In part 1, we explored the possibility of
life in the inner solar system. Out of all the terrestrial planets other than
Earth, it seems like Mars is the most likely place to habour life. Going beyond Mars, we start to leave the “goldilocks
zone” or the theoretical zone in our solar system where life could survive on the surface. We are now heading towards the gas and ice
giants, each with many unique and interesting moons. But can life be found this far out? I’m Alex McColgan, and you are watching
Astrum, and together we will go through the outer solar system and explore where scientists
think the most likely places to find life could be. The first planet after Mars is Jupiter. Jupiter itself is not at all hospitable to
life as we know it. It barely has any form of water, it doesn’t
have a solid surface, and the winds and convection forces on the planet would drag down any microorganisms
that tried to form in the tops of the cloud layer. The deeper you go into Jupiter, the more the
pressure and heat increases. The chances are very slim that life could
exist here in these extremes. However, Jupiter has some moons where the
conditions are much better. The biggest of Jupiter’s moons, called the
Galilean moons, are big enough to have differentiated interiors. Smaller moons and asteroids tend to just be
the same throughout, like a rock. However bigger moons will often have layers
and cores. The second of Jupiter’s Galilean moons,
Europa, is actually one of the most likely places to find life in the whole solar system,
but not on its surface. The crust of Europa looks extremely unusual
with these fault lines running all over. The crust is actually made of water ice, and
beneath this ice sheet is believed to be a liquid water ocean that spans the entire moon. Evidence of this is can be seen through the
rotation of the crust, which is thought to have moved by up to 80°, very unlikely to
have happened if the crust and core were solidly attached. Another piece of evidence is something that
has only just been confirmed in the old Galileo spacecraft data. Galileo actually detected water plumes or
geysers shooting water far into space when it passed by the moon very closely. In 2016, the Hubble team suspected they might
have imaged water plumes shooting 200km into space, and this rediscovered Galileo data
has confirmed it. NASA considers the prospect of life here so
intriguing that there will be a dedicated “Europa Clipper” mission due to be launched
in about 4 years’ time. The Europa Clipper will orbit Europa, passing
through the water plumes, sampling the water that is ejected. We are not expecting to find fish blasted
into space by these geysers, but the water samples will tell us what the conditions are
like under the crust, and if there really is a possibility of life down there. Future robotic missions that aim to reach
and traverse this ocean are still in the planning stages. Interestingly, while Eurpoa is the most likely
place to habour life around Jupiter; it is not the only moon that probably has an underground
liquid ocean layer. Three out of the four biggest moons of Jupiter,
Europa, Callisto and Ganymede all could have life under their surfaces. Callisto may have a water or ice layer up
to 300km thick. Ganymede has at least one water ocean layer,
but could also have several, all separated by sheets of ice. Ganymede is probably the second most promising
moon of Jupiter, as the bottom most water layer could be touching rock. Water/rock contact could be an important factor
for life to exist as the rock provides minerals. Ganymede is already the biggest moon in the
solar system, but data also suggests that its underground ocean could also be the largest. Ganymede will also be getting its own mission,
this time from ESA or the European space agency, and it will arrive about the same time as
the Europa Clipper. You may be asking ‘why do these liquid ocean
form under these moons?’ Surely being so far from the Sun means these
moons should be frozen solid? Well, along with tidal forces from Jupiter,
a predominant thought at the moment is that the moons generate heat through something
called “rossby waves”, also known as planetary waves. Rossby waves exist on Earth in its atmosphere
and oceans, and are caused by the inertia generated by the rotation of the planet. They are slow moving kinetic waves, but over
a whole planet or moon they can store a huge amount of energy. This means the subsurface oceans on the moons
could have a lot more energy than first thought, and may have currents and streams in them
just like in Earth’s atmosphere and ocean. Why do scientists think life could exist in
these oceans, away from sunlight and the goldilocks zone? Let’s remind ourselves of the factors needed
for life. Liquid water is present in these moons. Liquid water is essential because biochemical
reactions can take place in water. Water is also an excellent solvent that easily
dissolves and carries nutrients and other compounds in and out of cells. Life forms are made primarily of water. In fact, our human bodies are more than 60%
water. Conditions in the moons’ oceans would be
cold, but not too cold as water is able to be in a liquid state. Being this deep under the surface means it
