Having the sun as your neighbor is hard.
Searing hot temperatures, deadly
radiation, and the onslaught of meteor
collisions don't exactly make it the
most welcoming part of the solar system.
As you can imagine, these challenges
make Mercury a particularly tricky place
to get to. For this reason, even though
it's much closer to us than Jupiter or
Saturn, Mercury remains relatively
unexplored.
Still, scientists managed to send two
different missions out to study Mercury.
Mariner 10 flew by in the 1970s,
followed by NASA's Messenger in 2015,
which was the first probe to ever
successfully orbit the planet. And now,
as I speak, a third Mercury mission is
hurtling through the constellation of
Pisces, over 177 million km away from
Earth. Bey Columbo, a labor of love
between the European Space Agency and
the Japanese Aerospace Exploration
Agency is set to arrive on December 5th,
2025.
It will carry out a comprehensive study
of the planet's magnetic field, surface,
and internal structure.
But as they say, the journey is the
destination. And Bey Columbo's journey
is a long one.
I'm Alex Mccoan, and you're watching
Astramm. Join me today as we reveal what
Bey Columbo has revealed of Mercury so
far, why it's traveling farther than the
distance of Pluto to get there, and what
we're hoping to learn about the closest
planet to the sun.
On October 20th, 2018, Beep Columbbo
took off from the French Gana Space
Center, beginning its 7-year journey to
Mercury.
Its trajectory would take it around the
sun 18 times, including nine planet
flybys. The first was an extremely close
flyby of Earth on April 10th, 2020. Bey
Columbo came within just 12,700
km of our planet surface about the
distance from Copenhagen to Santiago,
Chile. Its journey will then take it on
two flybys of Venus and six of Mercury.
The last three of which are due to
happen in September, December, and
January of next year. In total, the
mighty spacecraft will cover a distance
of 8.5 billion km.
Taking such a long route was the only
way Beep Columbbo could slow down enough
before approaching Mercury's orbit. But
why is slowing down so important? Well,
if the spacecraft arrived with too much
speed, it would swing right past the
planet and get sucked into the sun's
powerful gravitational field instead.
Bey Columbo was designed to complement
and deepen the findings of its
predecessor, the Messenger probe. While
Bey Columbo carries many of the same
instruments as Messenger did, it is
equipped with higher caliber tools able
to obtain larger ranges of data.
Additionally, Messenger's orbit of
Mercury was rather one-sided as perapsis
was over the South Pole, meaning we have
great detail and mapping of the northern
hemisphere and almost non-existent data
on the southern hemisphere. Beep
Columbbo plans to rectify this by
examining Mercury from an orbital angle
more in line with the planet's equator.
Beep Columbbo is a very unique
spacecraft. It's made up of three
component parts, two probes and a
propulsion module which were launched
and travel together in a united
configuration known as the Mercury
Cruise System run by issa. The Mercury
Planetary Orbiter, or MO, will study the
planet's surface and interior, while
JAX's Mercury Magneettosphere Orbiter,
affectionately known as Mio, will study
the planet's suspiciously low magnetic
field that has baffled scientists for
decades. The name of Mio was actually
chosen by the Japanese general public.
It means a waterway, and according to
Jaxa, it symbolizes the scientific
milestones reached so far. and wishes
for safe travels to come. Mio and the MO
sit either side of an interplanetary
propulsion unit called the messenger
transfer module. Stacking the different
parts of the craft together like this
means most, but not all of the
scientific instruments on board will be
inoperative until Beyombo settles into
its orbit around Mercury. At that point,
the MO and MO will be jettisoned from
the spacecraft and go their separate
ways to study Mercury from different
vantage points. But don't worry, there's
still enough data being sent back to
keep us entertained. Why don't we take a
look back at how far this mission has
come, what it's already taught us, and
explore some of the amazing things it's
seen along the way.
[Music]
Beep Columbo's first encounter with
Mercury came in October 2021.
Passing just 200 km above the planet's
surface, it sent back some stunning
images. As it drifted past, Bey Columbo
snapped Mercury's northern hemisphere.
