Water is crucial for existence on Earth. Therefore, this fluid must be a necessity for life on other celestial bodies. For many years, the scientific community’s interpretation of habitability on different planets has been based on this premise.

However, the factors that render certain planets suitable for life might have minimal correlation with water. In fact, an entirely different kind of liquid could potentially sustain life in environments where water can scarcely survive. This is a possibility that MIT researchers explore in a study published this week in the Proceedings of the National Academy of Sciences.

Through laboratory experiments, the team discovered that a category of fluid termed ionic liquid can easily emerge from chemical components that are also anticipated to be present on the surfaces of various rocky planets and moons. Ionic liquids are salts that remain liquid at temperatures below approximately 100 degrees Celsius. The experiments indicated that a combination of sulfuric acid and certain organic compounds containing nitrogen yielded such a fluid. On rocky worlds, sulfuric acid may be a result of volcanic actions, while nitrogen-bearing compounds have been observed on numerous asteroids and planets within our solar system, hinting that these compounds might also exist in different planetary systems.

Ionic liquids exhibit extremely low vapor pressure and do not evaporate; they can form and endure at higher temperatures and lower pressures than what liquid water can manage. The researchers highlight that ionic liquid could create a nurturing environment for some biomolecules, like certain proteins that can maintain stability within the fluid.

The scientists suggest that even in planets that are excessively warm or that possess atmospheres with too low pressure to support liquid water, there could still be areas containing ionic liquid. Where there is liquid, there may be prospects for life, although likely not anything akin to Earth’s water-based organisms.

“We consider water essential for life because that’s what’s necessary for life on Earth. However, if we adopt a broader definition, we recognize that what we truly need is a liquid in which metabolic processes for life can occur,” states Rachana Agrawal, who conducted the study as a postdoctoral researcher in MIT’s Department of Earth, Atmospheric and Planetary Sciences. “If we now factor in ionic liquid as a possibility, this could significantly expand the habitability zone for all rocky worlds.”

The co-authors from MIT include Sara Seager, the Class of 1941 Professor of Planetary Sciences in the Department of Earth, Atmospheric and Planetary Sciences as well as a professor in the departments of Physics and Aeronautics and Astronautics, along with Iaroslav Iakubivskyi, Weston Buchanan, Ana Glidden, and Jingcheng Huang. Co-authors also comprise Maxwell Seager from Worcester Polytechnic Institute, William Bains from Cardiff University, and Janusz Petkowski from Wroclaw University of Science and Technology, in Poland.

A fluid breakthrough

The team’s research on ionic liquid stemmed from efforts to seek evidence of life on Venus, where clouds of sulfuric acid shroud the planet in a harmful mist. Despite its hazardous nature, the clouds on Venus may harbor signs of life — a hypothesis that scientists are set to investigate with forthcoming missions targeting the planet’s atmosphere.

Agrawal and Seager, who is directing the Morning Star Missions to Venus, were exploring methodologies to gather and evaporate sulfuric acid. Should a mission collect samples from Venus’ clouds, the sulfuric acid would need to be evaporated to unveil any residual organic compounds that could subsequently be examined for signs of life.

The researchers employed their custom, low-pressure system engineered to evaporate excess sulfuric acid, to trial the evaporation of a blend of the acid and an organic compound, glycine. They discovered that consistently, while the majority of the liquid sulfuric acid evaporated, a persistent layer of liquid always remained. They soon realized that sulfuric acid was chemically interacting with glycine, leading to a transfer of hydrogen atoms from the acid to the organic compound. This resulted in a liquid mixture of salts, or ions, known as ionic liquid, that maintains its liquid state across a vast range of temperatures and pressures.

This unexpected discovery sparked a new inquiry: Could ionic liquid arise on planets that are too hot and possess atmospheres too sparse for water to persist?

“From there, we expanded our imagination on what this could imply,” Agrawal remarks. “Sulfuric acid is present on Earth from volcanic activity, and organic compounds have been identified on asteroids and other celestial bodies. Therefore, this led us to ponder if ionic liquids could potentially generate and exist autonomously on exoplanets.”

Rocky havens

On Earth, ionic liquids are primarily manufactured for industrial applications. They rarely occur naturally, except in a unique instance where the liquid is created from the mixing of venoms from two competing ant species.

The team aimed to investigate the conditions under which ionic liquid could naturally form, and the range of temperatures and pressures suitable for its production. In the lab, they combined sulfuric acid with various organic compounds that contain nitrogen. In earlier research, Seager’s team had established that some of these compounds, which can be viewed as building blocks associated with life, exhibit surprising stability in sulfuric acid.

“In high school, you learn that an acid seeks to donate a proton,” Seager explains. “And surprisingly, we were aware from our earlier work with sulfuric acid (the key component of Venus’ clouds) and nitrogen-containing compounds, that nitrogen aims to absorb a hydrogen. It’s as though one entity’s waste is another’s treasure.”

The reaction could yield some ionic liquid if sulfuric acid and nitrogen-bearing organics were present in a one-to-one ratio — a ratio that was not the focus of previous investigations. For their new research, Seager and Agrawal blended sulfuric acid with over 30 various nitrogen-containing organic compounds, across a spectrum of temperatures and pressures, and then observed if ionic liquid emerged when they evaporated the sulfuric acid in different containers. They also applied the mixtures onto basalt rocks, which are known to exist on the surfaces of many rocky planets.

“We were quite amazed that the ionic liquid forms under such diverse conditions,” Seager states. “If you apply the sulfuric acid and the organic compound onto a rock, the surplus sulfuric acid seeps into the rock’s pores, but you still have a drop of ionic liquid remaining on the rock. No matter our approach, ionic liquid consistently formed.”

The researchers determined that the reactions resulted in ionic liquid generation at temperatures reaching up to 180 degrees Celsius and at extremely low pressures — significantly lower than that of Earth’s atmosphere. Their findings indicate that ionic liquid could naturally arise on other planets where liquid water is not viable, given the right conditions.

“We’re imagining a planet that is warmer than Earth, lacks water, and at some point in its history has had sulfuric acid, generated from volcanic emissions,” Seager notes. “This sulfuric acid must flow over a small accumulation of organics. Moreover, organic deposits are exceedingly common within the solar system.”

Subsequently, she adds, these resulting liquid pockets could remain on the planetary surface for potentially years or millennia, where they could theoretically act as small oases for simplistic forms of life based on ionic liquids. Looking ahead, Seager’s team plans to further explore what biomolecules, and essential ingredients for life, might survive and even flourish in ionic liquid.

“We’ve just unlocked a treasure trove of new research possibilities,” Seager adds. “It’s been quite an adventure.”

This research was partially funded by the Sloan Foundation and the Volkswagen Foundation.