Life on Titan
Titan is the only moon with surface liquids and prebiotic chemistry.
Titan, Saturn’s largest moon, remains an open question in the search for life. It is far colder than Earth, yet it is the only other world in the Solar System known to have stable liquids—rivers, lakes, and seas—on its surface. Its thick, chemically active atmosphere is loaded with carbon compounds. On the ground, there are bodies of liquid methane and ethane, and scientists suspect a layer of liquid water lies beneath the icy crust. Some researchers think these liquid mixtures could support a kind of prebiotic chemistry, possibly leading to living cells very different from those on Earth.
Data from the Cassini–Huygens mission revealed oddities in the atmosphere near the surface that might hint at methane-producing organisms, though non-living chemical or weather processes could also explain them. That mission was not designed to directly detect microbes or fully catalog complex organic compounds.
Titan’s atmosphere is a rich laboratory for prebiotic chemistry, driven by sunlight-triggered reactions in its upper layers. Cassini’s mass spectrometer identified a range of organic chemicals there, though the exact structures of many remain uncertain. Other compounds, like ammonia, polyynes, amines, and various hydrocarbons, are also thought to be present, along with small amounts of carbon dioxide and water vapor.
The surface is brutally cold at about 94 K (−179 °C). At that temperature, any water ice stays solid. Because of the cold and the lack of carbon dioxide, some scientists see Titan less as a likely home for life and more as a natural experiment for studying conditions before life arose on Earth. However, meteor impacts could create temporary “oases” of liquid water that last for centuries, enabling water-based organic chemistry. Still, the possibility of life using liquid methane and ethane as a solvent is not ruled out, and finding even primitive life there would have big implications for how common life might be in the universe.
In the 1970s, high infrared emissions from Titan led some to think its surface might be warmer, perhaps even Earth-like in cooler regions. But another explanation was that the upper atmosphere was heated by ultraviolet light absorbed by molecules like ethane and acetylene. Pioneer 11’s flyby in 1979 confirmed the surface is extremely cold, far below what is usually considered habitable.
- Surface temperature
- 94 K (−179 °C, −290 °F)
- Atmospheric compounds detected
- ammonia, polyynes, amines, ethylenimine, deuterium hydride, allene, 1,3 butadiene, carbon dioxide, water vapour
- Future surface temperature
- ~200 K (−70 °C) in 5–6 billion years
- Subsurface water layer depth model
- ammonia–water solution up to 200 km deep
- Nucleotide bases experiment
- five nucleotide bases and amino acids produced without liquid water
Lore & Background
In June 2010, scientists analyzing data from the Cassini–Huygens mission reported anomalies in the atmosphere near the surface which could be consistent with the presence of methane-producing organisms, but may alternatively be due to non-living chemical or meteorological processes. The Cassini–Huygens mission was not equipped to look directly for micro-organisms or to provide a thorough inventory of complex organic compounds. Titan's consideration as an environment for the study of prebiotic chemistry or potentially exotic life stems in large part due to the diversity of the organic chemistry that occurs in its atmosphere, driven by photochemical reactions in its outer layers. The Miller–Urey experiment and several following experiments have shown that with an atmosphere similar to that of Titan and the addition of UV radiation, complex molecules and polymer substances like tholins can be generated. In October 2010, Sarah Hörst of the University of Arizona reported finding the five nucleotide bases—building blocks of DNA and RNA—among the many compounds produced when energy was applied to a combination of gases like those in Titan's atmosphere. Hörst also found amino acids, the building blocks of protein. She said it was the first time nucleotide bases and amino acids had been found in such an experiment without liquid water being present.
Reader's Guide
Titan's significance in the search for life lies in its unique combination of surface liquids, a thick chemically active atmosphere rich in carbon compounds, and a likely subsurface liquid water layer. While the lack of liquid water on the surface has been cited by NASA astrobiologist Andrew Pohorille as an argument against life there—he considers water uniquely suited to promote self-organization of organic matter—other scientists such as Jonathan Lunine do not rule out life in an environment of liquid methane and ethane. Lunine has written about what discovery of such a life form, even if very primitive, would imply about the prevalence of life in the universe. Laboratory simulations have suggested that enough organic material exists on Titan to start a chemical evolution analogous to what is thought to have started life on Earth, assuming the presence of liquid water for longer periods than currently observable. Data published in 2012 from NASA's Cassini spacecraft strengthened evidence that Titan likely harbors a layer of liquid water under its ice shell. The detection of microbial life on Titan would depend on its biogenic effects, such as examining atmospheric methane and nitrogen for biogenic origin. Titan may become warmer in the future, with surface temperatures rising to ~200 K (−70 °C) as the Sun becomes a red giant, creating conditions that could persist for several hundred million years and be agreeable to exotic forms of life.
Did You Know?
- Titan is the only place besides Earth known to have liquids in the form of rivers, lakes, and seas on its surface.
- In October 2010, Sarah Hörst reported finding the five nucleotide bases and amino acids in an experiment simulating Titan's atmosphere without liquid water.
- Titan's surface temperature is about 94 K (−179 °C, or −290 °F).
More in Moons of Saturn, Part 2 1-24
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