obstacle that stands between [music] Pluto and its former glory.
Perhaps one of the things that makes
Pluto so fascinating is the sense of
mystery surrounding it.
After all, this was once considered our
furthest away planet, existing 5.9
billion kilometers from the sun on
average. And since we first spotted
Pluto in 1930, there has only been one
mission to explore this tiny object, New
Horizons.
So let's take a closer look at what the
New Horizons probe found and witness
some of the wonders that can be
discovered on this distant, enigmatic
dwarf planet.
New Horizons launched in January 2006
from Cape Canaveral Air Force Station.
Its primary mission was to photograph
and collect information on Pluto and its
moons Charon, Styx, and Nix, not to
mention Kerberos and Hydra, which hadn't
been discovered yet.
On the 14th of July, 2015, 9 years into
its mission, New Horizons made its
closest approach to Pluto, capturing a
breathtaking glimpse of its surface that
exceeded our wildest imaginings.
Ice mountains, nitrogen glaciers,
cryovolcanoes, and canyons.
But, given the immense distances
involved, transmission was limited to
just 1 to 2 kilobytes per second.
It took 15 months to down-link the full
data set, which caused a sensation every
time NASA released a new image.
As I look at these images today, my
sense of wonder hasn't dimmed. There's
so much information to unpack. It feels
a bit like working on a puzzle.
And like any good puzzle, the fuller
picture only reveals itself after you've
taken the time to piece it together.
Let's start with a unique feature called
the brass knuckles, located near Pluto's
equator.
It is comprised of six lowland regions
called maculae with an average width of
480 km, each roughly the size of
Iceland.
If we take a closer look, we can see
that they are interlaced with a network
of canyons.
From west to east, they are Cthun, Ala,
Balrog, Vukub-Came, Hun-Came, and
Meng-Po.
Each of these names come from a
different netherworld spirit from world
mythology.
So, for those Lord of the Rings fans
watching, you'll be excited to know that
Balrog Macula is a reference to that
wizard battling demon of Middle Earth.
We think the brass knuckles get their
dark color from a tar made of tholins,
which are hydrocarbons formed from the
interaction of methane and nitrogen with
cosmic rays.
Tholins are in abundance throughout
Pluto, such as here in the neighboring
whale-shaped region known as Cthulhu
Macula.
The reason the brass knuckles appear
separate is that each macula is divided
by towering uplands covered in ice.
These uplands stand several kilometers
tall and are likely made of frozen
nitrogen, methane, and carbon monoxide
on top of a bedrock of water ice.
On Pluto, water ice is strong enough to
form mountains because of its low
gravity.
Nitrogen ice, however, is much softer,
which is why it tends to flood the
lowlands on Pluto.
One can only imagine what these colossal
ice mountains must look like from the
canyons' perspective.
Nitrogen is the most common material on
Pluto's surface, 98% of it, in fact.
Nitrogen ice also accounts for a large
part of Pluto's most iconic feature, its
heart-shaped region known as Tombaugh
Regio, named after Pluto's discoverer,
Clyde Tombaugh.
The western lobe of the heart is a
smooth basin filled with nitrogen ice
called Sputnik Planitia,
while the eastern lobe is much rougher.
In this image, you can see the relief in
remarkable detail.
Scanning from west to east, you can
almost feel the difference with your
fingertips.
The eastern side looks pitted, like an
orange's skin.
Scientists think Pluto's winds are
carrying nitrogen from Sputnik Planitia
up into the atmosphere and depositing it
back into these eastern uplands, which
is why the albedo is so high.
Albedo being the measurement of how much
light a surface reflects.
Notice how the uplands have a bright icy
sheen due to all the nitrogen ice
deposited there.
We think that some of that nitrogen in
turn slides back into the lowlands via
glacial flow, meaning the western and
eastern halves of the heart have a
transactional relationship.
It's an unusual arrangement, but who are
we to judge?
[music] landscape features.
Over on the western side of Sputnik
Planitia is a towering mountain range
called Al-Idrisi Mons.
Here, the winds blow tiny particles of
methane ice towards the mountains and
deposit them back onto the surface like
grains of sand.
This is remarkable in itself, since
Pluto's atmosphere is very thin. We
didn't expect its winds to be capable of
this kind of transport.
