Titan is a large, in many ways planet-like, moon of Saturn.
Many aspects of Titan bespeak its astrobiological possibilities, although there are also drawbacks in this regard.
Titan is composed largely of rock and water ice. It receives very little sunlight. It has a dense, nitrogen-rich atmosphere that also include organic compounds. Furthermore, it is evidenced as likely having surface lakes of hydrocarbons (it’s surface is essentially too cold for liquid water, but hydrocarbons are liquid at much lower temperatures than water), and well as subsurface liquids that may be either liquid ammonia or a water-ammonia solution. Titan is also evidenced as having active weather, such as may include hydrocarbon rain and cryovolcanism. Many of these aspects of Titan suggest astrobiological possibilities, particularly the organics-rich surface liquids that interface with a dense, organics-rich atmosphere, as well as the energy and mixing/reacting effects of it’s chemically dynamic atmosphere and weather patterns.
On the other hand, Titan, being very cold, has essentially no surface or near-surface liquid water, although liquid water may exist deep within.
In addition to having subsurface liquids that might possibly contain liquid water (which raises interesting possibilities in itself), Titan is the only known astronomical body, besides Earth, that is believed to have surface liquids (though not water). Moreover, it has a dense atmosphere with which such liquids, or lakes, can interact, which combination could hypothetically be very conducive to microbial life. However, since these surface lakes lack water, any life in them would have to utilize hydrocarbons as a solvent, instead of water. All known life lives “in” water and uses water as the critical solvent; as such, any hypothetical life in the surface lakes of Titan would have to be a very different and exotic form of life as compared with known life. However, the possibility of life that uses hydrocarbons instead of water as a solvent, while highly speculative, is not beyond the realm of reasonable hypothetical speculation.
It has been (very speculatively) theorized (including by astrobiologist Chris McKay) that methanogenic (methane-producing) microbes may live in the surface lakes of Titan, using, as a solvent, not water as all Earth-based life does, but liquid hydrocarbons, such as methane, in the lakes of Titan (for some more overview information on this, see the Fosdick’s Astrobiology Series article on this subject).
An actual color image of the upper atmosphere of Titan (not it’s surface). The bluish upper portion is chemically dynamic, and the orange portion is a “smog”; the dense atmosphere of Titan, which may contact and interact with surface hydrocarbon lakes, is rich in both organic compounds and astrobiological intrigue. Public domain image. Source: http://photojournal.jpl.nasa.gov/jpeg/PIA06236.jpg
Image: Public Domain, http://photojournal.jpl.nasa.gov/catalog/PIA08409
The above is an actual mosaic image showing the heavily cratered surface of the northern hemisphere of Enceladus, moon of Saturn.
At just over 300 miles across, Enceladus is a small moon, yet large in astrobiological promise. Its ancient, crater-marked northern surface belies the energetic, dynamic nature of it’s southern polar surface and interior; there, evidence strongly suggests a subsurface ocean of liquid water, under a thick layer of surface ice, and possibly heated (and thus prevented from freezing) by tidal flexing related to the force of gravity from Saturn.
And the lie told by it’s northern surface even stands visibly
An artists conception of the Cassini-Huygens spacecraft in orbit around Saturn. Data from this spacecraft provides images of cryovolcanism on Enceladus, and strong evidence of a subsurface liquid water ocean. Image: public domain. Source: http://photojournal.jpl.nasa.gov/catalog/PIA03883.
betrayed by the dramatic story of its southern polar region: imaging performed by the Cassini-Huygens spacecraft during a 2005 fly-by of Enceladus confirms active surface cryovolcanoes (“ice volcanos”) spewing geyser-like jets of massive amounts of icy particles and water vapor out at enormous speeds from its interior, beyond its surface, and into space.
Recently mounting evidence suggests the presence of many of the “ingredients” required by known life in the likely subsurface ocean of Enceladus. In addition to the presence of stable, liquid water as a solvent for life, cryovolcanism and possible geologic activity increase available energy that could help drive life-friendly chemical reactions. Furthermore, simple organic molecules have been detected in the cryovolcano plumes, of types that, on Earth, support chemosynthetic microbes, such as methanogens (methane-producing microbes) that live near hydrothermal vents in Earth’s oceans.
