Everyone knows the name of the first of the
planets in our Solar System. Mercury was first seen in ancient times, and
carries the name of a Roman god. We’ve known of its existence for thousands
of years. It is not hidden in darkness, but thanks to
its position next to the Sun is bathed in brilliant light. And yet, it is the least explored terrestrial
planet, having only been visited by two probes in all of our spacefaring history. Why is that? Is it boring? What do we truly know about Mercury? What are its characteristics and history,
and how have we learned about it? I’m Alex McColgan, and you’re watching
Astrum. And in watching this supercut, you’re about
to find out. Join with me as I teach you everything you
might want to know about the first planet in our Solar System, Mercury. 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 colour. The first thing that I notice is that it actually
does have a colour. It's not just different shades of grey. And what else? Well did you know, for example, that Mercury
consists of approximately 70% metallic and 30% silicate materials. It’s actually more metallic than rocky. Because of this, Mercury's density is the
second highest in the solar system at 5.427 grams per centimetre cubed, only slightly
less than the planet with the greatest density - that of Earth at 5.515 grams per centimetre
cubed. If Mercury happened to be the same size as
Earth, that would mean it would have pretty much the same gravitational pull as its surface. But being the size that it is, its surface
gravity is only 3.7 metres per second 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. 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 mare like plains 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 out to several hundred kilometres in length. We’ll talk more about these later. One of the most distinctive things you'll
notice about Mercury is this huge crater on its surface called Caloris Basin, with a diameter
of 1,550 kilometres. The impact that created Caloris Basin was
so powerful it caused lava eruptions and left a concentric ring over two kilometres tall
surrounding the impact crater. At the antipode of Caloris Basin is a large
region of unusual 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 minus 173 degrees Celsius
to over 400 degrees Celsius. It never rises above minus 93 degrees on 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'll get back to later. The subsolar point reaches about 400 degrees
while on the dark side of the planet the temperatures are, on average, minus 163 degrees Celsius. 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 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 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 planets tail, and every planet
has this to a certain extent. Earth even does have an exosphere, but it
starts at 600 kilometres 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, which we’ll discuss in more detail later. 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 percent 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
which I find the most interesting about Mercury. Its orbit and its rotation. Mercury has the most eccentric orbits of all
the planets with its distance from the Sun ranging from 46 million kilometres to 70 million
kilometres. 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 3-2 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. It takes about 59 Earth days for Mercury to
rotate on its axis once, which is what we call a sidereal day. By pure coincidence, this is almost exactly
half its synodic period in respect to Earth, which is 116 days, so between conjunctions
of Earth and Mercury, Mercury rotates on its axis exactly twice. Historically, there was a big problem with
that. Because of this coincidence, we believed that
Mercury was tidally locked to the Sun for the longest time. You see, Mercury orbits closely around the
Sun, meaning it was always tricky for astronomers to get a good look at it for most of its year. When it finally got in a good viewing angle
from our perspective, we’d have a look at the face of the planet. 118 days later, we’d have another look during
this prime observation alignment, and see the same face again. So, to them, it showed that Mercury was tidally
locked to the Sun. What astronomers didn’t realise is that
Mercury had rotated exactly twice on its axis during this time. It wasn’t until radar observations of the
planet that we found out it does rotate slightly faster than it orbits. This 3:2 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 Mercurian 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 night-time 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 is rotating. At this point, the Sun's apparent motion stays
stationary. At perihelion itself, Mercury’s orbital
speed exceeds its rotational speed, so to a person actually standing on Mercury, the