Most likely, these methane particles are
uplifted by sublimating nitrogen ice, a
process which a substance changes phase
from solid to gas.
We think the sublimating nitrogen rips
tiny grain-sized particles of methane
ice, which carry up into the atmosphere
and settle closer to the mountains.
These methane deposits form a vast 2,000
square kilometer field made of
transverse dunes, somewhat resembling
those in China's Taklamakan Desert or
California's Death Valley.
But, unlike those deserts here on Earth,
these dunes form in frigid temperatures
of -230°
C.
The tiny grains of methane that form the
dunes are much lighter than sand, and in
Pluto's low gravity, would likely feel
like dust spilling through your
fingertips.
However, given the -230°
temperatures, I wouldn't recommend
trying.
The dunes aren't the only item of
interest in this corner [music] of
Sputnik Planitia.
On the western side of the mountains is
a deep trench, which you can see as the
dark blue band on this colorized
elevation map.
Based on our measurements, the peaks
adjacent to the trench vary from 200 m
to 1 km in height, while the furthest
peaks are much higher, reaching 3 km in
height, nearly as tall as the Rocky
Mountains.
This means the mountains [music] get
taller as you move further from the
trench.
Why is this significant? Well, we think
it might be connected to what's going on
underneath Pluto's surface.
You see, one of the most fascinating
revelations we've recently made about
Pluto is that it may have had,
>> [music]
>> and might still have, a vast subsurface
ocean.
This is a characteristic it would share
with several moons in our solar system,
such as Ganymede, Europa, and Enceladus.
[music]
The fact that Pluto could host liquid
water at all is remarkable when you
consider how far away it is from the
sun.
It would have to generate significant
heat from nuclear decay in its core.
Yet, Pluto is smaller than our own moon.
Given the diminutive size of its core,
you wouldn't expect it to generate a lot
of heat. So, the evidence for a
subsurface ocean took a lot of people by
surprise.
This is very interesting, but I'm sure
you're wondering what it has to do with
the mountains we just looked at.
Well, a 2019 [music]
study published at the Lunar and
Planetary Science Conference argues that
the Al Idrisi mountains have very likely
moved east through solid-state
convection.
This movement could have produced both
the deep trench on the mountain's
western side, as well as the uplift we
see in the rising peak elevations moving
from west to [music] east.
If this theory is true, these dynamic
processes happening on Pluto's surface
would seemingly support the idea that
there is a layer of liquid water beneath
its icy crust.
We see other evidence as well, in the
form of cryovolcanoes.
Wright Mons is one potential cryovolcano
we've looked at previously on this
channel, but here it is in the highest
resolution color view that the New
Horizons team assembled.
What you're seeing is a composite of
images taken by the probe's LORRI camera
and its Ralph multispectral visible
imaging camera.
Wright Mons is a massive structure made
mostly of water ice standing 4 km tall
with a gaping depression in the middle.
And it may have been active fairly
recently as evidenced by the lack of
impact craters in the surrounding area.
Scientists spotted another potential
cryovolcano to its southwest called
Piccard Mons and is even more massive
rising 7 km tall.
Sadly, it was already in darkness by the
time New Horizons was able to capture
it, so the images [music] aren't quite
as clear as those of Wright Mons, but
you can see it here in this colorized
topographic map.
It too has a huge depression in its
center.
The depressions of both structures are
reminiscent of calderas we see here on
Earth, but they are much larger relative
to the size of the volcanoes.
Looking closely, there appear to be
concentric rings surrounding the
depressions indicating perhaps a series
of eruptions followed by a retreat of
magma and subsequent collapse of the
vents.
Just to be clear, they haven't yet
confirmed that these are definitely
cryovolcanoes, but the literal mountain
of evidence is hard to deny.
From all of this, it is clear that Pluto
is a fascinating, beautiful place.
Hopefully, you now feel like you know it
a little better,
but none of this answers one key
question. Is it deserving of being a
planet?
The key to that question and why Pluto
stopped being a planet in the first
place all started in 1978
with one key discovery. It's time to
talk about Charon.
The story of Charon begins not with a
deliberate search, but with a mistake.
On the 22nd of June, 1978,
James Christy, an astronomer at the US
Naval Observatory, was performing a
routine but tedious task, examining
photographic plates of Pluto to refine
calculations of its orbit around the
Sun.
