Well everyone, here it is, your most asked for video: Titan.
And to be fair, I can understand the curiosity towards it.
It is the only moon with a substantial atmosphere.
There is clear evidence of stable bodies of surface liquid on it.
And best of all, mankind has visited it so I will be able to show a lot of real photos
and video footage!
I’m Alex McColgan, and you’re watching Astrum, and here is everything you could want
to know about Saturn’s biggest moon, Titan.
But let’s start from the beginning and give
some context to this remarkable planet-like place.
Titan is the sixth spherical moon from Saturn, and unlike Jupiter’s four Galilean moons,
in the Saturn system Titan is all by itself in its size.
The rest of Saturn’s moons are pretty small in comparison.
To give some idea of how big it is, Titan's diameter is 50% larger than Earth's Moon,
and it is 80% more massive.
In fact, it is the second-largest moon in the entire Solar System, after Jupiter's moon
Ganymede.
It does actually appear slightly bigger than Ganymede if you were to put them directly
side by side, but this is caused by Titan’s thick atmosphere which extends its apparent
diameter.
Even so, Titan’s real diameter is still larger than the smallest planet, Mercury,
but it’s only 40% as massive.
As its density is quite low for its volume, its gravity is reasonably weak, at only 0.14gs,
or 1.35m/s2, which is even less than our moon.
Due to Titan’s low density, it is thought that its composition is half water ice and
half rocky material.
And like other celestial objects this size, it is believed Titan has a differentiated
interior.
This means it has layers, and like a lot of other large moons, one of these layers is
thought to be a liquid ocean comprised of water and ammonia under the moon’s crust.
This liquid ocean is comparable to Earth’s “magma layer”, situated between the core
and the crust, which has been made liquid due to heat, pressure, and to a certain degree
– tidal forces.
The existence of this liquid layer was proven more likely as Cassini - the spacecraft orbiting
Saturn – discovered extremely low-frequency radio waves in Titan’s atmosphere.
Titan’s surface is not known to be a good reflector of low frequency radio waves, but
a liquid ice interior would be.
Another point is that surface features on the moon have shifted by up to 30km since
Cassini started observations, which could imply that the surface is not attached to
the core, but is rather floating on this liquid ocean layer.
And while there is no evidence of life on Titan, scientists do speculate that the conditions
could be right for there to be life in this subsurface ocean.
Unfortunately, if there was life to be found on the surface or below, we will have to wait
a while as there are no planned missions to check out this possibility, and Jupiter’s
Europa is a more likely candidate to be investigated for life in the foreseeable future.
The differentiated interior of Titan does not produce a magnetic field.
Titan is still quite protected from the solar wind though, as 95% of its orbit around Saturn
is within Saturn’s own magnetosphere.
Titan orbits Saturn once every 15 days and
22 hours, and has a rather large orbital eccentricity, which means the orbit isn’t so circular.
Its orbital plane is also at an angle.
But that doesn’t mean that Titan is likely a captured object.
Rather, like Jupiter’s Galilean moons, it is thought that Saturn also had several large
moons in the past, but most of these had been destroyed through big collisions which left
Titan the lone victor.
Saturn’s medium sized moons, like Iapetus and Rhea, are thought to be the remnants of
this tumultuous beginning.
Titan’s day, like the day on our Moon, is identical in length to its orbital period.
This means Titan’s rotation is tidally locked to Saturn and only ever shows one face to
the planet.
Not that visually it makes any difference; Titan’s hazy atmosphere completely blocks
the view of its surface from an outside perspective.
On the other hand you might just about be able see Saturn while standing on Titan, although
the view would be significantly obscured.
This does mean however that if you were to stand on one spot on Titan, Saturn would never
move in the sky.
Removing Titan’s haze, this is what it would look like.
This leads us on to one of the topics that
truly sets Titan apart from the rest of the moons in the solar system – its substantial
atmosphere.
I remember the first time I ever saw a photo of Titan; I was truly blown away as it never
occurred to my young self that a moon could even have an atmosphere.
I thought it must have been a new planet that they discovered recently or something!
To me, what looks odd about the atmosphere is how far it stretches into space.
When you see a picture of Earth, you see that the atmosphere has quite a tight fit around
the planet.
Titan on the other hand looks like it has a thick blanket all over it.
There are a number of reasons for this.
The first one is that Titan is a lot smaller than Earth, but its atmosphere is 1.19 times
more massive than Earths, or 7.3 times more massive on a per surface area basis.
The second reason is that Titan’s gravity is a lot weaker than Earth’s, meaning it
doesn’t pull it down as strongly.
The mass of the atmosphere actually means that the pressure at the surface is 1.45 atm,
45% more than the atmospheric surface pressure on Earth.
