores as
opposed to one single massive body.
The protostellar core was the earliest
phase in our sun's life that was
recognizably star-like, crunching itself
into a tighter and tighter sphere by the
ever clenching fist of its own gravity.
The core is surrounded by infalling gas
and dust. As more material accumulates,
the core becomes denser and hotter,
setting the stage for the next phase of
star formation, the protoar.
Once a protostellar core becomes
sufficiently dense and hot, it evolves
into a protoar. The central region has
heated up to the point that it begins to
emit light and heat from the energy
released by gravitational contraction.
But its core has yet to reach the
temperatures required to ignite nuclear
fusion in the core. As the protoar
continues to accumulate material and
grow in mass, the pressure and
temperature in its core increase
dramatically. Our sun developed through
this embryionic state of contraction and
heating for millions of years before
eventually building up its core
temperature to about 10 million° C at
which point hydrogen nuclei began to
fuse into helium and with that the sun
was born.
At this stage the innate angular
momentum of the surrounding material
that shrouds the infant star generates
rotation. A cloud of this size will
always have some slight rotation.
Nothing in the universe is still. And
conservation of angular momentum
dictates that the cloud must spin faster
and faster as it crunches down into
tighter space, similar to how a spinning
figure skater speeds up as they tuck in
their arms.
Around the equator of rotation, some of
the material is moving fast enough to
stay in orbit. But higher up and lower
down, the momentum is insufficient to
resist the protostar's ravenous pull and
falls helplessly into its jaws. And so
the cloud flattens out into a disc, a
protolanetary disc. This disc is a
cosmic lottery. As it cools and
coaleses, planets, moons, asteroids, and
comets will be relentlessly created and
destroyed. Some material will be robbed
of its momentum and tumble into the sun,
while neighboring matter will go on to
become part of the many bodies that make
up our solar system today.
With its new ability to fuse hydrogen
atoms into helium, the sun could finally
generate energy to push back against the
forces of gravity that sought to squeeze
it further. The energy streaming out of
the fusion reactions created an outward
radiation pressure that balanced the
inward gravitational pull of the gas,
allowing the young star to achieve a
stable state known as hydrostatic
equilibrium.
In this state, the star no longer
contracts under its own gravity and it
enters a stable phase of its life known
as the main sequence. This balancing act
between internal pressure pushing
outward and gravity crushing in is the
axis that defines every stage of the sun
and all stars lives. We saw it during
the initial collapse and here as it
settles into a stable main sequence star
and it will be the pendulum that
eventually swings towards its dramatic
death. For now, let's unpick the process
of nuclear fusion and how it's capable
of keeping an object as massive as the
sun stable and steady for billions of
years.
To understand how nuclear fusion occurs
in the sun, we need to examine extreme
conditions present within its core.
Today, temperatures reach around 15
million°
and pressures more than 250 billion
times that of the Earth's atmosphere. In
this blazing cauldron, hydrogen atoms
are stripped of their electrons, forming
a plasma composed of free protons and
electrons.
Under such intense conditions, these
protons, normally repelled by their
shared positive electric charges, are
forced into extremely close proximity.
The force that usually keeps them apart
is the coolum force, an electrostatic
repulsion between positively charged
particles. It's exactly the same force
that resists as you try to push the same
ends of a bar magnet together.
Overcoming this force is the primary
challenge for nuclear fusion to occur.
Despite the extreme kinetic energy of
protons in the sun's core, most
collisions are still not energetic
enough to overcome the coolum barrier.
Under normal conditions, protons need
immense energy to get close enough for
the strong nuclear force to bind them
together. This barrier is so significant
that physicists around the early 1920s
using a classical understanding of how
particles interact with each other
determined that fusion should not occur
at the temperatures and pressures found
in the sun's core. To solve the puzzle,
we needed a scientific model that has
come to explain many of the seemingly
impossible phenomena we observe in the
universe. Quantum mechanics.
The quantum mechanics revolution of the
1920s introduced the concept of
particles behaving both as particles and
as waves. One of these phenomena is
quantum tunneling where particles have a
certain probability of tunneling through
a barrier even when they lack the
classical energy needed to overcome it.
