Could Saturn's moon Titan host life, even without water?
When Kurt Vonnegut鈥檚 protagonist, Malachi Constant, travelled to the surface of Saturn鈥檚 moon Titan, he found the source of his destiny. New research focused on Titan, using data from the Cassini spacecraft and the Huygens probe, has added more credence to theories that聽Titan may also be another kind of source 鈥� a moon with potential to give rise to life.
by researchers at suggests that prebiotic chemical conditions may exist on Titan's surface due to the potential presence of polyimine chains. These molecules are capable both of staying flexible in extreme cold conditions and聽absorbing the small amounts of the sun鈥檚 energy that make it through Titan's murky atmosphere, thus combatting two major barriers that would inhibit prebiotic pathways to life.
The scientists' focus, however, was not on the origins of life. Rather, they set out to answer to an atmospheric puzzle: Titan's atmosphere contains聽hydrogen cyanide, believed to be a on Earth, but that molecule is absent from the satellite's surface. The hydrogen cyanide must react and transform 鈥� but what does it become on the surface?
In polyimine, lead researcher Martin Rahm found both a viable answer to the puzzle and a dynamic molecule that, if it does exist on Titan, could be a potential building block of life.
The research focused on proving, through聽rigorous, controlled modeling and testing that polyimine, which has already been identified in other studies as a polymer that readily forms from hydrogen cyanide, could exist on Titan's surface. But the聽researchers also acknowledge the origins-of-life implications of the research.聽
鈥淭he question of the origin of life on the Earth and the ubiquity of life in the cosmos are really very important questions,鈥� says Cornell planetary scientist Jonathan Lunine, an author of the study who has dedicated much of his professional life to studying Titan.聽鈥淯nderstanding the extent that other worlds and our solar system have life 鈥� those would be very profound discoveries 鈥� so the exploration of these other worlds are crucially important, and knowing what we might want to be looking for, that is the point of this kind of research.鈥�
Titan has been a particular point of interest for origin-of-life studies because of its atmospheric and geological similarities to Earth. Like Earth, Titan has聽lakes, rivers and seas. 聽
Yet instead of water, they are filled with liquid methane and ethane. And unlike what Dr. Rahm calls Earth's聽"warm and pleasant environment,"聽Titan鈥檚 temperatures are a frigid 290 degrees Fahrenheit below zero. There is another key difference: Titan has no liquid water, which has been considered the holy grail in the search for conditions that would sustain extraterrestrial life.
Robert Minard, a chemist who has worked on how hydrogen cyanide polymers could have led to the 聽but was not involved in this study, points to the absence of water as one of the key detriments to life pathways continuing on Titan, despite building blocks that could exist.
He emphasizes the analogous presence of nitrogen in Titan鈥檚 atmosphere and that of the early Earth, but says, 鈥渋f you want to have life, you also need water, and that鈥檚 where Titan fails.鈥�
Speaking generally from his own research and familiarity with Titan, Dr. Minard, a senior lecturer emeritus at Pennsylvania State University, says it is 鈥減ossible that the chemistry going on there could lead to self-replicating life forms, but it's difficult for me to see how that could happen because oxygen is such an important part of bio-molecules.鈥�
Rahm acknowledges this problem as well, but he says this could be part of our Earth-bias, an assumption we make 鈥渂ecause we live on planet Earth.鈥�
鈥淵ou'd have to have a different set of conditions, you'd have to imagine the processes happening without water,鈥� he says. This polyimine research could eventually indicate that life is not possible without water, despite prebiotic chemistry. But it also might not, he says: 鈥淭hat question has not been answered.鈥�
A model of what a on Titan might look like was proposed by another group of Cornell researchers, which also included Dr. Lunine, earlier this year.
In research, led by chemical molecular dynamics expert Paulette Clancy, the team worked outside the biological box to conceptualize oxygen-free cells that could metabolize and reproduce from Titan鈥檚 seas of methane.
Results of the two studies 鈥渁re very complementary,鈥� says Lunine. The cell structure modeled in the work with Dr. Clancy is essentially a container, he explains, and could potentially work in conjunction with polyimines that have fallen to the bottom of the sea. Or they could be separate, he muses, but both interesting pieces within the question of whether and how life could be built outside Earth.
To that same question, whether the polyimine molecule would actually absorb sunlight and instigate chemical pathways toward life is another unknown.
鈥淲hether or not there's a step after the forming of the polymer we don't know,鈥� says Rahm, who explains the complexity of predicting how the polyimine would act on Titan鈥檚 surface, when exposed to other compounds. 鈥淲e begin with this one polymer, it鈥檚 a first step. We don鈥檛 suggest that there would be aliens running around.鈥澛�
Although, he adds, 鈥淭hat would be cool.鈥�
What the team does hope for is further missions to Titan to test their proposal and those of others scientists working on research into "ocean worlds" like Titan or Europa.
鈥淚t's all in the service of the future exploration of Titan,鈥� Lunine says, 鈥渁nd trying to find out if this kind of chemistry does lead to life in multiple places in our cosmic backyard.鈥�
