The Recipe is Different, but Saturn’s Moon Titan has Ingredients for Life
These six infrared images of Saturn’s moon Titan represent some of the clearest, most seamless-looking global views of the icy moon’s surface produced so far. The views were created using 13 years of data acquired by the Visual and Infrared Mapping Spectrometer (VIMS) instrument on board NASA’s Cassini spacecraft. (Image Credit: NASA, JPL-Caltech, Stephane Le Mouelic - University of Nantes, Virginia Pasek, University of Arizona)
The Recipe is Different, but Saturn’s Moon Titan has Ingredients for Life
Catherine Neish is counting the days until her space launch. While the planetary geologist at Western University in London, Ontario, Canada isn’t space-suiting up for her own interstellar voyage, she is playing a key role in an international mission – dispatching a robotic drone to Saturn’s moon Titan – set to blast-off in 2027.
For nearly two decades, the global space sector has focused a majority of its funds and research on Mars, in search of the building blocks of life. And yet, there are dynamic worlds like Saturn’s moon Titan, which may actually have more going on biologically than the Red Planet.
In a recently published study, Neish – a member of Western’s Institute for Earth and Space Exploration (Western Space) – and her collaborators at the European Space Agency (ESA) used advanced imaging technology to investigate Titan. They found when impact craters are formed on Saturn’s largest moon, it exposes relatively fresh ‘water ice’ from Titan’s icy crust.
On Titan, atmospheric processes bury the ice under a layer of sand-like organic material. In Titan’s dry equatorial regions, the sand piles up; but at higher, wetter latitudes, surface streams erode the sand away.
It is difficult to assess what lies beneath Titan’s hazy atmosphere – unless of course, you have a multi-million dollar Visible and Infrared Mapping Spectrometer like ESA, which collected both light visible to humans and infrared light of slightly longer wavelengths during NASA’s Cassini mission.
“It’s wild. There’s no other place like Titan in the Solar System. There’s more sand on Titan per area than anywhere else.” said Neish. “And Titan has weather. It’s not unlike the Earth in that way. It’s just that the ingredients are all wrong. It has methane rain and streams cutting through the surface and organic sand getting blown around. It’s still very active just like it is here on Earth.”
These findings could prove beneficial in discovering ancient ecosystems frozen in the bottoms of impact craters and will also prove invaluable when preparing data analysis and monitoring techniques for the forthcoming Dragonfly drone mission to Titan.
As interest in Titan and other planetary bodies grow, Neish feels the global space sector is ready to start looking beyond Mars for the existence of life – even if the Red Planet remains the prime destination for now.
“I think more and more, we’re seeing a false equivalency between life and Mars. The recent findings about Venus and all the new things we’re learning about it once being an ocean world is another game-changer,” said Neish. “Finally, people are saying, in our search for life in the Universe, we really need to focus on a lot more places, and not just Mars. And that includes NASA sending the Dragonfly mission to Titan.”
NASA's Dragonfly Will Fly Around Titan Looking for Origins, Signs of Life
In 2019, NASA announced that our next destination in the Solar System is the unique, richly organic world Titan. Advancing our search for the building blocks of life, the Dragonfly mission will fly multiple sorties to sample and examine sites around Saturn’s icy moon.
Dragonfly will launch in 2026 and arrive in 2034. The rotorcraft will fly to dozens of promising locations on Titan looking for pre-biotic chemical processes common on both Titan and Earth. Dragonfly marks the first time NASA will fly a multi-rotor vehicle for science on another planet; it has eight rotors and flies like a large drone. It will take advantage of Titan’s dense atmosphere – four times denser than Earth’s – to become the first vehicle ever to fly its entire science payload to new places for repeatable and targeted access to surface materials.
Saturn’s moon Titan is, in many ways, similar to our very early Earth, and can provide clues to how life may have arisen on our planet. NASA’s Dragonfly, set to blast-off in 2027, will dispatch a robotic drone to explore Titan’s diverse environments from organic dunes to the floor of an impact crater where liquid water and complex organic materials key to life once existed together for possibly tens of thousands of years. Its instruments will study how far pre-biotic chemistry may have progressed. It will also investigate the moon’s atmospheric and surface properties and its subsurface ocean and liquid reservoirs. Additionally, instruments will search for chemical evidence of past or extant life.
“With the Dragonfly mission, NASA will once again do what no one else can do,” said NASA Administrator Jim Bridenstine. “Visiting this mysterious ocean world could revolutionize what we know about life in the Universe. This cutting-edge mission would have been unthinkable even just a few years ago, but we’re now ready for Dragonfly’s amazing flight.”
Dragonfly took advantage of 13 years’ worth of Cassini data to choose a calm weather period to land, along with a safe initial landing site and scientifically interesting targets. It will first land at the equatorial “Shangri-La” dune fields, which are terrestrially similar to the linear dunes in Namibia in southern Africa and offer a diverse sampling location. Dragonfly will explore this region in short flights, building up to a series of longer “leapfrog” flights of up to 5 miles (8 kilometers), stopping along the way to take samples from compelling areas with diverse geography. It will finally reach the Selk impact crater, where there is evidence of past liquid water, organics – the complex molecules that contain carbon, combined with hydrogen, oxygen, and nitrogen – and energy, which together make up the recipe for life. The lander will eventually fly more than 108 miles (175 kilometers) – nearly double the distance traveled to date by all the Mars rovers combined.
