A 1990 experiment to test whether we could discern life on Earth remotely
nature.com
nature.com
Life in other places might be radically different from our own, and we might be looking for the wrong things - but it is good to know that our search is at least able to find what we are looking for.
Even though we know that our atmosphere is maintained by life, nobody did really know if something else could create oxygen and methane. And betting on a specific light absorption spectrum is quite a hard thing to justify.
Today we know that those things are rare. And they are orders of magnitude more intense than the unexplained phenomenon we see on other planets. But at the time, we didn't have proof.
For mammals, the radio-emitting time is still only a 2-millionth of the total.
The popular conception is that our undirected, historical tv/radio signals might be detected by ETIs - and that therefore this is what our SETI efforts are looking for. In practice, the former is unlikely using known scientific principles, and the latter isn't really being attempted for the same reason.
Very possible. Something behind our imagination (e.g. Energy, Light based life forms etc.). But it is not unreasonable to expect something similar, based out of Carbon or Si that has a metabolism and senses.
Once I read a suggestion to look for life: look for an entropy source.
The only way alien civilizations will pick up each others' signals is if they're intentionally directed at those systems with highly focused emitters. If you're intentionally sending a signal in hopes another civilization will see it you're definitely not going to encrypt it. Compression also serves little purpose as there's no shared context to decompress the message.
By the same token, if humanity disappeared but left a bunch of encrypted hard drives around and aliens came and found them, they may have no hope of distinguishing the bits from random bits, but they'd be able to tell that no natural process encodes any kind of bits at all onto magnetic discs or flash drives or whatever else we manage to come up with.
The actual issue is what the other commenter said about the impossibility of detecting such low power radio at all.
historical forms of signal modulation like ook and fm don't look very much like white noise, because they're designed to be demodulated by circuits with 1-10 vacuum tubes, but more modern modulations can look pretty white. they'd look a lot whiter if they weren't constrained by regulation to strongly attenuate bands used by obsolete forms of signal modulation
I think even this is a bit of an assumption. I have a sort of thought experiment that what we call physics and what our ancestors saw as angels or spirits moving the heavenly bodies _could_ actually both be true.
Life as we know it is a source of entropy but if we take the view of God as an architect/clockmaker who created the universe as a (mostly?) deterministic machine with angels being his agents, it would stand to reason angels would be orderly as well rather than chaotic. Perhaps so orderly we would perceive their invisible involvement as natural laws rather than unusual miraculous intervention people usually expect from that sort of thing.
Of course it's unprovable and even if you proved it it doesn't really contribute much to our understanding of the natural world, but it's interesting to imagine a form of life so orderly that we don't even perceive it as life but as part of the machinery of the universe.
If it can't die it's not alive
And just because life on earth is fragile, it doesn’t mean all forms of life have to be.
When it comes to life made out of molecules, there aren't that many viable options for "base components".
For example, carbon based lifeforms like stuff on Earth can harvest energy from food without spending too much energy while remaining stable enough to avoid falling apart.
Silicon has somewhat similar properties, but its compounds require stricter conditions to be useful for hypothetical life.
Some amount of fragility is actually a good thing. Not for an individual, but for life in general. Animals harvest "building blocks and fuel" for themselves from other life forms that should be "fragile" enough to eat. All the advancements of evolution are possible due to mistakes in DNA replication that should be fragile enough for mutations.
That's why I mentioned silicon.
We have examples in popular culture already that are neither fragile nor organic: sentient machines.
Im not saying that the universe is full of Transformers and Terminators. Just that searching for silicon based life isn’t all that different to search for carbon based life when you consider that life is just a sophisticated machine and you can literally build a machine out of anything.
> digital 'computers' that can replicate themselves
By this definition we are just a few RepRap 3d printer revisions away from creating life.
> can you explain why that is not life?
As far as I know, the answer depends on ability of such a computer to become a competent Starfleet officer.
They included it because one of the scientific definitions of “life” and specifies the ability to reproduce.
https://en.m.wikipedia.org/wiki/Life
A RepRap wouldn’t satisfy every criteria though.
While it is certainly not impossible that there are forms of non-carbon based life out there, they would have to be utterly dwarfed by the number of carbon based ones.
Carbon's chemistry comes from three major factors:
(1) It forms four bonds readily. (2) It can form double- and triple-bonds readily. (3) It bonds strongly to itself in a configuration that allows it to form long chains.