Sun appears to move backwards. Four 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 note about Mercury's orbit
is that its inclined by seven degrees 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 seven
degrees, this only happens about once every 7 Earth years. The last thing we’ll discuss about the rotation
of Mercury is that its axial tilt is almost zero, with the best measured value as low
0.027 degrees. This is even smaller than that of Jupiter
which has been measured at 3.1 degrees. 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 new-borns, every single one of us was in this picture. But what was MESSENGER, and why was it important? Well, let’s start with a little context. When mankind first started sending spacecraft
out to explore the solar system, the first planet to be visited was Venus - our closest
neighbour - in 1962. Next was Mars in 1965, and then Jupiter in
1973. Only then came Mercury in 1974. And already this order might seem a little
odd. The closest distance between the Earth and
Mercury is 77 million km. In fact, it is the closest planet to us on
average. The closest distance between the Earth and
Jupiter is 588 million km, almost 8 times that. And Jupiter was visited again in 1974, twice
in 1979, in 1992, in 1995, in 2000, in 2007. Multiple missions were launched to Saturn,
Uranus, Neptune, to comets and asteroids… while Mercury got nothing for 30 years. Is this because it was deemed uninteresting? Did we discover everything there was to discover
about it with that single, first mission? No. That first mission was a flyby, and only mapped
about 40-45% of Mercury’s surface. Actually, the real reason is that Mercury
is one of the most challenging planets to visit in our entire solar system. Why? Well, as previously mentioned, Mercury exists
in a furnace. Due to its proximity to the Sun, its surface
temperature reaches highs of 430°C, so any probe visiting it would need to be highly
heat resistant. But that same proximity to the Sun means that
any probe launched towards it will accelerate faster and faster due to the immense gravitational
pull from our star, using rocket fuel against that would be like swimming up white-water
rapids. Combatting the Sun’s gravity required too
much fuel for a discovery-class spacecraft to carry, slowing down the spacecraft enough
to be caught up in Mercury’s orbit seemed impossible. It was a question of weight. Weight is a challenging limitation when it
comes to spacecraft. The heavier a craft, the larger a rocket needed
to get it out of Earth’s orbit, and the more expensive everything becomes. Scientists try to keep everything as light
as possible to reduce this cost. As fuel takes up precious weight allocations
that could go towards scientific instruments, scientists try to take only what is necessary
to help them complete their journey. However, for about 30 years, scientists could
think of no way to put enough fuel on a probe to get it to slow down enough to enter Mercury’s
orbit, especially if they wanted scientific equipment onboard too. So, after the success of Mariner 10’s flyby
missions of Mercury in 1974-1975, Mercury exploration was put on hold. But in 1985, an orbital mechanics expert named
Chen-Wan Yen realised that there was a way of getting a probe into orbit around Mercury
that didn’t need new technology. Instead, she had worked out a particular route
an orbiter could take around the solar system that would slow it down enough to enter Mercury’s
orbit with only a few course corrections. Rather than going straight to Mercury, the
orbiter would need to take a longer way. How long? Under Chen-Wan Yen’s model, a craft would
orbit the Sun about 15 times, flying past Earth once, Venus twice, and Mercury 3 times
before finally slowing down enough to enter its orbit on the 4th pass. All these planetary flybys would be essential
– by skimming the planets’ atmospheres, vital speed could be shaved off, from atmospheric
drag and due to the gravity of the planets. The entire route would cover a mammoth 7.9
billion km and would take 6 and a half years. Chen-Wan Yen’s findings were not immediately
picked up, but in 1998 NASA began to take an interest in the idea and after seeing the
feasibility of the route, they launched the MESSENGER probe in 2004. MESSENGER, or the Mercury Surface, Space Environment,
Geochemistry, and Ranging probe was only 1.8m long and 1.3m wide and weighed 1,100 kg. This is small and light for a typical NASA
mission. Just a comparison, Juno is 20m long. MESSENGER came equipped with a powerful thruster,
a ceramic heat shield to protect it from the Sun, two solar panels and a whole suite of
scientific equipment for imaging and measuring data from Mercury. Scientists hoped to take advantage of this
opportunity to learn as much as they could about the chemical composition of Mercury’s
surface, its geological history, its magnetic field, and its core (among other things). MESSENGER spent its first year in space making
