In 2017 I was lucky enough to live very close to the path of totality of the great 2017
solar eclipse.
I was able to view it in a secluded rural location far away from the parking lots full
of crowds that had come in from all parts of the country, and indeed the world to see
it.
In my setting of a secluded backyard set amidst a forest, I was privy to see how nature itself
reacted to the change in lighting.
It was not what I had expected, even after years of having seen partial eclipses and
photographs of totality.
Instead, earth became something else for a short time, and I felt that it was something
like what standing on an alien world would be like.
The black light-like conditions of a partially blocked sun, and the very strange calm that
came with totality, left the planet I have spent every moment of my life living on feeling
like it was not itself.
The birds went quiet, in fact everything went quiet, not even the evening insects were tricked
into thinking this situation was normal, all unfolding under an eclipsed sun, with its
corona stretching out around that no photograph can do justice to.
That experience got me thinking, and trying to imagine earlier periods in earth’s history
where it was not the planet it is today, but rather very much like you might expect other
worlds in the universe to be like.
At times in the history of this planet it has undergone profound changes to the point
that it’s hard to imagine how different it once was.
One of these periods of a very alien earth compared to today was the Hadean Eon.
The Hadean is the first period in which earth went through just after its accretion from
the sun’s protoplanetary disk.
This era is so named because the planet was a hellscape during this period, where earth’s
surface was very hot, and to varying degrees molten.
The naming of this period is informal, mainly because very little geologic evidence of this
period has survived, mainly some zircon crystals embedded in newer rock.
But where more material from this time does exist is on the moon, and what happened on
the moon during this period was probably very similar to what was happening down here on
earth.
For the moon’s geology, it can be divided into several periods.
The first is the Pre-Nectarian stretching from 4.5 to 3.9 billion years ago, and marked
a period of volcanism and constant impacts.
This then led to the Nectarian period, ranging from 3.9 billion years to 3.8, where the late
heavy bombardment was in decline.
Earth’s atmosphere at the time would have been made up largely of hydrogen and helium,
which would have been in a constant state of escape.
Large areas of earth would have been completely molten after the impact with theia that formed
the moon when earth was probably less than 100 million years old, during early stages
of the Hadean.
Imagine the aftermath of that, not merely an asteroid impact, like that which killed
the dinosaurs, but a much greater cataclysm of an entire minor plant smacking into an
ever growing major terrestrial world leaving behind two completely molten spots on the
surface stretching thousands of kilometers, in addition to rampant and widespread volcanism
of a scale not see on this earth today.
And this horrific scenario would have created something very alien to us today.
When you have such an enormous impact, a lot of rock gets vaporized.
This would have led to a period where Earth would have briefly had a rock vapor rich atmosphere.
And while transient, it would have had this bizarre altered atmosphere for several thousand
years, ultimately yielding to a hydrogen and water vapor atmosphere with a lot of carbon
dioxide.
This is thought to have led to a very strange situation on earth.
Earth would have accumulated a lot of water in its formation, so there would have been
plenty of water present, but not in the way we see it today.
Water in the atmosphere during this period would have been being lost like the hydrogen
and helium, and with an average surface temperature of 230 degrees celsius, or 446 degrees fahrenheit.
Even though you’d think this very hot world would have vaporized all the water, it actually
might not have and there may have been oceans already present due to very high atmospheric
pressure, thought to be above 27 atmospheres.
In fact, it’s thought that earth in this period may have been a complete water world,
before the continents rose up.
But also going on was that this high carbon dioxide atmosphere was dissolving into the
oceans, which can act as a carbon sink.
But earth had not yet settled into its carbon cycle, and changes in the newly formed surface
would have not been stable, instead the amount of carbon dioxide in the atmosphere would
have varied wildly at different times.
This early period of an ocean earth under very different conditions than what we have
now actually has some physical evidence in the form of zircon crystals from about 4.4
billion years ago that show clear evidence of two things.
The first is the presence of an atmosphere and liquid water, due to their geology.
But it’s been pointed out that earth must have cooled down for these crystals to have
formed as their geology requires lower temperatures.
