What would be easily missed, but undeniable, proof of life from non-advanced detectors?

Question

I need a way for a spacecraft to detect absolutely irrefutable proof of organic, but not intelligent, life on a planet, most likely through spectroscopy or another similar concept. The spacecraft is not in orbit around the planet (heliocentric and heading toward the star), but is in the same system. Assume a solar twin, but the planet could be of any format required, even a moon of another planet hosting the life if necessary. For plot reasons, I need this to not be something easily noticeable at first.

The spectroscope used for detection is probably a little bit behind modern day, but if there's some concept needed to stretch the capabilities I'd accept that too. If it helps with the timeframe, the delay in the realization should be at least a few days, but the data is going to be printed out on physical paper - no computer screens or automatic analysis tools available.

In short, what chemical compound(s) detectable on a planet from a spacecraft would be irrefutable, but not immediately obvious, proof of life?

See my previous question for additional context on the technology restrictions. (You might also be able to deduce the plot motivations I have for asking this question.)

Answer 1

Dimethyl Sulfide

In September and October 2023, there was news that the James Webb telescope had found this compound on a faraway planet. This is what gives the sea its particular smell, and there are basically only two sources of this substance on Earth: microbes and chemical plants.

If there is either of these sources on the planet (K2-18b, right at the bottom of Leo), then we'll have found life in another planet. Most likely it would be microbial, as the planet is a mini-neptunian hydrogen hell. But it may also be that we'll end up discovering a non-biological, non-artificial source for dimethyl sulfide as well.

There is also still a one in 50 or 60-something chance that the detection of dimethyl sulfide was an error, and some data that James Webb collected just a few days ago to confirm or deny that is still being processed.

But suppose we get confirmation it's biological. Imagine that, alien life found in our lifetime, and it's not fiction!

Anyway, your probe could be in a similar situation, where it has detected dimethyl sulfide through spectography. But it only had one good observation, so it needs a few more passes to confirm that it got the spectographic signature right.

Answer 2

I'm going to back up my comment with an answer, that still potentially fits.

Visual cues

Your astronauts take a series of photographs of the planet. None of them show anything extraordinary, the planet would have to be a mostly barren rock for them to not spot life immediately, so assume life only exists in the seas.

Only careful comparison of photographs reveals a visual change in the water that could only be an algae-like bloom, a shoal of proto-fish, or a really fricking big whale, etc. Something you mightn't 'detect' immediately. If comparing the photos over time isn't in your protocol, you might not notice it until you happen to put two photos next to each other. But in doing so, you see movement that is organic in nature.

Note that to leave the solar system and move onto another, they may have already committed to their new trajectory before their closest approach to the planet. So 'spotting it while near to the planet' and 'spotting it only after they've committed to leaving the system' aren't exactly mutually exclusive.

Answer 3

Frame challenge:

Not a chemical compound, but since you specifically outrule automatic analysis tools you could just resort to human error. Your scientist have to go over the thousands of chemical compunds detected by the spectroscopy by hand (or rather "by eye"). And eyes get tired.

If you ever read a book until deep in the night, you might have experienced reading a line, that doesn't make sense, only to realize that you just skipped a line. With a list of chemical compounds you do not have the "error correction" due to context because each line is just another compound.

Since they obviously know about this risk, the scientists have introduced a rule that makes in mandatory to have not only one but two scientist go over every printed list. But like most such "saftey measurements" this rule is lived laxly and if more interesting stuff is available to do the "recheck" is postponed and postponed again.

However the one guy, that always wants to keep every little rule, nags he others regulary to do their "rechecks" at least before leaving the system.

Answer 4

Oxygen is not a reliable indicator of life on it's own - but taken together with other facts about the planet it can be. Ideal as a late-realization tool for the plot.

NASA gives the photocatalytic activity of Titanium Oxide as an example of how even large ratios of oxygen might lifelessly exist in a planetary atmosphere - there is bound to be more such compounds. Also, water-vapor rich atmosphere + loads of UV equals oxygen, etc. .

So the spacecraft is nearing the planet, seeing lots of water vapor in the atmosphere with long range sensors, sun is UV-heavy, crew is primed to see non-life-indicating oxygen, sees it, no biggie. Spacecraft passes planet, oxygen content is confirmed. They do not pass through a veil of ultrathinned hydrogen though, which is a fact that rules out the UV-cleaving of water in the outer reaches of the atmosphere (hydrogen would need to escape into space in copious amounts to avoid recombination).

Taken together with not enough candidates for catalytic compounds detected on the planet's moon (=remainder of protoplanetary disc that has a very similar composition to the planet, but much more observable because no atmosphere) - e.g. via spectroscopy done during an observed impact ... life!