Searching for life in the Solar System has been the mission of this special four-part SpaceRip series.  So far, we have considered Mars, Europa, and Enceladus as possible homes to extraterrestrial organisms.  Now, we turn our attention to Saturn’s largest moon Titan.

Before the arrival of the Cassini-Huygens mission, at the end of 2004, Titan was shrouded from astronomers’ view by its opaque atmosphere.  As the Huygens probe descended on its parachute, it sent back photographs that showed a fascinating – and in some respects familiar – topography, including drainage channels and a possible shoreline.  When the probe landed, it sent back a photo of a frozen, rock-strewn plain and, as expected, transmitted data for only about 90 minutes before going silent.

View of Titan from Huygens as it descended toward the surface. (Credit: ESA/NASA/JPL/University of Arizona)

Looking for Life in All the Wrong Places?

Titan is larger than the planet Mercury and is primarily composed of water ice and rocky material.  It is the only object in the Solar System (other than Earth) that is known to have rivers, lakes, and seas on its surface.  It also has a dense, nitrogen-rich atmosphere supporting a climate that creates wind, rain, and seasonal weather patterns.  Sounds promising for life, doesn’t it?  Well, maybe, but there are some, uh, issues.

Titan’s Ligeia Mare, in false color, as seen from Cassini spacecraft. (Credit: NASA/JPL-Caltech/ASI/Cornell)

On Earth, the dominant liquid is H2O, which is necessary for life on our planet.  Three-quarters of our planet is covered by oceans, inland seas, and large lakes.  Water falls from the sky as rain, and rivers flow through the land areas.  Titan, on the other hand, is so cold (average temperature almost -180ºC) that its water is locked up in ice, and the only available liquids are methane and ethane.  So, no chance of life, right?  Not so fast.  This is where things get really interesting.

Expanding the Search

In the search for extraterrestrial life, among of the clues most investigating scientists look for are the usual suspects:  the presence of water, organic compounds and nutrients, energy sources, etc.  These perceived requirements, of course, are quite understandable, and often lead to a qualifying phrase after the word “life,” i.e. “as we know it.”  But what if there are organisms somewhere out there that bear little resemblance to life as we know it?

Artist’s rendition of ice floating on a methane lake on Titan. (Credit: NASA/JPL-Caltech/USGS)

Some astrobiologists hypothesize that a wholly different kind of life could have gotten started in the methane lakes of Titan.  Rather than breathing oxygen, as we do on Earth, such organisms would inhale H2, use acetylene to metabolize it, and exhale methane.  As far-fetched as this might sound, a 2005 paper by NASA planetary scientist Chris McKay (and H. D. Smith) and tantalizing findings, in 2010, by Professor Darrell Strobel at Johns Hopkins University concerning the possibility of methanogenic life add plausibility to such speculation.

Imagining a Return with Purpose

The best way to determine whether it exists on Titan would be to send another spacecraft there with a primary goal of testing the seemingly far-out hypothesis that methanogenic organisms might have gained a foothold.  There are a number of concepts for future NASA missions to Titan, including a hot-air balloon or drone flying in the atmosphere, landers that would float on the methane lakes, and even a submarine to explore beneath the surface of a lake.  However, none is currently on the docket, so Titan will orbit Saturn without another visit in the foreseeable future.

NASA/JPL-Caltech/Space Science Institute)