Universe Today
“Follow the water” has been a guiding mantra of astrobiology, and even space exploration more generally for decades. If you want to find life, it makes sense to look for the universal solvent that almost all types of life on Earth use. But what if life doesn’t actually need water to live or even evolve? A recent paper, available in pre-print on arXiv by researchers at MIT, including Dr. Sara Seager, and the University of Cardiff, proposes an alternative to water as the basis for life - ionic liquids (ILs) and deep eutectic solvents (DES). These liquids could allow life to exist in environments we had once thought were far too hot, too cold, or too barren to support life, and could dramatically change our search for it throughout the cosmos.
ILs are essentially salts that remain in a liquid phase at low temperatures - generally below 100°C. DES, on the other hand, are mixtures of compounds that have a much lower melting point than their individual parts, and are held together by electrostatic forces like hydrogen bonding and van der Waals forces. Both have physical properties that make them interesting from an astrobiological perspective.
They are extremely physically resilient. Most notably, they have vapor pressures that are orders of magnitude below water’s. Since that means they barely ever evaporate, they can persist as micro-droplets or thin films in a vacuum or on worlds with incredibly thin atmospheres. If a planet does have an atmosphere, they can also stay liquid across ridiculous temperature swings. For example, one specific ionic liquid ([NBu3H][HFAC]) has a melting point of a frigid -93°C.
Fraser discusses the concept of the “habitable zone”.Physical properties are great, but they’re useless if the building blocks of life can’t adapt to them. But according to a literature review done by the researchers, there’s a good chance life could. They found that 71% of tested proteins kept their folded structure in ILs with very little water. In addition, 65% of enzymes maintained their catalytic activity in the same solvents. And to prove part of their point, one particular enzyme called cellulase remained stable at a blistering 115°C.
Even more interesting, there’s some evidence of nature itself using these solvents in extreme cases. A species of ant known as the tawny crazy ant naturally synthesizes an ionic liquid to neutralize fire ant venom. In addition, certain types of “resurrection plants” that are evolved to survive extreme droughts, produce DES-like mixture of sugars and amino acids inside their cells that protects their proteins when water completely disappears.
What’s more, the building blocks for these solvents are available throughout the solar system. This research was in part inspired by a recent paper where Dr. Seager’s lab accidentally made an IL when mixing sulfuric acid (which make up a large percentage of the clouds of Venus) with nitrogen-containing organics. Mars has a ton of perchlorate and chloride brines, which, while toxic to life as we know it, could serve as chemical precursors for ILs and DES.
Fraser talks to Dr. Mary Voytek about NASA’s astrobiology vision.Even comets have examples of these precursors. As they heat up and cool down during their eccentric orbits, the protected liquid pockets could serve as crucibles for prebiotic chemistry. But perhaps most interestingly, rocky exoplanets that have lost all their water might still hold on to microscopic networks of these liquids in their crusts. It might not be enough to support an entire advanced civilization, but something equivalent to bacteria could survive indefinitely on even small amounts of these solvents.
That last finding is the basis for a speculative “water replacement hypothesis”. As a planet slowly loses water over millions of years, life could evolve to synthesize its own ionic liquid, like those tawny crazy ants. Eventually it could evolve so far as to completely internalize its new solvent that’s needed for its equivalent of complex organic chemistry.
While that’s great in theory, we’re not likely to be able to see any life forms with those features on an exoplanet anytime soon. But we could potentially do some testing here to see if the hypothesis is feasible. Trying to form ILs and DES out of the basic building blocks of other planets is one step. Testing if crucial biological structures like cell membranes can form in them is another. But if the theory holds up, the “habitable zone” around foreign stars might be vastly larger than we ever imagined. We might just need to look for different kinds of puddles.
Learn More:
S. Seager et al - Ionic Liquid Biospheres
UT - Exoplanetary Systems are Diverse. Our Search for Life Should Be the Same
UT - Life Might Be Difficult to Find on a Single Planet But Obvious Across Many Worlds