And comparing the two, you can see the extent of how far Titan’s atmosphere stretches
into space.
600km high is only the limit of the mesosphere.
Earth’s mesosphere on the other hand stops at 120km.
Even at a distance of 975 km, the Cassini spacecraft had to make adjustments to maintain
a stable orbit against atmospheric drag when it made its closest approach.
Like Venus, Titan is a “super-rotator”, meaning its atmosphere rotates faster than
the rotation of the planet.
This can especially be seen at the poles on the moon, each pole has a polar vortex that
rotates once every 9 hours compared with the moons rotation of 16 days.
The vortexes on each pole seem to be like permanent hurricanes.
So what does the atmosphere consist of?
And why is it orange in colour?
Well, the atmosphere is 98.4% nitrogen, the remainder being mainly methane and small amounts
of hydrogen.
There are also trace amounts of hydrocarbons from the break-up of methane in the upper
atmosphere due to UV light, and it is these hydrocarbons that are thought to give Titan
its orange hue.
This constant breakup of methane to hydrocarbons should have meant the moon ran out of methane
within 50 million years, a very short space of time compared to the age of the solar system.
This means there must be a source that replenishes the methane, the most likely candidate being
cryovolcanoes, although biological life has not been ruled out.
The methane in the atmosphere creates a greenhouse effect, without which the temperature on Titan
would be a lot lower.
Conversely however, the haze also reflects a lot of the sunlight, creating an anti-greenhouse
effect, which cancels out some of the potential greenhouse effect from the methane.
While Titan’s upper atmosphere gets 1% of the sunlight Earth does due to the distance
from Titan to the Sun, another result of this reflection of sunlight means the surface of
Titan only gets about 0.1% in the end.
The Huygens team likened the difficulty of taking photos at this light level to "taking
pictures of an asphalt parking lot at dusk".
All these things combined means that while it would be dark, a human would only need an oxygen mask
and to wrap up extremely warm to be comfortable while standing on the surface of Titan.
It really is cold on Titan.
-183c on average.
This means any water on Titan remains solid, and doesn’t ever melt, evaporate or sublime.
Then why then are there sometimes clouds on Titan?
Well, these are not water ice clouds, but rather methane clouds.
Which means yes, it can rain methane on Titan.
In fact, the temperature on Titan is just right for methane to be liquid.
Methane freezes at -182.5c and boils at -161.5c.
The temperature, combined with the surface pressure, got scientists very excited at the
prospect of there being hydrocarbon lakes or seas on the surface of Titan, similar to
water lakes and seas on Earth.
If there really were lakes on this moon, it would be the first time this had
ever been observed outside of Earth.
This was actually one of the main driving forces behind the Cassini-Huygens mission,
to see what there was under that thick atmosphere.
The Huygens probe, named after the astronomer who discovered Titan in 1655, was designed
to enter Titan’s atmosphere and land on the surface.
The possibility of even landing on an ocean was even taken into account during its design
process.
As the probe descended, its parachute was pulled and after an almost 3 hour journey
it finally rested on the solid surface of Titan.
Sadly, it wasn’t able to see any lakes, but what it did see confirmed that methane
lakes once existed, as Huygens landed on what appeared to be a dried up lake bed.
These stones you can see from the surface photos are rounded stones, much like pebbles
found in a river or lake on Earth.
Cassini, from the perspective of space, was able to confirm that methane lakes are still
found on Titan today.
Near the south pole, Cassini observed an area which was later confirmed to be a lake called
Ontario Lacus.
It is 20% smaller than its North American namesake, Lake Ontario, so in other words
it is still pretty big at 15,000 sq km.
On this side of the lake, you can see a smooth shoreline, eroded by waves.
On the west side, you can see the first evidence of a river and delta on Titan, meaning that
liquid hydrocarbons flow down higher plains to the lake, leaving delta deposits in much
the same process you would find on Earth.
Ontario Lacus is extremely shallow, only estimated to be between 40cm and 3m deep, the deepest
point likely to be just over 7m.
As Cassini radar mapped this lake, it found that lake did not have waves bigger than 3mm,
meaning the surface would appear like a sheet of glass or a mirror.
This doesn’t mean there can’t be bigger waves, unless the liquid is particularly viscous,
but the likelihood is that it was simply not a windy day as the observations were taken.
The atmospheric density and gravity on Titan should mean waves would be bigger on Titan
than they would be on Earth.
As the North Pole began to come out of a 15 year winter, another lake was discovered,
Jingpo Lacus.
As Cassini was passing by the moon, sunlight was reflected off the surface of Jingpo Lacus
like a mirror, directly into the view of Cassini.
Upon further observation, Cassini was able to detect further evidence of moving liquid
on Titan, as can be seen by these rivers flowing into the lake.
