Triton is an exceptionally unusual, although
often forgotten, moon. It has so many unique
characteristics, it makes it one of the most
interesting objects in the Solar System. But
because it is the largest moon of Neptune,
the planet furthest away from us, it also
means that we have only visited it once, very
briefly, as Voyager 2 flew by all the way
back in 1989, 30 years ago. But what did this
visit reveal? And what have we found out about
it since?
I’m Alex McColgan, and you’re watching
Astrum, and together we will find out everything
there is to know about the fascinating world
of Triton. First of all, let’s discuss where
Triton fits in to our solar system and its
local system. Triton is one of 14 known moons
of Neptune. 7 of these moons are regular moons,
or in other words, moons that orbit along
Neptune’s ecliptic with very circular orbits,
or orbits with very low eccentricity. After
these inner, regular moons, we get to the
irregular moons, the first of which is Triton.
An irregular moon is a moon that follows an
inclined, eccentric and often retrograde orbit.
This by itself is already where Triton is
set apart from any other spherical moon in
the solar system, it has an irregular orbit.
Triton orbits clockwise around Neptune as
Neptune rotates counterclockwise, and Triton
orbits at a 130° angle to the ecliptic of
the planet, although it should be noted that
its orbital eccentricity is close to zero,
its orbit is almost perfectly circular. All
other large moons in the solar system are
regular moons, orbiting the same direction
as the rotation of their parent planet. What
this heavily implies is that Triton did not
form alongside Neptune, but it is in fact
a captured object, specifically a captured
dwarf planet. No wonder then that it is by
far the biggest of Neptune’s fourteen moons,
comprising 99.5% of the mass found in Neptune’s
orbit. But how big is that in scales we can
relate to? Well, it is the second largest
moon in relation to its parent planet, second
only to Earth and its moon. While it is smaller
than our moon, it orbits closer to Neptune
than our moon orbits Earth, which means it
appears around the same size in the sky. It
is the 7th largest moon in the entire solar
system, and most interestingly, it is bigger
than Pluto. Pluto is often considered the
king of the Kuiper Belt, the biggest object
that we know of that formed there… until
we consider that Triton once ruled that area
before Neptune captured it. So, although Triton
is a moon of Neptune, it could also be said
that it is the biggest and most massive Kuiper
Belt object! Further evidence for this was
found as New Horizons passed Pluto in 2015,
suggesting Triton and Pluto share a near identical
composition, which supports the theory that
they share a common origin. Beyond Triton
are six other irregular moons, found much
further out. They are almost certainly captured
objects too, with unusually eccentric orbits
that take years to complete. They were probably
perturbed into these weird orbits by the gravity
of Triton. So, if Triton was a captured object,
how did that happen? Objects need to lose
momentum to be captured, otherwise they would
have enough momentum to escape. Well we can’t
know for sure, but the leading theory right
now is that Triton was once part of a binary
system, perhaps like Pluto and Charon. As
Neptune approached Triton and its moon, the
gravity from the encounter would have caused
the binary system to fall apart, with Triton’s
moon being slingshot away and Triton losing
enough momentum to be captured in orbit around
Neptune. I mentioned that Triton shares some
similar characteristics with Pluto. What exactly
does that entail? Well, they both have a predominantly
nitrogen ice surface with other ices mixed
in, like water and carbon dioxide. It has
quite a flat terrain, its topography never
varies by more than a kilometre, although
Voyager 2 did see ridges and troughs, plateaus
and ice plains. What you may find unusual
though is that it has very little in the way
of craters, this implies its surface is very
young and is constantly being renewed. Like
Pluto, it also has some reddish patches, which
is thought to be methane ice having reacted
to UV light from the Sun, producing what is
known as tholins, an organic compound that
has a supposedly tar like consistency. While
organic compounds do not mean life is present
there, organic compounds are the basic chemicals
from which life forms. Life likely couldn’t
exist on the surface of Triton anyway, as
its far too cold and the Sun far to dim to
support any lifeform that we can imagine,
but what’s interesting is what could be
found under Triton’s crust. Under Triton’s
surface is thought to be a rocky and metallic
interior, which gives Triton a reasonably
high density for a moon, at 2 g/cm³. Because
of this, and also due to the big step up in
size from the next biggest moon in the solar
system, Titania, it has more mass than all
moons smaller than it in the whole solar system
combined. The radioactive decay from the rocky
core could be enough to heat and power convection
in a subsurface ocean of water, much like
what is thought to be under the surfaces of
Europa, Enceladus and some other large moons
in the solar system. And just like Europa
and Enceladus, cryovolcanism is an active
process today on Triton. Liquid water in the
mantle erupts onto the surface like lava on
Earth. This is the main reason why the surface
is so young, it is being actively renewed
by liquid water erupting, and then freezing
on Triton’s surface. Some very young lava
plains have been identified, sparse and flat
regions, yet interestingly with a wall that
surrounds the plain. We call this a planitia,
or in other words, a solidified lava lake.