(1) is satisfied by other elements in its column on the periodic table, but silicon (the next element down in its group) and the following members (germanium, tin, and lead) fail (2) and increasingly (3). Silicon will form chains, but is reluctant to form double bonds; in general, double bonds become weaker for atoms further down the table. Silicon (and silicone, chains of Si-O bonds) are the most promising analogs but they have a lot of problems.
(2) is satisfied by most other light nonmetals, but those nonmetals mostly fail (3) and almost all fail (1).
The highly-electronegative oxygen doesn't really want to bond with itself (failing 3) and almost always takes a -2 oxidation state (failing 1).
Nitrogen actually does form four-bond atoms decently often (most notably the ammonium ion), so it somewhat satisfies (1) to some extent, but is so eager to form N2 that most polynitrogen compounds are wildly unstable to the point that "nitro" is a term even laypeople know is associated with explosives (failing 3).
Boron can form three bonds, allowing some of the complexity of (1), and will form nice polyboron compounds (3), but doesn't like forming double bonds (failing 2), and the bonds in boranes are so weak that they're mostly quite reactive (weakening 3).
Sulfur will form polysulfur chains (the most common form is an eight-membered ring), but sulfur (like its cousin oxygen) usually doesn't form more than two bonds except with extremely electronegative partners (like its effective total bond order of 3 in sulfur dioxide or 4 in sulfur trioxide).
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There's another problem here too: life is likely to form out of common elements in its environment, and carbon is just WAY more common than the alternatives. The mechanisms by which the elements are formed in stars very strongly favors elements with even atomic numbers (because they are mostly formed from helium-4 nuclei) and the burning processes peak at carbon/oxygen, neon, magnesium, and silicon.
As a result, carbon is very common. It's the fourth most common element in the Universe (after hydrogen, helium, and oxygen), and ~an order of magnitude more common than any of the elements discussed above except oxygen (which has basically no analogs to carbon chemistry).
> Astronomer William Keel explains:
> The cosmological principle is usually stated formally as 'Viewed on a sufficiently large scale, the properties of the universe are the same for all observers.' This amounts to the strongly philosophical statement that the part of the universe which we can see is a fair sample, and that the same physical laws apply throughout. In essence, this in a sense says that the universe is knowable and is playing fair with scientists.
There is no evidence that the nature of the electron or carbon atom is different in different parts of the universe. This would be detectable (if it was) by shifts in the spectral lines for atoms or differences in ratios of elements.
The law of gravity appears to be consistent throughout the observable universe. The physics of atoms and ratios associated with nucleosythesis have been shown to be consistent.
The constants of physics appear to remain constant over billions of years ( https://apod.nasa.gov/apod/ap050220.html - " Oklo by-products are being used today to probe the stability of the fundamental constants over cosmological time-scales" )
How do we know that distant galaxies are composed of matter rather than anti-matter? If equal quantities of each were produced in the big bang, might not some parts of the universe contain primarily matter and other parts primarily anti-matter? - https://www.scientificamerican.com/article/how-do-we-know-th...
https://www.nature.com/articles/nature.2012.11045 ("New molecular bonding mechanism predicted in exotic stars’ atmospheres")
Maybe one could speculate about a white dwarf star, cooled down to room temperature over astronomical eons, with some sort of life then evolving on its extremely magnetized surface. Its laws of chemistry would work differently.
It used to be that the thinking was that this was simply unlikely to the point of being impossible but since 2017 there has been some change in attitude towards especially the possibility of silicon based life.
