Smog Warning, Titan

Dr. Emma Bakes is a SETI Institute Principal Investigator who simulates complex chemical interactions in the silicon "laboratory" of her computer. Bakes focuses on Titan, and her work comprises one facet of the SETI Institute’s NAI lead team’s research on the co-evolution of life and its planetary environment.

Titan is Saturn’s largest moon and it is unique within our Solar System, being the only satellite that possesses an atmosphere.

Its atmosphere is smoggy, composed of a hydrocarbon haze, and it is this component that dominates its physical evolution, determining whether the moon ultimately boils or freezes, produces life or remains barren. The haze can also accelerate the formation of an oxygenated atmosphere by catalyzing the removal of hydrogen atoms from a planet’s atmosphere and by providing an excellent absorber of harmful UV radiation, protecting fragile macromolecules floating in the haze.

Scientists would like to know the origin of the atmospheric patches imaged on Saturn’s moon, Titan, as imaged by Hubble. Image Credit: Hubble Space Telescope/UA Smith

The most interesting point about simulations of Titan’s hydrocarbon haze is that this smoggy component contains molecules called tholins (from the Greek word, muddy) that can form the foundations of the building blocks of life. For example, amino acids, one of the building blocks of terrestrial life, form when these red-brown smog-like particles are placed in water.

When scientists analyze the building blocks of tholins by pyrrolysis, splitting up the tholins using plasma, scientists find a rich array of biomolecular building blocks such as pyrroles, pyrazines, pyridines and pyrimidines. All of these molecules have played an important role in the evolution of life.

Most coenzymes (molecules that are indispensable in powering biological metabolism) and vitamins contain a central heterocyclic ring (a ring composed of several hexagons or pentagons of carbon atoms), which is necessary to fulfill the biological function of the coenzyme. This observation has spawned the idea that coenzymes were formed early in the history of life and may have been present during the period of terrestrial evolution where life was about to begin.

Could they have originated as a product of atmospheric gas phase chemistry in the reducing, nitrogenated atmosphere of early Earth?

This is a question that Carl Sagan, Bishun Khare and many of their colleagues asked themselves two decades ago. As Sagan pointed out, Titan may be regarded as a broad parallel to the early terrestrial atmosphere with respect to its chemistry and in this way, it is certainly relevant to the origins of life.

Extreme Explorers' Hall of Fame
Huygens parachutes onto Titan. ESA’s Huygens probe descends through Titan’s mysterious atmosphere to unveil the hidden surface (artist’s impression) Credit: ESA

The biological function of pyrroles is extremely varied, and ranges from pheromones (chemical signal molecules involved in mating and defense) to plant hormones and even antibiotics. Without the hormones that help plants grow and develop, there would be no multicellular animal life on Earth due to a lack of oxygen renewal during photosynthesis.

One important class of multi-ring pyrrole derived products even bind to part of the deoxyribonucleic acid (DNA) helix, forming an integral part of its structure, certainly of fundamental biological relevance.

In another example, consider pyrimidines.

Three different components – a sugar, a phosphate, and either a purine or a pyrimidine organic base – make up nucleic acids. These three building blocks form a nucleotide, the basic building block of nucleic acids. Ribonucleic acid (RNA) contains a ribose sugar and the purines are adenine and guanine, while the pyrimidines are cytosine and uracil. The production of these building blocks is the first step towards the "RNA world".

The RNA world is important because it preceded the formation of DNA and ultimately, of all life. These life-forming structures are punctuated by nitrogen containing heterocyclics related to the types formed as the building blocks of aerosols in reducing atmospheres.

Did the early terrestrial atmospheric photochemistry yield building blocks that helped spawn the beginnings of the RNA world? This is certainly a tantalizing concept.

As a third example, consider pyridines, water-soluble enzymes known to bind metals and accelerate chemical reactions. Without these enzymes, life on earth simply could not exist because the chemical reactions required for its upkeep would proceed too slowly. Two pyridines fundamental to metabolism include Nicotinamide Adenine Dinucleotide (NAD) and Nicotinamide Adenine Dinucleotide Phosphate (NADP). These compounds are not tightly bound to the enzymes they operate and are termed co-factors as a result. While it would be extremely difficult to form these compounds during purely gas phase atmospheric chemistry, the nitrogenated heterocycles that form a fundamental part of their structure may be synthesized this way.

Building on the work of Sagan, Khare and their colleagues, we will investigate the importance of a hydrocarbon haze in accelerating the conditions necessary for life and the large molecules involved in its metabolism. A better idea of how life formed on Earth and how the transition was made from chemistry to biochemistry can be obtained by compressing thousands to millions of years of chemistry into one run of a computer model.

What’s Next

Scientists would like a better idea of how optically thick Titan‘s haze is, and how bright or dark its surface will be, to calculate camera exposure times. In addition, scientists are fine tuning their questions as they plan the Cassini observations.

Next summer, NASA’s Cassini spacecraft, launched in 1997, is scheduled to go into orbit around Saturn and its moons for four years. The piggybacking Huygens probe is scheduled to plunge into the hazy Titan atmosphere and land on the moon’s surface. The Huygens probe is geared primarily towards sampling the atmosphere. The probe is equipped to take measurements and record images for up to a half an hour on the surface. But the probe has no legs, so when it sets down on Titan’s surface its orientation will be random. And its landing may not be by a site bearing organics.

Related Web Pages

SETI Institute
When Eden Comes to Titan
Chronology of a Scientific Safari
Long, Strange Trips
Titan’s Icy Bedrock
Saturn– JPL Cassini Main Page
Alien Landers: Extreme Explorers Hall of Fame
Titan: Biological Birthplace?
Solar System Bodies: Titan (NASA JPL)
The Probe Mission (NASA JPL)
Why Titan? (ESA)