“Titan is unlike any other place in the solar system, and Dragonfly is like no other mission,” said Thomas Zurbuchen, NASA’s associate administrator for Science at the agency’s Headquarters in Washington. “It’s remarkable to think of this rotorcraft flying miles and miles across the organic sand dunes of Saturn’s largest moon, exploring the processes that shape this extraordinary environment. Dragonfly will visit a world filled with a wide variety of organic compounds, which are the building blocks of life and could teach us about the origin of life itself.”
Titan has a nitrogen-based atmosphere like Earth. Unlike Earth, Titan has clouds and rain of methane. Other organics are formed in the atmosphere and fall like light snow. The moon’s weather and surface processes have combined complex organics, energy, and water similar to those that may have sparked life on our planet.
Titan is larger than the planet Mercury and is the second largest moon in our solar system. As it orbits Saturn, it is about 886 million miles (1.4 billion kilometers) away from the Sun, about 10 times farther than Earth. Because it is so far from the Sun, its surface temperature is around -290 degrees Fahrenheit (-179 degrees Celsius). Its surface pressure is also 50 percent higher than Earth’s.
Dragonfly was selected as part of the agency’s New Frontiers program, which includes the New Horizons mission to Pluto and the Kuiper Belt, Juno to Jupiter, and OSIRIS-REx to the asteroid Bennu.
"Life Not as We Know It" Possible on Saturn's Moon Titan
Liquid water is a requirement for life on Earth. But in other, much colder worlds where water is a solid, life might exist beyond the bounds of water based chemistry.
Taking a simultaneously imaginative and rigidly scientific view, Cornell chemical engineers and astronomers in 2015 offered a template for life that could thrive in a harsh, cold world -- specifically Titan, the giant moon of Saturn. According to an article by Anne Ju of Cornell University, a planetary body awash with seas not of water, but of liquid methane, like Titan could harbor methane based, oxygen free cells that metabolize, reproduce, and do everything life on Earth does.
Their theorized cell membrane is composed of small organic nitrogen compounds and capable of functioning in liquid methane temperatures of 292 degrees below zero. The work is led by chemical molecular dynamics expert Paulette Clancy - Professor of Chemical and Biomolecular Engineering, James Stevenson - a graduate student in chemical engineering, and Jonathan Lunine - Professor in the Department of Astronomy.
Lunine is an expert on Saturn's moons and an interdisciplinary scientist on the Cassini-Huygens mission that discovered methane-ethane seas on Titan. Intrigued by the possibilities of methane based life on Titan, and armed with a grant from the Templeton Foundation to study non-aqueous life, Lunine sought assistance about a year ago from Cornell faculty with expertise in chemical modeling. Clancy, who had never met Lunine, offered to help.
"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?"
On Earth, life is based on the phospholipid bilayer membrane, the strong, permeable, water-based vesicle that houses the organic matter of every cell. A vesicle made from such a membrane is called a liposome. Thus, many astronomers seek extraterrestrial life in what's called the circumstellar habitable zone, the narrow band around the sun in which liquid water can exist. But what if cells weren't based on water, but on methane, which has a much lower freezing point?
The engineers named their theorized cell membrane an "Azotosome." Azote is the French word for nitrogen. “Liposome” comes from the Greek "lipos" and "soma," which means "lipid body." By analogy, "Azotosome" means "nitrogen body."
The azotosome is made from nitrogen, carbon, and hydrogen molecules known to exist in the cryogenic seas of Titan, but shows the same stability and flexibility that Earth's analogous liposome does. This came as a surprise to chemists like Clancy and Stevenson, who had never thought about the mechanics of cell stability before. They usually study semiconductors, not cells.
The engineers employed a molecular dynamics method that screened for candidate compounds from methane for self-assembly into membrane-like structures. The most promising compound they found is an acrylonitrile azotosome, which showed good stability, a strong barrier to decomposition, and a flexibility similar to that of phospholipid membranes on Earth. Acrylonitrile -- a colorless, poisonous, liquid organic compound used in the manufacture of acrylic fibers, resins and thermoplastics -- is present in Titan's atmosphere.
Excited by the initial proof of concept, Clancy said the next step is to try and demonstrate how these cells would behave in the methane environment. What might be the analogue to reproduction and metabolism in oxygen free, methane based cells?
Lunine looks forward to the long term prospect of testing these ideas on Titan itself, as he put it, by "someday sending a probe to float on the seas of this amazing moon and directly sampling the organics."
Stevenson said he was in part inspired by science fiction writer Isaac Asimov, who wrote about the concept of non-water based life in a 1962 essay, "Not as We Know It."
Said Stevenson, "Ours is the first concrete blueprint of life not as we know it."
Is an Exotic Life Form Consuming the Hydrogen and Acetylene on Titan?