The images, taken with the 1.5 m Kaj
Strand Telescope in Arizona, were not
considered high quality. In fact,
several plates had been marked as poor
or defective because they showed Pluto
not as a clean, sharp dot, but as a
strangely elongated blob with a distinct
bulge on one side.
To most, this would have been an
annoyance, an imperfection to be
discarded. Atmospheric turbulence or
improper telescope alignment could
easily cause such distortions, but
Christy had a specific background that
prepared him to see something others had
missed.
He spent years photographing double
stars and his trained eye recognized a
familiar pattern in the strange bulge.
He noticed a critical detail.
While Pluto's image was smeared, the
distant background stars on the very
same plates were perfect, sharp points
of light.
If the problem were with the telescope
or the atmosphere, everything in the
image would be distorted.
The fact that only Pluto was affected
meant the bulge had to be real. There
was something else there.
Christy began a deep dive into the
observatory's archives, unearthing
plates of Pluto from as far back as
1965.
Many of these, too, had been dismissed
as flawed, bearing notes like "Pluto
image elongated."
But now, these failures became the key
to confirmation.
Christy saw that the bulge
systematically moved around Pluto over
time. He and his colleague, [music]
Robert Harrington, meticulously tracked
its position across the years of
archived images.
They calculated that the bulge circled
Pluto once every 6.4 days, a significant
number because it perfectly matched
Pluto's own known rotation period.
This was the final piece of the puzzle.
The ghost on the photographic plate was
a massive companion.
A moon locked in a synchronous dance
with its parent world.
With the discovery confirmed, the new
moon was provisionally designated S/1978
P1.
As the discoverer, Christy had the right
to propose a name.
He chose to name the new moon after his
wife, Charlene, whose nickname was Char.
Inspired by his interest in physics,
where particles like protons and
electrons often end in -on, he added the
suffix to create Charon.
It was a personal tribute, but by an
almost unbelievable coincidence, it was
also a perfect mythological fit.
Only after proposing the name did
Christy learn that in Greek mythology,
Charon was a silent, spectral ferryman
who carried the souls of the dead across
the river Acheron into the underworld,
the realm ruled by the god Pluto.
The name was officially adopted by the
International Astronomical Union in
1986.
But this new found moon was far more
than just a companion. Its very
existence would force a complete
re-evaluation of the entire solar system
and in doing so would help topple Pluto
from its planetary throne.
The discovery of Charon immediately
began to unravel Pluto's mysteries and
the first revelation was a profound one.
By tracking Charon's orbit, astronomers
could finally apply Kepler's law to
calculate the mass of the system for the
first time.
The result was shocking.
Pluto was not a small terrestrial planet
as some had thought, but was incredibly
lightweight, about a sixth of the mass
of Earth's moon.
Charon itself was found to possess a
substantial 12% of Pluto's mass, a ratio
far greater than any other major moon in
the solar system. Earth's moon, by
comparison, is only about 1.2% of
Earth's mass.
This startling parity in size gives rise
to a unique dynamic that sets the pair
apart from every other planet-moon
system.
When a small moon orbits a large planet,
the center of their combined mass, or
the barycenter, lies deep within the
planet.
The moon makes a large orbit around this
point while the planet makes a tiny
wobble.
But Pluto and Charon are so close in
mass that their barycenter lies in the
empty space between them, approximately
960 km above Pluto's surface.
Pluto and Charon perform a perpetual
dance around a common invisible point in
space and that has implications.
Our moon is tidally locked to Earth.
That means it rotates on its axis in the
same amount of time it takes to orbit
us, which is why it always presents the
same face to us. From the moon's
surface, however, an astronaut would see
Earth rotate, going through a full
day-night cycle.
The Pluto-Charon system is an extreme
example of what is called mutual tidal
locking. Not only does Charon always
show the same face to Pluto, but Pluto
always shows the same face to Charon.
If you were standing on the
Charon-facing side of Pluto, the great
moon would hang motionless in the sky. A
colossal disc that never rises or sets.
If you were on the opposite hemisphere,
you would never know it was there at
all. They are locked in an eternal
stare-down across 19,640
km of space.
These two facts, the external barycenter
and the mutual tidal lock, led many
scientists to argue that the system
shouldn't be considered a planet and a
moon, but rather a double planet.