In the sun's core, quantum tunneling
allows a small fraction of protons to
effectively bypass the coolum barrier
without having to climb over it in the
classical sense. Even though these
protons do not have enough kinetic
energy to overcome the electrostatic
repulsion directly, quantum mechanics
gives them a chance to appear on the
other side of the barrier. To visualize
this process, consider a diagram of the
coolum barrier. In the diagram, you will
see a high energy barrier representing
the repulsive electrostatic force
between two protons at varying
distances. The energy needed to climb
over this barrier is represented as a
peak. The calculations of a classical
physicist would require the sun to have
temperatures high in the billions of
degrees C, thousands and thousands of
times higher than the sun's core to
hurdle this barrier. But quantum
mechanics allows for the probability
wave of a proton to tunnel through this
barrier, appearing on the other side
without ever going over the peak. This
tunneling effect is what enables fusion
to occur in stars like the sun, despite
the seemingly insurmountable coolum
barrier. Once a proton has tunnneled
through the coolum barrier, it gets
close enough to another proton for the
strong nuclear force to take over. The
strong nuclear force which is far more
powerful than the kulum force operates
only at very short ranges on the order
of phentometers where one phento is
equal to 10 ^ -15 m roughly the size of
a single proton. The strong nuclear
force binds the protons together. But
this is only the first step in a series
of nuclear events known as the proton
proton chain.
The proton proton chain reaction is the
dominant fusion pathway in the sun and
other stars of similar size accounting
for roughly 99% of the sun's energy
production.
It starts with two free roaming protons
combining to form dutyium, a heavy
isotope of hydrogen along with the
emission of a posetron and a neutrino.
This step is rare but essential as it
allows the fusion process to continue.
The dutarium then rapidly fuses with
another proton to form helium 3,
releasing a higher energy photon, a
gamma ray. In the final step, two helium
3 nuclei collide and fuse to form helium
4, releasing two protons and a
significant amount of energy in the form
of radiation.
While the proton proton chain is the
dominant fusion process in the sun,
there is another pathway at play
particularly in stars hotter and more
massive than our sun. This pathway is
known as the CN O cycle, short for
carbon, nitrogen, oxygen. In this cycle,
carbon, nitrogen, and oxygen nuclei act
as catalysts to fuse protons into
helium. The CNO cycle is more efficient
at higher temperatures and only
contributes a small percentage of our
sun's energy output. The energy produced
in these fusion reactions comes from a
small amount of mass being converted
into energy as described by Einstein's
famous equation E= MC².
In the fusion process, the mass of the
resulting helium 4 nucleus is slightly
less than the combined mass of the
original four protons. This missing mass
is not lost, but rather converted into
energy that radiates outward from the
sun. Every second, the sun converts
about 4.3 million tons of its mass into
energy. That's right. The sun is losing
mass all the time. To put this into
perspective, this is the equivalent of
the mass of about 3 million cars being
turned into pure energy every second.
This is the energy that ultimately
powers the sun and provides the light
and heat that reaches Earth.
But before these photons pouring out of
these reactions can reach the Earth and
everything else in the solar system and
beyond, they must first escape the
700,000 km of chaotic solar interior
that lies above them. To truly grasp the
complexity of the sun and its stratified
layers, let's follow a single photon, a
particle of light, from its creation
within a proton proton reaction all the
way to the surface. This journey is
anything but straightforward. In fact,
it will take thousands, if not hundreds
of thousands of years, for this photon
to complete its journey, passing through
several distinct layers of the sun, each
with their own unique properties and
behaviors.
As we've already found out, our photon's
journey starts in the core, a region
that spans about 25% of the sun's
radius, but holds nearly half of its
total mass. And as we've also heard, the
core is incredibly dense at around 160 g
per cubic cm. It's over 20 times denser
than iron. To put that into perspective,
if your phone was made of material that
dense, it would weigh as much as a
heavyduty sledgehammer
for our photon. This means that the
environment is so tightly packed with
particles, it cannot travel far before
being absorbed by a neighboring
particle. Of course, not long later,
that particle will pack the photons's
bags and send it out into the mele of
the core for another particle to catch
and release in some other random
direction. It's a bit like trying to
navigate through a dense crowd where
every few steps you bump into someone
and have to change direction.
This constant interaction of absorption
and remission is what gives the core its
chaotic nature, resulting in a journey
for a photon that is more of a random
walk than a direct path outward. This
random movement of photons is
reminiscent of Brownian motion where
particles suspended in a fluid move
erratically due to collisions with
smaller fastoving molecules. And there
is no preference for the direction that
a photon could fly back out of a
particle that had just absorbed it. In
theory, a photon could make it all the
way to the surface of the sun before
random chance directs it back towards
the core. To add to the complexity, as
photons are continuously absorbed and
remitted, the energy gradually decreases
from gamma rays to x-rays and eventually
to visible light as they move towards
the sun's outer layers.
Understanding the processes occurring
within the sun's core has been a
challenging endeavor for astronomers.
The core is completely obscured by the
outer layers of the sun, meaning we
cannot directly observe it through any
form of light. Instead, scientists have
developed ingenious methods to infer the
dynamics of the roaring inferno below.
One of the key techniques is the study
of heliosismology,
the observation of waves rippling across
the sun's surface. Just like
seismologists study earthquakes to
understand Earth's interior,
helioismologists measure the vibrations
of the sun's surface caused by sound
waves that travel through its interior.
