Water is key to life as we know it. But that doesn’t mean its key to life everywhere. Despite the fact that the ability to house liquid water is one of the key characteristics we look for in potentially habitable exoplanets, there is nothing written in stone about the fact that life has to use water as a solvent as opposed to other liquid options. A new paper from researchers at MIT, including those who are developing missions to look for life on Venus, shows there might be an alternative - ionic liquids that can form and stay stable in really harsh conditions.
Ionic liquids are basically a salt that is liquid at around room temperature, and contain entirely either negatively or positively charged ions. As a solvent for life, they have some pretty interesting properties. They are “polar”, meaning their charge is slightly different from one side to another, but they can also dissolve other polar molecules, including biomolecules like enzymes, which is a key factor in acting as a solvent for life.
They also exist at temperatures that are much too hot for liquid water to form, and they have very low vapor pressure, meaning they won’t evaporate even with miniscule amounts of atmospheric pressure on, say, a planet. If these features were taken into account, the habitable zone around the star would expand to include planets that would otherwise be considered too warm, or lacking in an atmosphere.
Fraser discusses the search for astrobiological sign with Dr. Mary VoytekLike much good science, this idea comes from a problem the researchers were having with an experiment they were trying to run. They were attempting to develop a system to scan for organic compounds in Venus’ atmosphere, which contains a lot of sulfuric acid. But as they were attempting to get rid of the sulfuric acid by evaporating it away to isolate the organic compounds, but every time they tried, a little bit of liquid would be left over. They eventually realized that the sulfuric acid must be reacting with the organic compound they were using (in this case, glycine), but creating an ionic liquid.
Such liquids don’t typically form naturally on Earth, with one weird exception of the mixing of the venoms of two competing ant species, but they can form when sulfuric acid washes over organic compounds. Scientists already know that there are plenty of organic compounds spread throughout the solar system, and likely throughout the galaxy. They have been found on everything from comets to Ceres. Sulfuric acid is also very common, and can be created by volcanic activity that likely happens on many exoplanets throughout the galaxy.
Given those abundances, the researchers decided to see if they could intentionally recreate the reaction that was frustrating their efforts to isolate organic molecules, and to do so in an environment similar to that found on some planets that would previously have been though to be barren. They set up an experiment on a slab of basalt, equivalent to a planetary surface, and introduced the sulfuric acid to the glycine at various temperatures and pressures and found that the ionic liquids were indeed created and stayed stable in those environments.
Video describing the details of ionic liquids. Credit - Alfa Chemistry YouTube ChannelGranted, the actual environment in space will be more harsh. Excess sulfuric acid would have to be taken away, which the researchers think can be done by it being absorbed into rock pores. The organic molecules and salts themselves would have to survive radiation-rich environments, but they could be protected by magnetic fields or those same rocks that whisked away the sulfuric acid.
This research is another step towards understanding the different forms of life that might exist out in the universe. While it is certainly not definitive proof that any such life does exist, it could be inspirational to the next round of astrobiologists in how they go about looking for their ultimate target.
Learn More:
MIT / Eureka Alert - Planets without water could still produce certain liquids, a new study finds
R. Agrawal et al - Warm, water-depleted rocky exoplanets with surface ionic liquids: A proposed class for planetary habitability
UT - Life on Earth Uses Water as a Solvent. What are Some Other Options for Life as We Don't Know it?
UT - Another Building Block of Life Can Handle Venus' Sulphuric Acid