Field of Science

Better extraterrestrial communication through chemistry: What do aliens want?

The search for extraterrestrial intelligence has traditionally hinged on detecting electromagnetic waves, most commonly radio waves but also infrared and x-ray radiation. But in the absence of knowledge about the specific nature of extraterrestrial civilizations, we need to explore all sources of communication possible and not just ones based on electromagnetic waves. Thus the message we would send or receive could and should include everything from symbolic signals to actual physical samples of material signifying the presence of intelligent life. SETI is an endeavor fraught with such momentous potential significance that it would be foolish to hinge it on physics alone. We need to employ other sciences in its service.

For doing this it’s extremely valuable to turn the question around and ask what we would do if we were to announce our presence. What kind if messages would we send to a potential ET listener? This line of questioning is valuable but it always includes the significant pitfall of suffering from anthropocentrism. It’s all too easy to believe that ET thinks just the way we do. Nonetheless, thinking from a human perspective opens the way toward understanding various potential forms of communication. So with the caveat that we should not constrain ET to fit our shoes too well, it’s worth pursuing this direction of thinking.

Assuming that the listening civilization is at least as advanced as ours and possibly more and assuming that they are actively listening and sending, the central requirement is that the message should be unambiguously construed as ‘artificial’ and not of natural provenance. The message should thus clearly seem artificially ‘designed’. This requirement is harder to satisfy than it seems. As we are well aware, there are numerous examples of natural entities which suffer from the ‘illusion of design’. Seashells, snowflakes, the myriad anatomical structures inside organisms and life itself all suffer from the illusion of design. No wonder that creationists and intelligent design proponents have seized on all of these and declared them to be the work of an intelligent designer. In fact, if we didn’t know better about the process of evolution and natural selection that has fashioned these complex structures, we too would think of them as designed, and indeed we did until Darwin came along and produced his great piece of work. Thus, when selecting a message to transmit, we need to be careful that it can be clearly distinguished between one which is natural but creates an illusion of design and one which must be actually designed by intelligent beings like ourselves. This requirement for making sure that a message looks designed has led the radio astronomy camp to suggest sending out messages that communicate prime number sequences. If after waiting for some time, we receive a message containing the next prime number in the sequence, we could be almost certain that the message was sent by beings who had discovered mathematics and factorization and who therefore could be considered ‘intelligent’.

Based on this background, I asked myself the following question as I drove on a particularly monotonous stretch of interstate highway:

‘As a chemist and especially as an organic chemist, how would I transmit a molecular message to an alien civilization such that the message would almost certainly be construed as designed by an intelligent being?’

Now organic chemists are well aware of differences between naturally occurring and artificially synthesized molecules. Chlorophyll, penicillin and quinine are examples of naturally occurring molecules while nylon, Viagra and LSD are unambiguously synthetic. Thus a chemist’s impulsive reaction might be to suggest sending samples of nylon or LSD out to potential ET listeners as decidedly ‘designed’ entities. But recall what we said about creating the illusion of design. Viagra may be man-made, but there’s really no reason why it cannot be made by nature in principle, even if it may be very unlikely in practice. Nature is wonderfully adept at producing an astonishing variety of molecular structures. For all we know, we might find Viagra someday as a vital communication molecule in some obscure marine sponge. To provide the strongest evidence of artificial design, we need to send a molecular message that is unlikely to be naturally designed not just in practice but also in principle.

That is when it hit me that we could make a good case for an unambiguously designed message by transmitting molecules that have all or many of their hydrogen atoms replaced by deuterium. Recall that deuterium (D) and tritium (T) are the two isotopes of hydrogen. But they are spread out exceedingly thin among the major isotope of hydrogen (H) that we all know and love. Hydrogen is the most abundant element in the universe but deuterium comprises 1 atom in about 6000 of hydrogen and amounts to only 0.02%, while tritium is even scarcer. These isotopic abundances of D and T are constrained by the fundamental laws of physics governing nuclear stability and are extremely unlikely to be different under any circumstances anywhere in the universe. Given the universal low abundance of D, the probability of, say, a molecule of benzene containing only D being synthesized naturally in the universe under any conditions is vanishingly small. On the other hand, organic chemists can and do make molecules containing D using their bag of chemical tricks. Thus, the discovery of a deuterated molecule in outer space should point almost unambiguously to an artificial origin. The molecule need not even be fully deuterated since even partial deuterium enrichment is very unlikely to occur naturally. Full replacement by deuterium would cinch the evidence, however.

Along with radio and infrared waves, we should thus also try to probe the presence of deuterated compounds in deep space. Fortunately we have several spectroscopic techniques to detect deuterium that include sensitive mass spectrometry methods. The problem with deuterium is that it might be hard to detect against the abundant background of normal hydrogen. Tritium could have been possibly used to circumvent this problem since its radioactivity would make it stand out against the background. Unfortunately the half-life of tritium is only 12 years so it’s useless as an emissary of interstellar communication.