the humans live out their lives at 300°, absolute, using the kelvin scale. there's lots of interesting behavior in matter down to 4° or less, and some solids up past 4000°. going further, human science doesn't really have a good handle on what kinds of complex interactions can occur with alfvén waves and plasma currents; they don't even know what's going on with prominently (heh) observable phenomena on the surface of the sun or how to stabilize a tokamak. but to be conservative let's consider just the 4°–4000° temperature range where we have solids and liquids
from that temperature range, covering three orders of magnitude, carbon-chain molecules have interesting and complex behavior over the range 200°–500°, more or less. just a factor of 2.5. 13% of the temperature range. below 200° (-73°C) pretty much all organic chemicals crystallize and become brittle and nonliving, though there are a few boring exceptions like methane. above 500° (230°C) they are unstable, breaking apart into elemental carbon and methane, again with a very few exceptions like teflon. the temperature range over which carbon chains are practical to synthesize is smaller still
a chemist on titan (94° absolute) would consider implausible the suggestion that living cells could be made out of carbon-chain molecules dissolved in water, a material that doesn't even melt until hellish temperatures nearly three times the boiling point of methane. a chemist on venus (740° absolute) would consider implausible the suggestion that living cells could be made out of such absurdly unstable molecules as carbon-chain polymers — much less with amide groups, which she knows as violently reactive with ordinary everyday substances like red-hot sulfuric acid. moreover, she would consider the pressure on earth's surface (1% of atmospheric for a venusian) to be a good approximation to the vacuum of space, constantly exposed to sterilizing levels of ultraviolet from the sun
(venus and titan specifically are not covered in vegetation or presumably in chemists, but there are astronomical numbers of other planets with conditions like theirs, and as sagan's paper pointed out, antarctica isn't exactly a jungle either)
there are interesting things going on over the other 87% of the temperature range, too. at room temperature the humans mostly know phosphates as minerals that are unusually hard to melt, but phosphoric acid just starts to polymerize above about 500° absolute and stays liquid past 1000°, polymerizing more and more enthusiastically; some common phosphates remain solid past 2000°. neat silica becomes a useful solvent at temperatures a bit higher than that, and silicates of sodium or potassium can be liquid down to room temperature if plasticized with enough water; silica is nearly as abundant as carbon. ammonia is liquid from 195° to 240°, a narrower range than water (273° to 373°, which is a 3:4 ratio to ammonia's 4:5) but not ridiculously so, and shares many of water's interesting properties, and we already know many interesting substances that can be synthesized in an ammonia solution, including many that are not stable at room temperature (and therefore might be in a useful reactivity range in ammonia's liquid range). and there are 1.7 orders of magnitude of temperature below ammonia's freezing point before you get to liquid helium and the cosmic background radiation. most of the universe's volume is in that temperature range, though not most of its matter, and if dyson's 'future of life in an ever-expanding universe' is correct, those temperatures and below are where our descendants will spend the vast majority of history
but the humans really only know a tiny fraction of the possible complex molecules at those temperatures, because working with them in their laboratories is difficult, expensive, dangerous, and unprofitable; molecules that explode when you heat them up to room temperature, or solidify into refractory ceramics when cooled anywhere near it, will not yield useful drugs, elastomers, cheap injection-molding plastics, or fuels, so research into them has mostly been pure, not applied. the exceptions are mostly high-temperature ceramics research focused on finding the strongest and most refractory materials, not interesting synthesis pathways that can operate inside white-hot zirconia crucibles
but don't let the utterly disorderly behavior of, say, nitrogen chains at the melting point of water fool you into thinking their chemistry is trivial; most compounds that are stable at 10% of that temperature haven't been discovered yet
(by the humans)
as the llenothians say, 'we don't know who discovered ammonia, but it wasn't a 𐇳𐇚𐇺𐇠𐇺'
(if you enjoyed this you might also enjoy my notes in https://dercuano.github.io/notes/2017-sap-allocation.html, which is less strictly factual)
https://en.wikipedia.org/wiki/Balanced_ternary has some enjoyable advantages over more conventional number systems; numbers are a lot shorter than in binary, but arithmetic is nearly as easy, and it accommodates negative numbers naturally and inherently rather than through a length-dependent hack like twos' complement. under certain plausible assumptions about your available hardware, it has about a 5.7% device complexity advantage over binary (https://en.wikipedia.org/wiki/Radix_economy), though those assumptions often do not hold in practice
so i thought it would make a good example of path-dependent divergences: this alternative system is arguably better than binary, decimal, vigesimal, or sexagesimal, but not by enough to make it worth the incompatibility in the real world
an extraterrestrial society with a different history, and thus no need for compatibility with c, ascii, ttl, and synchronous-logic eda platforms, might have taken a different path
Extremophiles are real world examples of the kind of life that still has some commonality with the rest of us and yet they are already skirting the edges of what's possible and they thrive in what wouldn't even begin to qualify as livable circumstances for others.
