I’m not an astronomer but have only a hazy understanding of Spectro-Polarimetric Imaging. So I’m phrasing my question the only way I know how. What biosignatures would the ROYGBIV color spectrum give to an astronomer looking for extra-terrestrial life? What would the color blue indicate? What color would indicate the presence of oxygen?
Referencing The Exoplanet Handbook under “Spectroscopic Indicators” of Extraterrestrial Life (Section 11.8.5, pgs 638-43), it identifies the 690 nm and 760 nm bans of diatomic oxygen (under the assumption that free oxygen is a strong indicator of life-like chemical processes), as well as the 1.27 µm band in the near infrared. Nitrogen doesn’t really have absorption spectra below the ultraviolet but methane has one at 7.7 µm (further into the infrared). Chlorophyll has varying absorption spectra across the visual range and the ‘red edge’ increase in reflectivity above 0.7 µm is considered a possible indicator, but who knows if extraterrestrial life would use anything like the chlorophyll molecule, especially if it evolved under different spectral conditions (a K or M type star), or indeed uses light from its central sun as a primary energy source at all.
Earth’s absorption spectrum has changed significantly over geological history, although exactly what it looked like at any point is a lot of guesswork based on geological and chemofossil analysis. We should also consider other potential spectroscopic biosignatures including chlorine, sulphur compounds, and perhaps more exotic molecules in the CHONPS chemical paradigm. Of course, ‘life’ that isn’t based on a carbon-nitrogen basis may have totally different atmospheric biosignatures, or indeed, not require an atmosphere at all, so we should really be pretty open-minded about how extraterrestrial life might fundamentally look and work.
Stranger
By the way, I’ve recently received but have not yet had time to read Lingam and Loeb’s Life In The Cosmos; from a quick flip through Chapter 5 (“Habitability: Planetary Factors”) and Chapter 6 (“The Quest for Biosignatures”) do seem to survey the current literature on the topic in more detail than The Exoplanet Handbook (to be expected given the focus) and with a minimum of Loeb’s more fanciful obsessions with UAPs and belief that Oumuamua is an alien spaceship. There is, of course, Sara Seager’s Exoplanet Atmospheres: Physical Processes but this is largely concerned specifically with Earth-like worlds almost to the exclusion of biosystems that may dissimilar. Most of the technical literature in the field is really focused on gas giants because they are more likely to provide readily observable spectra that gives insight into planetary compositions but the James Webb Space Telescope will provide a new range of spectra in the mid-infrared and at much greater sensitivity that may offer a wider range of detailed observations of planetary atmospheres.
Stranger
Thanks Stranger for your detailed response and references. I hope the works you referenced are all still available for oder online. I’ll start with Loeb’s [ Life In The Cosmos ]
Unfortunately, the colour blue is not really a reliable indicator of the presence of life. Two planets in our system (Uranus and Neptune) are coloured blue or blue/green, and according to the speculative Sudarsky gas giant classification system, there may be warm gas giants that are predominantly blue due to Rayleigh scattering.
The presence of oxygen may not be a particularly good indicator of life either. Wordsworth and Pierrehumbert published this paper in 2014, suggesting that oxygen could dominate on certain types of lifeless planets:
Even if planets with abiogenic oxygen are relatively rare, they could still skew the data significantly if life-bearing planets are even rarer. We could potentially detect many such abiotic oxygen worlds before detecting a true life-bearing world, if the latter are extremely uncommon. In the search for life, false positives (and false negatives) are both capable of producing misleading data.
Yeah, I’ve read that paper before; the problem is that it assumes a planet with a ‘pure’ H2O substrate, and that does mean pure because free oxygen will pretty much react with any element with an electronegativity < 3.0 (so, everything below and to the right of oxygen in the periodic table) even if it is bonded to something else, and it is difficult to conceive how a system with a large amount of oxygen and few other metals could form. Oxygen is the third most abundant element in our galaxy but virtually never found outside the presence of carbon with which it readily forms bonds.
In addition, for a planet to retain a high fraction O2 atmosphere would necessarily mean that it have relatively high density and (very probably) an active magnetic field, which suggest the presence of common heavier elements (iron, silicon, magnesium, sulphur, and at least traces of potassium, calcium, sodium, and phosphorous). Oxygen also readily dissolves in water (a fact that is crucial for the development of life on Earth) so even a layer of virtually pure water wouldn’t be enough to ensure that free oxygen doesn’t react with anything else; given that almost every mineral dissolves to some degree in water due to its polar nature, an ocean world covered in pure H2O is manifestly unlikely even with a very deep ocean. You’d actually need a completely non-reactive benthic substrate to prevent redox reactions that would prevent free oxygen being slowly produced by photolysis or other natural oxygen-producing process. So unless you have a planet coated in perfluorooctanesulfonic compounds, basic chemistry essentially assures that free oxygen will have something to react with.
If we saw a planet with a significant atmospheric content of free diatomic oxygen (or chlorine, or fluorine) we’d assume that either some life-like processes are occurring OR there is some massively energetic process that is constantly renewing the free content. (I shudder to think what life using fluorine as an oxidizer would be like but it would certainly present some exotic exobiochemistry.) If we did see a planet with some really energetic, non-biotic activity producing O2 like massive constant lightning or photolytic activity, we’d also expect to see a significant spectra of O3 which is essentially incompatible with stable organic compounds.
So, the paper presents an interesting hypothetical counterargument to free oxygen as a biosignature but it requires an environment with such constraints that seem physically improbable. Having just written that, I’m sure astronomers will soon find a planet with some exotic combination of conditions that spontaneously produces high oxygen concentrations and is covered with naturally occurring TeflonTM just because reality is perverse and likes to fuck with anyone who makes absolutist statements about speculative science. But it will be a very weird world for that to occur.