Known as the cool early earth hypothesis, it posits that while 4.6 to 4.4 billion years
ago things were very hot on earth, it may have cooled down for a time before the late
heavy bombardment.
This scenario may solve a mystery regarding the origins of life on earth.
During this cool period, the impact events that were going on may not have been as frequent
or heavy as has been assumed, but rather there may have been some kind of pause before the
late heavy bombardment.
This would have allowed for the cool period, and calm enough conditions for abiogenesis
to occur, which we find evidence of it having happened very early, and the emerging evidence
keeps pushing the date back further and further.
In fact, there may well have been several long periods in earth’s history where life
became possible, maintained it for millions of years, and then lost it again during a
period of bombardment.
This opens the way for a scenario where life might have arisen on early earth, but wasn’t
able to get far enough to get a foothold and was snuffed out by impacts, only for another
calm period to come along and abiogenesis occurred again.
You could term this a sputtering abiogenesis, where several occurrences of life on earth
have happened, but only ours got lucky and gained a foothold, and the others simply went
extinct in an impact because they hadn’t spread globally.
In other words, their geothermal vent or their tidal pool was hit, whereas ours got lucky,
and did not get hit and once our kind of life on earth got established, at least in its
microbial form, it became almost invincible and only the sun melting the surface of the
earth as its luminosity increases can ever kill it.
This also has implications on the notion of the shadow biosphere.
One version of that is that we could have an entirely separate microbial biosphere on
earth that independently arose on this world and is still here, and we simply lack the
ability to detect it because our methods are geared towards our type of life.
But we don’t understand abiogenesis very well, so another version is where abiogenesis
occurs periodically and simply gets eaten by the established microbes and can't ever
get a foothold.
This is something where we simply have to conclude that we don’t know enough to say
what was happening here 4.4 to 4.1 billion years ago in regards to life.
But the other mystery is plate tectonics.
Earth is unusual here, and we don’t know how common plate tectonics are on terrestrial
worlds.
Venus and Mars don’t seem to have it, and it plays a huge role in the carbon cycle vital
to life on earth now.
But it’s not clear just when it started.
When was earth’s surface completely molten, and when did it cool enough and plate tectonics
begin?
The answer may be in Hadean rock samples from Australia that seem to suggest plate tectonics
was present about 600 million years after the formation of the earth, or about 4 billion
years ago.
This would have affected the atmospheric carbon dioxide and sequestered it to the point that
earth would have become somewhat similar to how it is today very quickly in its history,
potentially favoring abiogenesis and survival of our brand of life on this world.
This may be the key to answering why earth was so hot, and then suddenly became so cool,
as the removal of greenhouse gasses were facilitated by the advent of plate tectonics, the advancement
of solid rock, and thus allowing for invincible widespread life to occur, and ultimately us.
But what’s really at the heart of this are, again, the zircon crystals.
These crystals are almost indestructible, and last in the geologic record indefinitely.
And like tiny time capsules, they preserve information about the conditions in which
they were subject and even reveal their age.
This is typically due to uranium.
As uranium decays, it turns to lead, and by measuring the uranium to lead ratio in zircon
crystals, you can determine their age.
But more importantly, by looking at the composition of the zircon crystals, you can even glean
hints about life itself.
This is due to the presence of carbon in the crystals.
Within the crystals carbon 12 and carbon 13 have been detected.
Life on earth tends to prefer carbon 12, so the ratio between carbon 12 and carbon 13
can reveal the presence of life when the crystal formed, which they tend to be layered.
All together, this seems to suggest that life on earth appeared literally at the very first
moment it could.
And while the evidence from the zircon crystals for life is still not agreed upon, contamination
can happen, so you need a bigger sample than we currently have, there remains a question
as to whether we’re actually seeing evidence of our family of life, or a completely different
extinct abiogenesis incident.
Incidentally zircons found in meteorites of Martian origin indicate two things, first
that Mars formed surprisingly quickly in the early solar system, and also that it had water
very early just like earth.
If life arose on Earth, there’s no reason it couldn’t have happened on Mars as well
given that the planets were under very similar conditions, and this may also have been the
case for Venus.