https://en.wikipedia.org/wiki/Extremophile
They're not going to start a Venusian equivalent of the royal society but they are life and I think it is important to note the fact that life doesn't necessarily have to be intelligent or even multi-cellular to be just as real as we are. And the thresholds for 'life' are probably a lot lower than for 'intelligent life'.
i don't think you can go that far from room temperature with nucleic acids. one thing that casts doubt on my thesis here is that though luca clearly used nucleic acids, surely (?) life didn't start that way. but we haven't found even undersea-vent extremophile life with a silicone-based genome or any other alternative chemistry, even in ecological niches where there would be no competition from carbon-based life. so,
- i could be just wrong;
- abiogenesis might have originally produced carbon-based life and then been unable to escape the shackles of that heritage, even in places like the parts of undersea vents that are too hot for carbon-based extremophiles;
- implausibly, those places do contain non-carbon-based life that nobody has recognized yet; or,
- more provocatively, abiogenesis might have happened somewhere extraterrestrial (venus, mars, europa) and only infected earth after already getting quite close to luca
life on earth does seem to have taken a long time, most of the planet's lifetime in fact, to get to the point of sometimes being able to prove theorems
That may simply be because even there carbon has the advantage and would call anything else 'food', so on the timescales that we observe life silicon based life might simply not have time enough to evolve.
It's an interesting question, and if life really revolves around a single atom (Carbon) being present that would in itself be yet another one of those things that makes you wonder what the chances are of life existing at all.
Judging by our own evolutionary timetable it may be that on the 'hot' earth other life forms were possible but as things cooled down they were no longer viable (I'm assuming that 'hot' life would evolve faster and 'cold' life would evolve slower, this may well be a wrong assumption), and as earth cools down further it may well be that things that are not viable right now may become viable. Life seems to have very few pre-requisites, energy and some specific molecules present in non-zero quantities and you're off to the races.
Also: nucleic acids may not be a pre-requisite either: life may well require them for bootstrapping but they may not be a requirement for all forms of life.
my thought with the undersea vents is that, if there was a diverse hadean or archean ecosystem of non-carbon-based life, some of it should have survived in the parts of the vents that are too hot for carbon-based life. maybe the cells that froze to death in boiling water would become food for carbon-based extremophiles, but the cells in the hotter parts of the rock should be safe
but we don't observe that, and i think that the most likely explanation is that there wasn't any silicone-based life in the hadean
nucleic acids are definitely not a requirement for all forms of life
i was saying that we don't understand the plasma dynamics that drives the solar phenomena we do observe, not that they are driven by life, so plausibly such plasma dynamics could also drive life in the same way that chemistry drives life here on earth
You could Define life broadly to include Stars or mountains, but then you just look for a new subset and name that would include life more similar to our own.
One definition of life that I think resists scrutiny is an object or unit which contains a transferable data and an algorithm to use that data. This would separate a cloud from a tree.
tax forms are similar but you can debate whether the algorithms are really in the tax form or in your head
would a hypothetical https://en.wikipedia.org/wiki/Mycoplasma_genitalium variety that harvested ribosomes from eukaryotic cells no longer count as 'alive'? it's already an obligate parasite
how about https://en.wikipedia.org/wiki/Nasuia_deltocephalinicola? it depends on its insect host for atp
In this case where manufacturing instructions are transmitted, you could have a system where neither component is consider alive by itself, but the system is.
One cell with DNA only, and another with ribosomes only would not be considered alive. However, a multicellular combination of them would be alive.
Obligate parasites will be alive in some cases, and not in others, depending on how they handle stored data (DNA), and what they rely on the host to do.
The problem is that all of the molecules in period 3 are much more massive than their carbon-family counterparts, which means that the energy required to construct said building blocks must be equally higher. If we assume a high-entropy environment exists for these molecules to form, the first obvious question is then: how come carbon-based life didn't appear in that environment as well and outcompeted Si-based life? Would it have to be high-pressure environments like inside gas giant planets, or high-temperature environments on planets much closer to the star? And if the environment is inhospitable to carbon-based life, is there sufficient stability in that environment to produce more than just the functional building blocks of life?
