Life can dramatically change a planet.
Make no mistake, life defines Earth.
It happened here during the great oxygenation event where the advent of photosynthesis gave
the atmosphere of earth its oxygen, but only after contributing to rusting the free iron
on its surface.
Whenever you see rust on a car , that’s the plants indirectly trying to eradicate
it.
This oxygenation event was deadly to much of the earliest life on earth, which was forced
to retreat into the depths of the planet to hold on.
On the surface, it became the dominion of the plants, a status quo that continues to
this day, but the atmosphere we breathe is not the original atmosphere of this world.
But Earth is not the only planet that has seen atmospheric changes, albeit for different
reasons.
We know that Mars once had a much thicker atmosphere, and liquid water on its surface.
This alone is interesting, even if Mars never spawned life.
The idea that two rocky planets in our solar system had a period of being able to sustain
liquid water, along with Venus as a potential third, coupled with the myriad ice shell ocean
moons strongly suggests that the most basic solvent of life must exist as a liquid everywhere
in the universe, and very commonly.
Indeed, we know that water ice is among the most common materials present in galaxies.
We’ve always tended toward the view that if Mars once had life, maybe as it became
increasingly hostile as it lost its atmosphere, that life may have retreated down into the
planet’s crust as the anaerobic microbes did here to escape the poisonous oxygen.
As such, it’s been suggested that Mars may still host a subterranean biosphere, a remnant
from a second abiogenesis in the solar system, or perhaps a cousin of life on earth, and
rather abiogenesis may not have happened originally on earth, but on Mars and was transported
here through panspermia.
It is in principle possible that our origins are not here, but Mars.
But Mars and Earth have related but different histories.
Geologically Mars is much more difficult to study than Earth.
We have a handful of Martian meteorites from generally unknown locations on that planet,
so there isn’t a good handle on context from those, and most of them are volcanic
basalts, but not much else.
That means that Mars’ geology can only be studied from orbit, and by geologically equipped
rovers.
What we can tell however is that Mars has gone through four major geologic periods,
which are named after locations on Mars where evidence of them is to be found.
The first has little evidence left but almost certainly had to have happened in a specific
way.
It’s called the Pre-Noachian period, and goes from about 4.5 billion to 4.1 billion
years ago.
Essentially, this is the period where Mars cooled enough to form a crust, but would have
been subject to a very high, very intense impact and cratering rate.
This period also seems to have seen the formation of a major feature of Mars, that its terrain
essentially breaks down into two types, the Northern lowlands, and the southern highlands.
Mars at this time would have had a very dense atmosphere created by the constant impacts
releasing gasses from the planet’s interior, along with water vapor that would have condensed
into an ocean but one that was very high temperature, and held so by high pressure.
This initial ocean may have even been global.
But as that ocean cooled, sometime around 4.4 to 4.3 billion years ago, the atmosphere
continually escaped or was locked up geologically and it is here we reach the point where life
hypothetically could have gotten a start.
That leads us to perhaps the most intriguing epoch of Martian geologic history.
The Noachian, which runs from about 4.1 to 3.7 billion years ago.
This period is named after a highland region known as Noachis Terra.
This period still saw heavy bombardment, indeed this period saw earth’s Late Heavy Bombardment.
But we know from Earth that life somehow appeared during the late heavy bombardment, as dicey
as it must have been in those days.
Life arose on this world under conditions much worse than those that killed the dinosaurs.
And a bombardment it was for Mars as well, the largest impact structures, basins, on
that world date from this period.
And there was heavy volcanism going on in areas of the planet, and indeed the famed
Valles Marineris canyon was formed during this period, which appears to be a huge crack
or fissure in the surface, or at least started out as one.
Volcanic activity releases gasses, and that would have contributed to the atmospheric
conditions of Mars at that time.
This may also have been a time of rain on Mars, and indeed many of the markers of the
action of liquid water on the surface of Mars such as valleys and apparent lake beds are
related to Martian rain.
But this was also a time where Mars had both liquid water, and dry land, resembling earth.
Mars also would have initially had a magnetic field, so crucial for life on earth, but this
period also saw that dynamo shut down, and thus the disappearance of that field.