one orbit around the Sun, before meeting back up again with Earth. This gave scientists a chance to test its
equipment on a known astronomical body, to make sure there weren’t any errors and to
make any adjustments as needed. MESSENGER took some photos of Earth and the
Moon, and also tested its other equipment to take readings of our atmosphere and magnetosphere. Fortunately, everything was working perfectly. As it began to head further sunward, MESSENGER
employed a clever technique to help reduce its acceleration towards the Sun. It used its solar panels to “catch” solar
radiation, like sails on a ship might catch wind. Solar radiation hitting an object actually
pushes it very slightly. While this force is very tiny, because MESSENGER’s
journey was so long, it really added up. Making the most of this phenomenon was one
of the ways MESSENGER saved propellant and decelerated naturally. The next notable landmark in MESSENGER’s
journey came in 2006, when it did its first flyby of Venus. Sadly, for scientists, this moment came at
a time where Venus was exactly on the opposite side of the Sun from Earth, which meant MESSENGER
was not in radio contact! It did take some photos of the planet which
it later sent, but otherwise performed no science. However, in 2007, it passed Venus again. At that time another spacecraft was orbiting
Venus – ESA’s Venus Express. MESSENGER and the Venus Express took the opportunity
to work together, performing the first ever simultaneous measurements of particle-and-field
characteristics of the planet. But then it was on to the main event: Mercury. MESSENGER made its first flyby of Mercury
on 14th January 2008, with everything going smoothly. The same was true of the second flyby. But during the 3rd flyby in 2009, something
went wrong. MESSENGER went into safe mode, which was designed
to protect systems on the craft in the event of an error. How disappointing to have come so far, only
for the mission to potentially fail during one of the final stages! MESSENGER remained in safe mode for what must
have been 7 hours of stress for all of the scientists involved. You see, MESSENGER had to pass through Mercury’s
shadow during this flyby, meaning it had to rely on its batteries for 18 minutes. Something wasn’t configured right in the
power management part of the software. Fortunately, MESSENGER’s computer reset
once power from the panels charged the battery, and it was able to continue with its mission,
swinging around the Sun one more time before finally entering orbit around Mercury on the
11th of March 2011. MESSENGER took up an elliptical orbit around
Mercury, alternating between as close as 200 km and as far away as 15,000 km. This is because Mercury acts sort of like
a giant sun mirror, radiating heat back into space. Remaining too close to Mercury was too hot
for MESSENGER, even with its heat shield (which was more designed to protect it from the Sun
– 7 times brighter by Mercury than it is on Earth), so moving further away every 12
hours gave it a chance to cool off. MESSENGER spent the next 4 years in Mercury’s
orbit, far exceeding scientists hopes and expectations for the mission, as they had
originally been planned for it to only last 1 year. Before launch, scientists had hoped that MESSENGER
would take at least 1,000 photos over the course of its lifetime. However, MESSENGER took over 200,000 photographs,
giving us a complete map of Mercury’s surface in high resolution and colour, as well as
photographing nearby comets, and other planets. On 25th December 2014 MESSENGER’s propellant
– so carefully saved up until that point – was finally about to run out. By this point, MESSENGER was orbiting a mere
25km from the surface of the planet. Scientists gave the thrusters one last burst
to extend its orbit for as long as possible, but on April 30th, 2015, MESSENGER crashed
into the surface of Mercury. After a journey that had lasted over a decade,
and had covered literally billions of kilometres, MESSENGER’s journey had come to an end. MESSENGER gave us a wealth of insights into
Mercury before it died. On board MESSENGER were a host of scientific
instruments, including a magnetometer to map out Mercury’s magnetic field, [Deleted,
sentences before and after are now merged and need new VO.] which is thought to be generated
by a dynamo effect in its molten core. Our fast rotation and tidal stretching from
our Moon keeps our core molten, but Mercury doesn’t have a moon, or a fast rotation. What it does have, however, is an eccentric
orbit, more so than any other planet. Gravitational strength increases and decreases
as it gets closer and further away from the Sun, so the tidal forces pull and squeeze
on the planet, the friction of which keeps Mercury’s core hot and the dynamo going. Unlike Earth’s, it is offset from the centre
by about 20% of the planet’s radius, and we don’t really know why. Its magnetic field is only about 1% as strong