These sound waves are influenced by the
temperature, density, and composition of
the sun's layers, allowing scientists to
construct models of what is happening
deep within.
Solar nutrinos, elusive subatomic
particles that I've talked about in a
previous video, are also released during
the fusion reactions in the sun's core.
Nutrinos interact so weakly with matter
that they can travel straight out of the
sun in just 2 seconds, hardly aware of
the tangle their photon counterparts are
caught up in. For decades, solar
neutrino detectors on Earth measured
fewer nutrinos than predicted by
theoretical models, leading to what was
known as the solar nutrino problem.
It wasn't until scientists discovered
that nutrinos change their properties or
flavors on their journey to Earth that
the mystery was resolved, confirming
both our understanding of particle
physics and the nuclear processes in the
sun's core. Next, the photon enters the
radiative zone, which extends from the
edge of the core to about 70% of the
sun's radius. Here, temperatures
gradually drop from 15 million° to
around 2 million°.
The density of matter also drops, going
from around the density of gold in the
zone's depths to less than water near
the top. The dominant energy
transportation method is still this
frenetic process of photons skittering
through the layer, carrying their energy
on average towards the surface. And as
the density of the plasma continues to
decrease as we move outward, our photon
has slightly more wiggle room capable of
bigger leaps between interactions. But
even here, outside of the core, the
sun's material is still so opaque that
even a few centimeters could block out
as much light as hundreds of meters of
water. We are able to envision such
precise models of the sun's interior
because of models like the standard
solar model or SSM. It is a mathematical
framework based on the principles of
hydrostatic equilibrium, energy
conservation, and radiative transfer.
Essentially using our understanding of
how materials and fluids behave here in
our laboratories on Earth to infer the
properties of a place out of our reach.
The SSM models how energy is passed
between particles in its complex
equations, describing how photons are
transported through the dense plasma.
This allows us to calculate how
transparent the solar material is to
radiation at different temperatures and
densities. And from there we can work
out where the radiative zone ends and
the next layer begins.
This is one of my favorite things about
mathematics. It allows us to explore
environments that we can never visit.
Almost like how quantum tunneling allows
protons to jump over the coolum barrier
and produce the photons we are now
following.
As our photon approaches the upper
boundary of the radiative zone, it
enters the tacoline, a thin transitional
layer where the radiative zone meets the
next layer, the convective zone. The
tacoline is an environment of extreme
contrast where the relative uniformity
of the radiative zone meets the boiling
bubbling convective zone.
The strange conditions in this extremely
thin layer likely plays a key role in
the solar dynamo that generates the
sun's magnetic field. The tacoline is
thought to twist and amplify the
magnetic field, shaping the sunspots and
solar flares that define the star
surface.
Zipping through the thin taco, our
photon arrives at the convective zone,
stretching out from about 70% of the
sun's radius to just beneath the visible
surface, the photosphere. The
temperatures have dropped to below 2
million° C. Now, the sun's plasma is no
longer dense enough to effectively
transfer energy by radiation. Instead,
the plasma becomes unstable and begins
to move in massive convection currents
much like boiling water in a pot. Hot
plasma rises towards the surface in
large cells called granules. These
granules with diameters of thousands of
kilometers surge upward under their
power of buoyancy at speeds of hundreds
of meters/s like a beach ball that you
force below the water rushing to the
surface as it squirms from under you.
The force required to move this volume
of matter against the sun's gravity is
astonishing. Imagine the force of 10
billion hurricanes and you'd be just
starting to get close to the energy
necessary. This convective process moves
energy much more efficiently than the
radiative zone, creating turbulent
rolling waves of plasma that churn
vigorously.
I have explained these convection cells
and the beautiful hypnotic patterns they
create on the sun's surface in a lot of
detail in this previous video.
This is a significant change in how
energy is transported. While photons
still interact with particles, the
primary mechanism of energy transfer in
the convective zone is no longer the
random absorption and remission between
particles, known as radiative diffusion,
but rather the movement of hot plasma
rising and cooler plasma sinking. This
convection motion carries the energy
outward more efficiently than in the
radiative zone.
Upon reaching the top of the convective
zone, the photon is carried to the
photosphere, the sun's visible surface.
Temperatures have now cooled to about
5,500°,
and the plasma has thinned enough for
photons to escape without being
constantly reabsorbed. It is from this
thin, roughly 500 km deep layer that the
light we see from the sun is emitted.
Above them lies only the sun's corona
which is itself a fascinating
environment but only around a billionth
of the density of the photosphere and
therefore not much of an obstacle to our
photon. It is incredible that an object
like the sun that many refer to as a
ball of gas can have such a sharp
boundary. But the photosphere is where
our photon finally breaks free,
traveling at the speed of light and
beginning its journey across the
vastness of space.
