Saturn’s moon Enceladus is among the best chances for detecting microbial life in the
solar system other than here on earth.
Like the similar moon Europa, these ice shell moons show clear interaction between their
surfaces and the oceans located beneath the ice.
This comes in the forms of plumes spraying water into space, but also surface features
that show that these worlds are almost craterless, meaning that they get resurfaced periodically
due to water from the ocean beneath making it to the surface and erasing everything,
at least briefly.
There are other ice shell moons in the solar system that show this kind of surface geology,
and some that do not, yet are suspected of harboring oceans much deeper below their frozen
surfaces.
But in the case of Europa and Enceladus, this water is shallow enough to spray into space
through cracks, geysers, effectively making their oceans much more accessible for life
searches than the other ice shell moons.
You basically fly your spacecraft through the plumes and take samples and see what’s
there, which is on the agenda for Nasa and the world’s space agencies.
But studying the compositions of the chemicals dissolved in those ice shell oceans is much
easier as we already have a data set from several different past exploratory spacecraft.
One such spacecraft was Cassini, a NASA flagship mission to Saturn.
That mission ended nearly 15 years ago when Cassini ran out of fuel and was sent on a
deliberate crash course to burn up in Saturn’s atmosphere to avoid any collisions in its
moon system, avoiding any potential contamination.
But the mountain of data it collected over its years of operation in the Saturn system
is still proving useful and surprising.
This time it’s the discovery of a key element for life.
Phosphorus.
This is the rarest of the six elements that our brand of life is based on and absolutely
requires.
There may be other possible biochemistries that replace phosphorus with arsenic, but
chemically speaking it’s just not as good and despite some past claims that didn’t
pan out we do not see this kind of life on earth.
This is a huge problem in a broader sense.
Phosphorus may well be the achilles heel for life, so much so that the lack of it is in
itself a strong potential solution to the Fermi Paradox.
Not enough phosphorus, no dice for life.
There are a number of associated problems with phosphorus.
The first is that some evidence shows, though it’s not yet supported with a huge sampling,
that phosphorus is rare and spotty in the milky way.
Some areas seem to have little of it, which may mean those areas cannot support life.
If that turns out to be the case, then it’s no wonder we see the great silence and we
do not see alien civilizations everywhere.
They might be rare just because phosphorus rich systems are rare.
That would mean that the search for life outside earth really translates to follow the phosphorus.
As an aside, phosphorus scarcity is already a problem even here on earth.
And it has been since at least the 19th century, when countries had to start importing bone
meal and many other things to get phosphorus into depleted soil.
While carbon rules the roost as far as life on earth, It’s interesting to compare nitrogen
and phosphorus, because they can deplete in the soil very readily.
If you don’t have nitrogen and phosphorus, you don’t have the basis of proteins and
you don’t have the scaffolding of DNA respectively, which limits life.
Nitrogen isn’t an issue, it runs out quickly, but the atmosphere is about 80 percent nitrogen
so it can either be artificially fertilized, or put back into the soil by the right microbes
that can fix nitrogen.
You can even rotate crops and do things like plant symbiotic plants like beans that carry
those nitrogen fixing microbes with them.
Phosphorus, however, has no such trick on earth.
You have to wait for fertilizer or geology, so either the earth crushes and dissolves
phosphorus into the environment on its own, or we do.
And even when it does that, a pretty solid fraction of it reacts and isn’t available
to biology.
This issue has been known for a long time, Isaac Asimov was making the point in the 1950’s
that phosphorus is life’s bottleneck.
In fact, it’s still a mystery within astrobiology and the origins of life as to just how life
on earth managed to scrounge up just enough phosphorus to get going and keep going.
The answer to that ranges all over.
Some theories say that it might have come from meteorites initially, in some local impact
with a phosphorus rich meteorite that unusually increased the levels where abiogenesis was
favored, wherever that may be.
Yet others say it had to be lakes where the phosphorus collected.
We just don’t know, but there had to be abundant phosphorus for the magic to happen
at all.
And back to Enceladus.
Cassini had made a number of passes through the geysers of Enceladus.
While not ideal for the job, it was able to take some measurements.
Scientists sifting through the mountain of data from Cassini and other sources found
that the key constituents of life were present.
These included Carbon, Hydrogen, Nitrogen, Sulfur and Oxygen.
