In an experiment exploring the chemical processes that might be going on in the hazy atmosphere enshrouding Saturn's largest moon, a University of Arizona-led team of scientists discovered a variety of complex organic molecules – including amino acids and nucleotide bases, the most important ingredients of life on Earth.

"Our team is the first to be able to do this in an atmosphere without liquid water. Our results show that it is possible to make very complex molecules in the outer parts of an atmosphere," said Sarah Hörst, a graduate student in the UA's Lunar and Planetary Lab, who led the international research effort together with her adviser, planetary science professor Roger Yelle.

The molecules discovered include the five nucleotide bases used by life on Earth to build the genetic materials DNA and RNA: cytosine, adenine, thymine, guanine and uracil, and the two smallest amino acids, glycine and alanine. Amino acids are the building blocks of proteins.

The results suggest not only that Titan's atmosphere could be a reservoir of prebiotic molecules that serve as the springboard to life, but they offer a new perspective on the emergence of terrestrial life as well: Instead of coalescing in a primordial soup, the first ingredients of life on our planet may have rained down from a primordial haze high in the atmosphere.

Oddball of the solar system
Titan has fascinated – and puzzled – scientists for a long time.

“It’s is the only moon in our solar system that has a substantial atmosphere,” Hörst said. “Its atmosphere stretches out much further into space than Earth’s. The moon is smaller so it has less gravity pulling it back down.”

Titan’s atmosphere is much denser, too: On the surface, atmospheric pressure equals that at the bottom of a 15-foot-deep pool on Earth.

“At the same time, Titan’s atmosphere is more similar to ours than any other atmosphere in the solar system,” Hörst said. “In fact, Titan has been called ‘Earth frozen in time’ because some believe this is what Earth could have looked like early in time.”

Scientists know that Titan’s haze consists of aerosols, just like the smog that cloaks many metropolitan areas on Earth. Aerosols, tiny particles less than a millionth of a mm, resemble little snowballs when viewed with a high-powered electron microscope.

The exact nature of Titan’s aerosols remains a mystery. What makes them so interesting to planetary scientists is that they consist of organic molecules – potential ingredients for life.

“We want to know what kinds of chemistry can happen in the atmosphere and how far it can go.” Hörst said. “Are we talking small molecules that can go on to becoming more interesting things? Could proteins form in that atmosphere?”

What it takes to make life’s molecules
For that to happen, though, energy is needed to break apart the simple atmospheric molecules – nitrogen, methane and carbon monoxide – and rearrange the fragments into more complex compounds such as prebiotic molecules.

“There is no way this could happen on Titan’s surface,” Hörst said. “The haze is so thick that the moon is shrouded in a perpetual dusky twilight. Plus, at -192 degrees Fahrenheit, the water ice that we think covers the moon’s surface is as hard as granite.”

However, the atmosphere’s upper reaches are exposed to a constant bombardment of ultraviolet radiation and charged particles coming from the sun and deflected by Saturn’s magnetic field, which could spark the necessary chemical reactions.

To study Titan’s atmosphere, scientists have to rely on data collected by the spacecraft Cassini, which has been exploring the Saturn system since 2004 and flies by Titan every few weeks on average. During fly-by maneuvers, Cassini has gobbled up some of the molecules in the outermost stretches of Titan’s atmosphere and analyzed them with its on-board mass spectrometer. Unfortunately, the instrument was not designed to unravel the identity of larger molecules – precisely the kind that were found floating in great numbers in Titan’s mysterious haze.

“Fundamentally, we cannot reproduce Titan’s atmosphere in the lab, but our hope was that by doing these simulations, we can start to understand the chemistry that leads to aerosol formation," Hörst said.

Titan: A window into Earth’s past?
In some way, Hörst said, the discovery of Earth’s life molecules in an alien atmosphere experiment is ironic.

Here is why: The chemistry occurring on Titan might be similar to that occurring on the young Earth that produced biological material and eventually led to the evolution of life. These processes no longer occur in the Earth’s atmosphere because of the large abundance of oxygen cutting short the chemical cycles before large molecules have a chance to form. On the other hand, some oxygen is needed to create biological molecules. Titan’s atmosphere appears to provide just enough oxygen to supply the raw material for biological molecules, but not enough to quench their formation.

“There are a lot of reasons why life on Titan would probably be based on completely different chemistry than life on Earth,” Hörst added, “one of them being that there is liquid water on Earth. The interesting part for us is that we now know you can make pretty much anything you want in an atmosphere. Who knows this kind of chemistry isn’t happening on planets outside our solar system?”

Source: University of Arizona