Next, the question of development speed is an interesting one. Of course, evolution isn't linear so everything here is conjecture, but just as a thought experiment: How long would it take for such an environment (inhospitable to carbon-based life, but capable of spawning silicon-based life) to produce single-celled organisms, and then for those organisms to meaningfully alter their biosphere (like Earth's Great Oxidation Event) so that we can detect those changes through our telescopes? Earth is 4.5 billion years old; oxygen started appearing in the atmosphere 2 billion years ago, but didn't appear in meaningful quantities until less than 1 billion years ago. That 3.5 billion years from planet formation to detectable biosphere signature for carbon-based life (n=1, of course). If we take a wild guess that silicon-based life is slower to develop by a factor 2, we'd have to find planets that are at least 7 billion years old (and have been stable for that time). Given that the lower bound for the age of the entire universe is 8 billion years, we're getting close to planets that are as old as the universe itself. Add to that that our telescopes are also looking into the past, the further out we look -- but 100 million years (the Virgo supercluster) seems negligible compared to the supposed 7 billion years projected incubation time: It seems rather optimistic to expect silicon-based life signatures to readily appear on our telescopes.
[0] and then there's also https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemi... -- where they suggest NH3 and its ionized counterparts could serve as a replacement for H2O, OH- and H3O+
I think we make a mistake by universalizing our perspective, so to speak. We define life by what we see around us and then assert, correctly and unassailably, that life must be made of the chemistry that we ourselves are made of, without generally understanding that the definitions are circular.
What gives?
...we should be able to recreate or predict some environments that would yield the same magic...
Why should we? Physics and chemistry derive from observation. Theory is descriptive.
1) Our experience with chemistry outside of the Earth environment is almost entirely theoretical. Only very expensive equipment can achieve, for only fractions of a second, in only microliters volume, extremes of temperature, pressure, magnetic fields, and never any environment with higher gravity than our own. We have no experimental access to, not to mention actual intuitive understanding of, the widely diverse environments that exist in the Universe. So, our experimental knowledge is lacking.
2) Were we disembodied minds with no actual experience of the world, could we predict the existence of terrestrial life from first principles if given complete access to current knowledge of terrestrial physics and chemistry? Probably not. We don't know what makes a good environment for life, other than that we look around on Earth and see it everywhere. So, our theoretical knowledge is inadequate.
3) Even what we perceive and define as life, sentience or civilization is limited by our lack of experience. If trees for example had a rich storytelling tradition, epics unfolding over centuries perhaps, how would we even discover that or understand them? Trees are far more relatable than aliens are likely to be given that we share a common ancestor. The concept of, say, wind patterns or waves or nebulae being alive or sentient is easily dismissed, but again, our definition of life is narrowly defined by our single data point, our interests and outlook conferred and constrained by our evolution. Is it useful? Is it dangerous? This is the sort of thing that informs even our ability to perceive something as worthy of attention. Before microorganisms were discovered, the concept of invisible critters was laughable. So, our understanding of what we're looking for is inadequate. I suspect that the evidence of alien life is all around, but it's just too different even to be interesting.
I can imagine people giggling to themselves coming up with the most understated name possible when publishing their results. Something like; "Strong indications of biological activity on rocky planet sol-3 detected by probe."
Go dolphins!
and the chronology also makes sense: they sent probes straight to mars and straight to venus on the expectation of life and everything, but then dashing hundreds of years of fantasy tropes and imagination upon arrival, the new goal was to expand the search to much further destinations pending ongoing public interest and location within orbit of planets with much larger orbits like Jupiter and everything beyond
so timing wise, there have only been a few years wasted in waned funding, and it was about time for the happenstance of that test to occur, logically
BTW, an oxygen atmosphere cannot be a requirement for life, as life on Earth created the oxygen atmosphere.
Also, in regards to life using electrical signals to process information, gaseous clouds would have to generate signals with lower entropy than white noise from exploding stars.
I can't remember specifically but at one point they encounter a planet where there is some process that is turning complete, and they realize an entire universe is simulated in the process with sentient beings that they can never communicate with.
I can't remember the details so I'm butchering it a little, but it's a great read.
Diaspora and Permutation City also touch on the topic of speeding up or slowing down conscious so that millenia pass in subjective seconds, or a subjective second passes in a millisecond, etc.
[1] https://timeteam.github.io/fiction/2016/12/21/wangs-carpets....
https://en.m.wikipedia.org/wiki/Rogue_planet
Under specific conditions, such a planet may even retain an atmosphere, have moderate surface temperatures (and thus, perhaps even liquid water), and life could exist on it if chemically-powered rather than using photosynthesis.
