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Recent findings have brought us closer than we’ve ever been to confirming life beyond the
Earth. As we wait for this to be confirmed or otherwise, let’s think about what this
will tell us about life everywhere, and also about our own imminent destruction.
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Three and a half billion years ago, two warm and watery planets twirled about a young G-type
star. On both of their surfaces, chemical soups bubbled and complexified and organized themselves
into tiny lipid balls that learned harvest energy, to duplicate themselves, to evolve.
Their methods were endlessly creative. On the smaller world, one lifeform learned to pluck a
spark of energy by reducing one iron compound into a slightly more stable one. They’d take hold in a
mineral-rich spot and spread outwards, a metabolic shockwave carving rings of vivianite into a damp
river delta. Who knows what else these clever little things might have become given time. How
much cleverer. But they didn’t have time. Their world was small. Its molten core cooled quickly,
hardened, stopped spinning. Its magnetic field switched off, and the delicate new biosphere
was laid bare to the abrasive solar wind. With the atmosphere lost, the world froze.
Its larger neighbor fared better for a while. It held its heat and so also its magnetism and
its atmosphere and its water and its carbon cycle. Life thrived, continued to complexify,
and eventually spawned some even cleverer little creatures whose metabolic shockwave was much,
much larger. They learned to travel between the worlds, or at least to send their robots. And so,
3.5 billion years later after some rings were carved in a river delta,
the artifacts of two interplanetary neighbors met. The robot of one touched the fossil of the other.
In that moment, the remaining lifeform understood two things: that, even now,
it was far from alone in this universe, and that its doom was closer than it imagined.
In case you didn’t guess, the small world is Mars and the large one Earth. The robot
is the Perseverance rover and the spots were the subject of a recent NASA announcement as
a possible biosignature. You may have heard the news. But this isn’t a news
piece. Here we’re asking the question: what does the potential detection of ancient life
on Mars or anywhere really mean for us? Why might it point to our doom?
I’ll say more about the finding in a minute. First, let me give you an alternate version
of this story that’s at least equally likely. Life only formed … only COULD have formed in
the larger world. Abiogenesis—the formation of life from non-life—requires such specific
circumstances that it’ll never happen twice in the one solar system, and may happen very
rarely in an entire galaxy. Now the robots still explored, but every pattern found in the rocks,
every odd atmospheric composition, turned out to be explainable by non-living processes.
They were phantoms of our fear of the truth—that we’re relatively alone in the universe.
We don’t know which of these stories are true—did Perseverance just discover ancient
life on our literal neighbor, meaning that life is extremely abundant in the universe?
Or will we never discover life because it’s insanely rare. It’s a hell of a thing to not
know which of these wildly different scenarios is true. That’s why the recent discovery by
NASA is so exciting. We’re on the precipice of possibly knowing one way or the other.
Let’s look at the science before we peer deeper
into the existential abyss of what this means either way.
The Perseverance rover landed in the Jezero crater on February 18th, 2021. The ancient
impact site was chosen to optimize the chance of finding evidence of past life. A critical factor
is evidence of past water. It’s pretty clear that Jezero was once a lake, with dry river beds
both entering and exiting. After meandering the crater basin, Perseverance found its way to the
Neretva Vallis region, part of the ancient river delta feeding the lake. Its sights were set on
the Bright Angel—an unusually light coloured outcrop in the region, apparently surfaces
with iron-rich sedimentary rock that washed in from surrounding regions billions of years ago.
And that’s where Perseverance stumbled on this spot. A collection of millimeter-scale
spots and rings that have been dubbed poppy seeds and leopard spots. The patterns sparked
immediate interest because they look for all the world just like deposits made by
Earth microbes—metabolic rings that expand as a colony grows.
So, Perseverance began its work. It drilled samples for Raman spectroscopy and bombarded
the ground to analyze reflectance and X-ray fluorescence. Then it sent the data home
via orbiting relay and the team on Earth who painstakingly analyzed the transmitted data for
many months. The team concluded that the rings and spots are some sort of iron phosphate,
while the pale interiors of the rings were an iron sulphide. Most likely vivianite
surrounding greigite. The team also found a strong association of organic molecules with
these patterns—and organic does not mean life, but rather the carbon-based molecules that life
tends to use. This all suggested a particular story - oxidation of organic molecules leading
to reduction of iron in the sediment into iron phosphates first, and then into sulphides.