To the average person, it might look
like our rocky, inert moon, but the
trained eye sees the Calino crater,
which could hold the key to deciphering
Mercury's crust composition. The
Lamontto crater hosts traces of volcanic
activity and mysterious pockets where an
unidentified compound in the planet's
crust is being lost to space, and
scientists have no idea how or why.
These images remind us just how
enigmatic Mercury is and how much we
have left to learn about it. Another
camera mounted on the spacecraft caught
Mercury from another perspective, one
that left the planet's volcanic history
on full display.
Previously lava flooded and over 250 km
wide, the Hayden Crater comes into view
along with the Pampuoo Facula, one of
Mercury's many bright spots, probably
formed by volcanic eruptions from
billions of years ago.
Even though Messenger had already shown
us Mercury's volcanic past, it's still
pretty amazing that this activity left a
mark visible after such an inconceivable
amount of time. And Bevy Columbbo didn't
just show us how the flyby looked, it
captured how the flyby felt.
You know how your phone has an
accelerometer in it that helps it know
when and how its orientation changes?
Scientists mounted a similar
accelerometer on Bey Columbo's MO. It
measured the movements and vibrations of
the spacecraft as it flew past Mercury,
creating a sort of sound map of the
flyby. Scientists then plotted that data
to frequencies us humans can actually
hear. So you can actually hear the
flyby. I'll play it for you now. Pay
attention to the two distinct sounds
towards the beginning. They represent
the changes in solar radiation pressure
when the spacecraft enters and then
exits Mercury shadow. The big clunk
towards the end is the sound of the
feebus instrument assuming its parking
position. Here's the clip.
[Music]
Pretty
[Music]
cool, right? I've got another one and
it's even better. Pepe Columbombo
captured magnetic field data during its
flyby that was also later deciphered and
sonified by scientists on the ground.
This one is a little more abstract than
the other, but we'll give it a go.
You're going to hear two synthesizers at
the same time. The first synthesizer
changes pitch with the changing
magnitude of the background magnetic
field. A lower pitch means more
intensity in the magnetic field. The
second synthesizer changes pitch with
the variations of the magnetic field
magnitude. A more turbulent magnetic
field is represented by a faster change
in pitch. The dotted vertical line shows
where the closest approach was. After
the closest approach, you'll hear
something that sounds like a DJ
scratching a disc. Lots of fast changes
in pitch. That right there marks when
the spacecraft crosses the magneetto
sheath where the magnetic field is
highly turbulent. Let's have a listen.
[Music]
[Applause]
On June 23rd, 2022, just over halfway in
its planned journey, Bebby Columbo
crossed paths with Mercury once again.
This time, the spacecraft turned its
gaze towards Mercury's largest impact
crater, Calleris Basin. At a whopping
1,550
km wide, it's actually one of the widest
known craters in the solar system.
Kaleris Basin was likely created by the
collision of a comet, asteroid, or other
celestial object some 3.9 billion years
ago. The highly reflective lava craters
on its floor make it easy to see as the
bright semic-ircular area of Mercury's
disc below the magneettometer boom. The
second flyby gave us beautiful images of
Mercury's craters like we'd never seen
before. Like this one of Hiani, a crater
embedded in smooth volcanic planes
potentially near an ancient volcano
which Bey Columbo plans to examine when
it finally settles into orbit.
Almost exactly a year later on June
19th, 2023, Beyblow reapproached Mercury
from a height of just 263 km. The flyby
images captured the 600 km long Beagle
Rupes, one of the many lobate scarves on
Mercury that was first imaged by
Messenger in 2008.
Other than our moon, Mercury is the only
planetary body in our solar system known
to exhibit this tectonic signature.
When the interior of the planetary body
cools, it leads to a global contraction
and surface expression of a thrust
fault, aka a lobate scarp. Beagle Rupes
has an especially pronounced curvature,
more so than other scarps on Mercury,
offering geologists a rare insight into
the planet's tectonic history. As a
result of the third Bey Columbo flyby, a
large 218 km wide peak ring impact
crater received the name Manley after
20th century visual artist Edna Manley.
