The recent claim for a detection of the molecule phosphine in the Venusian midlatitude cloud layers has brought the idea of chemical biosignatures back to the forefront of attention in our efforts to find signs of life elsewhere in the universe. Phosphine, the argument goes, is not generally expected to be produced, or to survive for very long, in the abiotic (nonliving) chemistry anticipated for this part of Venus’s environment. But on Earth phosphine is associated with organic matter (think swamp gas or sewage) and could be argued to be a biomarker or biosignature molecule of certain extreme organisms that might have cousins or convergently evolved equivalents lurking in the sulfuric acid clouds on Venus.
Not surprisingly there are a lot of caveats. A lot of caveats.
The skeptics’ list includes the challenge of identifying a specific molecule—even if it’s small and simple like phosphine with one phosphorus and three hydrogen atoms—with a single lonesome spectral line feature. Or the fact that we don’t actually know all of the chemical networks that are at play in Venus’s clouds (where droplets, gases and photochemistry intermingle as they stream around the planet at anything from 200 kilometers per second to 370 km/s)—a fact also noted by the researchers claiming the phosphine detection. Or that we don’t really know how microbes on Earth actually make phosphine: is it a direct product of metabolism or a consequence of metabolic products mingling with environmental chemistry? There is even evidence that ordinary iron, with phosphorus containing impurities, can react with sulfuric acid and produce plenty of phosphine at room temperature; an intriguing option for iron-rich meteoritic material raining onto acidic clouds.
But whatever is actually going on with Venus, it highlights a deeper question and challenge for the entire notion of using small molecules as biomarkers, especially when you can’t easily go and make in-situ measurements.
The simple fact is that lots of chemical processes make small molecules, but one of the things that differentiates life from other phenomena is that it consistently utilizes and generates complex molecules. And it does this in a way that arguable supersedes anything else that we know of in the universe.
So why don’t we look exclusively for large, complex molecules? The answer is that it’s incredibly hard, if not impossible, to do this remotely. Astrochemists are acutely familiar with this challenge. If you peer into distant nebulae using radio telescopes or far-infrared instruments, you can definitely pick up the signs of a multitude of molecular species. The problem is that their electromagnetic fingerprints are awesomely messy: great smears of overlapping, complicated features from the rotational, vibrational, flexing quantum energy states of covalently bonded atoms. Discriminating one chain of carbon atoms from another is also incredibly difficult because we don’t always really know what their spectral features should look like.
Small molecules are far easier to identify (although “easier” is a relative term). In exoplanetary science, astronomers and astrobiologists have spent a lot of effort to better understand how we might use the abundances of compounds like molecular oxygen, ozone, methane and carbon dioxide in distant planetary atmospheres as a way to “mark” worlds as likely harboring a biosphere. But researchers have also realized that we can be fooled by entirely nonbiological environments that churn out these compounds in unbalanced mixes—far from the expectations of chemical equilibrium based on the simplest thermodynamic rules.
There are certainly ways to steer partway around these issues. Measuring the precise proportions of several small molecules helps pin down the underlying possibilities, as does monitoring a planet over time, looking for seasonal changes and the pulsing dynamics that living systems seem to operate with.
In the end though, it’s super tricky. Even when you can go and collect samples directly, like on Mars or Venus, sniffing out anything but the simplest molecules is hard. If an ecosystem is sparse—as it might be in Venus’s cloud layers—you might have to spend a lot of time looking. Indeed, right here on Earth it’s taken us a shockingly long time to fully appreciate the presence and life cycle of atmospheric microbial life, and that’s when you can literally climb a mountain and grab samples.
All of this may sound rather depressing for our quest to find life in the universe. But I think that there is something extremely interesting buried in the very fact that all of this is so terribly difficult. It is truly fascinating that life, which to us on a day-to-day basis is such a vibrant and explosive phenomenon, is also so very elusive. That’s true of the microbial world here on Earth as much as for elsewhere. What is it that makes life simultaneously so colorful but hidden?
The answer may circle back to the statement I made previously, that life’s standout feature is that it uses and builds the most complex molecules we know of, from DNA to proteins. Complex molecules are naturally information-rich, in the truest most objective sense given by information theory and Shannon’s entropy. But they are also, in effect, deeply encrypted. DNA in a cell “decodes” itself with the help of an entire entourage of biochemical helpers. It’s taken us centuries to see how some of this works, and we’re far from finished.
We also know that information-rich, well-encrypted data become more and more indistinguishable from noise, from random and unpredictable junk. In other words, seen from a very high vantage point, life itself has to be—in some way—far more akin to noise than it is to simple, but recognizable, structure in the cosmos, whether microscopic or planet-sized.
One implication is that while small molecules might be practical to look for, and markers of interest, in the end what we’re really looking for are things that don’t readily explain themselves at all, yet clearly perturb an environment. We’re looking for the noise that isn’t quite noise.