Titan
Even more interesting than the atmosphere are the surface conditions. Like the Earth, Titan has lakes, rain and rivers. But unlike on Earth, the liquid is not water, but hydrocarbons. The lakes and rainfall are composed of liquid methane, with minor components of other hydrocarbons. The details of the complex chemistry taking place on the surface of Titan is beyond the scope of our investigation here, but the overall take-away here is that Titan has an active surface, with something resembling the water cycle on Earth. An additional positive for life is that the environment is rich in organics, the downside is that the temperature is −179.5 °C, which eliminates any liquid water chemistry. These conditions have led to significant debates about the possibility of life based on methane, existing on Titan.
In 2005 the Huygens probe actually successfully landed on Titan. The images it sent back during its descent were stunning proof of an active world full of flowing liquids; drainage channels are visible in the image on top of this page. On the 10th anniversary of the landing, the video on the right was released to celebrate the occasion.
In 2005, in an Article entitled “Possibilities for methanogenic life in liquid methane on the surface of Titan”, astrobiologists Chris McKay and Heather Smith discuss the possibility of hydrocarbon consuming life forms. They suggest that it is possible, but point to some significant problems:
Although we have shown here that the energetics of methane-based life on Titan may be favorable there are two important difficulties to consider related to such life forms. First, the low temperatures imply very low rates of reaction. However by the use of catalysts life can speed up any thermodynamically favorable reaction. The second, more problematic issue is the low solubility of organic substances in liquid methane. Because water is such an excellent solvent we have no experience with how life adapts and works with sparse solubility. It is possible that active transport and organisms with large surface to volume ratios could mitigate this problem.
Meeting such alien, methane-based life forms, would pose some additional problems. In 2010, astrobiologist William Bains painted this, not very pretty picture of a hypothetical encounter:
"Hollywood would have problems with these aliens," said Bains. "Beam one onto the Starship Enterprise, and it would boil and then burst into flames, and the fumes would kill everyone in range. Even a tiny whiff of its breath would smell unbelievably horrible.
Update December 2019: Despite the challenges for life, Titan is still consider quite Earth-like, having a geologically active surface with rain, rivers, lakes, and dune fields. And so NASA has selected Dragonfly, a rotorcraft-lander (essentially a nuclear-powered helicopter) as its next mission in the New Frontiers program.
From the missions' press release:
Launching in 2026 and arriving in 2034, Dragonfly will explore dozens of locations across Titan, sampling and measuring the composition of Titan’s organic surface materials to characterize the habitability of Titan’s environment and investigate the progression of prebiotic chemistry.
“Dragonfly is a bold, game-changing way to explore the solar system,” said APL Director Ralph Semmel. “This mission is a visionary combination of creativity and technical risk-taking that will help us unravel some of the most critical mysteries of the universe — including, possibly, the keys to our origins.”