Our nutrino friend that was created at
the same time, it's long out of the
Milky Way by now.
Since settling into its stable main
sequence phase, nuclear fusion has
altered the strength of the sun's
layers, gradually increasing its
brightness over billions of years. This
is because the fusion process inevitably
leads to changes in the composition,
density, and temperature of the sun's
core, which in turn affects the overall
behavior and the future of our star.
As hydrogen fuses into helium, hydrogen
gradually depletes in the core and
helium ash begins to build up. Unlike
hydrogen, which readily undergoes fusion
at the sun's current core temperatures,
helium requires much higher temperatures
to fuse. As the abundance of helium
increases, the core becomes denser.
Helium nuclei are more massive than
hydrogen nuclei. And as more helium
collects in the core, the overall
density increases. At the same time,
because helium is not fusing and
creating energy, it does not contribute
to the outward radiation pressure that
counterbalances the sun's immense
gravitational pull. The result is that
gravity will begin to dominate, causing
the core to contract further.
To prevent collapse under its own
gravity, the sun must increase its core
temperature. As helium builds up, the
hotter core turns up the dial on
hydrogen fusion because nuclear fusion
rates are extremely sensitive to changes
in temperature. This increase in fusion
releases more energy, which results in a
brighter star and temporarily restores
the balance between gravity and
radiation pressure. Through this
process, the sun's energy output has
increased by about 30% over its
lifetime. And this trend will continue
for billions of years as long as
hydrogen fusion dominates in the core.
In the radiative zone, the layers have
grown hotter and more opaque. The rising
temperature of the core means more
energetic photons are produced, which
are more prone to interacting with the
surrounding plasma and pinballing
around. As a result, this zone has
become less efficient at radiating
energy outward, trapping photons for
longer before they can continue their
escape. The convective zone has also
expanded slightly over time due to the
increased energy production.
As the sun continues to burn through its
hydrogen fuel over the next 5 billion
years, its core will eventually be left
overburdened with inert helium that can
no longer sustain fusion under its own
pressure. When this happens, the sun
will evolve into a red giant,
dramatically expanding and engulfing its
inner planets, including Mercury and
Venus and possibly the Earth. In this
phase, the core will contract and heat
up until it ignites in a helium flash,
starting the fusion of helium into
heavier elements like carbon and oxygen.
Yet again, swinging the pendulum between
the relentless gravitational hammering
and the resisting forces of energy from
the core. This process will create new
layers and cause the sun to swell even
further before shedding its outer layers
to form a ghostly remnant. a planetary
nebula. The core that remains will cool
and fade, becoming a white dwarf, a
dense earth-sized ember of a once raging
fire.
This transformation from a stable star
to a red giant and eventually a white
dwarf is a remarkable and complex
process with many fascinating details
which we will no doubt explore in a
future video.
But ending our journey from the depths
of the sun's ancient past to its modern
interior, let's return to that quiet
hill where we began, watching the colors
of sunrise. I hope the next sunrise you
see becomes that little bit more
special. I hope the pink and orange
photons scattered for one final time on
the molecules of Earth's atmosphere
before streaming through your pupils
will make you think of the incredible
voyage they have endured. Beginning with
a collapse of a colossal cosmic cloud
into an elementb building furnace,
struggling for hundreds of thousands of
years through a melting pot of plasma to
gift you with this beautiful scene. I
hope this exploration has deepened your
appreciation for that daily miracle we
often take for granted. And there's
still so much more to uncover about our
closest star, from the formations it
will undergo to the profound impacts it
will have on our solar systems future.
Thank you for joining me on this journey
through space and time. If you found
this journey as enlightening as our host
are, please give the video a like and
fuse it with your own thoughts down in
the comments below. Thanks for watching.
Beneath its swirling clouds, Venus is a
world of relentless heat, crushing
pressure, and volcanic planes stretching
for hundreds of kilome. A planet
reshaped by cataclysmic eruptions and
alien geology. Aside from a few
haunting, grainy photos from the
surface, Venus kept its secrets hidden
for centuries. No one had truly seen
what lay beneath the veil until Mellin.
On the 4th of May 1989, Space Shuttle
Atlantis launched its third flight from
the Kennedy Space Center. On board, it
carried a very special payload. The
first deep space probe launched by a
space shuttle. A probe bound for Venus
that was about to make history. For 4
years, the Mellan spacecraft ambitiously
studied our nearest neighbor, mapping
98% of its surface to a resolution of up
to 100 m. The images it sent back
shattered our assumptions of Venus
forever. And that's what I'm going to
show you today.