The original basin floor was flooded by
lava, given the crater its visibly
smooth interior texture. Baby Columbbo
will explore Manley Crater further from
orbit, seeking to measure its carbon
composition and detect other minerals
that may be present in order to learn
more about Mercury's geological history.
Gliding past Mercury on its most recent
flyby, Bey Columbo snapped a series of
farewell Mercury images. At 236 km from
the surface, Baby Columbbo appears to
hug the planet goodbye. Scientists back
home on Earth will have to wait another
16 months for the next set of images.
Now, these flybys are more than an
indulgence for our senses. They also
revealed important nuances of the
planet's particle environment and
magnetic boundaries. One of the
instruments aboard the MO probe is
Feebas probing of Herian exosphere by
ultraviolet spectroscopy.
Feebas uses UV emission from Mercury's
exosphere, the planet's outermost and
only atmospheric layer to determine its
composition, structure, and interactions
with the surface. It does this by
detecting specific types of light
emissions in the ultraviolet spectrum,
ranging from 55 to 350 nanome. By
capturing these light emissions across
the whole spectrum, it helps us figure
out what the exosphere is made of and
how its particles are spread out. As
Beep Columbbo emerged from Mercury
shadow, Feebas detected the presence of
hydrogen and calcium, confirming earlier
findings from Messenger that established
the presence of calcium in the planet's
exosphere, particularly around the
equator and on its night side.
Interestingly, this is the opposite of
the distribution observed for sodium and
potassium, which more readily
concentrate around the poles. Once in
orbit around Mercury, Feebas will be
able to send back much more detailed
analysis of Mercury's exosphere,
composition, and dynamics, including how
it changes with location and time.
Speaking of Messenger, back in its day,
it discovered ice on Mercury's North
Pole. Bev Columbo will continue this
legacy by continuing the search for ice
in permanently shadowed regions of high
latitude craters.
As you might know, Mercury is the only
planet in our solar system to have a
magnetic field similar to Earth's.
Thanks to its liquid iron core, Mercury
exhibits a dynamo effect, which creates
an innate magnetic field around the
planet. However, Mercury's magnetic
field is just 1.1% that of Earth's
magnetic field, leaving scientists
fascinated with how it's generated. One
of the big goals of the Bey Columbo
mission is understanding this magnetic
field better. So far, the only magnetic
activity of the Northern Hemisphere has
been mapped by NASA's Messenger mission.
Scientists hope that Bey Columbo will be
able to shed light on the dynamics of
the planet's magnetism in the southern
hemisphere. This magnetic field is of
great importance. By understanding it,
scientists aim to shed light on
Mercury's origins, its evolutionary
journey, and the current state of the
planet's interior.
Mo and Mio will travel through different
areas of Mercury's magneettosphere on
different time scales, simultaneously
measuring how the magnetic field changes
over time and space and its relationship
to the powerful solar wind. All this new
data from the Bey Columbo mission will
eventually be compared with global
magnetic field models from the Messenger
mission to build the most accurate
picture yet of Mercury's magnetic field,
which could unlock telling insights into
the planet's elusive past.
As we await Bev Columbo's next flyby in
September 2024, the journey to
understanding Mercury continues. The
spacecraft is gearing up for its next
major maneuver, a prolonged solar
electric propulsion thruster ark planned
to begin in early August and it should
last until midepptember.
Thruster arcs along with strategic
flybys help Epic Columbbo break against
the enormous gravitational pole of the
sun. This is the only way to
successfully insert into orbit around
Mercury when the time comes. Once in
orbit in December 2025, the real
exploration begins. Mercury's heavily
crated surface records a 4.6 billionyear
history of asteroid and comet
bombardment, which together with unique
tectonic and volcanic curiosities will
help scientists unlock the secrets of
the planet's place in solar system
evolution.
Beb Columbbo is set to remain in orbit
around Mercury for one year and if all
goes well might get the green light for
an extra year on the mission. Scientists
hope to study the planet like never
before. From its composition to its
geoysics, its atmosphere and
magneettosphere, both its history and
the history of other inner planets like
it, including Earth. Until then, we've
got a few more flybys to look forward to
and the haunting pictures they're bound
to capture for us Earthlings back home.