If through future study we can determine if either of those planets also spawned microbial
life in separate instances of abiogenesis, then it becomes a virtual certainty that the
universe teems with at least microbial life.
There would be countless billions, or more, living worlds in the observable universe that
we’d know had to be there, even if we couldn’t directly see them.
And that gets even better if life were found in the ocean of one of the ice shell moons.
This would also up the chances of a previous abiogenesis on earth having occurred, or may
have occurred multiple times during the Hadean and Eoarchean periods of Earth’s history.
And this also applies to Venus and Mars, which may have seen multiple abiogenesis and extinction
events, or have hosted shadow biospheres of their own.
Or the very complicated question of panspermia, where Mars might have had an original abiogenesis
that was then deposited on Earth, only for the first Martian abiogenesis to go extinct
and a completely separate one to later arise.
But just how abiogenesis occurred remains a mystery, though there are some interesting
insights that have come to light.
One is that scientists think that abiogenesis wasn’t any one single event, but a process
of evolution coming from prebiotic organic chemistry, and then on to molecular self-replication,
and then eventually things like cellular membranes emerge and eventually the process yields something
that can be deemed alive.
This would have all occurred under the conditions of the Hadean eon.
Given that the conditions on earth were so radically different then from today, can earth
under its modern conditions still produce abiogenesis?
As strange as it may seem, possibly not.
It may be that something about the conditions of the Hadean eon allowed for the chemistry
of abiogenesis, but our habitable modern world may lack that which could be a reason why
we don’t see life appearing randomly all the time on earth.
One interesting related tidbit here is a mysterious ancient organism known as LUCA.
LUCA, or the Last Universal Common Ancestor, is the latest organism that all life on earth
today descends from.
Studies of the genomes of archaea and bacteria show that all life on earth has inherited
355 genes from LUCA, and studies of those genes show that LUCA was likely an anaerobic
microorganism that existed over four billion years ago.
There are believed to have been earlier organisms than LUCA, it’s just the last common ancestor,
and given that they were anaerobic, it points squarely to them living in the environment
of a hydrothermal oceanic vent.
In fact, fossil evidence from Quebec also supports this, not that long after the formation
of earth’s current oceans 4.4 billion years ago.
This is of course good news for the ice shell moons like Europa and Enceladus, because if
life begins at a hydrothermal vent in an ocean environment, then they both are suspected
to have such vents.
If life begins at hydrothermal vents, then they are nothing close to a rare phenomenon
in the universe.
Microbial life in this case should be everywhere.
But of course there are competing theories, such as the idea that life began in a tidal
pool, in which case you need a moon for life to arise, or that life arose within clay where
certain types of chemistry related to life are known to occur.
Further still is the Iron-Sulfur world hypothesis where life arises on the surface of iron and
sulfuring bearing minerals such as pyrite.
There are many more, including my favorite, the radioactive beach hypothesis where the
highwater mark of an early beach accumulates radioactive materials such as uranium, possibly
forming a natural nuclear reactor, and giving the energy needed for the chemistry of life
to get going.
This situation may have led to what’s known as the RNA world, where self replicating RNA
molecules led eventually to DNA in conjunction with the appearance of cellular walls and
structures, which it is not yet clear how any of that occurred.
And there is also the role of viruses, which probably arose in the RNA world.
But the point is, with so many proposed pathways for abiogenesis to occur, it could be that
there are several viable paths for it to happen, which again ticks a box that the universe
teems with microbial life.
In the end the mysterious Hadean eon shows us that a very alien version of earth spawned
life, not the world we currently know.
And the first life, the anaerobes, can’t even tolerate the oxygen that hallmarks the
current atmosphere.
They had to retreat to anaerobic environments to survive the onslaught of the plants as
they poisoned the atmosphere with the oxygen we currently breathe.
And in a sense, we come originally from very alien conditions right here on earth, and
the palette of just what can happen with life on exoplanets may be wider than we imagine.
Thanks for listening, I am futurist and science fiction author John Michael Godier currently
plugging the excellent and interesting Eastern Border podcast, link in the description below,
where I recently guested for a discussion of cold war era soviet and american futurism
and how they differed in their viewpoints 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.