We also can see caldera, which is the collapse
found at the centre of a cryovolcano, where
lava plains formed from. It is thought that
the water from these eruptions would have
also brought minerals from the underground
oceans onto the surface, perhaps even being
the source of the tholins and organic matter
I mentioned earlier. If this is the case and
organic compounds are found in the subsurface
ocean, it means that there’s a possibility
that conditions are right for life to have
been able to form there. We also see long
lines permeating over the surface, these are
likely faults caused either by tectonic activity,
or freeze-thaw weathering processes. If we
look at some more of the Voyager 2 images
of Triton, we can see the results of some
recent eruptions. You’ll notice what looks
to be dark deposits on the surface, in cone
or funnel like shapes up to 150km long. However,
these smaller eruptions may not originate
from the mantle itself. Voyager 2 spotted
some plumes reaching 8km high, but these are
thought to because of a solid greenhouse
effect within the moon’s icy crust. Imagine
the surface of Triton consisting of a clear
ice which has settled on dark deposits like
tholins. The Sun shines through the ice, warming
the darker, more absorbent tholins beneath,
which sublimes a pocket of ice under the surface.
As the ices sublime, the pressure builds in
the air pocket until the surface above the
pocket gives way, causing an eruption. This
eruption also takes the darker deposits with
it, spreading them out on the surface again.
If this is the case, a very similar process
has been seen on Mars’ poles with carbon
dioxide ices and darker deposits under the
ice layer. This process can only exist because
of one thing, Triton has an atmosphere, although
not as thick as scientists were initially
expecting. Triton’s atmosphere is thin,
only 0.014 mbar, about the equivalent of 80km
up on Earth, although like Pluto, this density
varies through seasonal changes. Since Voyager
2’s observations, Triton’s atmosphere
has become denser, as the surface has warmed,
evaporating a little of the nitrogen ices
on the surface. However, when Voyager 2 passed,
Triton’s atmosphere was still dense enough
to support weather up to 8km above its surface.
In this image, you can actually see clouds
on Triton, and going back to the image of
the plumes, you can see the deposits from
the eruptions all end up facing the same direction,
due to a prevailing wind in that region of
the moon. Like Pluto, Triton’s atmosphere
is hazy, the cause of which is thought to
be hydrocarbons in the atmosphere not yet
broken down into tholins by UV light from
the Sun. The constant depositing of organic
compounds through cryovolcanism, ices evaporating
and freezing again through seasonal variations,
and a weather rich, active atmosphere makes
Triton a very dynamic world, unlike most other
moons in the solar system. It is more a dwarf
planet than a moon, likely a sibling of the
more famous Pluto in the Kuiper belt. All
these factors combined make it one exceptionally
unusual moon. And there we have it! Everything
you could want to know about Triton. Did you
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be sure to check out my other videos on the
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