Could life exist elsewhere within our solar system, and if it does, where would be the best place to start looking?
The most obvious solution is to look to Mars, our closest and most similar celestial neighbor. A new study at Northern Arizona University, however, asks the question: “What if we were to look a little further?”
In his presentation “Searching for Life in the Rivers and Lakes of the Outer Solar System” at this year‘s Flagstaff Festival of Science, Gerrick Lindberg talked about his research into what it takes for life to form and how this could occur in the outer reaches of our solar system.
With the project, Lindberg and his group are tackling the concept of life as we know it and reevaluating what the essential components of the formation of life are.
“Oftentimes when you hear this idea presented, it will be presented in Earth analogues … but I want to broaden this little a bit to think about how could other places in the solar system be a situation that we could also use to understand the formation of life," he said.
Traditionally, the essential components for the formation of life are thought to be energy, complex molecules and liquid, specifically liquid water.
“Water is an important media to move things around. Based upon the life that we know, it’s hard to imagine how it happens without water,” Lindberg said, “In my approach to this, I’ve relaxed the necessity of water, just because you can imagine other liquids where things can happen and we can see molecules that we might be interested in from our understanding of our life on Earth that could then occur there.”
The impetus of the idea come from the incredible images of Pluto returned by NASA’s New Horizons mission in 2015. Lindberg is an associate professor of chemical physics at NAU who moved to Flagstaff in 2014. Then he met Jennifer Hanley and Will Grundy, two planetary scientists at Lowell Observatory. It was through his collaboration with them that he was introduced indepth to NASA’s New Horizons mission. The incredible results of that mission led to the idea that liquid of some kind could be present on the surface of Pluto.
The images of Pluto captured by New Horizons show features that suggest the presence of flowing liquids. With an average surface temperature between -400 and -360 degrees Fahrenheit, it is hard to imagine how liquid could exist on the icy world.
“If we look at the molecules that are present there, and we have a very good inventory of what molecules are there because New Horizons has spectroscopic instruments that allows us to look at what’s there," Lindberg said. "All of these molecules should be solid at this point.”
Lindberg asserts that the key to determining how liquid could be present on the surface of Pluto lies in a phenomenon known as eutectic behavior. A eutectic mixture is a mixture of two substances whose freezing point is lower than that of either of the two individual components.
Lindberg explains: “If you have this sort of phenomena going on there, we could have mixtures of molecules that all should be solids in their pure forms at the temperatures that we see on Pluto, but maybe when we mix them together we start to see them forming liquids in the conditions that occur on Pluto’s surface.”
In order to predict these behaviors, Lindberg and his group have developed thermodynamic models to analyze the freezing point depressions of different mixtures of the molecules present on Pluto’s surface. These experiments have led them to identify two mixtures that Lindberg predicts could be liquid near Pluto’s conditions -- the first being a mixture of nitrogen and methane, the second a mixture of nitrogen and carbon monoxide.
The catch with these predictions is that they rely on an ideal approximation of molecular interaction.
“The thermodynamic model that we’ve developed hinges on something in thermodynamics called ideal solution behavior. What the ideal solution approximation states is that we are assuming that two different molecules interact with each other in the same way that they would interact with themselves," Lindberg said.
He added: “Two molecules of different types will never interact with each other in the same way.”
To account for these differences in the way that molecules interact, Lindberg and his team utilize a method known as molecular dynamic simulations. This process involves using computer simulations to recreate the interactions between the different molecules in a solution to see how closely their interactions align with the ideal approximation. This allows them to predict more accurately which solutions are likely candidates to exist in a liquid state on Pluto’s surface.
To further analyze how the building blocks of life could form and develop outside of the Earth’s ecosystem, Lindberg looks to Saturn’s largest moon, Titan, the only other place in our solar system known to have persistent lakes. These lakes, however, do not consist of water like the ones on Earth, but are instead composed of liquid nitrogen and hydrocarbons, like ethane and methane.
Titan’s lakes are an appealing ecosystem to study because they are home to complex molecules that we don’t see on Earth.
“If you’re on Titan, there are a lot of molecules that exist there that can’t occur here because we have a lot of oxygen in our environment and oxygen likes to react with almost everything," Lindberg said. "Titan is kind of interesting because you get things that stick around and go through all these different processes that you don’t see here. It turns out that lots of molecules have been observed on the surface of Titan, and other places in the outer solar system. Many of these have been implicated in the formation of biological molecules.”
With this information, Lindberg and his team began running experiments to see how these molecules would interact in an environment like the lakes of Titan.
This was done by running molecular dynamic simulations -- much like the ones used in the Pluto research.
Lindberg explained: “We decided to put many of these into models of these Titan lakes to see what would happen.”
When they performed the simulations they began from a totally random mixture of complex molecules and hydrocarbons like those found in Titan’s lakes. What they found was that a number of these complex molecules grouped together into clusters.
“We’re seeing these things really tightly cluster together with each other, and they really like to be separated from the surrounding Titan lake environment. So these are prime locations for thinking about the sorts of chemistries that might be interesting for the formation of even larger molecules that might be relevant for life.”
The research performed by Lindberg and his associates has shown that even on the frigid surfaces of celestial bodies like Pluto and Titan, under the right conditions the building blocks of biological molecules can form. The answer may not be too far off: In 2026, NASA is planning to launch the Dragonfly Mission, which will arrive at Titan in 2034 to search for signs of life on Saturn’s icy moon.
Tristan Donnelly is a NASA Space Grant intern for Northern Arizona University for 2021-22.