Imagine yourself standing on the surface
of Mercury, but not the Mercury you
usually picture. Not the scorched
sunblasted daytime inferno. Instead,
you're here during the planet's
incredibly long night. The sun set weeks
ago, and it won't rise for weeks more.
Above you, the stars are stark and
brilliant against a perfect black,
untouched by atmosphere haze. The ground
beneath your heavily insulated boots is
rigid, radiating away the day's heat
into the vacuum of space. It's a silent,
desolate landscape. But tonight, you've
brought special equipment, not just for
survival, but for observation. You point
a sensitive detector tuned to a specific
wavelength of light towards the anti-
solar point in the sky. The direction
directly opposite where the sun lurks
below the horizon. And you see it
something faint, ethereal, almost
unbelievable.
Stretching away from the planet,
reaching impossibly far into the
blackness is a ghostly elongated glow.
It's a whisper of light invisible to the
unaded human eye. A delicate structure
millions of kilometers long trailing
behind this tiny battered world like the
train of a spectral gown. Mercury, the
seemingly inert little rock closest to
the sun, is doing something deeply
strange in the dark. It's venting a tail
into space. A tale made not of gas or
dust in the way we usually think of
comet tales, but of something far more
specific and intriguing. Sodium atoms.
I'm Alex Mccoan and you're watching
Astramm. Join me today as we delve into
one of the solar systems more subtle and
surprising phenomena, the vast invisible
sodium tail of Mercury. We'll explore
how this bizarre feature was discovered,
investigate the complex processes that
create and shape it, and understand what
this ghostly appendage tells us about
the innermost planet and its
relationship with the sun.
When we think of planets with distinct
features, Mercury rarely comes to mind.
It's the run of our planetary family, a
tiny cratered world baking in the sun.
But in 2001, everything changed.
Scientists have been using specialized
sodium filters to observe Mercury since
the mid 1980s when it was found in the
planet's exosphere. That's exactly what
the team at Macdonald Observatory in
Texas were doing when they spotted
something strange. A faint glow
extending far beyond Mercury's tiny
disc. Initially, they didn't believe
their own eyes. They thought it must be
the result of an equipment malfunction
or data processing errors. But after
thoroughly checking and double-checking,
the astonishing truth emerged. Mercury
was trailing an enormous structure
through space. A tail of sodium
stretching almost 2.5 million km behind
it. For perspective, that's 700 times
the width of Mercury itself.
The discovery was humbling. Mercury
isn't some distant obscure object. It's
one of our closest cosmic neighbors.
We've been observing it since ancient
times and even visited it in the 1970s
and the 2000s thanks to NASA's Mariner
10 and Messenger spacecraft. And yet,
this colossal feature remained
completely undetected until the 21st
century.
The discovery was a big deal because it
challenged our classification of what
can have a tail in our solar system.
Suddenly, Mercury wasn't just a bland
cratered world, but a planet with a
dynamic feature rivaling the most
spectacular comets.
So, how is Mercury's tail different from
comet tails? And why would this planet
have a sodium tail specifically?
The answer is the fascinating result of
Mercury's unique position, composition,
and the sun's fierce influence.
Comets are kind of like snowballs
hurtling through space. They are mainly
made of ice, which sublimates as the
comet approaches the sun, leaving behind
a trail of ionized gas and dust
particles. In contrast, Mercury is a
rocky planet with minimal if any ice.
Scientists think there may be ice hidden
in permanently shadowed craters at its
poles, but these frozen deposits remain
stable precisely because they never
receive direct sunlight. So, Mercury's
tail isn't formed through the
sublimation of ice into gas. It's made
of sodium. But why?
To understand that we need to take a
look at its proximity to the sun. At
perihelion, Mercury comes as close as 47
million km from the sun where it's
exposed to solar radiation up to 10
times more intense than what we receive
on Earth. The solar wind is absolutely
torrential.
Imagine a constant stream of charged
particles flying at 1.6 6 million kmh
from the sun straight at you. When those
particles strike, they physically knock
out other atoms on Mercury's surface in
a process called sputtering.