This ambiguity became a central exhibit
in the heated debate over planetary
definitions in the early 2000s. The
discovery of Charon provided the
critical data, Pluto's low mass and the
system's strange dynamics, that forced
astronomers to confront the fact that
the solar system was more complex than
our simple categories allowed.
The International Astronomical Union
even formally considered a proposal to
classify Pluto and Charon as a double
planet in 2006, though the idea was
ultimately set aside amidst the larger,
more controversial decision to
reclassify Pluto as a dwarf planet
instead.
So, how did that happen?
Prior to 2006, the term planet was used
fairly loosely to refer to large objects
in our solar system and beyond. A term
rooted in traditions that predated our
modern understanding of the universe.
But while astronomers were gaining a
greater appreciation for just how
sizable Charon was compared to Pluto,
they were also discovering other objects
out beyond the orbit of Neptune: Quaoar,
Sedna, Makemake, and Eris.
Of these, at the time, Eris was thought
to be larger than Pluto, although more
exact measurements since then have found
it to be slightly smaller at 2,326
km in diameter at its equator compared
to Pluto's 2,377
km.
Still, it is more massive.
So, if Pluto was a planet, would these
planets, too?
This question caused the International
Astronomical Union to convene and come
up with a new definition of a planet,
one that could remove all the confusion.
And it laid out three key criteria that
a planet had to meet. One,
a planet had to orbit the Sun.
Two, it had to [music] have enough mass
for its gravity to form it into a
roughly round shape. And three, it had
to clear the neighborhood around its
orbit.
It was the last criterion that was the
fatal blow for Pluto as a planet,
because while Pluto has managed to form
into a roughly round shape and does
orbit our Sun, its tiny mass had not
been enough to clear out its orbit of
thousands of other large objects like
the plutinos, Orcus, or Lempo.
While finding Charon had not been the
final nail in Pluto's coffin as a fully
fledged planet, its discovery was a key
domino in a cascade that led to Pluto's
eventual demotion.
Its fascinating gravitational dance with
Pluto has linked it inextricably to the
dwarf planet, and 2006 was not the first
time it had a massive impact on the
dwarf planet's history.
So, how did such a strange system come
to be?
For many years, the leading theory for
the formation of the Pluto-Charon system
was a scaled-down version of how our own
moon was born, the giant impact
hypothesis.
In this scenario, a massive Kuiper Belt
object slammed into a proto-Pluto in a
cataclysmic collision.
The impact would have blasted a huge
cloud of debris into orbit, which then
gradually coalesced to form [music]
Charon.
This model successfully explains the
system's high angular momentum and
Charon's nearly circular orbit.
However, as our understanding [music]
of planetary materials and our
computational power grew, this model
started to show cracks.
Pluto and Charon are not molten bodies
of rock, like early Earth. They are
small, rigid worlds made of rock and a
great [music]
deal of water ice.
These materials behave very differently
under the stress of a cosmic collision.
More recent and sophisticated
simulations have given rise to a new,
more [music] elegant theory.
This model proposes a much lower
velocity grazing impact, less of a
planetary shattering and more of a
cosmic kiss.
Instead of obliterating each other, the
proto-Pluto and proto-Charon collided
gently enough that they became
temporarily stuck together, spinning
through space as a single snowman-shaped
object for a few hours before tidal
forces pulled them apart again.
They separated, but not completely,
remaining forever bound by gravity in
the binary system we see today.
This kiss and capture scenario has a
powerful advantage over the giant impact
model. It explains how both bodies could
have survived the encounter largely
intact, preserving much of their
original internal structure and
composition.
This aligns perfectly with observations
that show Pluto is denser and more
rock-rich, about 70% rock, than Charon,
which is only about 55% rock. Suggesting
they formed as separate bodies that were
not thoroughly mixed together in a fiery
collision.
Crucially, this formation story does
more than just explain the system's
orbit and composition. It provides the
key to understanding Charon's entire
geological history.
The initial impact, and more
importantly, the immense tidal friction
generated as the two bodies stretched
and pulled on each other during their
separation, would have deposited a
tremendous amount of heat deep inside
both worlds.
This injected energy is the starter
pistol that fired off Charon's
geological evolution.
Trapped beneath a crust of ice, this
heat would melt the interior, creating a
subsurface ocean, and setting the stage
for the planetary scale drama that would
later unfold across its surface.
