Historically, we’ve assumed that extraterrestrials would likely conduct their metabolisms like we do; use water as a solvent; be built from cells and utilise the carbonyl group to metabolise; exploit chemical-energy gradients; use nucleic acids for genetic functions and proteins for catalytic functions.
That first trait has turned the astrobiology eyepiece on those bodies known or likely to hold liquid water: currently subsurface Mars, beneath the icy surfaces of Jupiter’s Galilean moons Europa, Ganymede and Callisto and maybe on Saturn’s moon, Enceladus.
It’s an understandable fixation, especially when you consider that the first stars to die flung the same well-known elements carbon, oxygen, nitrogen, hydrogen throughout the young universe. In the youth of the Milky Way, those elements were necessarily confined to the galaxy’s centre, where the earliest superstars exploded. Our own solar system formed around nine billion years later. Life took around four billion years to materialise on Earth, but it could already have been flourishing at the centre of the Milky Way long before that.
Life, by definition, must be able to metabolise, and replicate. In our world, it does so by biochemical pathways that often exploit temperature and other gradients for motive power. Carbon is widespread, and eminently open to forging more complex, yet predictable, molecules.
But there is no evidence that’s the only formula just the only known formula. What if life were able to utilise some non-aqueous solvent? Pursue alternative chemistries? Dispense with DNA? Proteins, lipids and amino acids might either be unrecognisable to us or absent altogether, replaced by metabolisms
Phosphorus is a component of DNA, RNA and ATP and is essential for processes of replication and the transfer of energy in living cells. However, arsenic is in the same group as phosphorous so similar that it’s one of the reasons arsenic is toxic to humans and can be readily substituted for phosphorous for some functions in microorganisms such as certain marine algae. But scientific evidence for its substitution in DNA is still incomplete. beyond our comprehension or experience.
Any number of structural molecular alternatives are possible that could theoretically support life. Chemical reactions between non-carbon compounds are not especially far-fetched, and even if it relied on water as a matrix for its biomolecules, extraterrestrial life might not need it as a solvent. The trick is to open our minds and entertain an alien architecture so different that it could have been overlooked by the blunt instruments of our life-detection technology.
For instance, we tend to assume that liquid water is a prerequisite for life, but it cannot exist on the surface of Mars because the ambient pressure is too low. There, water ice turns directly to vapour without going through a liquid phase. If we were looking for an ideal solvent on Mars, we’d go no further than formamide, which has a wide liquid temperature range there.
Similarly, ammonia is liquid at much lower temperatures than water, and stays that way over a far greater temperature range at planetary surface pressures. In fact, if we put water aside for just a few moments, we’d come up with a half-dozen better alternatives on the This NASA Hubble Space Telescope image shows the colorful “last hurrah” of a star like our Sun. Ultraviolet light from the dying star then makes the emitted gases glow. back of a napkin: dihydrogen, ethane, helium, methane, hydrogen sulphide, sulphuric acid…
Similarly, we tend to assume that extraterrestrial “plants” must photosynthesise, absorbing, as they do in our back yard, all visible spectra except green. But there’s nothing to say that other suns necessarily emit the same radiation, at the same wavelengths, as our own. In which case, such plants would have to employ a very different chemical pathway.
The colour of a planet says everything about what might or might not live there; one look at Earth and you know it’s mostly liquid water, while green is the colour of life itself. Theoretically, any light from deep violet through to near-infrared could power photosynthesis. In our world, black is a void, but on any planet orbiting a red dwarf the stellar equivalent of a 20-watt bulb plants might need to be black to absorb as much of its feeble light as possible. Vegetation on a planet orbiting a young red star might survive only under water or ice, protected from its searing ultraviolet flares. By the time that star becomes a supergiant, like Betelgeuse or Antares, plants could be white, to reflect as much of the star’s glaring radiation as possible.
But, for a probabilistic certainty, extraterrestrial life is proving maddeningly uncertain. Early last December, NASA teased journalists by hinting of a major breakthrough in a press release. “NASA will hold a news conference at 2 PM EST on Thursday, December 2, to discuss an astrobiology finding that will impact the search for evidence of extraterrestrial life.” Clearly, they must have finally found affirmation of the alien kind?
Well, no. At the news conference, Felisa Wolfe-Simon, a NASA astrobiology research fellow at the US Geological Survey, announced: “We’ve cracked open the door to universe.” Her team had found a bacterium on the bed of California’s Mono Lake that could apparently replace the usual phosphorus in its molecular structure with highly toxic arsenic.
OK, that’s fairly interesting; we’d always thought that all life depended on the same six elements—oxygen, hydrogen, carbon, nitrogen, sulphur and phosphorus. Human, haddock or housefly, we all need these ingredients to build DNA, proteins and lipids, and so intrinsic are they that alien-hunters have, with classic human narcissism, made them the prime target of their intergalactic search.
But at the bottom of Mono we have GFAJ-1, an organism which illustrates that life refuses to stick to the recipe. It’s learned a neat trick it can utilise phosphorus or arsenic—but it’s a sleight of hand; the two elements are chemically very similar. So similar, in fact, that our cells can’t always tell them apart, which is what makes arsenic toxic. (Arsenic-based compounds break down in water, so cells fail and die.)
And that, claimed the NASA paper’s many critics, is precisely what proves it to be flawed. Wolfe-Simon’s team grew the bacterium in the lab, denying it phosphorus and watching as it switched to arsenic for fuel instead. They then extracted its DNA, and found arsenic in there too, but, critically, they analysed it in water. Harvard microbiologist Alex Bradley was one of a chorus that pointed out that, because arsenic compounds quickly fall apart in water, the DNA should have broken down too. But it stayed intact, possibly because the bacterium had somehow survived on the minuscule traces of phosphate clinging to the salts it was fed on. It’s possible; microbes thrive in the Sargasso Sea a region of water in the North Atlantic bounded by gyres where phosphate levels are 300 times lower than Wolfe-Simon’s lab cultures.
“I couldn’t believe how bad the science was,” Dr Rosemary J. Redfield, of the University of British Columbia, told The Guardian. “I don’t know whether the authors are just bad scientists or whether they’re unscrupulously pushing NASA’s ‘There’s life in outer space!’ agenda,” she later blogged.
So all we really have in GFAJ-1 is a microbe that might have been munching on a murder weapon, and further evidence for those that charge NASA with drumming up hype to justify its budget.
What it does make clear, however, is that the answer you get depends on the question you ask, and we could be hopelessly off-track.
Where any pattern can copy itself, “life” is possible, and patterns pervade our universe, in radio and gamma waves, in gases and fluids.
Life could be exploiting those patterns, and the energy that drives them, right under our noses. It could be gaseous itself, or lurk, latent, in invisible radiation. Such talents would make practically every planet habitable.
GFAJ-1 was a crock of sorts, but it reminded us to expect the unexpected. Life has repeatedly amazed us with its adaptive ingenuity here on Earth. We should be much more willing to suspend our beliefs about the form it may take elsewhere.