Picture a bunch of fridge magnets
sitting on your fridge. They don't fall
on their own because the fridge holds
them in place. But then, say you throw a
bouncy ball at the fridge. When it hits,
the energy spreads out and shakes some
of the magnets. If enough energy makes
it to the weakest magnet, more energy
than is what's keeping it stuck, it will
pop off and fall down. That's exactly
what's happening in a sputtering
collision cascade. Ions from the solar
wind shower Mercury, creating high
energy impacts that dislodge the weakest
link atoms from the surface of the
planet, just like the bouncy ball
dislodged the weakest fridge magnet. It
just so happens one of the most weakly
bound elements on Mercury's surface is,
you guessed it, sodium.
Messenger data show that during periods
of high solar wind activity, these solar
wind particles rain down on Mercury's
surface and result in a 50% rise in
sodium group ions in the planet's
atmosphere. This indicates that stronger
solar wind activity enhances surface
particle interactions leading to a
greater release of sodium into Mercury's
atmosphere and hail. But there is more
to the story.
In addition to solar wind, Mercury is
also tormented by micrometeoroids
and without an atmosphere to slow them
down, even dustsized particles strike
the planet with tremendous force.
These tiny space rocks vaporize surface
materials on impact, releasing even more
sodium atoms. We're essentially watching
a process of space erosion. A planet
that is slowly disintegrating over
billions of years right in our cosmic
front yard.
Some researchers have noticed
enhancements in the tail that seem to
correlate with times when Mercury is
predicted to pass through denser streams
of interplanetary dust like debris from
comet anchor. This would boost the
contribution from micrometeoroid impact
vaporization temporarily increasing the
amount of sodium injected into the
exosphere and subsequently fed into the
tail. But the shaping of the sodium tail
isn't primarily due to the solar wind
blowing it back, as is largely the case
for a comet's ion tail. Instead, the
dominant force sculpting Mercury's
sodium tail is solar radiation pressure.
This is a subtle but powerful effect.
Photons, despite having no mass, carry
momentum. When a sodium atom in
Mercury's exosphere absorbs a photon of
light, it gets a tiny push in the
direction the photon was traveling away
from the sun, it then quickly remits a
photon in a random direction, causing
negligible recoil on average. However,
it's immediately ready to absorb another
photon coming from the sun. This
continuous process of absorbing solar
photons and receiving tiny directed
pushes effectively acts like a gentle
but constant wind pushing the sodium
atoms away from the sun. Imagine
countless tiny solar sails. Each one a
sodium atom being persistently nudged by
sunlight. This relentless pressure
accelerates the atoms anti-sonwood,
overcoming Mercury's weak gravity and
stretching them out into the vast
elongated tail structure observed from
Earth. Because radiation pressure acts
directly away from the sun, the tail
reliably points in the anti- solar
direction.
But as I mentioned, despite being so
big, it's not actually visible to the
naked eye. So, how did scientists spot
it?
When sodium atoms absorb energy, they
become excited or their outermost
electron jumps to a higher energy level.
When this electron falls back to its
ground state, it emits photons at a very
specific wavelength, 589
nanome.
Light at this wavelength has a distinct
color you've definitely seen before.
It's the color of old street lights.
This process is happening constantly in
Mercury's tail. However, the tail is
hidden from the human eye and from
standard telescopes. To reveal it,
astronomers use specially designed
filters called sodium Dline filters that
isolate light at precisely 589 nanome.
These filters block all other
wavelengths, allowing the faint glow of
sodium to stand out against the black
background of space. By deleting all
other wavelengths of light, scientists
were able to see the sodium tail.
Mercury's sodium tail varies
dramatically throughout its 80-day orbit
around the sun, which is highly
elliptical. When Mercury reaches
perihelion, the increased solar
radiation dramatically intensifies the
rate of sodium ejection from its
surface. You might expect the tail to
reach peak visibility at perihelion
since that's where the solar wind and
radiation are strongest. However, it
actually reaches peak 16 days later. Let
me explain.
Since Mercury's orbit is highly
elliptical, its orbital speed also
varies a lot from 39 km/s when it's
farther from the sun to 59 km/s when
it's closer. Through repeated
observations, scientists noticed that
Mercury reaches its maximum radial
velocity after slingshotting past the
sun, exactly 16 days after perihelion.
