The search for life elsewhere has long focused on what we are most familiar with on Earth—in other words, “life as we know it,” or organisms which are carbon-based and require water to survive. However, a growing number of scientists are now thinking that alternative forms of life are possible, ones which have never been seen on Earth, but could flourish in other types of alien environments. A new study from Cornell University addresses this very question, demonstrating a form of microscopic life which would be possible on Saturn’s largest moon Titan.
Liquid water is essential to life on Earth; Titan also has large bodies of liquid on its surface, but they are composed of methane/ethane instead of water. At the extremely cold temperatures on Titan’s surface, water can only exist as rock-hard ice. It has long been thought that life would be impossible under those conditions, but new research is showing that alternative forms of biology could actually thrive. These putative lifeforms would be methane-based, oxygen-free cells which could metabolize and reproduce just like their carbon-based counterparts. The cell membrane would be composed of small organic nitrogen compounds and capable of functioning in liquid methane temperatures of 292 degrees below zero.
The new study is led by chemical molecular dynamics expert Paulette Clancy, the Samuel W. and Diane M. Bodman Professor of Chemical and Biomolecular Engineering at Cornell. The co-author of the new paper is Jonathan Lunine, the David C. Duncan Professor in the Physical Sciences in the College of Arts and Sciences’ Department of Astronomy.
“We’re not biologists, and we’re not astronomers, but we had the right tools,” Clancy said. “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?’”
Lunine is, however, an expert on Saturn’s moons and an interdisciplinary scientist for the Cassini-Huygens mission which is still orbiting Saturn.
The team wanted to know what kind of organism could possibly exist in Titan’s methane seas and lakes. They came up with the azotosome (“nitrogen body”), a theoretical cell membrane which would be based on methane instead of water. Life on Earth is based on the water-based phospholipid bilayer membrane which contains the organic matter of every cell. A vesicle made from this type of membrane is called a liposome. Such an azotosome would be composed of nitrogen, carbon, and hydrogen molecules, all of which are already known to exist in Titan’s seas and lakes. Surprisingly, they showed the same flexibility and stability as liposomes on Earth. A form of life which mimics cells on Earth, but in a very alien kind of environment. Even the extreme cold isn’t a problem, as these azotosomes are well suited to survive in it.
The team also searched for candidate compounds from methane which could assemble themselves into membrane-like structures. An acrylonitrile azotosome seemed to be the best candidate, since it showed good stability, a strong barrier to decomposition, and a flexibility similar to that of phospholipid membranes on Earth. Acrylonitrile is also already known to exist in Titan’s atmosphere.
Now that the team has shown that such cell structures could theoretically exist on Titan, the next step is to figure out how metabolism and reproduction might occur. As do many others, Lunine would like to investigate this directly on Titan itself, by sending another mission to explore Titan’s seas, “someday sending a probe to float on the seas of this amazing moon and directly sampling the organics.”
From the paper:
“The availability of molecules with an ability to form cell membranes does not by itself demonstrate that life is possible. However, it does direct our search for exotic metabolic and reproductive chemistries that would be similarly compatible under cryogenic conditions. As our understanding of conditions that could nurture extraterrestrial life expands, so does our probability of finding it, perhaps within the liquid methane habitable zone.”
The findings are an exciting example of how life elsewhere may have evolved and adapted to environments very different to those on Earth. As paper co-author James Stevenson, a graduate student in chemical engineering, stated, “Ours is the first concrete blueprint of life not as we know it.” Stevenson was inspired by science fiction writer Isaac Asimov, who wrote about the concept of non-water-based life in his 1962 essay, “Not as We Know It.”
Along with Europa and Enceladus, which both have liquid water oceans beneath surface crusts of ice, Titan may turn out to be another good place to search for life elsewhere in the Solar System.
The new paper is available here in Science Advances.