I'm Alex Mccoan and you're watching
Astramm. Join me today as we explore the
stunning images of Venus. NASA's Mellan
mission sent back. The surprises they
revealed about Venus's violent past and
the mysteries that still linger
unanswered to this day.
Mariner 10 snapped some of the first
pictures of Venus in 1974,
capturing Venus's dense cloud formations
on its way to Mercury. A year later, the
Soviet Union's Veneer program landed a
spacecraft on the rocky planet, sending
back the first and only images from
Venus's surface. They show a desolate,
eerie landscape, but these images only
captured data from tiny portions of the
planet's surface. The lander's cameras
could only see a few meters in any
direction and were destroyed within
hours by Venus's crushing pressures.
In the 1970s, the Pioneer Venus Orbiter,
also known as Pioneer Venus 1 or Pioneer
12, provided some radar mapping data,
but its resolution was low and its
images incomplete.
Imagine trying to map the Earth, but
Google Maps didn't let you zoom in
further than 75 km.
New York City would just be one pixel,
so pretty hard to make an accurate map.
Additionally, Pioneer 12 didn't map the
northern and southernmost parts of
Venus. That's like a map of Earth with
most of the Arctic and Antarctic circles
missing.
NASA decided they could do better. They
wanted to see it all. Landforms,
tectonics, impact, and chemical
processes, erosion, and even get to know
Venus's interior. So they mounted an
ambitious mission to capture the entire
surface of the planet in ultra high
detail along with a gravitational map
that would give insights into what lies
beneath. That mission was Mellin.
Now before I show you Mellin's photos,
you need to understand three key
engineering innovations that made me
Mellin's mission so groundbreaking.
First, to penetrate Venus's dense
atmosphere, Mellin improved on Pioneer's
basic radar ultimatry.
Engineers used a 3.7 m high gain Voyager
antenna and components from other past
missions to create a synthetic aperture
radar that collected multiple readings
of each area from different orbital
positions. These combined readings
simulated observations from a much
larger antenna, revealing unprecedented
surface details.
Second, Mellin followed a polar orbital
path, scanning Venus in north south
strips during each of its 3-hour
15-minute orbits. In total, it completed
six 243-day
mapping cycles. Cycles 1 to three mapped
the planet surface and cycles four to
six captured gravitational field data.
By the end of its first cycle on the
15th of May 1991, Mellin exceeded its
70% mapping goal, covering 83.7%.
By the end of cycle 3, it had mapped 98%
of Venus at 100 to 250 m resolution. The
probe ended up sending home more data
than all of NASA's previous planetary
missions combined, a total of 150 GB. By
today's standard, it admittedly isn't
that impressive. Your phone probably has
more storage capacity, but for the
early9s, it was quite something.
Finally, after its third mapping cycle
in September 1992, Mellin transmitted a
continuous radio signal to NASA's deep
space network based on how the probe's
acceleration changed as it orbited the
planet. This would produce a change to
the radio signal via the Doppler effect,
allowing scientists to decipher Venus's
gravity field. This revealed information
about the planet's interior structure
and density distribution.
The images I'm about to show you are 10
times sharper than any previous radar
images taken from Earth or orbiting
spacecraft at the time and 250 times
sharper than Pioneer 12's data. For the
first time, we actually got to see what
was lurking under those thick, noxious
clouds. Let's take a look.
Where previous missions had only shown
vague outlines, Mellin revealed a world
in exquisite detail, revealing for the
first time the surface characteristics
of our next door neighbor. This mosaic
took a decade to put together, with the
bulk of the radar data coming from
Mellin's readings. Let's zoom in even
further and take a little tour.
When you approach Venus, you immediately
notice the lack of craters. Compared
with the moon or Mars, it's pretty
untouched with Mellin only identifying
940 impact craters across the entire
planet. Many small impact craters are
missing because their meteors burn up in
Venus's atmosphere. And the other
craters we did find didn't look as
expected.
Take these three craters for example.
How danalova and agleones. On impact,
debris from the surface is ejected and
forms the pale yellow blanket-like
formations around each crater. Crater
ejector leave behind distinctive marks
and shapes that help scientists
determine the angle and energy of impact
and how long ago it happened, the
composition of surface and subsurface
materials, and much more. What's
interesting about these crater ejector
is they all show sineuous flows in a
flower-shaped pattern, not unlike lava
flows. They are made of molten rock and
remain this way for way longer than
expected due to Venus's high surface
temperatures.
The Adivar Crater is another great
example of Venus's unique ejector
patterns. On airless worlds like the
moon, ejected material from the impact
follows an arked ballistic path. But on
Venus, the ejected debris interacts with
the planet's dense atmosphere, creating
powerful winds that sweep the material
outward in groundhugging flows. This
creates unusual streaks like the
horseshoe-shaped one we see here, which
are only found on Venus. Finally, the 90
km wide Adams crater has a 600 km
outflow tail likely made of impact melt
or magma which can remain in its liquid
form for much longer due to the high
temperature and pressure on Venus.
