Titan

Data bank: Titan

This much is known about Titan, by far the largest of Saturn’s several dozen moons and the second largest moon in the entire solar system.

With a diameter of 5,150 kilometers, Titan is bigger than the planet Mercury and only a shade smaller than Jupiter’s largest satellite, Ganymede. Titan is the only moon in the solar system to possess a substantial atmosphere. Indeed, Titan’s atmosphere is 50 percent denser than Earth’s at ground level.

That atmosphere is composed mainly of nitrogen, laced with hydrocarbons such as methane, ethane and propane, plus nitrogen-carbon compounds such as hydrogen cyanide, cyanogen, and cyanoacetelyne. Shine sunlight on such an atmosphere and you get the same result you would in Los Angeles or Tokyo or Mexico City: photochemical smog, induced by solar ultraviolet light. Titan is a smog-covered world. Its predominantly orange coloring is due to this smog, which blankets Titan and makes it necessary for observations of its surface to be done in infrared wavelengths, which penetrate the smog, rather than visible light, which does not.

The incoming solar ultraviolet light, together with energetic electrons from nearby Saturn’s powerful magnetosphere, produce complex chemical reactions high in Titan’s thick atmosphere. Organic polymers called tholins are created, to drift downward deeper into the atmosphere and eventually fall onto the moon’s surface: black snow.

Laboratory experiments on Earth showed that tholins, when dissolved in liquid water, yield amino acids, which are the building-block molecules of proteins and thus fundamental to life.

Orbiting more than a million kilometers from Saturn, which in turn lies twice as far from the Sun as Jupiter and ten times farther from the Sun than the Earth does, Titan’s surface temperature averages -183° Celsius. Titan is cold, too cold to have liquid water on its surface—except when a region might be heated temporarily by a volcanic eruption or the impact of a meteor. Or if the water is mixed with an antifreeze compound, such as ammonia or ethane derivatives.

Titan’s density is not quite twice that of water, which means that its body must be composed largely of ices—frozen water and/or frozen methane—with perhaps a small rocky core beneath a thick icy mantle.

Despite Titan’s low temperature, liquid droplets of ethane can form in its atmosphere and rain down onto the frigid surface, collecting as lakes or perhaps larger seas. There are streams of ethane (or ethane-laced water) carving out channels across the ground of ices. Several sizable seas of hydrocarbon-crusted liquid methane dot the moon’s surface.

Titan rotates on its axis in slightly less than sixteen Earth days, the same period as its orbit around Saturn. Thus Titan is “locked” in its rotation so that it always presents the same face to its planet, Saturn, just as our Moon presents the same face to Earth. But even a “locked” moon wobbles slightly in its orbit, and Titan’s rotation is perturbed slightly by its sizable neighbors, the moons Rhea and Hyperion, each of which is close to 1,500 kilometers in diameter. Titan rocks slightly back and forth as it orbits Saturn, a ponderous wobbling that creates strange tides in its hydrocarbon seas.

A world rich in carbon, hydrogen and nitrogen. A world where raindrops of ethane and sooty flakes of tholins fall from the smoggy sky. A world that contains rivers and streams of ethane or ethane-laced water, and methane seas. Although it is a very cold world, a primitive form of microbial cryogenic biology was found to exist on Titan’s surface by the earliest automated probes from distant Earth. Could there be a more sophisticated biosphere, perhaps deeper underground?

And there are large swaths of dark material carpeting parts of Titan’s surface. Early probes showed that they are rich in carbon compounds. Fields of frozen petroleum? Patches of solidified hydrocarbons? Swales of black tholin snowbanks piled on ground that is too cold for them to melt?

Or something else?