In other words, that's when it's
traveling away from the sun at its top
speed, creating optimal conditions for
the sodium tail to extend and become
more observable.
But why would higher speed lead to a
brighter tail? Two words, Doppler shift.
When we look at the spectrum of light
from the sun, we see dark lines called
frownhoffer absorption lines where
certain wavelengths of light are being
absorbed by elements in the sun's
atmosphere before the sunlight is
emitted. The sun's atmosphere contains
sodium just like Mercury's tail which
absorbs all the sodium specific light
before the light travels to Mercury.
But the faster mercury travels away from
the sun, the more the sunlight is
Doppler shifted or stretched. This means
the wavelength gets longer or redder,
the entire sunlight spectrum is shifted,
including the frown huffer lines. So the
sodium specific light is no longer
blocked and is available for the tail to
absorb and scatter. The result, the tail
gets brighter and we can see it from
Earth.
So, if astronomers wanted to spot
Mercury's tail, they just need to look
up on day 16 past perihelion, right?
Well, it's not that simple. There are
three other main factors at play to
consider. Firstly, Mercury must be at a
sufficient elongation from the sun in
Earth's sky so that it isn't caught in
the glare. Secondly, it can't be behind
the sun from Earth's perspective. And
finally, it must be at the perfect angle
for Earth-based telescopes to view its
anti-ward direction. This astronomical
alignment only occurs during very
specific periods, roughly every few
years. And to think the scientists at
Macdonald Observatory caught it
basically by coincidence. Talk about
perfect timing. Some of the most
spectacular images of Mercury sodium
tail have come from observatories
utilizing coronagraphs,
instruments that block the sun's direct
light, allowing the faint glow of the
tail to become visible even when Mercury
is relatively close to the sun in our
sky. Each successful observation
represents a triumph of astronomical
planning and precision. These rare
glimpses show a dynamic that changes
dramatically with Mercury's position
relative to the sun, stretching,
shortening, brightening, and dimming in
a cosmic dance governed by orbital
mechanics and atomic physics. But this
story stretches beyond Mercury itself.
It has provided an unexpected window
into key processes occurring throughout
our solar system and beyond.
Firstly, it gives us a glimpse into
planetary evolution. While the current
rate of surface material loss is too
small to significantly alter mercury in
human time scales over billions of
years, this process could potentially
affect its surface composition,
gradually depleting sodium and other
easily liberated elements. We could use
this knowledge to infer what might have
happened to older planets in other solar
systems and understand their life cycle
better. Secondly, and perhaps most
excitingly, the discovery has profound
implications for exoplanet research.
Many star systems contain hot Jupiters,
massive planets orbiting extremely close
to their stars. If Mercury can generate
a 2.5 million km tail, these giants
likely sport even more dramatic
features, potentially shedding
significant mass through similar
mechanisms.
The detection methods developed to study
Mercury's tail are now being applied in
the search for similar phenomena around
distant worlds. By looking for the
distinctive spectral signatures of
elements being stripped from exoplanets,
astronomers may gain new insights into
their composition and environment. In
fact, we have already detected similar
sodium tails around our own moon,
suggesting this phenomenon may be common
around airless bodies exposed to solar
wind. Mercury's sodium tail represents a
perfect opportunity for studying planet
star interactions throughout the
universe. By better understanding this
phenomenon, we gain crucial insights
into how stars shape and erode their
planetary systems over billions of
years.
Bey Columbbo mission has already
completed six rapid flybys of Mercury.
It's set to enter into orbit in December
2025 where it will spend at least one
Earthy year studying Mercury's internal
composition and its impressive
exosphere. This will provide data on the
sodium tail in unprecedented detail.
promising new discoveries about this
fascinating phenomenon.
Mercury's elusive tale reminds us that
even after centuries of astronomical
observation, fundamental discoveries
about our closest cosmic neighbors are
still being made. What other secrets
might our cosmic neighborhood still be
hiding? The universe never stops
surprising us. All we need to do is keep
looking.
[Music]
Everyone knows the name of the first of
the planets in our solar system. Mercury
was seen in ancient times and carries
the name of a Roman god. We've known of
his existence for thousands of years.