We also know that sugars and amino acids are easy to make, as shown by the Miller-Urey
experiments where you put in the nutrients, simulate the atmosphere of early earth, add
an energy source and out come the nutrients of life.
We also find amino acids, the building blocks of life, in meteorites.
The stuff of life is everywhere, we see organic compounds pervading the universe.
Only phosphorus stands as a question.
What was missing at enceladus however was phosphorus.
That element teams up with sugars to form the skeleton of DNA, and the process for just
how DNA came about on Earth is still yet another great mystery.
But you at least need the constituent parts, and here’s the good news.
Recent findings based on the Cassini data set have shown that Enceladus has plenty of
Phosphorus.
In fact, it’s in crazy high amounts based on interpretations of the Cassini data.
Its oceanic phosphorus content is 500 times that of Earth’s oceans.
So, that’s it.
Enceladus now satisfies the biggest obstacle as far as the chemical components of life.
If you have phosphorus, you’ve got the gasoline to run the car as it were.
But there are some things that might cast some questions on this.
The detections were not actually from a plume directly, but Saturn’s E ring, which is
strongly thought to be generated by Enceladus's plumes but that’s not completely proven
and the dynamics of it aren’t completely understood.
Still, plumes are the likely culprit.
But it turns out, it’s a good form of phosphorus, it’s orthophosphate which organisms can
use for growth.
It’s interesting because these findings actually contradict an earlier model that
suggested that Enceladus’ oceans would be depleted of phosphorus, though the author
of that paper was actually among the first to say that these new findings are a game
changer and they happily overturned his hypothesis.
Honest science at work indeed.
That’s how we learn and progress
So here’s another oddity about Enceladus.
It seems orthophosphate would dissolve at a much higher rate in Enceladus’ ocean than
anyone previously thought.
This could literally be the phosphorus moon, and earth the phosphorus poor restricted life
world to the tune of having 1000 times less bioavailable phosphorus than Enceladus has.
This also all but confirms that Enceladus’ oceans are in contact with bedrock, and are
dissolving chemicals from it.
Previous evidence has also strongly suggested volcanic vents doing it.
But, there remains a huge question and an even bigger mystery not often discussed outside
of the depths of the history of discussion within astrobiology.
The question is, we don’t actually know how old Enceladus is.
Here, we have conflicting data.
In 2016 a paper focused on the dynamics of the orbits of Saturn’s moons showed that
they should not be as they are due to tidal effects, for any satellites inward of Titan
in the Saturn system.
This would mean that all of those moons, including Enceladus, are young perhaps forming as little
as 100 million years ago.
It’s worth noting here that Saturn’s rings are transient, thought to only be 400 million
years old, and will be gone in about 100 million years.
Geologically speaking, things happen in the saturn system.
On the other hand there is some evidence that Enceladus’ ocean is at least a billion years
old, well long enough for life to have arisen there so far as we know.
So pick your contradictory data set on that one.
But within this lies the even bigger, very obscure mystery.
Take a look at Enceladus’s cracks.
These are presumably made up of material that welled up from the ocean below and onto the
surface.
It looks like blue ice, not unlike what you might see in arctic regions here on earth.
And, you find microbial life in those regions.
But look carefully at the cracks on Europa.
These again seem to be due to the same processes, two very similar ice shell moons should look
roughly the same, but why the dramatic difference in coloration?
This is an open question because we don’t actually know what’s causing that or what
it’s made of.
What we do know is that Europa is as old as the solar system, 4.6 billion years.
It’s had as much time to develop life as earth had.
Enceladus maybe not.
Rewind to 2002.
Planetary geologist Brad Dalton looked at the infrared signature of this staining and
did a comparison with algae that thrive around hot springs here on earth.
The signatures were similar.
So he subjected a variety of earthly microorganisms to conditions similar to that of Europa in
a lab here on earth.
Again, the infrared signatures were similar to the Europa staining, but not exactly in
infrared.
Some have suggested that mixtures of salts might be the culprit, but no combination of
salts has ever reproduced the results.
But here’s the oddity.
In the visible light spectrum the staining just so happens to be the same color as a
very prominent earth bacterium.
E. Coli.
Thanks for listening!
I am futurist and science fiction author John Michael Godier currently giving another shout
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