as Earth’s, but this still has an impact on deflecting a lot of the solar wind around
the planet. However, due to it being closer to the Sun,
the solar wind pressure is a lot greater here than it is around Earth. Add a weak magnetic field to the mix, and
the magnetosphere around Mercury is compressed closely to the planet’s surface. Earth’s, on the other hand, extends many
times the diameter of the planet away from the surface. Interestingly, these factors make the magnetosphere
of Mercury highly dynamic. What does this entail? Well, for one, reconnection events are 100
times more common around Mercury than around Earth. Reconnection events occur when magnetic field
lines snap together as the charged solar wind pushes against a planet’s magnetosphere. When this happens, it allows a few of these
charged particles to break into the planet’s magnetosphere, entering a region of plasma
in the planet’s magnetotail. The flows you see in this simulation in the
plasma region are from reconnection events. Another feature of the magnetosphere that
MESSENGER detected was energetic bursts of electrons, producing hundreds of thousands
of electron volts of energy. As MESSENGER orbited Mercury, it picked up
thousands of these events, and mysteriously, they were mainly localised in the northern
hemisphere, and were compressed towards the planet along the sun facing side. This is still an ongoing field of study; however,
scientists believe these electrons have been accelerated through breakdowns in the magnetotail,
and they follow the direction of the magnetic field around from the south pole to the north. MESSENGER also hosted a wide array of spectrometers. Spectrometers are important for detecting
the composition of mineral deposits on the surface without having to actually take a
sample. Spectrometers can also be used to detect the
particles in the atmosphere. Now, Mercury doesn’t have an atmosphere
per se, but as previously mentioned it has an exosphere, or an extremely tenuous atmosphere. It is so thin that the particles within it
don’t interact with each other. But what MESSENGER found out about this exosphere’s
relationship to the surface really surprised scientists. Mercury is covered with volatile substances,
it isn’t just a fried, rocky planet. It seems to be covered in potassium, magnesium,
sulphur, sodium and chlorine, at a higher level than any other terrestrial planet, and
much higher than on our Moon. The fact that its volatile ratios have more
in common with Mars than with Earth and Venus have completed disproved a lot of solar system
formation theories that existed before MESSENGER arrived at Mercury. These volatiles are blasted by radiation from
the Sun, more so at the equator than near the poles, which may explain why on the surface,
some substances, like potassium, are more abundant in the northern hemisphere than around
the equator. It is much hotter on Mercury around its equator
than at the poles, so the potassium there would have been heated enough that much of
it has been lost from the surface to the exosphere. Now, the exosphere contains a lot of the particles
you would also find on the surface, like sodium, potassium, and the others I mentioned. This exosphere is not at all stable. Solar wind picks up and carries away a lot
of charged particles, and solar light pressure also pushes a lot of the neutral particles
away. Were it not for the processes that replenish
the exosphere, Mercury would lose it all to space over a relatively short time frame. While most substances certainly do come from
the planet’s surface, it also contains other elements, like hydrogen and helium, which
cannot be found there. So where did they come from? Well, as you may know, the Sun is made predominately
of hydrogen and helium, and interestingly, the solar wind carries these particles to
Mercury. Some of the solar wind actually gets caught
up in the exosphere and stays for a while. As far as we know, this is the only major
source of hydrogen and helium in the exosphere. The particles that do get stripped away have
been spotted by MESSENGER too. In these images, we see calcium, an unknown
process of which means it’s much more prevalent in the exosphere during the planet’s dawn
than dusk, and magnesium streaming away from the night side of the planet. In fact, Mercury’s tail has been known about
for a while. In these images, sodium ions are lit up as
they stream away from the planet, making Mercury look like a comet. Incredibly, if you were to look up into the
night sky on Mercury, you would actually see a faint yellow glow, reminiscent of city lights
on Earth. This tail is seasonal. The eccentric orbit of Mercury means that
its distance to the Sun varies throughout its year, and as it orbits its orbital speed
also changes. So, the time of greatest sodium emission is
actually when Mercury is at its middle distance from the Sun. There was one other curious substance found
