In 1980 Voyager One photographed Saturn’s far out, large moon Titan as a hazy orange ball, its weighty nitrogen-rich atmosphere more than one hundred kilometers high, loaded with many organics–methane, ethane, ethylene, acetylene, benzene, carbon monoxide, carbon dioxide, hydrogen cyanide, cyanogen(C2N2), acetonitrile(CH3CN)–even polycyclics and ionic hydrocarbons. An impressive list! Surely enough to start life going. Well, maybe.
Then, beneath the hazy smog, Cassini and its Huygens Lander delivered a surprise–continent-like dark patterns, river-like branched channels, basins and low plains. Equatorial sand dunes surrounded irregular highlands made of granular ice and organic compounds, not silicates as on Earth. Since meteors needed to be fairly large to make it through Titan’s smog, craters numbered less than 12.
The portrait of Titan rapidly grew more interesting. Various-sized lakes of ethane appeared at the polar regions, one as large as Lake Superior. Soon, an undersurface ocean, like Europa’s, became a real possibility. Evidence for alkaline water beneath the solid frozen surface (-180°C) included radar images, the disconnect between Titan’s rotational period and surface rotation, and volcanism spewing ammonia water. The latter’s resurfacing activity added to that produced by sporadic methane/ethane rains released in torrents from blowing clouds in the dense atmosphere.
So who’s out there? Some critters like the Europaians? Could be. There are plenty of organics, a dense atmosphere, and liquid in ethane/methane ponds, lakes and rivers—even an undersurface ocean of ammonia-water, providing opportunity for life to develop in benthic (ocean bottom) and icy ceiling alkaline environments. If Titan had a surface ocean in its early days–even though the sun’s light provided only 2% of what Earth gets–microbial films might have evolved along with more advanced reeds, scrapers and fungi, but they had to be able to tolerate alkaline conditions and lots of hydrocarbons. Perhaps they are still hanging on, as scum on the sides of channels or the bottom of ethane lakes.
[frame type=”lifted” align=”none”][/frame]
As Titan’s surface froze solid and the ethane/methane rains began, the two different environments mentioned above would have developed housing for the original hydrophobes or hydrocarbon-tolerant aquatics postulated by Irwin and Schulze-Makuch in the book Cosmic Biology.
The most challenging requirement for life to survive on Titan may not be the hydrocarbons and energy challenges so much as the extreme cold, which guarantees that any metabolism would be quite slow. Low reaction rates would also mean less mobility, and the puny amounts of oxygen in Titan’s atmosphere and the lack of free compounds for oxidation-reduction reactions don’t help.
However, there is hope for life out there. Evolution finds a way, as our extremeophiles illustrate. On Earth there is a fungus of the genus Fusarium that degrades hydrocarbons in conditions with little oxygen and water. Its membranes protect it from its hydrophobic environment. In Trinidad, microbial life has been found in a liquid asphalt lake. Bacillus cereus degrades n-hexadecane in a Chinese oilfield, and anaerobes are known to degrade petroleum sludge.
Though the biochemistry may be like nothing we know, three basic conditions for life on Titan could be met. 1)Boundry membranes could be formed from Titan’s insoluble tholins (polymeric molecules formed by UV irradiation of organics and nitriles), which precipitate out or rain down on the moon. Silanes offer another possibility. In the ammonia-water of the undersurface lakes, lipophilic membranes would make sense.
2)Variable molecules that transmit needed information for life could “easily” form, like hydrocyanic acid nitriles, courtesy of the UV light and ionizing radiation in the upper atmosphere.
3)Energy to power simple, microbial life could be farmed from sources like atmospheric carbon monoxide and ions or water soaring out from Enceladus’s south pole. With its triple bonds, acetylene in Titan’s upper atmosphere could react with hydrogen to produce methane and energy. Such methanogenesis would also be useful on a subsurface ice ceiling or ocean bottom near the metallic core of Titan.
It’s a stretch—but so are we. While the space explorers labor over their heaps of data, we can enjoy the ride into the improbable, until they find more surprises.