For 37 years after its discovery, Charon
remained little more than a fuzzy
companion to Pluto.
But on the 14th of July, 2015, NASA's
New Horizons spacecraft flew past the
binary pair, transforming them from
distant points of light into vibrant,
complex worlds.
The images New Horizons sent back from
Charon were stunning.
They revealed a world that had, at some
point in its distant past, torn itself
apart.
Charon's surface isn't smooth. It's
dominated by a system of vast tectonic
faults, enormous ridges, towering
cliffs, and deep valleys that scar its
surface.
The most spectacular of these is a great
belt of chasms that wraps around the
moon's equator.
This system, which includes the
informally named Serenity Chasma, runs
for at least [music] 1,800 km and
plunges to depths of up to 7.5 km.
To put that in perspective, [music] it's
more than four times as long and nearly
five times as deep as Earth's Grand
Canyon.
These are not canyons carved by rivers,
but colossal pull-apart faults, evidence
that the entire crust [music] of Charon
was once stretched to its breaking
point.
What immense force could cause a whole
world to expand and rupture?
The answer lies with the heat from its
formation.
That initial energy, supplemented by the
slow decay of radioactive elements in
its rocky core, was likely enough to
melt the water ice deep inside Charon,
creating a vast liquid water ocean
beneath a solid, frozen crust.
For a time, Charon was an ocean world.
But all small bodies in the outer solar
system eventually lose their primordial
heat to space.
As Charon cooled over millions of years,
the subsurface ocean began to freeze.
Here, a peculiar property of water
becomes critically important.
Unlike most substances which contract
when they freeze,
water expands.
As the ocean turned to ice, its volume
increased, pushing relentlessly outwards
on the brittle, overlying crust from
below.
The surface stretched and strained until
it could hold no longer.
The crust of Charon ruptured, creating
vast canyon systems that still span the
dwarf planet today.
As one scientist put it, Charon tore
itself apart at the seams like Bruce
Banner becoming the Hulk.
Today, Charon appears to be geologically
inert.
Its period [music] of great upheaval is
long past.
Because it lacks a thick atmosphere
[music] or ongoing geological activity
to erode or bury these features, the
evidence of this ancient cataclysm is
well preserved. The surface of Charon is
not just a landscape, it's a frozen
snapshot of the death of its ocean.
The freezing of Charon's ocean did more
than just break its [music] crust. It
also paved over half the world. The New
Horizons images revealed a tale of two
hemispheres with dramatically different
stories.
The northern hemisphere, informally
named Oz Terra, is an ancient, battered
world with rugged, high-standing
terrain, heavily cratered and
crisscrossed by the vast network of
tectonic faults and canyons created by
the expanding interior. This is the
original crust of Charon, frozen in its
fractured state.
But to the south, the landscape
transforms.
This is Vulcan Planitia, a vast, smooth
plain that's starkly different from the
tortured lands of the north.
This region is far less cratered,
indicating that it is a much younger
surface.
It sits at a lower elevation than Oz
Terra, and its surface is marked by
features that look like frozen flows and
broad swells.
Scientists believe Vulcan Planitia is
one of the most extensive examples of
cryovolcanism
in the solar system.
These weren't volcanoes of molten rock,
but icy slush.
As the subsurface ocean froze, the
expansion not only cracked the crust,
but also pressurized the liquid water
below, which was likely mixed with
ammonia that acted as a sort of
antifreeze.
This pressurized cryomagma was forced
upward through the newly formed cracks
and fissures, erupting onto the surface.
Icy lava flooded the southern
hemisphere, creating the smooth young
plains of Vulcan Planum.
Scattered across these plains are
several large, isolated mountains such
as Kubrick Mons, [music]
which appear to sit in strange
depressions or moats.
These may be colossal blocks of the
older northern crust that broke off,
were carried along like icebergs on the
cryovolcanic flows, and left stranded as
the plains froze solid around them.
Perhaps the most visually arresting and
mysterious feature on Charon is its
North Pole.
Capping the top of the moon is a large,
dark, reddish-brown stain. A feature so
prominent that the New Horizons team
informally named it Mordor Macula, after
the black land from The Lord of the
Rings.
The color is almost certainly caused by
a class of complex organic molecules
called tholins, the same ones we saw on
Pluto earlier.
