Venus, the second planet from the Sun, is often called Earth’s sister planet. If extraterrestrial observers on a remote exoplanet were to analyze our solar system with the same methods used by observers on Earth today, the two planets would appear strikingly similar. Both are rocky, with nearly identical diameters and masses, and both exist within or near the solar system’s habitable zone. However, only one of them is known to support life.
A significant difference between the two planets—and a likely reason for the first—is their atmospheric compositions. Earth’s atmosphere comprises approximately 78% nitrogen and 21% oxygen, whereas Venus’ atmosphere consists of more than 96% carbon dioxide. In the distant past, volcanic activity released this carbon dioxide, triggering an uncontrollable greenhouse effect. This process, coupled with Venus’ proximity to the Sun, has driven its surface temperature to a searing 500°C (900°F).
Harold Morowitz and Carl Sagan first observed in 1967 that although Venus’ barren surfaces may be inhospitable to life as we know it, its clouds present “an entirely different story,” according to Morowitz. The upper atmosphere of Venus contains low levels of water vapor and cloud regions characterized by extreme temperatures and pressures. These conditions could potentially support some types of terrestrial microorganisms and have led scientists to investigate the clouds of Venus. In the 1970s, these clouds were found to be primarily composed of sulfuric acid, which is considered incompatible with life. Nevertheless, a controversial detection of phosphine—gases found in Venus’ clouds that could be produced by microbes on Earth—has prompted some astrobiologists to reevaluate this notion of habitability. This has opened discussions on potential habitability.
Previously, researchers established that biomolecules such as the nucleic acids forming DNA can remain stable for up to one year in sulfuric acid concentrations ranging from 81% to 98%. To advance this research, scientists at the University of Chicago have recently tested whether more complex organic structures can also form in concentrated sulfuric acid.
They began with a set of carbon-based molecules known as lipids. Lipids serve as the foundation of cell membranes, acting as a barrier to the external environment and regulating what enters and exits the cell. The research team contended that cell membranes are essential for life, especially under extreme conditions like those present in Venus’ clouds. Thus, they evaluated whether simple lipids could create membranous structures called vesicles in concentrated sulfuric acid.
Membrane lipids feature one side that is attracted to water, known as the hydrophilic side, and another that repels water, termed the hydrophobic side (Figure below, left). The hydrophilic side consists of long carbon chains, referred to as tails, while the hydrophobic side comprises charged compounds known as polar heads. In cell membranes, lipids are arranged in bilayers, with hydrophilic tails oriented inwards and hydrophobic heads facing outwards (Figure below, right). The research team selected simple, commercially available lipids with tails of 10 or 18 carbon atoms and polar heads of trimethylamine, sulfate, and phosphonate. These tailed lipids were chosen for their solubility and ability to form membrane structures due to their hydrophobic nature.
Illustration of a single simple lipid (left) and stacked lipids forming a cell membrane structure (right). Created by the author.
To assess the lipids’ resilience against sulfuric acid, various concentrations of each 10-carbon lipid were incubated in 1%, 30%, and 70% sulfuric acid for a minimum of 1 hour at room temperature. Utilizing a method that evaluates molecular structures based on their magnetic properties, they examined how increasing acid concentrations affected the lipids. Results indicated that trimethylamine and phosphonate lipids remained stable in up to 70% sulfuric acid, although around 20% of the sulfate head degraded.
The researchers then explored whether the lipids could form vesicles in these sulfuric acid solutions. They prepared lipid mixtures across varying concentrations in 70% to 90% sulfuric acid, measuring the particle size of the lipid-acid mixture using light scattering techniques. They discovered that a 50/50 blend of 10-carbon or 18-carbon lipids produced particles comparable in size to typical vesicles in 70% and 80% sulfuric acid solutions, with these particles maintaining stability even after a week.
Upon examination under a high-powered microscope, the lipid particles formed foam-like vesicles. Lastly, numerical models illustrated that the charged ends of lipid and acid molecules interact at the molecular level to help stabilize the vesicles and prevent the entry of acid.
The researchers concluded that simple lipids can create stable membrane-like structures in sulfuric acid concentrations similar to those found in Venusian clouds. They recommended that future studies conduct laboratory experiments to validate the molecular model and ascertain whether lipid membranes can effectively block sulfuric acid. These scientists are beginning to formulate a clearer picture of the potential types of life that could exist within the cloud layers of Venus, although that picture remains largely incomplete.
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Source: sciworthy.com