For any intelligent civilization, the advantages of sending out deuterated molecules would be many. For one thing, virtually any all-D molecules would do the trick. Simple molecules containing D are as unlikely to have been naturally synthesized as complex molecules. As noted above, even all-D benzene could be a signature of intelligent life. So would all-D methane. Using simple heavy water (D2O) is another option. This wide berth in picking the exact molecular structures frees us up to focus on optimizing other important properties of the molecules like resistance to the rigors of outer space (extreme heat, cold, radiation etc.). Since even simple D containing molecules would serve our purpose, chemists won’t have to go to great lengths in terms of the actual synthesis. Plus as mentioned above, even partial enrichment in D would work.

This strategy of transmitting isotopically enriched molecules could be extended to other elements. What about carbon, the element of life? The most abundant isotope of carbon is carbon-12 (C12). C13 makes up about 1% of the rest while radioactive C14 comprises as little as 0.0000000001%. Just like T, C14 is a potentially valuable but unfortunately useless isotope because of its half-life of 5700 years. That leaves C13 as the ideal candidate. Similar to D, the probability of a molecule naturally enriched in C13 is tiny, and therefore the discovery of a C13 enriched molecule would also strongly suggest an artificial origin. But C13 has other advantages. It is a magnetically active isotope of carbon which can be detected using Nuclear Magnetic Resonance (NMR) spectroscopy; organic chemists use it all the time in deducing the structures of complex molecules by enriching them in C13. A molecule enriched in C13 would not only signify an artificial origin but it would also provide the added bonus of revealing its own identity through NMR spectroscopy. Just like a message containing prime numbers would reveal knowledge of mathematics, a message containing a swarm of C13-enriched molecules would reveal knowledge of chemistry and NMR spectroscopy. Quite adequate for concluding the presence of intelligence.

I propose therefore that the search for extraterrestrial intelligence should include the search for molecules enriched in deuterium, carbon-13 and minor isotopes of other elements in addition to more traditional signals like electromagnetic radiation. If we wished to communicate our existence and intelligence to other civilizations, we would not constrain ourselves to physics and astronomy but would also employ chemistry, biology and every other tool at our disposal. The discovery of intelligent life in the universe is too important to be left to the vagaries of a single or a few approaches. One of the signs of intelligence is the ability to make the most out of diversity. We can only expect other intelligent civilizations to behave accordingly.


  1. Wouldn't gravity kind of defeat this plan a bit? Or are you suggesting to enrich the solar system to such a degree that it can be picked up by spectroscopy from afar? If the latter, wouldn't we need to ever so slightly scale up our production?

    I'd also think that if we're going down the anthropomorphic path, we should stick with whatever we first came up with. If you're studying the universe (both natural and artificial), most information seems to be encoded in radiation, which increases the chance that someone out there is looking for it.

  2. Long-chain carbon molecules have been detected in space; some of these deuterated molecules could likely be more volatile. Microwave and radio spectroscopy can detect molecules from afar but I was also including actual capture and spectroscopic analysis of such molecules. The idea of seeding the solar system with such compounds is not bad, but that would likely create so much noise that it would preclude us from detecting communication from other civilizations; scaling up production of deuterated molecules may not be difficult, however. I was also considering equipping unmanned spacecraft with deuterium-detecting equipment. Radiation will certainly continue to be a crucial part of the search, but expanding the possibilities could only help in my opinion.

  3. I wonder if a hybrid molecular/radiation type approach would be good. The SETI folks talk about the 21 centimeter hydrogen line as a good place to look for encoded messages, as it's a universal reference, and the spectrum is rather clear there.

    I wonder if we could use isotopic radiation lines instead. Blasting out a signal at several D2O specific wavelengths would communicate (1) this is a non-natural signal (2) we're advanced enough to perform isotopic enrichment/quantum mechanical calculations to figure out which wavelengths we should send and (3) water is important to us.

  4. This makes me wonder if anyone is already combing through existing data from satellites, telescopes and such to find such molecules.

    Anyone know where I can get access to the data from these instruments?

  5. @RM: That's a good idea. I wonder how much noise you would get at that frequency.

    @elise: Another good point. Maybe some literature searching is warranted.

  6. I like the idea and the article. but another question would be how fast you can transport molecules. radio signals have the advantage that they are at light speed

  7. nature has some pretty bizzare isotopic enrichment/production methods. you would have thought that a uranium fission reaction would be a 'signature of intelligence' but then again there was oklos.

    What if by chance you had a planet where the surface temperature and gravity were just calibrated in exactly the range so that the thermal velocity of h2 was above the escape velocity but not dh?

  8. Isn't the 21 cm hydrogen line an awful place to encode messages? I mean, since the universe isn't terribly transparent at that frequency? I like contact's idea - 21cm * pi. However, even sending a signal at exactly this frequency is somewhat silly, as the doppler effect will cause this frequency to be mismatched when it arrive at its target.

  9. chemDroid:

    If you find that the atmosphere of a planet has a high concentration of DH, but almost zero percentage of H2, you'd probably have a hint that there is a natural process going on which creates this rare situation. I do not think this kind of signal would be confused with signals possibly created by intelligent life.


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