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.
