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!
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The third and final part of this series will be released in two weeks’ time, where we
will look at the most likely places to find life outside the solar system.
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