Titan, the large moon of Saturn (larger than Mercury), is unique among outer solar system satellites in possessing a thick atmosphere. It shares a characteristic property of Earth in the respect of sustaining surface liquids.¹ Other targets of astrobiologists like Europa and Enceladus could only conceivably have life deep in subsurface oceans, but Titan might have “cold little ponds” of organic molecules that inspire the imaginations of origin-of-lifers. But how do its overall characteristics measure up as a potentially habitable object? 

One of the highlights of my time with the Cassini team was being among the first to see images of Titan returning from the spacecraft. The orbiter’s first close pass of Titan occurred three days after Saturn Orbit Insertion on July 2, 2004, allowing its radar mapper, ultraviolet imaging spectrometer (UVIS), composite infrared spectrometer (CIRS), and visible and infrared mapping spectrometer (VIMS) to look through the haze at close range for the first time. Faint outlines of large, dark formations were first seen that looked like continents. The RADAR instrument began mapping these features.

Uncannily Familiar

A year later, when the European Space Agency’s Huygens Probe descended to the surface, the playback of recorded images after the 90-minute time delay astonished us all with earth-like vistas of river channels, plains and cliffs that resembled the coastline of California (watch a video of the descent below). When the parachuting craft hit the ground with a soft thud, its camera took a final picture of a flat surface littered with rounded football-size boulders of water ice that must have tumbled down river channels following flash floods. The landscape looked uncannily familiar.

Subsequent radar mapping and imaging at a variety of wavelengths coupled with experiments with the radio science subsystem (RSS) revealed a world of shallow continents, sand dunes, and large polar lakes. These observations heightened the interest of astrobiologists in the possibility of life there.

Cassini melted into Saturn on September 15, 2017 for its Grand Finale, but the data is safely stored on terrestrial computers I helped administer (until I was laid off in January 2011 for sharing information on intelligent design). Now all the data has been distributed widely to researchers around the world. This month (July 2024) several papers on Titan were published. We can organize the current and previous findings around requirements for habitability.

Ten Measures of Habitability

Magnetic field. Titan orbits mostly within Saturn’s giant magnetic field but spends 5 percent of its travel outside the field. At those times it is exposed to the solar wind, UV light, and cosmic rays. If life existed on Titan, it would be subject to a significant radiation hazard at those times, similar to that experienced on Mars, despite Titan’s much greater distance from the sun.²

Temperature. Titan’s surface is extremely cold, -179 °C (almost -300 °F). Ethane and methane are liquid at this temperature, but the ice is as hard as rock.

Atmosphere. Titan has 1.5 times the atmospheric pressure of Earth, but it is almost all nitrogen (94.2 percent), with some methane (5.6 percent) and trace amounts of hydrogen and argon. The methane creates a haze of “smog” that obscures the surface at visible wavelengths. The atmospheric methane provides just enough greenhouse effect to keep the nitrogen gaseous. When depleted, the entire atmosphere would freeze and fall to the surface. One planetary scientist I conversed with at JPL put an upper limit of 100 million years for all the methane to be depleted. Unless there are other sources of methane, this remains a puzzle.

Interior. The interior of Titan is thought to consist primarily of water ice with a liquid layer of salty water and a silicate core (see NASA diagram). Scientists have not detected tectonic plates on Titan.

Weather. Cassini detected methane clouds that cover about 1 percent of the moon but expand during precipitation conditions. River channels detected at numerous places on the surface indicate that Titan has occasional cloudbursts of methane sufficient to erode icy mountains and flow down channels to flat areas, but it is unknown how frequently those occur.

Surface. Very few impact craters were observed on Titan, indicating that geological processes erase them as on Earth. Two potential cryovolcanoes were proposed. A big surprise was to find longitudinal dunes belting Titan’s equator up to about 30 degrees north and south latitude. The “sand” is thought to be ice particles coated with organic molecules precipitated from the haze. Prevailing winds are apparently strong enough to loft the particles into the atmosphere where they fall as dunes similar to the parallel dunes in Namibia. The dunes are about 300 feet tall, more than half a mile wide, and thousands of miles long, covering 13 percent of Titan’s surface.

Lakes. Large lakes of methane with irregular borders were detected at the north pole of Titan, with another large one near the south pole. Planetary scientists believe that some of the methane in these lakes migrates from pole to pole during Titan’s 29-year seasons, causing the shorelines to grow and shrink each Titan year. The methane evaporates in the warmer summer and seeks a cold trap at the opposite pole. Depending on the depth of these lakes, scientists estimate they contain much more methane and ethane than all of Earth’s proven oil deposits. The surfaces of these lakes are very smooth; however, this month, by interpolating and combining RSS data from different directions, scientists detected some roughness possibly indicating surface waves a few millimeters high.³ Other research found evidence of coastal erosion caused by waves.⁴ Despite the lakes, Titan is much drier than Earth.

