[BLOG-LIKE POSTING] On the latest on lakes and life on Titan

[rfmcdonald]

It's been known for a while that for all of its differences from Earth, Saturn's planet-sized moon Titan is Earth-like in that it has surface lakes and seas which just as on Earth constitute key elements of a planet-wide hydrographic cycle. Titan's lakes and seas are rather different from Earth's, of course, since Titan is a cryogenic environment (average global temperatures around minus 180 degrees Celsius) with a dense nitrogen-methane atmosphere, and Titan's lakes are liquid hydrocarbons (ethane and methane). Somewhat paradoxically, Titan is also a desert environment, warm enough that most bodies of liquid on Titan's surface are located in Titan's polar areas. (The first lake discovered was Ontario Lacus, a south-polar sea I've mentioned here in the past not only because of its being a namesake of Toronto's own lake.)

The discovery of surface bodies of liquid in Titan's equatorial region by the space probe Cassini makes things even more interesting.

[I]mages show dark regions that appear to be pools of hydrocarbons around the moon's equator.

"We detect evidence for the presence of a tropical lake with an area of 2,400 square kilometers [927 square miles]—as large as the Great Salt Lake in Utah—with a depth of at least a meter [three feet]," said study leader Caitlin Griffith, a planetary scientist at the University of Arizona in Tucson.

"Our work also suggests the existence of a handful of smaller and shallower ponds, similar to marshes on Earth, with knee- to ankle-level depths."

Based on the polar lakes and other evidence, scientists think methane—the main component of natural gas—might play a similar role as water on Earth, cycling between the atmosphere and the surface.

But until now, Titan's lower latitudes were thought to be dry, filled mostly with rippling sand dunes.

Scientists had previously seen only hints of moistness in Titan's tropical areas. For instance, in 2004 the European Huygens lander set down near the moon's equator and captured views of what appeared to be rain runoff.

In addition, later Cassini images turned up evidence of storms in this supposedly parched region.

Still, Griffith and colleagues think the newfound lakes can't be explained by rainfall, since a number of circulation models say that significant bodies of liquid shouldn't be stable at the moon's tropics based on precipitation alone.

"Any liquid deposited in the tropical surface evaporates quickly and eventually is transported by Titan's circulation to the poles, where the large polar lakes appear," Griffith said.

"Lakes at the poles are easy to explain, but lakes in the tropics are not."

One possibility, Griffith said, is that "our detection of tropical lakes suggests that a subterranean source of methane may flood the surface and dampen the ground, in essence creating oases on Titan."

The use of the term "oasis" may have been intentional. A recent post at Supernova Condensate explores the possibility that Titan's environment might be hospitable enough for life.

The idea has been considered for some time now of whether Titan might be able to support some kind of life. In fact, the idea was given quite some consideration by one Carl Sagan back in the 1970s. Since then though, ideas have developed that perhaps any prospective Titanian organisms might be methanogenic. The mechanism put forward by McKay and Smith in 2005 is that, in the absence of liquid water, an organism might metabolise molecular hydrogen and acetylene to give methane:

C2H2 + 3H2 --> 2CH4

Life could potentially use a reaction like this to acquire energy comparable with the way methanogenic bacteria work here on Earth. The result would be drastically depleted H2 and acetylene (or ethane, or other larger hydrocarbons) at the surface, and an abundance of CH4. Interestingly enough, in 2010 some detailed calculations implied that there may well be a lack of hydrogen at the surface of Titan, and despite how easily it forms in Titan’s upper atmosphere, the Huygens probe could barely measure the ethane at ground level because it was so scarce. The latest discovery of methane lakes in regions of Titan where rain isn’t normally seen to fall would appear to add some further weight to this .

We, of course, shouldn’t get too excited just yet. There are still quite a number of missing pieces to this puzzle. Indeed, other reviews of the data have suggested that such methane-based life may be quite unlikely. All the same Titan’s methane has to be replenished somehow. Exactly as with the methane on Mars, Titan’s methane is destroyed by sunlight hitting the moon’s upper atmosphere. If not replenished, in a few tens of millions of years it would be all gone.

The ground on Titan is seemingly a lot like wet sand, damp with liquid methane. Huygens managed to detect plenty of methane after it landed, despite the instrument it was using being embedded several centimetres into the ground. Indeed, it’s quite likely that Titan is a “muddy” little planet with plenty of wet ground and humid air near its surface. This is coupled with the fact that Titan’s lakes are expected to be ephemeral in nature, appearing and disappearing over time.

All in all, while there’s no way we can definitively state that there is any kind of life on Titan, the evidence so far is compelling, it has to be said. In closing, a final thing to consider is the temperature. Yes, Titan is cold. Very cold. And both chemical and biochemical reactions would occur very slowly there. But remarkably, at least one species of Earth microbe, named colwellia, has been found to be capable of both surviving and metabolising at temperatures which would make Titan seem… well… genuinely tropical. Perhaps the door shouldn’t be fully closed on the idea of methanogenic titanian life just yet….

(A related post at Supernova Condensate was skeptical of the idea that Titan's unusual atmospheric content is a unique artifact of our time. The mediocrity principle widely adopted by astronomers militates against the assumption that we're lucky enough to see something that rare in bodies with histories so much longer than humanity's recorded history.)