The paper does incidentally make an astute observation, namely that an atmosphere rich in N2 is an apparent anomaly. The reason for the composition of the Earth’s atmosphere at 78% diatomic nitrogen is not understood, and it seems to be key to life as we know it, because although we are generally described as “carbon-based”, nitrogen is equally as important right down to the nucleic acids that code for proteins and adenosine triphosphate that powers activity at the cellular level. So, if we saw a planet with high concentrations of N2, we might suspect an environment amenable to life. However, as noted previously, diatomic nitrogen doesn’t have any characteristic absorption spectra above 0.1 μm, so nowhere in the visual spectrum and challenging to find in the irradiance of a M- or K-type main sequence star and not even that easy to measure about a G-type star.
Stranger
As for “most sophisticated telescope on the market” if you are implying something that can be bought off the shelf, there are none. This is the realm of very high-end observatories. James Webb, for example.
In fact, to look for absorption spectra of Earth-like worlds, you pretty much have to use a space-based telescope because you need to be free of the absorption of our atmosphere.
Stranger
One possibility is a thick ice mantle below the liquid ocean, as suggested by Leger et al. Covering the rocky core with a high-pressure ice mantle would reduce oxygen uptake by the planetary surface to a minimum.
I just skimmed through that paper and found a number of possible issues but the first one that occurs to me is how it would be possible to maintain a pure-ice ‘mantle’ beneath a liquid ocean. I’m sure there is some balance of pressure and temperature conditions that could achieve that, and you could find some path of planetary evolution where a Neptune-like world forms the substrate, migrates into a habitable zone, blows off its atmosphere except for water vapor, and comes to the stated condition but at least at first glance it seems like a very knife-edge type of scenario that would occur very infrequently and probably not be stable for very long on a planetological time scale.
This seems more like a case of special pleading to hypothesize a scenario of a planet with an abiotic oxygen-rich atmosphere than a statistically probable condition even in a small subset of rocky planets. You see a lot of this kind of thing in the literature because noodling about edge cases is interesting (and creative; I read a paper about a hypothetical world that rained liquid silicon, iron, and nickel!) but it requires a lot of mental articulation to get there. The point remains that if we saw a planetary atmosphere with strong spectra for diatomic oxygen we would be very interested in the potential for some kind of self-organizing and non-equilibrium thermodynamic (i.e. life-like) system. It would not be definitive evidence in and of itself, but absent of other mechanisms to maintain a rich free oxygen atmosphere you would be looking at very unique conditions compared to our understanding of planetary evolution.
But then, that understanding has been evolving radically in the past couple of decades of observational evidence, and as I said, the perversity of reality probably means that James Webb will start finding a bunch of inexplicably oxygen-rich planets that will create a cottage industry of such theorizing. Aside from the fact that we’re actively disrupting our own biosphere with reckless abandon, it’s a great time to be a planetologist.
Stranger
Did you mean below and to the left? I’m not a chemist, but the noble gasses for example are below and to the right while iron, for example, is below and to teh left.
Uh, yeah, that’s correct. I’m somehow solid on “port” and “starboard” but I’m endlessly mixing up Amy Adams and Ilsa Fisher, Czechoslovakia and Yugoslavia, and left and right.
Stranger
There are probably aliens out there (sulfur-based, maybe) who would say the same thing about oxygen-breathing life.
And even if an abiotic-oxygen water-world could exist, such a world would probably have some other detectable signatures (like, say, very large amounts of water in the atmosphere). So if we detect oxygen without those other signatures, that might mean something.
Well, yeah, we’d never hinge any declaration of a discovery of exoplanetary life on a single indicator. Well, YAHOO! News might, but the International Journal of Astrobiology or Nature are probably going to demand more evidence and peer review. An atmosphere with a strong O2 absorption spectrum would be very interesting but not as conclusive as, say, indications of complex biochemical reactions, radiative spectra of fusion propulsion systems, or microwave transmissions of the popular Sens-O-Vid “I Affective-Lust-Impulse ZeeZeeBoo!”
Stranger
Well precisely. There are no ocean worlds with 200km deep oceans and high-pressure ice mantles in our system, but they may be relatively common elsewhere. If we found one in another system it would have a lot of water in its atmosphere. We would probably be able to distinguish between such a planet and a life-bearing Earth clone, perhaps by density alone.
It would be more difficult to analyse a planet with an exotic exobiochemistry, especially one which results in an excess of a different gas or fluid. Maybe any planet with an excess of any reactive atmospheric component should be considered a potentially life-bearing world, not just one with oxygen (or methane).
Would a visible light / roygbiv telescope even be appropriate? My understanding is that the spectral lines would be shifted because of relativistic Doppler effect. For most exoplanets, would there be usable spectral lines in the visible spectrum or would they all end up in uv or ir? Too much I don’t know to calculate it myself.
For anything close enough that you’d have any hope whatsoever of detecting planets, the Doppler shift will be too small to worry about. I mean, you’d still be able to detect it (that’s part of how you’d be detecting and studying the planets to begin with), but it’d be much smaller than the difference between red and orangeish-red, never mind all the way to UV or IR.
You still probably wouldn’t use visible light, or at least wouldn’t restrict yourself to it. You’re looking for useful or interesting spectral lines, wherever in the spectrum they happen to lie. Some interesting spectral lines are in the visible range, true, but some are elsewhere. This is why astronomical photographs are usually false-color: An ordinary consumer camera uses filters corresponding to what we see as red, green, and blue, but an astronomical camera will use filters corresponding to whatever spectral lines the astronomer considers relevant (and just which lines those are will depend on what sort of thing you’re looking at and why you’re looking at it).