The scarcity of these craters suggests
Venus's surface is actually quite young.
Indeed, gravity data collected by Mellin
supports the hypothesis that Venus
underwent a global resurfacing event
approximately 300 to 500 million years
ago. So, what could have caused such a
dramatic event?
Volcanoes. Lots and lots of volcanoes.
We knew Venus had volcanic features
before Mellin, but we had no idea that a
staggering 85% of the planet is covered
in volcanic flows like these pancake
domes 65 km wide with broad flat tops
less than 1 km tall. They likely formed
from highly viscous lava which later
cooled and withdrew leading to the
cracks and pits you can see in the
image.
or these sprawling lava flows stretching
for hundreds of kilometers over the
fractured plains beneath two large
shield volcanoes, Sapasmons and
Martmons.
By studying the varying shapes and
patterns of these volcanic planes,
scientists can identify different
volcanic formation processes from
localized shield volcano eruptions to
massive regional lava flooding events.
Speaking of Martmons, let's take a
closer look. Towering at a dizzying
8,000 m tall, it's almost as large as
Mount Everest. It's also a shield
volcano characterized by its broad,
gently sloping sides. They are typically
formed by fluid lava flows like the
Mount Aloa in Hawaii.
If you think Matt Mons looks as awesome
as I do and you want to carry around a
bit of Venus's fire to spice up your
day, then you should check out the
wallpapers we've made over on Patreon
for your phone, desktop, or tablet.
Shield volcanoes are not the only kind
of volcano you'll see on Venus. Small
volcanic domes like these are also very
common features on the planet's surface.
Unlike shield volcanoes, volcanic domes
are formed by small, bulbous masses of
lava too thick to flow very far. As it
slowly exits the vent, the lava piles up
and cools on the sides. This cluster of
volcanoes sits between rough lava flows
in the east and smooth lava flows in the
west, indicating two different types of
lava overlapping, though the exact
process remains unknown.
The abundance of lava flows and volcanic
features on Venus confirmed that
volcanism played a dominant role in
shaping Venus's surface and gave rise to
the theory that the planet might still
be volcanically active to this day. In
2024, researchers who were revisiting
Mellan's archival data found that two
volcanoes actually erupted during the
probe's watch in the early 1990s.
This supported a historic finding from
the previous year that Martmon's had
also erupted in 1991.
So what's going on beneath the surface?
What is Venus's mantle like?
A hint to those answers lies in the
surface corona. Like this 500 km wide
Yaveen corona.
When hot upwelling magma from the depths
of a planet's mantle reaches the surface
or the lithosphere, it leaves its mark.
On Earth, our tectonic plates shift over
this magma hot spot, creating volcanic
chains like Hawaii or flood bassults.
But Venus only has one single tectonic
plate. So mantle plume activity pushes
the surface upward before collapsing as
the magma cools. The result, these
corona.
Despite being hundreds of millions of
years old, many corona appear well
preserved. There are around 500 of them
widely distributed across the planet in
patterns inconsistent with plate
boundaries and their characteristics
suggest a lithosphere that is too thick
or strong to break into mobile plates
like Earth's.
All this evidence suggests a unique
style of lithospheric deformation where
the crust stays largely in place but is
pushed, pulled, and deformed by forces
below. This stagnant lid planet model
might also play a role in Venus's
runaway greenhouse effect. On Earth, our
tectonic plates carry out vital
processes like burying oceanic carbon.
When it rains, atmospheric carbon
interacts with mountain rocks to create
new minerals, which slowly make their
way to the oceans and eventually find
themselves stored in seabed sediment.
This sediment is then subducted when an
oceanic plate sinks the seabed back into
Earth's magma layer, a process that
regulates Earth's temperatures and the
concentration of carbon dioxide in our
atmosphere.
Since Venus's crust is stagnant and
devoid of tectonic activity, this
regulatory process can't take place.
Instead, heat builds up beneath the
surface and greenhouse gases are not
effectively removed from the atmosphere.
The hotter the interior becomes, the
more unstable the system until the whole
lithosphere overturns in a global
resurfacing event and the cycle begins
again.
We aren't exactly sure when the last
resurfacing event occurred on Venus, but
NASA's technical report following Mellin
suggests it happened about 300 million
years ago.
Another curious symptom of Venus's
unique tectonics and mantle activity are
the tesseray that we can see in the
highland regions of the planet. They are
distinct geological features
characterized by highly deformed terrain
with a tilelike appearance. They cover
about 8% of Venus's surface and are
primarily found in the highland regions.
