Saturn’s largest moon Titan may be the most fascinating piece of real-estate in the Solar System right now. Not surprising, given the fact that the moon’s dense atmosphere, rich organic environment and prebiotic chemistry are thought to be similar to Earth’s primordial atmosphere. As such, scientists believe that the moon could act as a sort of laboratory for studying the processes whereby chemical elements become the building blocks for life.
These studies have already led to a wealth of information, which included the recent discovery of “carbon chain anions” – which are thought to be building blocks for more complex molecules. And now, thanks to data from the the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, a team of NASA researchers have detected the presence of acrylonitrile, another chemical elements that could be the basis for life on that moon.
The study that details their findings – titled “ALMA detection and astrobiological potential of vinyl cyanide on Titan” – was published in the July 28th issue of the journal Science Advances. In it, the team explains how data from the ALMA array indicated that large quantities of acrylonitrile (C2H3CN) exist on Titan – most likely within the moon’s stratosphere.
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As Maureen Palmer, a researcher with the Goddard Center for Astrobiology and the lead author on the paper, indicated in a NASA press release: “We found convincing evidence that acrylonitrile is present in Titan’s atmosphere, and we think a significant supply of this raw material reaches the surface.”
Also known as vinyl cyanide, acrylonitrile is used here on Earth in the manufacture of plastics. In the past, it has been speculated that this compound could be present in Titan’s atmosphere. However, it was only recently that scientists became aware of the possibility that it be the basis for living creatures within Titan’s rich organic environment – with its steady supply of carbon, hydrogen, and nitrogen.
This is based on a study that was conducted in 2015, where a team of Cornell scientists sought to determine if organic cells could form in Titan’s harsh environment. Given that the moon experiences average surface temperatures of -179 °C (-290 °F) and the atmosphere is predominantly nitrogen and hydrocarbons, lipid bilayer membranes (which are the foundation of life on Earth) could not survive there.
However, after conducting molecular simulations, the team determined that small organic nitrogen compounds would be capable of forming a sheet of material similar to a cell membrane. They also determined that these sheets could form hollow, microscopic spheres that they dubbed “azotosomes”, and that the best chemical candidate for this sheets would be acrylonitrile.
Such a material would be capable of surviving in liquid methane and at extremely cold temperatures, and would therefore be the most likely basis for organic life on Titan. As Michael Mumma, the director of the Goddard Center for Astrobiology, explained:
“The ability to form a stable membrane to separate the internal environment from the external one is important because it provides a means to contain chemicals long enough to allow them to interact. If membrane-like structures could be formed by vinyl cyanide, it would be an important step on the pathway to life on Saturn’s moon Titan.”
For the sake of their study, the Goddard team combined 11 high-resolution data sets from ALMA, which they retrieved from an archive of observations that were used to calibrate the array. From the data, Palmer and her team determined that acrylonitrile is relatively abundant in Titan’s atmosphere, reaching concentrations of up to 2.8 parts per billion. They also determined that it would be most common in Titan’s upper atmosphere.
It is here that carbon, hydrogen and nitrogen could chemically bond from exposure to sunlight and energetic particles from Saturn’s magnetic field. Eventually, the acrylonitrile would make its way down through the cold atmosphere and condense to form rain droplets that would fall to the surface. The team also estimated how much of this material would accumulate in Ligeia Mare – Titan’s second-largest methane lake – over time.
Finally, they calculated that within every cubic centimeter (cm³) of its volume, Ligeia Mare could form as many as 10,000,000 azotosomes. That roughly ten times the amount of bacteria that exists in the waters along Earth’s coastal regions. As Martin Cordiner, one of the senior authors on the paper, indicated, these findings are certainly encouraging when it comes to the search for extra-terrestrial life in our Solar System.
“The detection of this elusive, astrobiologically relevant chemical is exciting for scientists who are eager to determine if life could develop on icy worlds such as Titan,” he said. “This finding adds an important piece to our understanding of the chemical complexity of the solar system.”
Granted, the study and the basis for its conclusions are quite speculative. But they do show that within certain established parameters, life could exist within our Solar System well-beyond the limits of our Sun’s “habitable zone”. This study could also have implications in the hunt for life in extrasolar systems. If scientists can say definitively that life does not need warmer temperatures and liquid water to exist, it opens up immense possibilities.
In the coming decades, several missions are expected to go to Titan, ranging from submarines that will explore its methane lakes to drones and aerial platforms that will study its atmosphere and surface. Already, it is expected that they will obtain valuable information about the formation of the Saturn system. But to also discover entirely new forms of life? That would truly be Earth-shattering!
Further Reading: NASA, Science Advances