“Liquid Salt” Instead of Water? A New Lead in the Search for Life on Other Planets

Scientists suggest that life on other planets could arise not only in water but also in other solvents—for example, so-called ionic liquids—or "liquid salts." Such liquids could be stable at high temperatures and low pressures, they argue in new research published in PNAS.

Until now, astrobiologists have used the "follow the water" doctrine when searching for unknown life in the universe. They have focused on planets that are neither too cold nor too warm, and that could potentially have liquid water and a thick atmosphere (to prevent water from evaporating).

Now, scientists in a publication in PNAS propose a broader view – life could develop somewhere in space in solvents called ionic liquids, which can form spontaneously even on planets previously considered inhospitable.

"Ionic liquids are substances that remain liquid even at high temperatures and low pressures, even near vacuum. Our experiments show that ionic liquids can be formed from materials readily available on distant planets, such as sulfuric acid and nitrogen-containing organic compounds. This offers a potential pathway for the emergence of life on hot, waterless planets with thin atmospheres," we read in the publication by Rachana Agrawal and the team of Professor Sara Seager from MIT in PNAS. One of the corresponding authors of the paper is Dr. Janusz Pętkowski from the Wrocław University of Science and Technology.

When scientists consider the origins of life on Earth, they often invoke the concept of a "primordial soup." Life as we know it requires an environment in which molecules can freely move and react with each other. This is impossible in either a solid or a gas. In what liquid—besides water—can life-giving molecules dissolve? Scientists are suggesting a new idea: ionic liquids.

Water, as a liquid, partially dissociates—some of its molecules break down into positively charged H⁺ and negatively charged OH⁻, but most remain as neutral H₂O molecules. However, there are substances that, in the liquid state, consist entirely of ions, charged molecules. Liquid salts of various compounds are ionic liquids. Thousands of such substances are known, for which the liquid phase is achievable at low temperatures (table salt becomes an ionic liquid only at 801 degrees C).

Ionic liquids are used in industrial chemistry, including in the production of batteries and pharmaceuticals. It's known that some enzymes and proteins produced by organisms are stable in ionic liquids. Ionic liquids are produced, for example, by certain species of ants fighting with each other. "But we know few examples from nature, because this topic is rarely studied," commented Dr. Pętkowski in an interview with PAP, suggesting that one could, for example, try searching for ionic liquids in the tissues of desert plants, where they could protect the plant from water loss. Now, scientists want those searching for extraterrestrial life to also take an interest in ionic liquids.

Where did this idea come from? Dr. Pętkowski is the deputy scientific director of the MIT-Morning Star Missions to Venus initiative, planned private space missions to Venus. Before such a mission can happen, however, a great deal of research is needed. "Because this planet's atmosphere is rich in sulfuric acid, we tested whether any organic compounds, such as amino acids, could survive in this corrosive substance. However, to safely study them with scientific instruments during space missions, it is necessary to evaporate the acid," Dr. Pętkowski told PAP. It turned out that after evaporating the excess sulfuric acid from the sample, a liquid remained that could no longer be evaporated. "It was a simple ionic liquid," he said.

In this way, scientists have shown that simple ingredients such as sulfuric acid and organic nitrogen compounds are sufficient to form such a "liquid salt." They have also shown that ionic liquids form under various conditions, including on the surface of basalt, a typical volcanic rock found on terrestrial planets.

Because ionic liquids do not evaporate, even small amounts of them can provide a stable environment for chemical reactions – you don't need entire oceans of liquid, just small droplets that can very slowly accumulate the substances needed for life to arise.

This discovery significantly expands our understanding of what a "habitable planet" might look like. Until now, such planets have been almost exclusively associated with the presence of liquid water. The team's conclusions suggest that it's worth expanding the search to include warm, rocky exoplanets with very thin atmospheres, or even celestial bodies without atmospheres. Such places may be too hot for water, but ionic liquids can survive there for a very long time, for example, in rock crevices or shielded by magnetic fields.

"In the search for extraterrestrial life, it's worth keeping an open mind. Let's be prepared for the fact that life on other planets, both within our solar system and beyond, may be completely different from what we have on Earth. Applying our Earthly template to the phenomenon of life may not be the right approach and may unnecessarily limit us," concluded Dr. Pętkowski.

Ludwik Tomal (PAP)

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