The second biggest lake on Titan to be discovered is called Ligeia Mare.
Found in the north polar region of Titan, it is bigger than Lake Superior on Earth with
a surface area of 126,000 sq km.
While parts of this lake are reasonably shallow, the average depth is a lot deeper than Ontario
Lacus at 50m, and some parts of it could reach depths of over 200m.
Plenty of rivers can be seen flowing into the lake, and there are large islands found
around this area here.
A particularly curious observation Cassini made, dubbed the “magic island”, is the
appearance and disappearance of what appears to be an island.
Although scientists are unsure exactly what happened here, the theories are that it could
be silt suspended in the lake, bubbles, or subsurface ice rising to the surface as the
lake warmed during the moon’s spring.
But still, very curious!
The largest lake on Titan, at 400,000 sq km is the Kraken Mare.
As you can see, the lake is split up into two main parts, separated by a small stretch
quite similar to Earth’s strait of Gibraltar.
Its nickname is the “Throat of the Kraken”.
Because of tidal forces and the size of the lake, it is thought the tides change by about
1m and so this strait may have strong currents and even whirlpools.
The Kraken Mare is also quite deep in comparison to Ontario Lacus, but isn’t any deeper than
170m.
So we know about the lakes on Titan now, but what other interesting surface features might
it have?
Well, plenty actually.
Titan’s surface is quite young, as young as 100 million years old, which means its
surface must be geologically active.
Some scientists believe the dirty ice crust is substantially rigid, although there’s
also evidence to suggest there is tectonic activity on the moon, possibly caused by tidal
forces with Saturn.
The main factor of a renewed surface, however, is likely to be the same thing that produces
the methane in the atmosphere – cryovolcanoes.
Now this is pretty interesting, you know how magma on Earth is pretty hot, but when it
comes out of the ground it freezes, well Titan has the same thing, except its magma is water
and ammonia.
And when it comes out of the cryovolcanoes and spills over the land, it too freezes to
renew the surface.
Because a water and ammonia blend is a lot less viscous than lava, it flows further than
lava on Earth.
This means mountains are more flat, and will never reach the heights of volcanoes on Earth.
While it is hard to confirm specific cryovolcanoes on Titan due to the obstruction caused by
the atmosphere, the most likely candidate is Sotra Patera, found on the Southern Hemisphere.
In this image height has been exaggerated by a factor of ten, but it gives a good idea
of the sides of the dome, and the 1.7km deep pit, the largest that we know of on Titan.
The force necessary for this to erupt would have had to be incredible, and while it doesn’t
appear to be active now, it is being actively monitored.
The fact that lava on Titan is a mix of water and other minerals means the surface could
be compared to dirty ice.
Because we know that Huygens bounced and wobbled in a certain way as it landed on Titan, we
have a rough idea of what the consistency of the surface could be like.
Scientists have referred to it as soft damp sand.
Another theory is that it is like snow with a thin crust on the top.
Imagine walking on frozen snow, it you’re careful you can walk on a solid surface, but
if you stomp too hard you will sink in quite deeply.
Titan is believed to be something like this.
Titan’s highest mountains come in the form of ridge belts, like the Rockies in America.
These ridge belts could also be a form of cryovolcanoes.
The largest mountain on Titan can be found in one of these mountain ranges, known as
Mithrim Montes, and is 3,337m high.
Interestingly, mountains this tall are thought to be topped with methane snow!
Also found on Titan are many gorges, valleys and dunes.
Using infrared cameras to see the surface from space, what can also be seen quite prominently
are these large patches of dark terrain.
Originally, these dark patches were thought to be seas until the Huygens probe landed
on one of those areas, known as Shangri-La.
They could well have been seas in the past, but now they are plains of dark mineral deposits.
Similar to the Namib Desert on Earth, they appear as windswept dunes in some places.
Overall, Titan can be compared to Earth in a lot of ways.
Scientists think that Titan shows signs of what early Earth could have been like, only
much colder.
It’s fair to say that Titan is remarkably interesting, and I can only hope it gets its
own mission one day.
Cassini has done a great job, but it was never a Titan orbiter and its mission will soon
be over.
What I would find extremely fascinating is to explore more of its surface, and hopefully
it won’t be long until a new mission to do that will be approved.
Until then, here was everything you could to know about Titan.
As you might guess, this is a start of a new series I’ll be doing called “Our Solar
System’s Moons”.
It will be much the same format as the “Our Solar System’s Planets” videos, and at
some point, I will also do an “Our Solar System’s Dwarf Planets” where I will do
the next most requested video, Pluto plus a lot more.
So there’s still a lot to look forward to on this channel!
I hope you’ll subscribe so you don’t miss out, and I’ll see you next time.