Titan: A Tale of Two Worlds

Titan: A Tale of Two Worlds
This radar image of the surface of Saturn's moon Titan was acquired on October 26, 2004, when the Cassini spacecraft flew approximately 994 miles (1,600 kms) above the surface. Brighter areas may correspond to rougher terrains and darker areas are thought to be smoother. This image highlights some of the darker terrain, which the Cassini team dubbed "Si-Si the Cat." This nickname was chosen after a team member's daughter, Si-Si, pointed out that the dark terrain has a cat-like appearance. Image
(Image: © NASA/JPL)

As wesearch for extraterrestrial life, Earth in some sense always provides our framework. The data indicate that life does in fact exist on this planet, and it existedhere sometime before about 3.5 billion years ago (give or take a couple hundredmillion years). However, thanks to plate tectonics and other pesky processes,we're missing some critical information about this early time. This includes informationabout how life got started on Earth.

Titan maycome to our rescue. As the largest of Saturn's moons--much larger than our ownmoon, larger even than the planet Mercury--Titan has several striking similaritieswith Earth. For one, it has a substantial, largely nitrogen atmosphere. Foranother, it supports an active volatile cycle. And the effects of thisvolatile cycle make Titan appear 'surprisingly Earth-like' (as scientists withthe Cassini-Huygens mission have been saying). For example, images show whatare interpreted to be convective clouds forming and then dissipating, apparentlyraining out onto the surface and producing what looked for all the world (punintended) like terrestrial rivers and lakes.

But Titan alsohas significant dis-similarities from Earth, at least as we know ittoday. Instead of a water-based volatile cycle as on Earth, Titan cycleshydrocarbons. At Titan's temperature, water is about as solid as rock onEarth, but hydrocarbons--which are gaseous under terrestrial conditions--canevaporate into the atmosphere, re-condense into rain, and fall veeeery slowly(Titan's gravity is about 1/7th of Earth's) to the surface, eitherto pond or flow over the surface as a liquid. A summer cloudburst on Titanwould be something like standing in a cloud of droplets of liquid natural gasat -300 degrees Fahrenheit. In addition to N2, Titan's atmospherecontains significant amounts of methane, and ethane and other hydrocarboncompounds have been detected.

One thingTitan's atmosphere does not contain is oxygen (which is probably just aswell--oxygen + hydrocarbons is highly flammable). The story of the rise of oxygenon Earth is not well understood. Oxygen-rich habitats must have developedaround the time of those cyanobacteria that produce oxygen (>2.7 Ga), butbefore that time, that is, during the Archean (3.8-2.5 Ga), life must have developedunder oxygen-poor conditions. So how does that relate to Titan? Well, itmeans that Titan is like early Earth in not having oxygen. It's also good for astrobiologistsinterested in exploring the chemical, possibly pre-biotic, pathways that mayhave lead to the development of biota on early Earth.

Thisexploration takes a variety of forms. Some of it is theoretical modeling,trying to understand the large-scale movement and resultant combinations ofmaterials that occur on Titan's surface. For example, in some equatorialregions, Cassini imagery shows what look like giant wind-blown dunes. Thesedunes seem to be composed preferentially of fine-grained organic sediments, whichmay facilitate interesting (i.e., biologically relevant) chemicalinteractions. These organic sediments form by UV photolysis and recombinationof hydrocarbons in Titan's atmosphere, a process that gives Titan its orangehaze (kind of like an L.A. smog on steroids). After these complex organicmolecules are formed, they settle out of the sky onto Titan's surface to formdunes, fill up depressions, line river channels, and generally add to Titan's airof being a giant 'Rorschach inkblot test.' (In support of this statement, Irespectfully cite the fact that an area of dark terrain in an early Radar imagewas referred to by the Cassini Team itself as 'Si-si the cat.' [See Image.]

Anotherapproach to discovering pre-biotic chemistry of Titan and early Earth isexperimental. This approach has a long history, dating back to the Miller-Ureyexperiment, in which gases believed to represent the atmosphere of early Earth werecruelly subjected to shock treatment (ie., simulated lightening) as a test tosee how life might have arisen on Earth. Two decades, later in the late1970's, Carl Sagan and Bishun Khare (who is now, appropriately, a scientist atthe Carl Sagan Center), did a similar style of experiment using gases found inTitan's atmosphere. They created dark, reddish gunk, which they termed 'tholin'(from the Greek word for 'muddy'). These types of experiments requirecontinual 'updating' as we learn more about the true conditions--and the actualgas species--that existed on early Earth and exist on Titan. A recent study,the most faithful yet to Titan's conditions, used a deuterium lamp to simulate atmosphericUV radiation of methane and measured the properties of the resultant aerosols.

This workis all focused on understanding Titan as an analogue for pre-biotic Earth. Asmall and courageous group of scientists have suggested that life may exist onTitan even today. This runs contrary to the maxim that life as we know itrequires liquid water, of which Titan has none, but is based on the idea that non-aqueouslife might arise through analogue chemistry. For example, ammonia--like water--isa polar molecule and dissolves other molecules--including hydrocarbons--withelectrical charge.

Continuedresearch here on Earth may inform us about this possibility for life on Titantoday. Likewise, continued spacecraft investigation of Titan may tell useabout life on Earth in the ancient past. In the exploration for life, Titanand Earth symbolize spatial and temporal symbiosis.

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