It's 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 space fairing 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 Mccoan and you're watching
Astramm. 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.
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
g per cime cubed, only slightly less
than the planet with the greatest
density, that of Earth, at 5.515
g per cm 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. 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 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°
C 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-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 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
magneettosphere 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.
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.
It takes about 59 Earth days for Mercury
to rotate on its axis once, which is
what we call a cidurial day.
By pure coincidence, this is almost
exactly half its cenotic 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 tidily 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 tidily locked to
the sun. What astronomers didn't realize
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 that it does
rotate slightly faster than it orbits.
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.
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 neighbor, 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 Earth and
Mercury is 77 million km. In fact, it is
the closest planet to us on average.
The closest distance between Earth and
Jupiter is 588 million km, almost eight
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. The first mission was a
flyby and only mapped about 40 to 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.
Combating 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 tried to only
take 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 on board,
too. So after the success of Mariner
10's flyby missions of Mercury in 1974
to 1975,
Mercury exploration was put on hold. But
in 1985, an orbital mechanics expert
named Chen Wan Yen realized 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 go a longer
way. How long? Under Chen Wang Yen's
model, a craft would orbit the sun about
15 times, flying past the Earth once,
Venus twice, and Mercury three times
before finally slowing down enough to
enter its orbit on the fourth pass. All
these planetary flybys would be
essential. By skimming the planet's
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 1/2 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 geocchemistry and ranging
probe was only about 1.8 8 m long and
1.3 m wide and weighed 1,100 kg. This is
small and light for a typical NASA
mission. Just a comparison, Juno is 20 m
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 magneettosphere.
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
when 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 it
performed no science. However, in 2007,
it passed Venus again. At that time,
another spacecraft was orbiting Venus,
issa'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 the 14th of January 2008 with
everything going smoothly. The same was
true of the second flyby. But during the
third 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
the scientists involved. You see,
Messenger had to pass through Mercury
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, seven 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
planned for it to only last one 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 surface in high resolution and
color, as well as photographing nearby
comets and other planets.
On the 25th of 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 25 km from
the surface of the planet. Scientists
gave the thrusters one last burst to
extend its orbit for as long as
possible. But on the 30th of April 2015,
Messenger crashed into the surface of
Mercury. After a journey that had lasted
over a decade and had covered literally
billions of kilome, Messengergers's
journey had come to an end.
Messenger gave us a wealth of insights
into Mercury before it died. Onboard
Messenger were a host of scientific
instruments, including a magneettometer
to map out Mercury's magnetic field,
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
Maitri 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
center 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 that 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
the planet's magneettosphere.
When this happens, it allows a few of
these charged particles to break into
the planet's magneettosphere, entering a
region of plasma in the planet's magneto
tail. The flows you see in this
simulation in the plasma region are from
reconnection events.
Another feature of the magneettosphere
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 localized
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 magneetto tail 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 actually
having to 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, sulfur, 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
completely 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 the equator than 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 predominantly 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.
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 vapor.
This could come from cometry tails as
they passed by, or it could come from
the 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 Arosibo 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 kg.
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 unique
feature 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 scarves that were evidence of its
gradual cooling.
These scar show that Mercury is
contracting, and from messengers data,
Mercury has contracted by over 14 km in
diameter since its formation, a lot more
than 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 issa's Bey
Columbo 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, 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 honor
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 or 100,000 more viscous than
water. For a point of reference, this is
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
1,300°.
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 Raditi is a
relatively young crater likely under a
billion years old with wellpreserved
walls and a floor relatively clear of
other later impacts. It's large over 25
km in diameter. Notice how rough the
hills are around the crater and yet
inside is a smooth plane. This is no
coincidence. Originally, the terrain
inside Radit Ladi 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 cooled, it formed the
smooth plane 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 the
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 realize that
there are numerous other craters on
Mercury that similarly must have been
filled with lava. Just look at Rustelli
where crags of mountain can be seen
poking up through the smooth lava layer
or copland
polygotus
or Rakmaninov.
