Scientists preparing a mission to evaluate the potential habitability of the clouds above Venus report a chance laboratory discovery of ionic liquid formation on rocky planets, potentially expanding their habitable zones.
Many rocky, Earth-like exoplanets have previously been considered inhospitable to water-based life forms due to their orbital distance from their host star, which prevents the sustained presence of liquid H2O on their surfaces. According to researchers with the Massachusetts Institute of Technology (MIT) behind the unexpected discovery, these planets could potentially support theoretical, rudimentary life forms that use ionic liquid instead of water to metabolize energy.
“We just opened up a Pandora’s box of new research,” said Sara Seager, study co-author and Professor of Planetary Sciences in the school’s Department of Earth, Atmospheric and Planetary Sciences. “It’s been a real journey.”
An Accidental Ionic Liquid Discovery Motivated the New Investigation
According to a statement announcing the team’s research, Seager and Rachana Agrawal, who led the study as a postdoc, were conducting experiments in preparation for the upcoming Morning Star Missions. As project leader, Seager was examining how to collect and evaporate sulfuric acid from the clouds of Venus so any residual organic compounds could be studied for signs of biological activity.
In those experiments, the research team was evaporating solutions of sulfuric acid and the organic compound glycine in a customized, low-pressure system. According to the statement, every experiment revealed that while most of the liquid acid evaporated, a “stubborn layer” of liquid residue remained. Tests revealed that the acid was receiving hydrogen atoms from the organic compounds, which turned them into a fluid mixture of salts and acid that resisted evaporation.
Called an “ionic liquid,” the hardy mixture resists evaporation even in extremely low-pressure environments. Since Seager’s previous work had also found that organic molecules can remain highly stable in an ionic liquid, this unique ability motivated the duo to consider whether it could persist on the surface of planets considered too warm and with atmospheres too thin to support surface liquid H2O.
“From there, we took the leap of imagination of what this could mean,” Agrawal explained. “Sulfuric acid is found on Earth from volcanoes, and organic compounds have been found on asteroids and other planetary bodies. So, this led us to wonder if ionic liquids could potentially form and exist naturally on exoplanets.”
Experiments Confirm Liquid Formed Under Exoplanet-Like Conditions
To begin, the researchers designed a series of experiments simulating various potential exoplanet surface conditions, encompassing a range of surface temperatures and atmospheric pressures. According to the statement, this involved mixing sulfuric acid in vials with “various nitrogen-containing organic compounds.” Over 30 different compounds were tested.
In parallel experiments, the team also mixed the ingredients on the surface of basalt rocks, similar to those found on many exoplanets. Seager noted that, in theory, the organic compounds could “donate” a hydrogen atom the same way an acid wants to donate a proton.
“We knew from our past work with sulfuric acid (the main component of Venus’ clouds) and nitrogen-containing compounds, that a nitrogen wants to receive a hydrogen,” the researcher explained. “It’s like one person’s trash is another person’s treasure.”
After examining the test vials and the basalt rock samples, the team found that their concept was correct. Although all the sulfuric acid had effectively evaporated away, a small amount of fluid residue remained. Tests confirmed it was an ionic liquid. Seager said the results were consistent across a large number of their different simulated low-pressure environments, including those simulating the surface of a rocky exoplanet.
“We were just astonished that the ionic liquid forms under so many different conditions,” Seager says. “If you put the sulfuric acid and the organic on a rock, the excess sulfuric acid seeps into the rock pores, but you’re still left with a drop of ionic liquid on the rock.”
“Whatever we tried, ionic liquid still formed,” Seager added.
Further analysis revealed ionic liquid producing reactions “up to 180 degrees Celsius” and at pressure “much lower than that of the Earth’s atmosphere.” The team believes these results suggest ionic liquid could form naturally and persist on the surface of a planet incapable of supporting liquid water.
How Findings Affect the Search for Life on Other Planets
In the conclusion of the study, Seager and Agrawal note that, on Earth, ionic liquids are typically made only for industrial purposes and occur naturally in one unusual circumstance involving an exchange of venom between two rival ant species. Still, because their experiment showed it can form naturally and offer a “hospitable environment” for some biomolecules, the team proposed a set of exoplanet conditions that might lead to ionic liquid supporting biological life.
“We’re envisioning a planet warmer than Earth, that doesn’t have water, and at some point, in its past or currently, it has to have had sulfuric acid, formed from volcanic outgassing,” Seager explained. “This sulfuric acid has to flow over a little pocket of organics. And organic deposits are extremely common in the solar system.”
In the team’s ideal scenario, pockets of ionic liquid could persist on the planet’s surface for an extended period, “potentially for years or millennia.” They note that the presence of such a liquid could “theoretically serve as small oases for simple forms of ionic-liquid-based life.” However, they also note that ionic liquids would likely not support a type of life form that “resembles Earth’s water-based beings.”
“We consider water to be required for life because that is what’s needed for Earth life,” Agrawal said. “But if we look at a more general definition, we see that what we need is a liquid in which metabolism for life can take place. Now, if we include ionic liquid as a possibility, this can dramatically increase the habitability zone for all rocky worlds.”
The study “Warm, Water-Depleted Rocky Exoplanets with Surface Ionic Liquids: A Proposed Class for Planetary Habitability” was published in Proceedings of the National Academy of Sciences.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.