As some of the oldest visible surface
features on Venus, they date back about
750 million years. The USSR's Venera 15
and 16 missions first identified these
formations in the 80s, but Mellin
brought them to life in spectacular
detail, like this photo of Alpha Regio,
one of the most prominent tesser on
Venus. Scientists still aren't sure
exactly how tesseray form, but the two
leading theories indicate they are
likely made of volcanic rocks or they
are part of Venus's continental crust,
deformed through compression or
expansion. If so, they'd be gateways
into Venus's geological past and
possibly even show us what the climate
was like billions of years ago.
Fascinatingly, Mellin actually managed
to capture the first evidence of active
tectonics occurring on another planet in
the solar system in Venus's Aphroditi
Terror, a continental area roughly the
size of Africa. The image on the left
was taken in late November of 1990
during Mellin's first trip around Venus.
The image on the right was taken 8
months later as the spacecraft flew over
the region for the second time. See the
bright flow-like area that appeared.
It's likely the result of a landslide
which was possibly triggered by a Venus
quake.
On the 13th of October 1994, after
completing one of the most successful
deep space missions ever, the probe made
its final descent into the atmosphere of
the planet it had studied so devoutly
over the last 4 years. As it plunged
towards the surface, it continued to
transmit data about Venus's upper
atmosphere until it was consumed by the
intense heat and pressure.
Over 30 years later, Mellin is still our
primary source of information on Venus.
And as any good mission, it raised as
many questions as it answered. Was there
ever water on Venus? If there was, would
Venus have been habitable?
How has the planet evolved through time?
Is Venus still volcanically active
today? Together, NASA's upcoming Veritas
and Da Vinci missions will set out to
answer them and many more. Set to launch
in 2028 to 2030, they will revolutionize
our understanding of Venus once again.
Da Vinci Plus will study Venus's clouds
during two gravitas flybys and also send
a descent probe deep into Venus's
atmosphere to get a better understanding
of its gases, chemistry, and water
history.
Even though it isn't expected to survive
the landing, the probe will collect
valuable data and images on its
hour-long descent to the planet's
scorched surface. Meanwhile, the Veritas
spacecraft will be a Venus orbiter,
similar to Maven, currently orbiting
Mars. Over the three Earth years it
spends circling our neighbor, its job
will be to search for evidence of
ongoing volcanism and tectonic activity
by measuring heat coming from the
surface and analyzing the composition of
Venus's tesseray.
These findings will help us understand
just how different or similar Venus and
Earth are. Could our neighbor really be
a window into our future? Or is it more
like our hot evil twin?
Mercury,
the solar system's closest planet to the
sun.
Everything I'll show you today will be
an actual picture or video image of
Mercury from the Messenger probe. We'll
discuss Mercury's orbit and rotation,
its physical characteristics,
its surface conditions, and the magnetic
field and magneettosphere of the planet.
I'm Alex Mcolen, and you're watching
Astramm.
Stick with me on this video and you will
learn almost everything you could want
to know about this tiny yet fascinating
planet.
Now, when you think about the physical
characteristics of Mercury, I'm sure you
imagine it being the closest planet to
the sun, but also that it's this giant
rock floating in space. You wouldn't be
too far wrong with that, but it is much
more interesting than what you may first
think. For example, when I look at
Mercury, I do think of our moon, but
Mercury actually is visually more
appealing than our moon. Look at it in
its true color. The first thing that I
notice is that it actually does have a
color. It's not just different shades of
gray. And what else? Well, did you know,
for example, that Mercury consists of
approximately 70% metallic and 30%
silicut materials. So, it's actually
more metallic than rocky. Because of
this, Mercury's density is the second
highest in the solar system at 5.427 427
g per cime cubed, only slightly less
than the planet with the greatest
density, that of Earth at 5.515
g per cime cubed. If Mercury happened to
be the same size as Earth, that would
mean it would pretty much have the same
gravitational pull at its surface. But
being the size that it is, its surface
gravity is only 3.7 m/s squared. If you
were to compare its gravity to Earth, it
would look something like this.
This means the surface gravity of
Mercury is only slightly less than what
it is on Mars. And considering that Mars
is a much bigger planet, that just says
something about the density of Mercury.
But before we leave the subject of
Mercury's size, I want to show you one
last comparison. That of Ganymede and
Titan against Mercury. Now, Ganymede is
the solar system's biggest moon and also
the biggest moon of Jupiter. While Titan
is Saturn's biggest moon and the second
biggest moon in the solar system. These
two giant moons are bigger than Mercury,
as you can see here, but their masses
are far less.