Rakmaninov is particularly interesting
as here you can see the strong
indicators of lava bubbling up through
from beneath the surface to the center
of the crater. Take a look at the
strange crinkled cracks forming a rough
circle inside the central crater. Such
craters are a signal that a slower
outpouring of magma pushed up from
beneath the surface, breaking the plane,
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. Let's look at the valley
known as Ankor Valis.
Here you can see clear signs of smooth
lava flow, but this time moving like a
river. The lava traveled 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 swaths of the planet and
turning them the more orangey color we
see today.
Scientists have begun to recognize this
telltale orange color 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 northeast of Rakmaninov are
likely formed by volcanic activity. When
Messenger flew over this area in 2015,
it took detailed photos 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 that
you'd have to contend with on Mercury,
but burning ash falling from the sky.
And that was just the karma 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 center.
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 the 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
teardropshaped.
However, the ray's 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 66 km in diameter, the amount of
force exploding upwards that would have
been necessary to carve out this crater
and scatter its remnants for kilometers
all around 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 the thick
toxic gases emitted with the eruptions.
And even the ground you could stand on
might at any moment explode under 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 two
missions to Mercury with just one more
on the way. Mariner 10 in 1974,
Messenger in 2011, and Bey Columbo 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
color. Mercury's color is not actually
monochrome and is smattered with
speckled grays, 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 on to them
later.
Beyond that, you most likely notice the
craters. Much like the moon, Mercury is
covered with craters as ancient pieces
of space debris crash 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° of the
crater wall has retained its shape, the
remaining 270° 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 the bottom
right corner? Was it something to do
with the angle the impact is struck at?
We don't really know. And that's one of
the things that is so intriguing about
Mercury. 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 400 km across. Their
lighter color is a sign that the
material kicked up from under Mercury's
surface is less graphitri 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 limes 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 spiderweb lines.
Scientists racked their brains for
years, but now it seems that this
mystery has been solved. In 2018, a
scientist called Tapan Sabuala was
scouring the internet and discovered
that a group of students had managed to
recreate the spidery line pattern of
crater rays. Sabuala was excited but
also confused. Why were these students
able to manage what other scientists had
not? Interestingly, he realized that
this was an example of scientists being
too neat. Before performing their tests
in lab conditions, Sabuala 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 the 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
know how these rays were formed. Crater
rays do not care about the speed of the
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 400 km? That was just the
smaller ones. Look at the ray system
originating from the crater known as
Hokusai.
These rays must have been created from
incredible impact as their lines stretch
almost entirely around Mercury's
surface, which by the way has a
circumference of 15,000 km.
And although not quite as large, the ray
system of the crater Dusi covers over
1,000 km.
While the moon also has ray systems,
they are 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 his 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
body block it.
Yet, this explanation cannot explain
this last formation. Take a look at one
of the most fascinating craters on
Mercury, Apollodoris, and its
surrounding Pantheon fossai.
At first glance, you might think there
is nothing unusual about the crater
Apollodoris. Yes, those fractures
running out from the center are a little
unusual.
There are a few features that are odd
here. To begin with, the fractures and
the surrounding radial fractures bear a
strange resemblance 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 makeup.
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° C. But if that
was so, why has this not happened in
other places? About half the impact
should be hitting Mercury's night side
at least.
When we take a closer look at
Apollodoris, the crater you might assume
is the cause of the fractures, we notice
something even stranger. Apollodoris is
not quite the epicenter.
While it's pretty close, it doesn't
actually line up. This might suggest
that Apollodoris and the spidery
fractures of the Pantheon fossai are
actually unrelated.
Whatever caused this phenomenon may have
happened only to be later hit by an
asteroid near to but not on its
epicenter. But if that's so, what caused
Pantheon fossai? The intriguing thing is
that we don't know. Evidently, something
created this fractured glass shape but
left no crater. Might that imply that
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 out on
Mercury's surface and many other
fascinating insights to be gleaned as
science advances.
When issa's Bey Columbo arrives at
Mercury in 2025, it will begin another
extensive study of the planet and
perhaps then we will have the answers.
Beep Columbbo will uncover the
characteristics of Mercury's
magneettosphere and exosphere and we'll
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 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 systems 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.
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