Redefining Habitable Zones with Liquids: Can We Live Without "Water"? - The Ecosystem of Space Unlocked by Ionic Liquids

Is Water a "Standard" Rather Than an "Absolute"?

"Life requires liquid water"—this premise, etched into astronomy textbooks, has been significantly challenged by a research team from MIT. They have demonstrated the possibility that "non-water liquids" can naturally form and persist on rocky planets in the universe. The key lies in "ionic liquids," a group of substances that remain liquid below 100°C, have very low vapor pressure, and do not easily evaporate.MIT News

The Venus Cloud Trigger: A Chance "Drop" Left Behind

The story begins in a laboratory preparing for Venus exploration. The research team was testing a procedure to evaporate sulfuric acid, the main component of Venus's clouds, and analyze the residue. However, when mixed with nitrogen-containing organic compounds (e.g., glycine), a stubborn "layer of liquid" remained even after most of the sulfuric acid had evaporated. Upon investigation, they found that hydrogen from the sulfuric acid had transferred to the organic compounds, forming a mixed salt solution—an ionic liquid. This serendipitous observation led to the hypothesis that liquids could form even in environments where water is not viable.MIT News

Experiments Illustrate "Chemistry of Rocky Planets"

The team combined over 30 types of nitrogen-containing organic compounds with sulfuric acid under various temperatures and pressures, and even tested them on basalt. They confirmed that ionic liquids could form at high temperatures of 180°C and at much lower pressures than Earth's, and that liquid droplets could remain on the surface even when acid was absorbed into the rock's pores. This suggests that wherever volcanic gas-derived sulfuric acid meets organic compounds common in meteorites, "oases of liquid" could potentially form.Phys.org

Expanding the Habitable Zone with "Alternative Solvents"

The strengths of ionic liquids lie in their "resistance to evaporation" and "broad stability range." While water is vulnerable to high temperatures and low pressures, ionic liquids can more easily remain liquid under such conditions. Moreover, it is suggested that some biomolecules, such as certain proteins, can exist stably within ionic liquids, potentially serving as "stages for metabolism" in extreme environments. By broadening the definition of habitability to include the presence of liquid phases where metabolism could occur, the theoretically "habitable" regions of rocky planets could significantly increase.MIT News

Context: The Trend of Exploring Solvents Other Than Water

The debate over considering "solvents other than water" as candidates for life is not new. For example, Titan, with its seas of methane and ethane, or ammonia and supercritical carbon dioxide, have been explored as part of a diverse "liquid universe." Recently, reviews and theoretical frameworks have been established, and efforts to systematically evaluate "alternative solvents suited to the environment" continue. The value of this achievement lies in presenting a concrete example of "naturally occurring salt liquids on the surface of rocky planets" within this trend.Liebert Publishing

From Venus to Exoplanets: The Intersection of Observation and Exploration

While Venus's clouds are seas of sulfuric acid, organic compounds are widely distributed across small bodies and planets in the solar system. If volcanic sulfuric acid comes into contact with organic deposits, patches of liquid can form. Professor Seager, who led the research, is also involved in Venus atmospheric exploration (Morning Star Missions), which motivated the experiments. Future challenges include examining which biomolecules can function within ionic liquids and designing observational indicators (such as spectral features) to suggest the "likelihood of ionic liquids" on distant planets.MIT News

How to Measure? The Reality from the Observation Side

From an observational standpoint, directly detecting "the liquid itself" is challenging. However, if reactions between volcanic sulfuric acid and organic matter progress, changes in the derived ion species or surface optical properties could serve as clues. Additionally, recent suggestions of atmospheric presence even on ultra-hot rocky super-Earths indicate that chemical cycles could occur in hellish environments. Accumulating such case studies will provide testing grounds for the ionic liquid hypothesis.Reuters

Social Media Reactions: A "Two-Phase Mixture" of Enthusiasm and Caution

Immediately after the news release, the spread began on X (formerly Twitter) and Threads, starting with posts from Phys.org and MIT News. Proponents welcomed it as a "redefinition of the habitable zone" and an "expansion of the target population for observation." Meanwhile, skeptics questioned the practicality of biochemistry in ionic liquids, asking about constraints such as toxicity, viscosity, and diffusion. Indeed, Phys.org's official X post received both favorable quotes and threads questioning the roadmap for verification. On Threads, comments suggested that if liquids remain under high temperatures and low pressures, the surface evolution models of dry planets should be reconsidered.X (formerly Twitter)Threads

Expert Perspectives and Emerging Counterarguments

The fact that this study was published in PNAS indicates it passed peer review, but it remains a presentation of the "possibility of existence" in experiments and theory. Critical perspectives include (1) the frequency with which reactants meet in near 1:1 stoichiometric ratios in natural environments, (2) the time scale over which formed droplets can withstand weathering, radiation, and dust accumulation, and (3) the feasibility of material transport and metabolic diffusion in high-viscosity environments. These will likely be the focus of the next experimental designs and modeling efforts.MIT News

Yet the "Map" Has Expanded

Expanding the definition of habitability from "regions where water can be liquid" to "regions where some liquid phase allows metabolism" also impacts observational strategies. This opens up the possibility of exploration on warm, low-pressure rocky planets previously excluded by "water conditions." The research team described it as "opening Pandora's box," but this is not a metaphor for chaos. Rather, it means that a "box of research challenges" has opened, diversifying the intersection of planetary surface chemistry and life chemistry.Phys.org

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