And this is wild because it’s exactly what certain Earth microbes do. They extract energy via redox
reactions of minerals, grow and spread in the process, often leading to similar structures.
Alternative explanations do exist—the same reactions can happen in high heat or acidic
environments, however this once warm, wet river delta shows no signs of either. Nor of potential
geothermal sulphur sources that would be the other explanation for the greigite.
This all sounds pretty exciting. But it also sounds familiar. We’ve seen these
headlines before—scientists find biomarkers for life,
only to find abiotic—non-living—processes that can cause the same thing. Or, at best, for the
potential biomarker to hang as a mere potential without new evidence to confirm or refute.
So let’s take a look at some of the previous "maybe aliens"
to see if there’s a difference to this one.
A little over a year ago, NASA reported the tentative JWST detection of dimethyl
sulphide in the atmosphere of a planet orbiting a red dwarf 124 light years
away. This molecule is a major bioproduct of microbial metabolism on Earth. There’s
no known abiotic mechanism to produce it at least in the very high quantities needed to
explain this observation. However, further observations have not made that detection
any more confident—it remains very tentative despite further effort.
In a similar vein, in 2020, a team of astronomers reported the detection of phosphine in the
atmosphere of Venus. Again, phosphine is a common microbial byproduct, and so the team claimed this
as a potential biosignature. The Venutian surface is pretty uninhabitable, so the idea
here is floating colonies at altitudes closer to Earth conditions. And then there are these
shifting dark patterns in the venutian atmosphere, which Carl Sagan proposed could be microorganisms
back in 1963, and which seemed to show unusual UV absorption characteristics in a 2019 study.
So life on venus!
The problem is, neither the phosphine detection has not been confirmed,
and there are potential abiotic mechanisms for both the phosphine and for the dark streaks.
There are plenty more would-be biosignatures, but let’s go back
to Mars—or at least to a chunk of it that ended up as a meteorite on Earth after,
presumably, being ejected from its home planet by another impact. Scientists found
a peculiarly cylindrical mineral formation in this meteorite that they claimed might be
a fossil microbe. Chemical analysis is tricky in this case because of Earthly contamination,
so the speculation is all around the shape of the thing. But then, being the kill-joys they’re paid
to be, other scientists pointed out abiotic means of getting the same shape structures.
So, the common thread here is that previous “potential biosignature” discoveries either
have clear abiotic explanations or have not yet been verified as definite discoveries,
or both. The case of the Bright Angel find feels different. There really isn’t yet an
obvious abiotic alternative—at least not yet. Some new geochemistry could
certainly be at work, or maybe some freak circumstances veiled the hot,
or acidic, or sulphurous history of this region, confusing our story. But previous biosignature
candidates have all been quite a bit more tentative or more abiotically explainable.
At this point, it’s not possible to assess the likelihood of this being real.
The only thing that’s possible right now is a kind of gut check. On one hand,
almost every potential discovery that promises to revolutionize our entire understanding of
the universe turns out to be wrong. These sorts of false positives are expected.
At this advanced stage in the development of science, truly revolutionary discoveries are very,
very rare, and false positives only have to be slightly less rare to show up more often than the
real thing. It’s a combination of this reasoning and a sort of cultivated skepticism that leads
me to expect that some brilliant geochemist will find a boringly abiotic explanation.
On the one hand, the NASA experts who spent a year analyzing the evidence do seem … unusually
excited. And as maybe another data point, we’re doing an episode on it when we don’t
cover every “potential biosignature.” This one seems special. Special enough
that it’s got me and others thinking about the implications. So let’s talk about that.
Currently, we know of one place in the universe where life appeared—Earth. And we know that life
appeared quickly, which makes it tempting to think that life emerges easily given the right
conditions. Even if “right conditions” means only very Earth-like planets, there are many
billions of such planets in our galaxy alone. This implies that life is very common in the universe.
Or … not. There are strong arguments for why the early emergence of Earth life tells us very little
about the broader abundance of life. They come down to selection bias and anthropic reasoning.
