This is Pluto, a strange and beautiful world so remote that the Sun looks merely like a bright
star in its firmament. The icy dwarf planet, which sits at the edge of the Kuiper Belt a
disc-shaped region of dispersed bodies beyond the orbit of Neptune looks like a fever dream of
towering peaks, nitrogen ice glaciers and winding canyons. It’s extremely tiny, much smaller than
our own moon and far more colourful. Its surface hue varies from white to red to burnt orange
and charcoal black. As a contrastive body, it is second in the Solar System only to Saturn’s moon
Iapetus. Yet until recently, even high-powered telescopes like Hubble could make out just a
few blobs of colour on it. Pluto is so small and distant, being nearly 5 billion kilometres from
Earth at its closest approach, that we only had a vague sense of what it looked like. However,
that all changed in 2015, when NASA’s New Horizons space probe transmitted the first up-close images
of Pluto ever taken and they were astonishing. Veterans of this channel may remember previous
videos I released on the New Horizons mission, but those were just the tip of the Plutonian glacier,
so to speak. When it comes to Pluto, there’s a lot to talk about, and we still have many more
questions than answers. So, what else can we learn about Pluto? What surprised scientists, and how
are recent studies changing our understanding of the dwarf planet’s past and present?
I’m Alex McColgan, and you are watching Astrum. Join me today as we return to Pluto
and highlight some of the most incredible features New Horizons saw on its flyby.
New Horizons launched in January 2006 from Cape Canaveral Airforce 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 July 14, 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 or 2 kilobytes
per second. It took 15 months to downlink the full dataset, 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 kilometres
(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 Krun, Ala, Balrog, Vucub-Came, Hun-Came and
Meng-p’o. Each of these names comes 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 colour 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 neighbouring 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 kilometres 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 canyon’s 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 a nitrogen
ice plain 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 their 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? After all, every heart has its secrets.
Over on the western side of Sputnik Planitia is a towering mountain range called Al-Idrisi Montes.
Here, the winds blow tiny particles of methane ice toward 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, and 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, the process by 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-kilometre 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 minus
230 degrees Celsius. 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 fingers.
However, given the minus 230-degree temperatures, I wouldn’t recommend trying.
The dunes aren’t the only item of interest in this corner 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 meters to one kilometre in height, while the furthest peaks are much higher,
reaching 3 kilometres in height nearly as tall as the Rocky Mountains. This means the
mountains 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, 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. The fact that Pluto could host liquid
water at all is remarkable when you consider how far 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 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 mountains’ eastern side, as well
as the uplift we see in the rising peak elevations moving from west to 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 colour 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 kilometers 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 it’s
even more massive, rising 7 kilometres tall! Sadly, it was already in darkness by the time
New Horizons was able to capture it, so the images aren’t quite as clear as those of Wright Mons, but
you can see it here in this colourized topographic map. It, too, has a huge depression in its centre.
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, we haven’t yet confirmed
that these are definitely cryovolcanoes, but the literal mountain of evidence is hard to deny.
So, there we have it, another group of incredible features New Horizons discovered on Pluto.
And given the surprising revelations about its possible subsurface ocean, you can be sure that
there will be a lot more research on the subject. What more discoveries do you think we’ll make?
Could we one day find life in Pluto’s Ocean? Will we send a probe to another of the dwarf planets
in the Kuiper Belt, such as Eris? It’s been proposed, but the spaceflight alone will take
over 20 years! All the more reason to appreciate the wonderful images New Horizons captured.
Thanks for watching and see you next time.