For us, altering the composition of our atmosphere is a matter of pollution, an unfortunate byproduct
of our current technology.
But, for an advanced alien civilization, and someday ourselves, atmospheric alteration
could be one of the most useful tools in a terraforming tool kit, that is to say to take
an uninhabitable planet, such as Mars, and turn it into a fully habitable second earth.
But in doing something of that scale would we make ourselves visible to others in the
universe?
And, if so, have others done this and can we see them?
We are getting increasingly advanced with our observational equipment, such as the upcoming
James Webb Space Telescope, and that promises to allow us to look closer at exoplanets and
through various techniques in spectroscopy begin to study their atmospheres.
This opens up the possibility of detecting atmospheres that are altered by life, either
biologically or technologically.
In fact, if any alien civilizations are aware of Earth and its life, the most likely way
they found out about it is not our television signals as was envisioned in novels like Contact;
it's only been a hundred years or so that we've been broadcasting so any civilization
that might see that would have to be really close.
However, earth's weird oxygen rich atmosphere complete with methane screams life and has
done so for billions of years, meaning that anyone in this galaxy could be aware that
there is life here.
But that's just life, a biomarker as opposed to a technosignature.
However, if one sees an atmosphere altered by other chemicals or circumstances not so
easily created by nature then you have a possible technosignature; in other words a planet with
a civilization actively polluting its atmosphere if they are low on the technology scale, or
you may also see a planet with such an impossibly weird atmosphere that it could only have been
intentionally made that way.
We have two obvious candidates in our solar system for terraforming.
Mars and Venus.
Both are thought to have had liquid water at some point early in their history, and
might have initially hosted the conditions for life to arise.
Trouble is, they were in the wrong location to maintain those conditions, and in Mars'
case was just too small anyway.
Say we terraform those worlds in the future.
Any alien civilization studying our solar system would see two counterintuitively habitable
planets along with an obviously habitable earth.
Scientifically speaking, Venus should be a runaway greenhouse effect hell world, and
Mars a cold desert world.
Yet, the aliens would notice that they aren't.
Coupled with calculating the age of the sun, they would know that such a strange state
of planetary affairs might, but not necessarily, be indicative of a civilization.
Likewise, as our telescopes improve and we begin to study the atmospheres of exoplanets
in detail, if we saw that kind of an arrangement with habitable planets, we would want to take
a closer look at these worlds for other chemical technosignatures to gather information so
we could conclude beyond reasonable doubt that we were seeing an alien civilization's
activities.
This topic is covered in a 2014 paper by Henry Lin and colleagues, link below.
This paper focuses on the most easily detectable chemicals by the James Webb Space Telescope,
though only in one specific case: earthlike planets orbiting white dwarfs.
These chemicals are the CFCs, though there are others, such as nitrous oxide that could
point to industrial production by aliens.
CFCs are a group of chemicals that can be produced in nature, but in general are only
produced in large quantities industrially, so if you see them in a spectrum they are
an imperfect, but still fairly strong indicator that a civilization might be present.
But it might also imply that the civilization is primitive and in our own stage of development,
with inefficient industrial processes polluting their atmosphere.
Or maybe not.
What of greater civilizations and their use of CFCs?
In a paper by M.F.
Gerstell and colleagues, they note that CFCs also happen to be among the best greenhouse
gases there are, in fact they absolutely shame carbon dioxide in this regard.
They could, hypothetically, be used to warm Mars fairly rapidly.
You essentially load compressed CFC's onto delivery rockets and crash them into Mars
regularly until you get to concentrations high enough to raise the temperature just
enough to kick off a greenhouse effect.
Eventually Mars' frozen carbon dioxide releases into the atmosphere as things warm up and
at some point, voila, you have a warm planet ready to be further altered to be habitable
for humans, though good luck forming an ozone layer anytime soon.
But what does this say of alien civilizations?
If we do see CFCs in an exoplanet's spectrum would it necessarily indicate a reckless civilization
industrially polluting their world, or might it also simply indicate that we're looking
at a terraformed planet?
Thanks for listening!
I am futurist and science fiction author John Michael Godier currently terraforming my backyard
to produce more oxygen because I've been neglecting to pull the weeds and be sure to check out
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explorations into the interesting, weird and unknown aspects of this amazing universe in
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