is also a protected environment from cosmic and solar radiation. However, there needs to also be essential
chemicals and an energy source, which we can’t guarantee exists at the bottom of these oceans. Let’s compare these moons to life on Earth. Previously, scientists believed that the Sun
was the source of all energy for life on Earth, but as we explored the bottom of our own oceans,
we found ecosystems that were completely independent from the Sun. The organisms there relied on chemicals and
energy released by hydrothermal vents. Bacteria there use chemosynthesis, not photosynthesis
to create energy for themselves. These bacteria coat the vent, where amphipods
and copepods graze on them directly. Above them in the food chain are snails, shrimps,
crabs, fish, eels and a host more. Here, the temperature is hot, and the pressure
is enormous. If something similar to hydrothermal vents
exists at the bottom of these moons’ oceans, and life isn’t unique to only Earth, this
is a good a place as any to look. Beyond Jupiter, there are three more planets
and their moons. Like Jupiter, the planets are very unlikely
to contain life. However, some of their moons also share the
same characteristics with the moons of Jupiter. Particular moons of note are Rhea, the second
largest moon of Saturn, Titania, the largest moon of Uranus, Oberon, the second largest
moon of Uranus, and Triton, the largest moon of Neptune. The most exciting moon with these characteristics
though is Enceladus, a moon of Saturn. It has extremely active geysers which spew
250kg of minerals and water into space per second at over 2,000kph. It ejects so much material that it has formed
a ring around Saturn called the E ring. Cassini, a spacecraft that used to orbit Saturn,
was able to pass through these water plumes, and detected carbon, hydrogen, nitrogen and
oxygen, all key components of life. There is definitely heat being generated under
the ice crust, as can be seen in this heat map. These are the tiger’s stripes of Enceladus. The evidence of hydrothermal activity, water,
and essential chemicals mean that this tiny moon could be the most likely place in the
whole solar system to find life. Sadly, we are far from proving any of this. Enceladus does not have its own approved mission,
and it will be at least 4 years until we get orbiters launched that will go to Jupiter’s
moons. Actually exploring the oceans is still a long
way off. I can understand the problem though of getting
a robot that deep into a moon, but it’s a little disheartening to think we won’t
see it happen for at least 30 years. Beyond the planets and their moons, we have
dwarf planets like Pluto, Eris and Sedna. If they follow the patterns we see in the
larger moons, they too could have liquid water oceans under their surface. But we are very far away from being able to
prove that too. There are just two more curious places to
look for life in the solar system. The first is Titan, the largest moon of Saturn. It is extremely cold, and so is dismissed
by some as uninhabitable. However, it is unusual from any other moon
in the solar system in that is has a thick atmosphere with methane in it. In fact, the temperature is just right that
liquid methane can form on the surface. The moon actually has a methane cycle similar
to Earth’s water cycle. There is evidence of seas, lakes, and rivers
of methane and ethane on the surface of Titan. Other factors essential to life also exist
there, including chemicals and minerals on the surface, plus the moon orbits mostly within
Saturn’s magnetic field, which means it is protected from solar and cosmic radiation. Theoretically, lifeforms could exist that
replace water with liquid hydrocarbons. Such hypothetical creatures would take in
H2 in place of O2, react it with acetylene instead of glucose, and produce methane instead
of carbon dioxide. Titan has been compared to primordial Earth. NASA had proposed a mission to investigate
the lakes of Titan in the form of a boat, but again this was sadly turned down. Personally, I think that the lakes of Titan
would be the most interesting to investigate, and even seems a lot easier than burrowing
a several kilometres to reach a subsurface ocean. Maybe a space agency in the future will revisit
this idea. The last place to look in the solar system
for life is on comets. A long standing theory is that life has propagated
through the galaxy on the backs of comets, although it is quite an outside possibility. The Philae lander that visited the comet 67P/Churyumov-Gerasimenko
in 2014 was originally proposed to have life detecting instrumentation on board. Sadly, this idea was laughed out of court. But as it turns out, the comet is rich with
organic material and there are clumps that resemble viral particles. These findings are difficult to explain with
prebiotic chemistry. For now though, the concept of life on comets
is heavily disputed and so it will remain a remote possibility. Thanks for watching! This Astrum video is brought to you by Skillshare. Skillshare is an online learning community
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Skillshare for free. To sign up, go to Skl.sh/astrum. The third and final part of this series will
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