Looking back at the stars, with the realization that we are looking at entire worlds distant from our own like so many grains of sand, we can’t help but imagine what may be staring back. A planned flyby of the moon Titan once again has drummed up interest in the possibility of extraterrestrial life, very different from our own. While this particular moon of Saturn, a point of interest due to its vast lakes of methane, contains no water, astronomers at Cornell University have proposed a picture of what creatures from Titan might be like – very different from the carbon based life forms that exist on Earth.
Using both the power of imagination and a prominent scientific vision, in the burgeoning proto-scientific field that is astrobiology, the study of potential life processes on other planets, this team of chemical engineers proposed a way in which cells that are methane based can reproduce and acquire nutrients without the need for oxygen – carrying out the same life processes that organisms do on Earth.
They modeled a cell membrane, made up of small organic nitrogen compounds which would be capable of replicating in liquid methane temperatures as low as 292 degrees below zero, what scientists estimate to be the temperature of Titan’s lakes. Their work was published in the journal Science Advances on Feb. 27. Paulette Clancy, who specializes in molecular chemical dynamics led the effort, alongside his first author James Stevenson, who is currently a graduate student in chemical engineering. The paper’s co-author was Jonathan Lunine, who serves as the David C. Duncan Professor in the Physical Sciences in the College of Arts and Sciences’ Department of Astronomy at Cornell, where past faculty members include the legendary science educator Carl Sagan, who himself dabbled in astrobiology.
Lunine’s field of interest is the moons of Saturn. He served as an interdisciplinary scientist on the Cassini-Huygens mission, the first mission to land on Titan, and discover its methane-ethane seas back in 2004. Fascinated with the prospect of methane-based lifeforms on Titan, he received a grant from the Templeton Foundation for studying non-aqueous life. Clancy and Lunine met after the latter sought help last year with chemical modeling from Cornell faculty members.
“We’re not biologists, and we’re not astronomers, but we had the right tools,” Clancy recalled. “Perhaps it helped, because we didn’t come in with any preconceptions about what should be in a membrane and what shouldn’t. We just worked with the compounds that we knew were there and asked, ‘If this was your palette, what can you make out of that?’”
On Earth, all life is dependent on the phospholipid bilayer membrane, a strong, yet highly permeable vesicle that is water-based and shelters the organic matter that defines the makeup of every cell. Out of these vesicles come liposomes. Until now, the majority of astronomers interested in the possibility of extraterrestrial life hunted for the circumstellar habitable zone of each system, a narrow band surrounding a star where temperatures would be ideal for liquid water. The Kepler telescope has singled out several thousand of these planets throughout the universe thought to bear similarities to conditions on Earth. Rarely have they considered the possibility of lifeforms based on other elements such as methane, which contains a substantially lower freezing point than water.
The new concept is called an “azotosome,” derived from “azote,” the French word for nitrogen, and a play on “liposome” – “soma” meaning the body.
Their azotosome bears components of nitrogen as well as carbon and hydrogen molecules which exist throughout the vast cryogenic seas of Titan, regions thought to contain waves and even islands that appear and disappear under the currents. So far, the working model exhibits a similar stability and flexibility to the liposome – a surprise as neither engineers involved with the project had had much familiarity with cellular stability, building the model in the way they worked on semiconductors.
In order to commence this new application of molecular dynamics, the researchers routinely did a search on potential compounds of methane that would allow for self-assembly into membrane-like structures. The best compound they were able to find is an acrylonitrile azotosome, which maintained good stability, as well as a strong barrier to decomposition, and also had considerable flexibility, allowing for fluid molecules to enter and exit its walls – just like phospholipid membranes on Earth, therefore making it possible for organelles to develop which would later lead to living beings. Acrylonitrile – which is an organic compound that is colorless and poisonous is routinely used to manufacture acrylic fibers, resins and thermoplastics – is present in Titan’s atmosphere.
Clancy, intrigued with the launch of their concept, has said that their next step will be to simulate an environment in which their model cells would interact in the way of breathing, eating, and reproducing within a methane environment. Meanwhile, Lunine is eagerly anticipating the next mission to Titan which should kick off some time by the 2020s, soon followed by ones that will successfully sample the lake material.