in Mercury’s exosphere that scientists really weren’t expecting. Water vapour. This could come from cometary tails as they
pass by, or it could come from ice deposits MESSENGER detected around the planet’s poles. Surprisingly, water ice can exist on this
scorched planet, but only at the bottom of permanently shadowed craters, forever protected
from directly interacting with the Sun’s light rays. The Earth-based Arecibo Radio Telescope had
already detected highly reflective regions around the poles, and as images from MESSENGER
came in, these regions matched up with regions of permanent shadow at the bottom of large
craters. Estimates put the amount of water ice found
on Mercury at a quadrillion kgs. This isn’t huge by Earth standards, but
it would be a significant boost to any future colony there to have that much water accessible. There were some other surprising features
found on Mercury’s surface too. Hollows were found dispersed all over. This is a feature unique to Mercury. While we aren’t completely sure what causes
them, they may be volatile substances sublimating, and they are unique to Mercury simply due
to the proximity of Mercury to the Sun. They seem to be an active geological process,
apparently some of the youngest features on the planet, and they are certainly not the
result of meteor impacts. There is much more going on on Mercury’s
surface – ancient dried up lava flows, evidence of volcanic activity. Craters from massive asteroid strikes that
warmed its surface. And, surprisingly, the thin scarps that were
evidence of its gradual cooling. These scarps show that Mercury is contracting, and from MESSENGER's data, Mercury has contracted by over 14km in diameter since its formation - a lot more than what was expected. All these findings have thrilled scientists,
yet even though we would barely know anything about Mercury were it not for MESSENGER, somehow
this mission isn’t that well known among the general public. Perhaps ESA’s BepiColombo mission, already
on its way to Mercury right now, will better capture the public’s imagination when it
arrives in 2025. Let’s go back to when the planet was warmer. So warm in fact, that it becomes necessary
to ask an important question: What happens when a planet melts? For Mercury, the closest planet to our sun,
this is no idle question. At the risk of it being understated, Mercury
is a very hot planet. With daytime temperatures reaching an incredible
430°C, the temperature of some wood-burning fires, the rocks and dust on Mercury’s surface
bake beneath a blistering heat that pushes them towards their limits. It’s not the hottest planet in the solar
system – that honour goes to Venus, thanks to its thick atmosphere – but it’s certainly
up there. The Mercury we know today has actually cooled
considerably over the years. There is ice at its polar caps, and we discussed
the signs on its surface that show it has contracted over time as its interior became
colder. So, what was it like back then?When rock is
sufficiently heated, its solid structure breaks down and it turns into the gloopy, viscous
liquid known as magma, with a viscosity – or “runniness” – 10,000 to 100,000 times
more viscous than water. For a point of reference, this is a similar
viscosity to tomato ketchup, although I would not recommend putting this on your food. Depending on the rock type, magma forms at
temperatures of at least 600°C, but potentially as high as 1300°C. So, for Mercury to have
begun to melt, we know that it must have reached at least these temperatures. In spite of being much less runny than water,
lava can still travel for great distances before stopping. This is because once the surface of lava hardens,
it forms an insulating layer that keeps the rest of the lava within protected so it can
flow freely. How do we know this happened on Mercury? The clues can be found in craters like Raditladi. Scientists estimate that Raditladi is a relatively
young crater – likely under a billion years old - with well-preserved walls and a floor
relatively clear of other, later impacts. It’s large – over 25km in diameter. Notice how rough the hills are around the
crater, and yet inside is a smooth plain? This is no coincidence. Originally, the terrain inside Raditladi was
likely about as rugged as the hills around it. So why is it so smooth now? The answer is lava. When lava is left on its own, it will try
to form the flattest surface possible, just like water does if you put it in a bowl, as
it is dragged down under the effects of gravity. The same happened here: an asteroid crashed
into the planet’s surface, and the crater quickly filled with lava. Once the lava inside cooled, it formed the
smooth plain you see here. But where did this lava come from? There are two theories. The first is that the impact of the meteor
triggered a creeping volcanic eruption, as magma from beneath the surface rose up through
cracks to fill the basin. The second explanation is that the surface
