Our knowledge of what creates life is pretty limited, considering the only example we've ever found and examined is life here on Earth. The only life we know of, whether microbes or mammals, needs water and carbon. But one potentially habitable moon in the solar system doesn't have much of either.
Saturn's moon Titan is the only moon in the solar system that has a thick and complete atmosphere, one filled with methane and nitrogen. If there's any life there, it would look very different than life on Earth. The moon has large lakes and circuitous rivers of liquid methane and ethane, and with a surface temperature around -290 degrees Fahrenheit, sustaining any living thing on Titan would be extremely difficult. However, a paper published today in the journalScience confirms the detection of a chemical in Titan's atmosphere that could, just possibly, allow life to evolve on this mysterious moon.
The chemical, vinyl cyanide, was discovered on Titan by astronomer Maureen Palmer and a team of researchers at NASA Goddard. In the right conditions, vinyl cyanide could form cell membranes similar to those found in life on Earth—except the cells of organisms on Earth are mostly water. Vinyl cyanide might sound like a new heavy metal rock band, but it's long been suspected that any biological life on Titan would necessarily contain this chemical. The team found the substance mainly in the upper atmosphere, tangled within the haze that envelops the moon.
"The most exciting part for me from an astrobiology perspective is that a couple years ago some other researchers did computer simulations where they were studying if you were going to make cell membrane in Titan conditions, what would be possible?" says Palmer. "And they found that vinyl cyanide would form the most stable membrane, so it's exciting that we actually found it."
The team made the discovery by analyzing calibration data from the Atacama Large Millimeter Array (ALMA) observatory in Chile. The array often uses Titan to calibrate its instruments because the brightness of the moon is known so precisely. Often, the researchers just happen to point the array to Titan before conducting scientific observations of galaxies and other deep space objects. Hidden within this calibration data was information about the chemistry at Titan. After pairing the ALMA data with additional spectral information from the Cassini spacecraft, the team was able to provide the first confirmed detection of vinyl cyanide in Titan's atmosphere.
Vinyl cyanide is a chemical that, in computer simulation models, was able to maintain stable cellular walls using nitrogen and methane instead of water. The base for all cells on Earth, even for the extremophiles living in nitrogen-heavy environments, is water. Most cellular membranes on Earth also include fat-like molecules called lipids, but these lipid layers can't survive in cryogenic methane. On Titan, while a basic cell structure may look similar, it's components would have to be wildly different. The water in our cells would be swapped out for liquid methane, and researchers have no idea what life may look like as a result.
"The main difficulties with Titan and the possibility of life is that it's so cold there that instead of having any liquid water, the surface has bodies of liquid methane," says Palmer. "On Earth, all of our life is water based. There's water in all of our cells, so we really don't know if it's possible for life to exist using some other solvent."
In addition to astrobiologists like Palmer, researchers who study the atmosphere of Titan are also thrilled by the discovery. Sarah Horst, for example, is a professor of planetary sciences at Johns Hopkins University, where she runs a laboratory to study the atmosphere and haze of Titan.
"We were super excited about it. It's one of the heavier molecules that we've been able to see on Titan, and so that's exciting. The fact that is has nitrogen in it is great for helping us understand the chemistry there," she explains. "Nitrogen-bearing molecules are important for life as we know it. It's important for life on Earth and understanding the possibility of life on Titan."
Because Titan has an atmosphere and weather systems, there's a chance the vinyl cyanide that resides in the upper atmosphere could rain down through the haze and condense onto the rivers and lakes that crisscross the surface. This process could possibly result in the right conditions for the chemical to create cells—but right now, it's hard to know if this process is happening. To find out, we need to go back to Titan.
Part of the reason scientists are so interested in studying Saturn's largest moon is that it may provide us a glimpse of what Earth could have have looked like early in our home planet's formation. Palmer is especially interested in this area of research as it relates to habitability.
"Titan has been suggested as being similar to the early Earth before there was oxygen, because Titan doesn't have O2 , or molecular oxygen, or there's not enough to be noticeable. In that sense, it's similar to Earth before there was photosynthesis. So that's interesting if we want to have more of an understanding of the development of life on Earth."
Currently there are no missions planned to Titan, though that could change with the next round of NASA mission proposals. The Cassini mission at Saturn, which has been studying the Saturnian system for more than 13 years, will end in September. So for the foreseeable future, scientists around the world will rely on ALMA and other ground-based observatories to develop a better understanding of this strange, wonderful, hazy orange planetary body.
In the meantime, Palmer and her team are hoping their work will inspire other scientists to create freezing, Titan-like conditions in their labs. If researchers are able to form a full vinyl cyanide cell in the laboratory, it has the potential to rewrite our knowledge of what life in other parts of the solar system could look like.
So the question remains, is Titan habitable? For now, the answer remains a mystery. But it's clear there are enough interesting processes taking place on the large orange moon to justify more study of Titan and a spacecraft mission all its own. At the very least, using Titan as a proxy to develop an understanding of the chemical complexities of the early Earth could prove useful when trying to understand how life came to exist on our own planet, still a mystery in and of itself.
But, as Palmer points out, there's nowhere quite like Titan. "It's a chemical laboratory that's been running for billions of years. I find the possibilities for life there really exciting."