If you look closely at Mercury's
surface, you'll see its appearance is
similar to that of our moon. It shows
extensive marike planes and heavy
cratering indicating that it has been
geologically inactive for billions of
years. But it obviously was geologically
active at one point because one of the
distinctive features of Mercury's
surface is the presence of many narrow
ridges extending up to several hundred
km in length. It's believed that these
were formed as Mercury's core and mantle
cooled and contracted over time when the
crust had already solidified.
And one of the most distinctive things
you'll notice about Mercury is this huge
crater on its surface called Calaris
Basin with a diameter of 1,550
km. The impact that created Calleris
Basin was so powerful it caused lava
eruptions and left a concentric ring
over 2 km tall surrounding the impact
crater. At the antipode of Caleris basin
is a large region of unusually hilly
terrain known as the weird terrain. If
you compare this region to the rest of
Mercury, you can see why it would have
this name. So what's it like on the
surface of Mercury?
Well, to start with, the surface
temperature is hugely different all
over.
It can range from -73°
to over 400°.
It never rises above - 93° at the poles
though because there's no atmosphere
retaining the heat. This means that
there's quite a big difference between
the equator and the poles. But this
variation is also due to its orbit and
rotation, which we will get back to
later.
The subsolar point reaches about 400°
while on the dark side of the planet the
temperatures are on average -63°.
Because Mercury is too small and hot for
its gravity to retain any significant
atmosphere over long periods of time,
it's not able to retain any of the heat
it gets from being so close to the sun,
which is why the dark side of the planet
is so much colder than the side facing
the sun. Mercury however does have an
exosphere which is like an extremely
thin atmospheric-l like volume
surrounding the planet. Molecules in an
exosphere are gravitationally bound to a
planet but the density is so low that it
can't behave like a gas because the
molecules don't collide with each other.
In this picture you can see the
Messenger probe's view of Mercury's
exosphere. When solar wind hits the
planet, it rips off certain atoms out of
the exosphere. And what's left is this
trail of atoms going into space. We call
this the planet's tail. And every planet
has this to a certain extent. Earth even
does have an exosphere, but it starts at
600 km above the surface. It's really
the point where space and the atmosphere
meet. Now, in the case of Mercury, this
exosphere is not at all stable. Atoms
are continuously lost and replenished
from a variety of sources.
NASA has been able to confirm that
craters at the north pole of Mercury
contain water ice. Mercury also has
something which Mars lacks, an actual
magnetosphere or a magnetic field all
around the planet.
It is only about 1.1% as strong as
Earth's, but it's still strong enough to
deflect a lot of the solar wind around
the planet.
Now, we're going to get to one of the
things I find the most interesting about
Mercury, its orbit and its rotation.
Mercury has the most eccentric orbit of
all the planets with a distance from the
sun ranging from 46 million km to 70
million km.
Now, this is something a bit hard to
imagine, but bear with me. Mercury takes
about 88 Earth days to complete an orbit
around the sun. It also has a 32 spin
orbit resonance of the planet's rotation
around its axis. This means it spins
three times around its axis for every
two times that it orbits around the sun.
So although it takes about 59 Earth days
for Mercury to rotate on its axis once,
which is what we call a cidurial day,
this 32 orbital resonance means that if
you were actually standing on Mercury,
it would appear that one day from
sunrise to sunrise or what is called a
solar day is two Mercuran years.
Standing on Mercury, that would look
something like this. you would see the
sun rise relatively fast and then as it
approaches midday it slows down and even
starts going backwards before continuing
on again to sunset. As you can see that
took a whole year which means a
nighttime on Mercury also takes a year.
The sun starts going backwards in the
sky because approximately four Earth
days before perihelion, the speed in
which Mercury travels along its orbit
equals the speed in which it's rotating.
At this point, the sun's apparent motion
stays stationary. A perihelion itself,
Mercury's orbital speed exceeds its
rotational speed. So to a person
actually standing on Mercury, the sun
appears to move backwards. 4 days after
perihelion, the sun's normal motion
resumes.
You can see this even clearer from a
top- down perspective of Mercury. Twice
a day on one of its poles, the sun seems
to pause and then continue on again.
Something else to notice about Mercury's
orbit is that it's inclined by 7° to the
plane of Earth's orbit. As a result of
this, we can only see Mercury transit in
front of the sun when it's directly
between us on Earth and the Sun itself.
And because its orbit is inclined by 7°,
this only happens about once every seven
Earth years. The last thing we'll
discuss about the rotation of Mercury is
that its axle tilt is almost zero with
the best measured value as low as
0.027°.
This is even smaller than that of
Jupiter, which has been measured at
3.1°.
And finally, do you want to see Earth
from Mercury? Well, here we are just a
couple of pixels across. This photo was
taken from the Messenger probe several
years ago, and barring the newborns,
every single one of us was in this
picture.
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