within the crater got so hot due to the impact of the meteor, that it pushed the already
hot rock crust over the tipping point into melting. This kind of lava is known as impact melt. The true explanation is likely a combination
of both. Now that we know that smoothness is a sign
of lava flow, we suddenly realise that there are numerous other craters on Mercury that
similarly must have been filled with lava. Just take a look at Rustaveli, where crags
of mountain can be seen poking up through the smooth lava layer: Or Copland. Polygnotus. Or Rachmaninov. Rachmaninov is particularly interesting, as
here you can see the stronger indicators of lava bubbling up through from beneath the
surface to the centre of the crater. Take a look at the strange crinkled cracks
forming a rough circle inside the central crater. Such cracks are a signal that a slower outpouring
of magma pushed up from beneath the surface, breaking the plain, then cooling, then pushing
up and then cooling again under the effects of Mercury’s fluctuating temperature. Here, and in many of these impact craters,
the collisions from space triggered deep volcanic activity from within Mercury’s shell. But lava didn’t just flow within the craters. Take a look at the valley known as Angkor
Vallis. Here you can see clear signs of smooth lava
flow, but this time moving like a river. The lava travelled from high to low ground,
until it eventually poured into the basin next to it. Flows like these ended up filling massive
seas, taking up vast swathes of the planet and turning them the more orangey colour we
see today. Scientists have begun to recognise this tell-tale
orange colour as a sure sign of volcanic activity, and from it a more detailed picture has begun
to emerge of conditions on early Mercury, that make it even less hospitable. Areas like this one to the north east of Rachmaninov
are likely formed by volcanic activity. When MESSENGER flew over this area in 2015,
it took detailed photographs of it, and found the surface to be covered in a fine dust:
Upon review, it was obvious what this dust was – volcanic ash, that must have fired
out of vents and covered the terrain around it. NASA scientists likened it to snow – “fiery,
hot, angry snow”. So, it wasn’t just lava flowing beneath
your feet you’d have to contend with on Mercury, but burning ash falling from the
sky. And that was just the calmer volcanoes. The final indicator of volcanic activity on
Mercury hints at eruptions so destructive that whole chunks were scooped out of the
planet. Take a look at this crater Navoi. This is no impact crater. When a crater is formed onto a hard surface,
one that’s not sufficiently hot to melt into lava, a central peak is usually formed. This is because when the crater walls suddenly
find themselves exposed, gravity suddenly exerts itself on all that loose particulate,
which rushes down the walls of the newly scooped out basin towards the centre. Once there, having built up momentum, it comes
crashing into all the rocks and landslide that is sliding down from the other side of
the crater. The two sides meet, and all that momentum
and energy forces them to keep moving in the only direction they can – up. You see this same effect more clearly when
you throw a large rock into water. The water of the newly formed basin rushes
in to fill the gap, but then crashes into water from the other side and all of it shoots
upward in a powerful secondary splash. But unlike water, the sand and loose rock
of a crater does not level out, but forms a central peak. Depending on what angle the meteor impacted,
this peak is either perfectly rounded, or possibly tear-drop-shaped. However, the raised central formation of Navoi
is neither of these things. As scientists looked at this, they came to
the conclusion left, that this crater was not formed by an impact at all. Instead, it had been carved out through the
force of an erupting volcano. At 66km in diameter, the amount of force exploding
upwards that would have been necessary to carve out this crater and scatter its remnants
for kilometres all around it must have been truly massive. So there you have it. Meteors raining from the sky, tipping the
rocks they landed on over the melting point. Volcanoes bursting forth, either filling the
landscape slowly with bubbling magma in lakes and fiery rivers, or choking the air with
burning ash – not that there was any air to begin with, beyond thick toxic gases emitted
with the eruptions. And even the ground you could stand on might
at any moment explode beneath your feet. This is what it was like when a planet was
melting. Mercury is quiet now. As near as we can tell, there are no longer
any active volcanoes on the planet. Although the sun still bakes down on it, the
unbridled fury that raged beneath its surface is now calm and soothed. Yet, for all those who know how to look, the