Such a planet would be an excellent vehicle to carry life from one star system to another. And another plus: out of a star system's influence sphere (while free-wandering through space), it could have very stable conditions over long periods of time. Which doesn't hurt in the formation or evolution of life.
True, there's a lot of (big) if's there. But the universe is biiiiiigggg so I wouldn't rule it out.
They should have taken photos of the night side of Earth instead. It would have been hard to miss the bright lights everywhere.
The night-side view is merely looking for surface-generated radiation with less interference from external radiation.
Aliens might not do that. Their cities might be fairly dark at night.
You are also assuming "intelligent life", and that it is sufficiently advanced to have created cities and some form of light bulb. (this could include a lantern). Without that you have to rely on other signs of life.
Hmm.. satellites? A planet with 'advanced civilization' would have satellites or other objects orbiting. Also devices that fly - so some radar can pick a plane in the sky? its Transponders? Scan the sea for large floating objects?
On the dark side one should be able to observe lights, and the same locations during the day should have cities (and not i.e. forest fires or volcanoes)?
On the light side one would be able to 'zoom in'/scan the surface and see structures or other shapes that would imply some manipulation of the surroundings on large scale.
How about antennas picking up multiple signals (radio, tv, etc.?)
For the case that it's pre-cities/pre-industry, good luck... Good luck zooming in and spotting 10 'planet-lings' (from Earthlings) with their planet-goats crossing a valley.
Having played A LOT of Starcontrol II though I can imagine that sending small crafts to the surface would help verify suspicions.
SETI is our attempt at the radio wave interpretation. The Sagan paper gives us non-radio detection methods as well (spectography, interpretations of reflected ligjt, and so on).
[1] https://news.stanford.edu/press-releases/2022/05/02/gravity-...
https://www.youtube.com/watch?v=NQFqDKRAROI
...along with a different paper, where they are going for 1km resolution:
Even better, is it predictable that by a few thousand years ago we would probably be getting to space about now? Does this actually give credibility to the idea that aliens may visit and watch us from up close now? Perhaps there are probes arriving now-ish that were dispatched thousands of years ago.
ed. - but it requires high-res direct imaging of the planet
clouds fly, volcanoes emit radiation, icebergs float
> Having played A LOT of Starcontrol II though I can imagine that sending small crafts to the surface would help verify suspicions.
the humans didn't discover deep subsurface bacteria until the 01990s; they'd been living in small crafts on the surface for millions of years without ever noticing the kilometers of life below the top few meters
they didn't even know about archaea until the 01970s
reasoning from fictional evidence merely leads you to mistake the authors' beliefs (or even narrative tropes they know to be false) for objective fact
For example, passing light through a hydrogen gas cloud will remove some red/IR wavelengths from the end result.
We can measure those missing wavelengths and figure out what gasses it's passed through
See https://exoplanets.nasa.gov/discovery/how-we-find-and-charac...
In any case it boggles my mind that we can do things like that for planets thousands of light-years away from us.
Lesson that the blinders of consensus rarely see what’s new.
I love this example so much because the method was so new and spoke to a key truth. If methods are useful then mundane results are to be expected but still need to be tested. One mentor once said to me that the whole point of an experiment is to test what you think you know. Testing ground truths helps mitigate confirmation biases.
I'd been living here my whole life when it flew over, and hadn't had much luck detecting such signs either.
More seriously though, I wonder if the same tests could have detected anything a few 1000 or even million years after life first evolved?
I'd like to see a followup to this article in the current time - that considers what we could detect on nearby star systems. What can we see at 5 ly, 10, 20, 100. I think someone must have already published some description of what we could see, but haven't come across it. Seems like I read they were looking for signs of air pollution, figuring they might follow our path? We seem to barely be able to see anything through the atmosphere from the planets that pass at just the right point between their sun and the earth.
One or two markers would be inconclusive but a bunch of different ones would give a high probability of life existing here. It's why the "Dark Forest" concept is fundamentally silly. There's no hiding Earth's biosignatures.
If the arbitrarily advanced aliens have a long enough measurement baseline and are within 150 or so light years they could also see the technosignatures of the Industrial Revolution with different air pollutants. If they're within about 60-70 light years they'd start seeing evidence of radioactive isotopes from above ground nuclear testing.
Was it? I dunno, it seems kind of obvious? Like, that should have been the plan from the very beginning of the mission.