Even still, Mars would have conditionally been habitable for the duration of this age,
but less so as time progressed.
Next comes the Hesperian age at 3.7 to 2.9 billion years ago.
This age saw a drop in cratering rates, and somewhat of a drop in volcanism but not by
that much, with as much as 30 percent of the surface of Mars getting renewed by volcanic
plains.
The rain continued, but it was acidic and created extensive sulfate deposits and overall
altered surface geology.
Temperatures plunged, and the planet’s water began to freeze, but with abundant volcanic
heat this led to steam eruptions and massive transient flooding.
And then it ended, and that’s when life on the surface of Mars became unlikely.
For completeness, the last and current era in Mars’s geologic history is the Amazonian
period, ranging from 2.9 billion years to now.
This is the relatively inactive Mars we see today that preserves an extensive record of
its past geologic history.
This is the frozen desert Mars we’re now so familiar with.
Weathering is slow, mostly by wind, though it’s thought that there have been occasional
periods of a warmer, wetter Mars that were short lived due to changes in axial tilt and
its orbit.
In fact, even today some observations seem to indicate that Mars is actually coming out
of an ice age, and may see warmer temperatures in the future, but not by that much and liquid
water on the surface of Mars instantly boils away or freezes.
Unless you somehow terraform it, the oceans of Mars are not returning.
Early in this period Mars saw late volcanism, even on Olympus Mons, and indeed Mars may
still occasionally be volcanically active to some degree.
But whether there is life still holding out on Mars is an open question.
But Mars hints that something may still be going on.
In this regard, Mars has actually proven to be really mysterious.
Hints of life first came in the form of the 1970’s Viking Labeled Release experiments
which were designed to detect soil microbes on Mars essentially by dumping in some nutrients
into a soil sample to see if anything ate it.
Something did.
Indeed, the experiment was verified across both landers.
This question remains open however, in that we can’t know from the data whether we were
seeing a chemical reaction mimicking metabolism, or actual life.
But this life would be very different if it exists because it’s so near the surface
that it’s hard to see anything resembling our life being able to live there.
If Mars life exists, it seems a better bet to look deep rather than shallow.
But the mysteries didn’t stop there.
In 2018 seasonal plumes of methane were detected on Mars.
Methane can be formed geologically, but seasonally is a little bit harder to explain.
Further, Formaldehyde has been detected, which means that Mars is more geologically active
than we know, or it may hint at subsurface life.
And there are the still controversial findings in Martian meteorites that may hint at a biological
origin as they do somewhat resemble fossils.
But, there’s a huge problem here.
It may very well be that ancient life on Mars may have had no choice but to poison itself
and cause its own self-extinction.
The problem goes back to the beginning of this video.
Life can change a planet’s atmosphere, in Earth’s case it was for the better for the
eventual rise of intelligent life with our oxygen hungry brains, but ultimately not great
for the anaerobic microorganisms.
But that’s Earth, Mars may have been in a sufficiently different boat where the changes
to that world may have been worsened by the presence of life.
A recent study paints a picture of Mars about 4 billion years ago.
Life could have existed subsurface, consuming hydrogen and carbon dioxide, and producing
methane as a byproduct, all while being protected by the Martian soil.
As temperatures dropped, that life would have needed to retreat deeper and deeper to avoid
the increasing cold.
That mix of hydrogen, Carbon dioxide and methane, all heat trapping gasses may have compounded
the global cooling happening at Mars.
This leads to a tale of two hypotheses in astrobiology.
One is known as the Gaia hypothesis and it’s been around since the 1970’s, first advanced
by James Lovelock and Lynn Margulis.
This hypothesis rests on the notion that life is self-regulating and that life in general
reaches a kind of equilibrium of maintaining environmental habitability on a life-bearing
world like earth.
So think of it like this, if you end up with too much carbon dioxide in the atmosphere,
it gets compensated for by increased plant growth and activity.
As you can imagine, in recent years this idea has lost some favor given that on this planet
there is a giant elephant in the room altering any sort of natural equilibrium, human industrial
civilization.
The other is the Medea hypothesis, and it’s much more recent, advanced by Peter Ward in
2009.