evidence of what once was is still there, locked in the geological record. It’s the scars that tell the story of a
violent past. When something is as incredibly difficult
to get to as Mercury, it is extremely tricky to study. Which is one of the reasons why in all of
human history, there have only ever been 2 missions to Mercury, with just one more on
the way. Mariner 10 in 1974, MESSENGER in 2011 and
BepiColombo due to arrive in 2025. Of these prior two missions, only MESSENGER
went into orbit around Mercury, and so it is the only mission to ever give us close
up shots of Mercury’s surface. And crazily enough, some of the formations
we’ve seen on the surface are still unsolved mysteries even more than a decade on, while
other formations give us hints at the raw primal power of the early solar system. So let’s finish by taking a look at some
of those mysteries and formations, and see what answers we can find. When you look at the surface of Mercury, there
are a few features that immediately jump out at you. First, its colour. Mercury’s colour is not actually monochrome,
and is smattered with speckled greys, creams and beiges, with lighter sections and lines. These darker sections are believed to indicate
high levels of graphite, the same material used in pencils! And the lighter sections? Well, we’ll get onto them later. Beyond that, you most likely noticed the craters. Much like the moon, Mercury is covered with
craters, as ancient pieces of space debris crashed down on the unprotected planet with
a roughly even distribution. These offer fascinating insights into the
planet’s violent history. You can get a sense for how old a crater likely
is by how sharp its crater rims are. Sharp and crisp rims are likely a lot more
recent than the older, rougher rims that have had more time to erode down due to the natural
processes happening on the planet. Sometimes asteroids strike within the same
place as older collisions, creating overlapping craters of differing ages, such as the craters
here. But it is in the difference between these
older and younger craters that we get our first fascinating clue about the surface of
Mercury. It is an active, flowing place. Although there is no real atmosphere to produce
the weathering we would imagine, evidently things on the planet’s surface do not remain
static on an astronomical time scale. As previously mentioned, Mercury is cooling,
and as it does so its surface bunches together in kilometer-long scarps. But Mercury’s surface is not just crumpling. It is also smoothing out. In this crater, there is evidence of slumping
having taken place. While about 90 degrees of the crater wall
has retained its shape, the remaining 270 degrees has slipped further into the crater
bottom, breaking away from the rest of the rim under the force of its own weight. Scientists are not entirely sure why this
happened to only some of the crater and not all of it. Is the soil particularly hard in that bottom
right corner? Was it something to do with the angle the
impactor struck at? We don’t really know. And that’s one of the things that is so
intriguing about Merury – there is still so much more to discover. Here's another interesting phenomenon. In my videos about the Moon, I mentioned crater
rays, or ray systems. These spidery lines that radiate out from
certain craters are a prominent part of Mercury’s surface as well, with some stretching over
400km across. Their lighter colour is a sign that the material
kicked up from under Mercury’s surface is less graphite-rich, or at least is a different
chemical composition to the Sun-exposed surface. But did you know that for a while, scientists
could not account for how these spidery lines were formed? When they tested different weights, consistencies
of terrain, and speeds of impact, they were unable to recreate these patterns in lab conditions. Whatever they tried, the material they kicked
up would always return back down in a consistent circle, not thin, spider web lines. Scientists wracked their brains for years,
but now it seems that this mystey has been solved. In 2018, a scientist called Tapan Sabuwala
was scouring the internet and discovered that a group of students had managed to recreate
the spidery line pattern of crater rays. Sabuwala was excited, but also confused. Why were these students able to manage what
other scientists had not? Interestingly, he realised this was an example
of scientists being too neat. Before performing their tests in lab conditions,
Sabuwala and other researchers had always prepped the experiment by smoothing out the
sand their test asteroid was impacting into. The students had not done this step, leaving
their test surface rough. This made sense in hindsight, as it more closely
mimics the rough terrain on the surface of an alien planet. And as it turns out, this was the entire key
to how these rays were formed. Crater rays do not care about the speed of
impact, the angle or the composition of the crust. They only care about the surface shape and