Water. Since biomolecules need to interact, and water is the best medium, it is highly unlikely that proteins and nucleic acids, if they were to form somehow, could function in a perpetually frozen state, except perhaps in temporary melt pools after a large meteor strike. For this reason, astrobiologists do not expect to find life on the surface but postulate it might arise in a water ocean under the crust. 

Carbon. When exposed to the solar wind, the atmospheric methane (CH₄) ionizes to CH₃^– which recombines into ethane, C₂H₆. The ethane falls to the surface in liquid form where it accumulates. This is why scientists before Cassini had predicted a global ocean of ethane half a kilometer thick, but such was not seen on radar images or by the Huygens probe (which was designed to float, just in case). The falsification of that prediction remains an unexplained puzzle. Regarding Titan’s carbon budget, the polar lakes are primarily methane with some ethane detected in the channels leading up to the lakes. Spectral analysis by Cassini and Huygens instruments indicated some complex hydrocarbons including propane, butane, benzene, acetylene, and PAHs (polycyclic aromatic hydrocarbons), and nitriles like hydrogen cyanide (HCN), cyanoacetylene (CH₃CN), butyronitrile (C₃H₇CN), and a few others.

Elemental composition. Except for the presence of carbon, hydrogen, and nitrogen in the atmosphere and on the surface, Titan’s interior and hypothetical ocean would be lacking in essential elements for life as we know it: iron, calcium, sodium, potassium, phosphorus, sulfur, chlorine, and trace metals used by enzymes in complex organisms: copper, zinc, magnesium, manganese, molybdenum, and cobalt. Perhaps some of these elements could be delivered by meteorites that could melt the ice in ponds temporarily for the formation of some amino acids. 

Unfortunately, it would be too little, said Catherine Neish from Western University in Ontario, Canada, last February 2024 in a paper in Astrobiology.⁵ Even if a quantity of glycine (the simplest amino acid) equal to a bull elephant’s mass somehow found its way down into the hypothetical ocean each year, it would be too diffuse to sustain life. She concluded pessimistically that most likely Titan’s interior is non-habitable, because “It’s hard to have both the water and carbon needed for life in the same place.” And if this best location for extra-terrestrial life in the solar system fails, the other locations fare worse.

Other icy worlds (like Jupiter’s moons Europa and Ganymede and Saturn’s moon Enceladus) have almost no carbon on their surfaces, and it is unclear how much could be sourced from their interiors. Titan is the most organic-rich icy moon in the Solar System, so if its subsurface ocean is not habitable, it does not bode well for the habitability of other known icy worlds.

Our Exceptional Home

The unmanned space program combined with the ability to observe conditions on faraway exoplanets with space telescopes have given design enthusiasts a large dataset for comparing Earth to other bodies. The data have established that the physical properties of planets and moons (rotational axis, orbital eccentricity, length of day and year, atmosphere, elemental composition, type of star, radiation environment, etc.) vary to wide extremes. This implies that factors affecting habitability are contingent, not determined; they can take on arbitrary values over a wide range of possibilities.

As factors required for complex life multiply, like those explicated in Michael Denton’s Privileged Species books and in the upcoming edition of The Privileged Planet, the case for Earth’s exceptionalism grows, because hard data permit calculations for the probability of each factor separately and in combination. Independent factors have probabilities that rapidly multiply into extreme improbabilities. Some proponents of the “Rare Earth” hypothesis conclude that Earth could be unique in the universe.

We have engineers at NASA and ESA to thank for these findings, but in my opinion, the press offices would do well to discontinue claims that the presence of water alone motivates the search for life. Instead, characterizing the nature of real worlds out there — all lifeless — helps us appreciate and protect the immensely improbable and beautiful treasure that is our own biosphere.

Notes

1. Not counting some lava lakes on Jupiter’s moon Io, which would be of no interest to astrobiologists due to the high temperatures.
2. See, “Cassini Catches Titan Naked in the Solar Wind,” NASA, January 27, 2015.
3. Poggliali et al., Nature Communications, July 16, 2024. See also Cornell Chronicle and an article at Space.com.
4. Palermo et al., “Signatures of wave erosion in Titan’s coasts,” Science, June 19, 2024. See also MIT News and New Scientist.
5. Neish et al., “Organic Input to Titan’s Subsurface Ocean Through Impact Cratering,” Astrobiology, February 20, 2024.