It’s not surprising that we observe our home planet as having life because the alternative is
impossible. But that only puts us on a planet with life, not on one where life formed early—which,
again, we wouldn’t expect if life is improbable. But there’s another factor to consider. The
formation of life might be extremely sensitive to the conditions of the planet. In some places it's
pretty improbable, in others much more probable. The latter may be quite rare,
like the Earth. Depending on how this biogenesis probability depends on conditions,
it might be that only near-perfectly tuned planets get life, and they get it early.
This is the argument made, for example, by David Speigel and Ed Turner, they show that
the early emergence of life on Earth does not guarantee abundant extraterrestrial
life and that's the case if the prior probability of that abiogenesis is very
sensitive to initial conditions.
David Kipping adds to the analysis the fact that significant time is
needed for life to spawn a sentient species,
and that this timespan starts to bump against the maximum habitability of Earth-like planets.
He finds that the early emergence of life on Earth is perfectly consistent with life being rare.
But all of this changes if we find one more incidence of life in the universe,
and it changes massively if we find it on Mars. That’s a huge hit to the hypothesis that the
initial formation of life is improbable, and to the idea its probability is super
sensitive to the conditions. If life formed on Mars then life is not overly picky about where
it forms. And if it did so quickly on Mars then that short timescale is not biased by
the need of lots of time to then form us, as was the case for Earth. The Speigel and Turner paper
also quantifies this in light of the discovery of early life on Mars. In short, the discovery
of ancient life on Mars short-circuits the anthropic arguments for life being very rare.
That said, intelligent and technological life may still be very rare, and our non-observation
of advanced species arising before us on the galactic scale IS evidence of that rarity.
This leads to an awkward conclusion. If primitive life is extremely common,
and if it almost never reaches a stage capable of being seen across the stars,
then something impedes that development after the abiogenesis stage.
This idea is often framed as the “great filter”--one or more steps
in the development of civilizations that are so ridiculously hard to cross that very few
potential starting points actually make it all the way to star-hopping sci-fi mega-civilization.
And the question has always been, is humanity one of the lucky few that has passed its great filter?
Or is that filter still ahead of us? Was that hard transition abiogenesis, or the development
of complex cells or big brains, or all of these together? Or is it the survival from digital age
to space age that’s so damn tricky, due, perhaps, to species almost invariably wiping themselves
out. We don’t know, but the more we rule out great filters in the past, the more likely it
is that the great filter is in the future. Nick Bostrom and others have articulated this argument.
Currently, the probability of abiogenesis on a given suitable planet could be as small as
one in many, many billions—because anthropic selection allows such a rare phenomenon to be
observed by the observers it produces. Even if that’s one lifeform per galaxy or universe. Or
the probability of abiogenesis could be closer to one—100%--which means many billions of incidents
of abiogenesis in this galaxy alone. But the filter has to be there somewhere. We don't
see aliens. If we eliminate early filters, then later filters have to pick up the slack.
Abiogenesis could do a lot of work in pushing the filter into our past, but an observation
of life on Mars reduces the filtering effort of abiogenesis by a factor of up to many billions.
If we have found life on Mars, at the very least we should stop taking for granted that
we’ll survive into the future, because there’s now even stronger evidence of our existential risk.
I should add that there’s another story that allows us to keep feeling special— and
that's panspermia. It could also be that life formed only once and early on either Earth or
Mars and then early microbes hitched rides on impact ejecta to travel between the planets.
Then we’re back to wondering if we’re alone, and we also get to wonder if we’re Martians.
But putting existential anguish and planetary identity issues aside, let’s
allow ourselves to be excited at the potential discovery of our extinct planetary neighbors,
at least for a bit. Did we find life on Mars? We can absolutely know for sure
whether the Bright Angel patterns were biological when we can collect those samples. Perseverance
packaged the samples and leaves them waiting for a upcoming mission designed to return them to Earth.
That mission was cancelled in the last White House budget, so, there’s that. There was some
verbal backtracking after the NASA announcement but the retrieval of those samples is pretty
unclear. It seems to me that in these times of division, our first discovery of life on another
world could be just what we need to reignite some collective pride. And maybe to get on
with our real job of cooperatively exploring and maybe even contacting a life-filled space time.
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