In a recent paper published in the journal Astrobiology, Steven Benner from the Foundation for Applied Molecular Evolution in Florida makes the case for a genetic approach to life detection. His starting point is that any kind of life, whether on Earth or elsewhere, would require a genetic code that enables Darwinian evolution. This requires that any code-containing molecule should be able to generate copies of itself, that some of the copies should be imperfect, and that those imperfections should themselves be replicable.
Our own DNA does all that, of course, but a genetic code does not necessarily require DNA. Any kind of molecule with repeating backbone charges would in principle be suitable. The repeating charges could be negative, as with phosphate groups in DNA, but they could also be positively charged, like, for example, ammonium groups.
DNA has two other properties that make it well-suited to Darwinian evolution: (1) changing the encoded information does not affect its physical properties (for example, no matter what its information content, the molecule’s solubility in water stays the same), and (2) the information-carrying units are approximately the same size, which contributes to structural stability. We would expect a genetic molecule on an alien world to have the same properties, whether or not it’s DNA.
These insights should help in planning future life detection missions in space. The amount of genetic material contained in samples gathered during such a mission (such as in water retrieved from an icy moon’s geyser) would likely be fairly small. Nevertheless, since polymers with repeating negative charges are strongly attracted to polymers with repeating positive charges, genetic material should be detectable even in very small amounts.
Benner suggests that something as simple as a waveguide with charges imprinted on the surface would be sufficient for detecting alien genetic molecules. And a detection system using spectroscopy would involve no reagents and no moving parts, which is ideal for any robotic mission. Using fluorescent tagging of DNA would increase the sensitivity even more, and might allow the detection of life at concentrations a million times less than what we see in Earth’s oceans. For these reasons, Benner proposes that instruments to detect molecules carrying genetic information should be the first tools used in the search for extraterrestrial life, particularly in near-term missions to water worlds like Enceladus and Europa.
As it stands today, proposals for missions such as the Enceladus Life Finder (ELF) do not intend to use Benner’s approach. However, it would be worthwhile to investigate whether his suggested techniques could still be applied to later missions. This would require testing the instruments on Earth first.
In principle, Benner’s approach is very powerful because it is generic, and doesn’t require much a priori knowledge about the alien world to be investigated. We don’t know if alien life would contain DNA, but it probably would use a molecule with similar properties if the available solvent is water, which undoubtedly would be the case for Mars and the icy moons.
Searching for other kinds of biosignatures such as organic compounds or homochirality (molecules with the same “handedness”)—which are often proposed for life detection missions—could easily lead to results that are difficult to interpret and inconclusive. Benner’s approach could be a marked improvement.
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