One can’t help but be chilled by the naming of these two hypotheses.
Gaia in Greek Mythology is a personification of Earth, as the ancestral mother of all life.
Indeed, in the equivalent ancient Roman Mythology the name is Terra, as in Terrans of science
fiction fame in many stories, or Terra Ferma etc.
In contrast, Medea was a figure from Hesiod, Grand daughter of Helios, the sun, figuring
in the Jason and the Argonauts myth, and chilling in contrast to Gaia represents a mother that
murders her own children.
That name fits the medea hypothesis unusually well for a scientific name based on the ancient
Greeks.
The idea is that life itself is inherently self-destructive, more than it is self-regulating.
Ward pointed out that mass extinction events on earth don’t really look very self-regulating.
The great oxygenation event certainly wasn’t, it was really the opposite, it was life being
its own worst enemy.
Only through abundance and stability has life continued on here.
But for Mars, that may not have been available and a very different trajectory forced it
down a different path, though perhaps as earth’s habitability reaches an end, it may see such
a collapse as well.
While this work does not preclude the persistence of subsurface Martian life managing to hold
on until today, It’s not hard to imagine life running into problems the deeper it tries
to retreat early in the history of Mars.
Go too deep, and it’s too hot, go too shallow and it all freezes and thus wholesale extinction
may occur so long as there aren’t any conditions that offer a reprieve.
All of this decline may have been partly precipitated by the presence of life on Mars.
And that’s one of the more spooky solutions to the Fermi Paradox.
Most life is microbial, and it’s inherently unstable and self-destructive on any worlds
that aren’t just perfect to sustain life long enough for the magic of complex life
to happen, remember it took earth life nearly two billion years to make the leap from prokaryotic
to eukaryotic life.
Had the conditions of earth not been stable enough for that to happen, almost 2 billion
years of relative stability, earth would have only ever been a planet of the simplest microbes.
If that’s the rule of the universe, then here’s a scenario.
Simple microbial life is common, it could be everywhere, but very transiently because
it’s inherently prone to self-extinction and poisoning its own environment.
This could even extend to ice shell moons, where the advent of life creates a noxious
environment just by its chemistry.
Only very lucky, situational perfect planets like earth can pull it off long enough for
complexity to develop.
Those tolerances would depend greatly on where in the habitable zone of a star a planet is,
what conditions it presents such as plate tectonics and whether photosynthesis ever
develops, and also the presence of a magnetic field that sustains rather than shuts down
like that of Mars, but also the size of a planet.
Like Mars, it can’t be too small, but there is also an upper limit where it can’t be
too large which is thought to be about 2 earth masses for the super earths.
So you’re left with not just a goldilocks zone around stars, but goldilocks planets
that can produce complex life.
But then it needs to get to intelligence, which took earth almost 4 billion years.
Here we are, finally, but had anything happened in that very long period of relative stability,
had anything strayed just outside the parameters that allowed for us, whatever those may be,
and we wouldn’t be here.
And then there is the question of our own future survival and avoidance of extinction
as an intelligent species.
This really starts looking like the great silence, doesn’t it?
Perhaps that great silence is what we should expect to see.
And the ultimate solution to the question of are we alone boils down to a very simple
answer.
No, not always, but the natural result of it when it happens is extinction with very
few exceptions from microbial life to civilizations.
The great filter may simply involve the realities of life itself, at its earliest stages, and
from then on out it’s nothing but challenges to continue to exist.
And someday, sooner or later, you no longer do.
Thanks for listening!
I am futurist and science fiction author John Michael Godier currently commenting about
how that script certainly went unusually dark at the end.
It’s me, it happens.
I’m an author.
We do that.
But never fear, there are always the immortal machine civilizations that nothing can destroy
except the heat death of the universe!
All it takes is one past civilization in the Milky Way to have developed that and sent
it out von Neumman self-replicator style and oh yeah, we’ve got something to interact
with when we get out there and now you see why I don’t want to go to space and be sure
to check out my books at your favorite online book retailer and subscribe to my channels
for regular, in-depth explorations into the interesting, weird and unknown aspects of
this amazing universe in which we live.