how rough it is. Knowing that this is how these lines are formed,
it really opens your eyes to the scope of some of the impacts that have struck Mercury
in its past. Remember I mentioned how some of these ray
systems stretch over 400km? That was just the smaller ones. Just look at the ray system originating from
the crater known as Hokusai. These rays must have been created from an
incredible impact, as their lines stretch almost entirely around Mercury’s surface
– which, by the way, has a circumference of 15,000km. And although not quite as large, the ray system
of the crater Debussy covers over 1000km. While the Moon also has ray systems, they
usually smaller than these. In fact, one of the main visual distinguishing
aspects of Mercury are these giant ray systems. These show us that one of the formative processes
that explain Mercury’s unique surface is incredibly powerful bombardments. Seeing as the Sun is so nearby, objects caught
in this intense gravitational pull would crash into Mercury with far more force than Mercury
could produce with its own gravity. Mercury’s gravitational pull is so weak
compared to the Sun that Mercury cannot normally capture objects as moons – they get pulled
past, instead. This is one of the reasons why visiting Mercury
is so difficult for spacecraft. But that’s not to say that Mercury can’t
stop such an object reaching the Sun, it just has to bodyblock it. Yet, this explanation cannot explain this
last formation. Take a look at one of the most fascinating
craters on Mercury – Apollodorus, and its surrounding Pantheon Fossae. At first glance, you might think that there
is nothing to unusual about the crater Apollodorus. Yes, those fractures running out from the
centre are a little unusual. There are a few features that are odd here. To begin with, the fractures, and their surrounding
radial fractures bear a strange resemblence to fractured glass. Glass fractures in this way due to its hard
but brittle qualities. As you have seen, most other craters we have
seen on Mercury do not follow this pattern. The crust of Mercury does not tend to fracture,
but instead sprays in ray systems or just leaves perfectly round craters. This is in keeping with a looser material
make-up. Sand does not fracture when hit. We do not see this fracturing anywhere else
on the planet either, so something unusual is clearly happening here. Was the surface of Mercury particularly cold
and hard when this impact occurred, thus making it more brittle? Mercury’s nights can get as cold as -180
degrees C. But if that was so, why has this not happened in other places? About half the impacts should be hitting Mercury’s
night side, at least. When we take a closer look at Apollodorus,
the crater you might assume is the cause of the fractures, we notice something even stranger. Apollodorus is not quite the epicentre. While it’s pretty close, doesn’t actually
line up. This might suggest that Apollodorus and the
spidery fractures of Pantheon Fossae are actually unrelated. Whatever caused this phenomenon may have happened,
only to later be hit by an asteroid near to - but not on - its epicentre. But if that’s so, what caused Pantheon Fossae? The intriguing thing is that we don’t know. Evidently, something created this fractured
glass shape, but left no crater. Might that imply this is the result of not
something hitting it from above, but pushing its way up from beneath? Perhaps this is the result of immense volcanic
activity, suddenly pressing up and cracking the crust? That’s just my guess. There are still many mysteries to be found
on Mercury’s surface, and many other fascinating insights to be gleaned as science advances. When ESA’s BepiColombo arrives at Mercury
in 2025, it will begin another extensive study of the planet, and perhaps then we will have
the answers. BepiColombo will uncover the characteristics
of Mercury’s magnetosphere and exosphere, and will take a clearer look at its geology
and composition. But until then, scientists will continue to
pour over the data we have. For now, Mercury endures; baked in its solar
furnace. It has survived there for millions of years,
and will likely survive for millions of years, in spite of all that the Sun and the solar
system have to throw at it. The hellish conditions of its environment
might make it challenging to get to, but there is no denying its resilience, shown in its
charred, cratered beauty. Perhaps one day we will know all there is
to know about the first of the Solar System’s planets. But that day is not yet. In spite of it being the most illuminated
planet in the Solar System (thanks to its location), there is still plenty of light
to shed on Mercury. Thanks for watching. If you enjoyed this supercut, be sure to check
out my others in this playlist here. A big thanks to my patrons and members. If you want to support the channel and have
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