Titan's Hazy History and the Potential for Life

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Saturn’s moon Titan is the only moon in our solar system known to possess an atmosphere of any significance.

Ten times thicker than Earth’s, Titan’s atmosphere extends nearly 370 miles (600 km) above its frigid surface. It’s a literal chemical factory, where nitrogen and methane are zapped by the sun’s ultraviolet rays and transformed into organic molecules, some of which descend to the moon’s surface while others rise up above the clouds, creating a bluish high-level haze of hydrocarbons.

Titan’s atmosphere forms an opaque orange shroud that covers it and hides many of its surface features from view, keeping much of its details a mystery until the arrival of the Cassini-Huygens spacecraft in 2004.

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Cassini’s instruments were able to pierce Titan’s cloud cover to reveal a world much more Earth-like than moon-like, with weather and rain feeding rivers, streams and lakes all across its surface. In fact Titan is the only place in the solar system where we have discovered liquid on the surface!

The only difference — and it’s a big difference — is that, 800 million miles away from the warmth of the sun, the liquid on Titan is not water but methane. With temperatures of nearly -300 degrees Fahrenheit water has long since frozen as solid as rock on Titan’s surface while methane rains down from the clouds… falling slowly in large drops, methane fills streams and rivers that eventually flow into vast shallow lakes.

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Dry stream beds are coated with rounded pebbles and stones, hinting at flash methane floods that must repeatedly flow over them, eroding them and the entire landscape around them.

Were it not for the cold temperatures, a time-traveling space explorer could draw many similarities between the Titan of today and the ancient Earth… which is precisely what makes Titan so fascinating to planetary scientists and astrobiologists. If life could evolve on a once-inhospitable Earth, is it possible that it could have also evolved on Titan?

Time — and lots more research — will undoubtedly tell, but Titan is definitely on the short list of places scientists are looking for extraterrestrial life.

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Of course, the million-dollar question now is how and why does Titan, a moon slightly larger than our own located hundreds of millions of miles away, even have an atmosphere that’s so similar to one presumably found on early Earth?

The answer may lie in asteroids.

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Much of the water on Earth may have come from ancient asteroid collisions, according to research by Josep M. Trigo-Rodriguez of the Institute of Space Sciences in Barcelona, Spain, and Javier Martin-Torres at the Center for Astrobiology in Madrid, Spain.

Even though Earth and Titan formed in very different regions of the solar system, where different raw materials were available, they ended up with some of the same elements on their surface and in their atmospheres. These were most likely delivered by asteroids, the pair surmise, as well as by comets to a smaller extent.

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During a period known as the Late Heavy Bombardment, between 4.1 and 3.8 billion years ago, water-rich asteroids and comets inundated the inner solar system from the icy outer regions, delivering the water — and thus oxygen — to the then-oxygen-poor planet we now call Earth. Other important elements were deposited as well, like carbon, nitrogen and hydrogen. These and other volatile elements became the foundations of our oceans, our atmosphere, and eventually the building blocks of life itself.

Titan, located in the already ice-rich reaches of the solar system, was likewise struck by water-bearing comets and asteroids. Outgassing and accretion by collision helped develop a similar atmosphere on both worlds, the difference being that while Earth’s atmosphere eventually evolved into the one that oxygen-breathing animals like insects, dinosaurs, birds and eventually humans could breathe (with the help of mild climates and the existence of liquid water) Titan’s chilly temperatures make liquid water impossible, replacing its Earthly role with methane and hydrocarbons.

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Yet at one point the atmospheres of the two worlds were probably very similar, lending scientists to ask the question “if life could develop on one, could it also have developed on the other?”

“We see Titan as a natural oasis of remarkable astrobiological significance to understand the environment in which origin of life took place on Earth,” Trigo-Rodriguez says. “It seems that a plausible scenario to build life consists of a dense atmosphere, where small particles like organic haze and meteoric metals could act as catalysts for the formation of more complex organic compounds from simple precursors such as carbon monoxide and methane, thus promoting increasing complexity.”

Finding the answer to this will require more exploration of Titan’s surface.

“We need a surface exploration with a lander-style mission,” Martin-Torres says. “We’re still missing the most important data.”

Read more in the Astrobiology Magazine article by Shaun McCormack.

Image credits: Top image: Titan as seen by Cassini on October 12, 2010. NASA/JPL/SSI. Raw image color-composited by Jason Major. See more Cassini images on the Imaging Team’s site here. Middle image: Artist’s concept of a hydrocarbon lake on Titan. Credit: NASA. Bottom right: Artist’s concept of an asteroid impact. Credit: NASA.