In 2010, two research efforts based on data from NASA’s Cassini spacecraft scrutinized the complex chemical activity on the surface of Saturn’s moon Titan. While non-biological chemistry offers one possible explanation, some scientists believe these chemical signatures bolster the argument for a primitive, exotic form of life or precursor to life on Titan’s surface. According to one theory put forth by astrobiologists, the signatures fulfill two important conditions necessary for a hypothesized "methane-based life."
One key finding shows hydrogen molecules flowing down through Titan’s atmosphere and disappearing at the surface. Another maps hydrocarbons on the Titan surface and finds a lack of acetylene.
This lack of acetylene is important because that chemical would likely be the best energy source for a methane-based life on Titan, said Chris McKay, an astrobiologist at NASA Ames Research Center, Moffett Field, CA, who proposed a set of conditions necessary for this kind of methane-based life on Titan in 2005. One interpretation of the acetylene data is that the hydrocarbon is being consumed as food. But McKay said the flow of hydrogen is even more critical because all of their proposed mechanisms involved the consumption of hydrogen.
"We suggested hydrogen consumption because it's the obvious gas for life to consume on Titan, similar to the way we consume oxygen on Earth," McKay said. "If these signs do turn out to be a sign of life, it would be doubly exciting because it would represent a second form of life independent from water-based life on Earth."
To date, methane-based life forms are only hypothetical. Scientists have not yet detected this form of life anywhere, though there are liquid-water-based microbes on Earth that thrive on methane or produce it as a waste product. On Titan, where temperatures are around 90 Kelvin (minus 290 degrees Fahrenheit), a methane-based organism would have to use a substance that is liquid as its medium for living processes, but not water itself. Water is frozen solid on Titan's surface and much too cold to support life as we know it.
The list of liquid candidates is very short: liquid methane and related molecules like ethane. While liquid water is widely regarded as necessary for life, there has been extensive speculation published in the scientific literature that this is not a strict requirement.
The new hydrogen findings are consistent with conditions that could produce an exotic, methane-based life form, but do not definitively prove its existence, said Darrell Strobel, a Cassini interdisciplinary scientist based at Johns Hopkins University in Baltimore, MD.
Strobel, who studies the upper atmospheres of Saturn and Titan, analyzed data from Cassini's composite infrared spectrometer and ion and neutral mass spectrometer in his new paper. The paper describes densities of hydrogen in different parts of the atmosphere and the surface. Previous models had predicted that hydrogen molecules, a byproduct of ultraviolet sunlight breaking apart acetylene and methane molecules in the upper atmosphere, should be distributed fairly evenly throughout the atmospheric layers.
Strobel found a disparity in the hydrogen densities that lead to a flow down to the surface at a rate of about 10,000 trillion trillion hydrogen molecules per second. This is about the same rate at which the molecules escape out of the upper atmosphere.
"It's as if you have a hose and you're squirting hydrogen onto the ground, but it’s disappearing," Strobel said. "I didn’t expect this result, because molecular hydrogen is extremely chemically inert in the atmosphere, very light and buoyant. It should float to the top of the atmosphere and escape."
Strobel said it is not likely that hydrogen is being stored in a cave or underground space on Titan. The Titan surface is also so cold that a chemical process that involved a catalyst would be needed to convert hydrogen molecules and acetylene back to methane, even though overall there would be a net release of energy. The energy barrier could be overcome if there were an unknown mineral acting as the catalyst on Titan's surface.
The hydrocarbon mapping research, led by Roger Clark, a Cassini team scientist based at the U.S. Geological Survey in Denver, examined data from Cassini's visual and infrared mapping spectrometer. Scientists had expected the sun's interactions with chemicals in the atmosphere to produce acetylene that falls down to coat the Titan surface. But Cassini detected no acetylene on the surface.
In addition Cassini's spectrometer detected an absence of water ice on the Titan surface, but loads of benzene and another material, which appears to be an organic compound that scientists have not yet been able to identify. The findings lead scientists to believe that the organic compounds are shellacking over the water ice that makes up Titan's bedrock with a film of hydrocarbons at least a few millimeters to centimeters thick, but possibly much deeper in some places. The ice remains covered up even as liquid methane and ethane flow all over Titan's surface and fill up lakes and seas much as liquid water does on Earth.
"Titan's atmospheric chemistry is cranking out organic compounds that rain down on the surface so fast that even as streams of liquid methane and ethane at the surface wash the organics off, the ice gets quickly covered again," Clark said. "All that implies Titan is a dynamic place where organic chemistry is happening now."
The absence of detectable acetylene on the Titan surface can very well have a non-biological explanation, said Mark Allen, principal investigator with the NASA Astrobiology Institute Titan team. Allen is based at NASA's Jet Propulsion Laboratory in Pasadena, CA. Allen said one possibility is that sunlight or cosmic rays are transforming the acetylene in icy aerosols in the atmosphere into more complex molecules that would fall to the ground with no acetylene signature.
"Scientific conservatism suggests that a biological explanation should be the last choice after all non-biological explanations are addressed," Allen said. "We have a lot of work to do to rule out possible non-biological explanations. It is more likely that a chemical process, without biology, can explain these results – for example, reactions involving mineral catalysts."
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