Tholins are common in the outer solar
system, but their concentration at
Charon's pole is unique.
The key to understanding why they are
here is to find the source of the
methane.
There are two competing theories.
The leading hypothesis is [music] that
the methane is a gift from Pluto and its
atmosphere rich in nitrogen and methane
that's slowly escaping into space.
As Charon orbits, its gravity captures
some of this lost gas.
During Charon's long, dark polar winter,
which lasts for more than a century,
temperatures plummet to below minus 240°
C.
At these extreme temperatures, the
captured methane freezes directly onto
the surface in a process known as cold
trapping.
When the pole finally swings back into
sunlight, the accumulated methane ice is
bombarded with UV radiation, which cooks
it into the reddish tholins that stain
the pole.
Charon and Pluto are then a dynamic,
interconnected system with one world
actively painting the surface of its
companion across thousands of kilometers
of space.
But a competing theory suggests that the
methane was an inside job, that it
originated from Charon's own interior.
The massive cryovolcanic eruptions that
formed Vulcan Planitia would have also
released enormous quantities of gas,
including methane that were dissolved in
the subsurface ocean.
This would have created a temporary
atmosphere on Charon, and just as in the
first hypothesis, methane would have
migrated to the coldest parts of the
moon, the poles, and frozen out.
Over billions of years, this native
methane deposit was then irradiated to
form Mordor Macula.
This theory elegantly ties the origin of
the polar cap directly to the other
major geological events that shaped the
moon.
Whether the methane came from Pluto's
atmosphere or Charon's own interior,
Mordor Macula is a visible testament to
the inescapable connection between these
two distant worlds, a direct product of
their shared history and environment.
Charon's journey began with a smudge on
a photographic plate that was nearly
dismissed as an error.
It ends with a complex and dramatic
world, a world of continent-spanning
canyons deeper than any on Earth, vast
plains of ancient ice lava, and a
mysterious red-stained pole. It's
impossible to tell the story of Pluto
without Charon.
It was Charon's orbit that gave us the
first true measure of Pluto's mass,
revealing it as the king of a new class
of worlds.
It was Charon's existence that pointed
to a shared, violent birth that sculpted
both bodies and ejected them with the
heat that would fuel their evolution.
It's Charon's fractured surface that
tells a story of a lost ocean, a
geological history that provides a
crucial context for understanding the
forces that may still be churning deep
inside the more active Pluto today.
And it's Charon's red pole that may
serve as a permanent record of Pluto's
atmosphere bleeding out into space over
the eons.
>> There is one final point I want to
consider in our story of Charon and
Pluto. The discovery of this tidally
locked neighbor might have started the
process that saw Pluto relegated to the
position of largest of the dwarf
planets,
but will that always be the case? Will
Pluto one day regain its position as a
full planet in spite of its strange
relationship with its similarly sized
moon?
There are some grounds to [music] think
so.
Setting aside letters from 10-year-old
girls to NASA administrators, there were
a lot of pleases in that letter, to be
fair. There is something a little off
about the current definition of a
planet. To be a planet, an object needs
to have cleared its neighborhood, the
orbital track around the sun, that the
would-be planet traverses.
But, there are 10,000
asteroids near the Earth and 100,000
Trojan asteroids in Jupiter's orbit.
Strictly speaking, if Pluto failed its
definition on this count, shouldn't we
shouldn't Jupiter?
Now we've seen the rich and complex
geological processes that take place
across Pluto's surface. There are some
scientists who believe that this
complexity should be taken into
consideration when deciding whether
something is a planet or not.
They argue it should take precedence
over the requirement that the planet
should have cleared out its orbit.
So, with the administrator of NASA
seemingly in favor of the idea,
it's possible that scientists may one
day revisit the definition.
After all, the only thing that truly
makes an object a planet is enough
people agreeing that it is.
The term planet is entirely a human
definition. And if we decide that we
want to redefine it based on new
information or considerations, there's
no reason why that couldn't happen
again. It's happened once, after all.
So, it will be very interesting to see
what Isaac Asimov meant by "We're
looking into this." But, whether it's a
planet or not, Pluto will continue
drawing [music] our fascination, our
curiosity, and our awe. A rose by any
other name will still smell as sweet,
after all. Apparently, the same thing
goes for the beauty and charm of the
place we call Pluto.