I am under the impression that SETI looks for radio waves of technological civilizations, and astronomical spectroscopy looks for the chemical composition of planets.
But on earth, long before intelligent civilization started, single celled organisms altered the atmosphere. The gases on our planet have been altered by life, and this happened long before multicellular life evolved, let alone intelligent life and technology.
Can you use spectroscopy to determine if the chemical composition of a faraway planet has life on it, even if the life isn’t intelligent or capable of technology? Or can you not tell because you cannot tell what the baseline atmosphere of a dead planet should be?
A large amount of free oxygen would strongly suggest life, since free oxygen would normally be expected to react with other elements and not persist unless something was constantly replacing it. Although it would be hard to rule out some sort of highly unusual chemical environment without more evidence.
Methane + free oxygen would be an even stronger indication - it means something is continually generating both gases, otherwise they’d react with each other and one of them would disappear.
All spectroscopic observations will be like this - the best we can hope for is something strongly suggestive. It won’t be like in 2010 (the movie) where they look at a spectrum and say “it’s chlorophyll!!”
Not just oxygen; a high composition of any oxidizing agent, such as chlorine, fluorine, hydrogen peroxide, nitrous oxide, et cetera, would indicate some pretty energetic chemistry going on. However, it does not necessarily follow that ‘life’ (by any definition we would recognize) must be present; simply that there are non-equilibrium thermodynamic processes occurring. Without other independent observations of other life-like phenomena, and especially that which would be limited to highly technological civilizations (such as coherent light or gravity waves, spectral emissions on very specific bands not associated with natural phenomena, et cetera) it would be premature to call out life as the most likely cause. Indeed, we might not recognize extraterrestrial life up close unless it was really obviously reproducing in front of us.
Although you are correct that SETI looks for radio transmissions, such transmissions would have to be in fairly close proximity for us to receive them. Our most powerful transmitters could, if so directed, send a message that would be coherent against the radio background at no more than about 150 light years to a ground based receiver the size of the Aricebo radio telescope. A larger and more sensitive receiving array in an orbit may be able to detect signals at significantly greater distances but the degree of signal loss and degradation over interstellar distances makes this an impractical means of communication. For a sufficiently advanced civilization transmitting signals via coherent gravity waves would make far more sense. Of course, we have no way to detect gravity waves smaller than those generated by interacting pairs of supermssive black holes, so we could be awash in the extraterrestrial equivalent of I Love Lucy reruns and never know it.
Free oxygen can persist under certain conditions, so those would have to be ruled out before you can be sure it comes from life. For instance, if the surface of the planet is covered in ice, such as Earth was during the Cryogenian Period, then oxygen will not have any unoxidized rocks to combine with. So oxygen generated by photodissociation of water molecules would build up in that atmosphere.
Oxygen would also build up on a deep-water ocean planet; this is not really any different to an ice planet apart from the temperature. Even in our solar system several icy moons have thin oxygen atmospheres.
Note that we have sent quite sophisticated probes to Mars and we still aren’t 100% sure if there’s life there or not. Maybe hiding somewhere underground or near the poles. And we know it’s atmospheric composition even better than a spectrograph would.
Then there’s the suspected oceans under certain satellites like Europa. Might be life there. We have good spectra from it too.
Life is tricky. Any assumption of what the signs of life should be will likely turn out wrong. Earth is apparently quite special.
You can rule stuff in if you detect a lot of something like oxygen. (And even then there will be reasonable doubters.) But ruling it out is another thing.
But is existence period a very interesting aspect of a thing? (Like whether god exists or not. If he’s not making miracles happen it doesn’t make a difference either way.)
See all the exoplanets. Based on some extremely limited data points we’re getting wild stories about super earths and the weather on hot jupiters. The fact is that we don’t know anything about these planets other than an extremely rough approximation of their mass and size. After the novelty wears of it’s hard to care.
The operative word here is “thin”; the Galilean moons have atmospheres on the order of 10[SUP]-12[/SUP] bar, hardly enough to show up in spectrographic analysis even at close range much less at interstellar distance. To find a planet with a thick non-reducing oxygen- (or at least oxidizer-) rich atmosphere such as Earth would let us know that there are at least non-equilibrium thermodynamic processes occurring which are indicative of life-supporting activity. But it would not by itself be conclusive.
Well, perhaps in terms of the science fiction-inspired search for habitable worlds (which are likely to be vanishingly rare) it might be hard to care, but for planetologists the range of worlds that have been found in orbits, characteristics, and configurations which are not only unexpected but completely in violation of current hypotheses of planetary system formation it is fascinating and as revolutionary as natural selection was in the 1860s or plate tectonics was in the 1950s. It represents a fundamental shakeup of the entire field with vast opportunities to speculate on new hypothesis, and in the absence of higher fidelity data scientists in the field are having to rely upon statistical assessment, analytical modeling, and pure creativity to come up with plausible explanations for how novel planetary bodies and configurations could come to be.
Finding extraterrestrial life would be interesting, of course, and despite the protests of some, hardly unexpected, as we are either a result of normal electrochemical processes that must occur all over the Universe with relatively great frequency, or we are genuinely uniquely and divinely created, a singular remarkable instance of serendipity; a brief perusal of history will show that while the latter assumption is popular but unerringly wrong, from the geocentric universe to the special property of élan vital, the former ends up being reality. But even if we don’t find life in nearby observable systems, just the wide variety of planetary bodies and different stable configurations is worthy of extensive study, and will hopefully spur on better space-based telescopes for longer range and higher fidelity observation.
Just to put that number in perspective, 10[sup]-12[/sup] bar (10[sup]-9[/sup] millibar) is considered ultra-high vacuum. That’s better than vacuum chambers used to test satellites and spacecraft.
No, it would not. Photodissociation on a deep-water ocean planet would be sufficient to create an oxygen-rich atmosphere, since the hydrogen would escape (assuming the escape velocity was similar to Earth’s) but the oxygen would have nowhere to go. Remember that at the bottom of a deep-water ocean there would be a layer of high pressure ice, which would act as a barrier to absorption of oxygen by rock.
See
or for a more technical look at the subject, A Leger et al. Ocean Planets
Hmm; the first link there is to a planet with a magma ocean, not a water ocean; that is a class of planet I hadn’t considered capable of sustaining an oxygen-rich atmosphere. Well, you live and learn.
That doesn’t really mean much. What in the spectrum of Mars indicates that there might be life? And the better statement is : There is no conclusive evidence of life on Mars, now or ever. If we never find life there, we will still not be 100% certain that there is no life there.
Some methane has been discovered on Mars. First found by orbiter and then by Curiosity. That may or may not come from life. On Earth a lot of it does, mostly generated by bacteria including bacteria in the digestive tracts of mammals.
Personally, I really doubt there’s any life on Mars, although there may have been millions of years ago when the planet was warmer, wetter, and had more air. Finding the evidence of that will be non-trivial.