Titanic Moon: Prebiotic Chemistry from Saturnian Turn
Titan is Saturn's biggest moon. A little larger than the planet Mercury, it is also one of the biggest moons in the solar system (only Jupiter's moon Ganymede is larger). The astronomer Carl Sagan described Titan as, "one of the most fascinating and instructive worlds illustrating prebiological organic chemistry." Sagan stated that, on Titan, we could see the synthesis of complex organic molecules happening right before our eyes.
|Hubble Space Telescope 1994
discovery of Titan's "continent"
photo credit: Hubble Space Telescope Institute
That's because the orange haze that completely surrounds Titan is an organic chemistry experiment in action. Cosmic rays, ultraviolet light from the sun, and charged particles trapped by Saturn's magnetic field all initiate chemical reactions in Titan's atmosphere, causing chains of hydrocarbons to form. Titan's atmosphere very much resembles the oxygen-free, primordial atmosphere of early Earth. Titan's atmosphere is about "95% nitrogen, 5% methane with traces of other hydrocarbons, hydrogen cyanide (HCN) and other cyano- compounds," said Dr. Robert Minard, Senior Lecturer in Chemistry at Penn State University.
Minard, and his collaborators with the Astrobiology Institute and Penn State, have recently developed a new diagnostic method that has improved our understanding of Titan's atmospheric chemistry.
To probe for carbon and hydrogen in more complex hydrocarbons on Titan, the technique employed a combined thermal and chemical analysis, called GC-MS. The term GC-MS refers to gas chromatography (GC) - to separate various components like ammonia or cyanide - and mass spectroscopy (MS) - to understand their various identifying mass and molecular weights. GC-MS is a powerful analytical tool to probe gaseous elements and ions. It renders a kind of chemical fingerprint of what conditions might be like in the atmosphere. It has allowed the researchers to demonstrate that Titan's atmosphere contains elements that are similar to the degradation products from a polymer of hydrogen cyanide (HCN).
|Computer image of HCN: a building block of early biomolecules. The results of this work by Robert Minard at Penn State University are to appear in the journal Geochemistry, and the particular advance in testing for biomolecules relied on a modified procedure for thermochemolysis which degrades and derivatizes (high molecular weight) macromolecular materials before analysis by GC-MS.
HCN polymer is a black solid believed to have played a major role in the emergence of life on the earth 4 billion years ago. HCN plays a crucial role in primordial organic synthesis, for it is a building block of such basic elements of life as amino acids and purines (parts of nucleic acids like DNA; specifically, adenine and guanine).
This analytical method provides strong evidence that HCN chemistry is an important aspect of Titan's atmosphere, and that HCN polymer is a major component of the orange colored haze.
In an experiment designed to simulate the chemistry going on in Titan's atmosphere, degradation/GC-MS analysis was also carried out on HCN polymer and on a reddish brown polymer created by heating an 80:20 mixture of nitrogen and methane. "The similarity of the products from both materials demonstrated that Titan's brown haze is probably a hydrocarbon/HCN polymer composite," said Dr. Minard.
Titan's atmosphere has intrigued astronomers for years. When Voyager I passed by Titan in November 1980, scientists were hoping to get a close look at what they believed to be a complex and intriguing world. What they saw instead was a moon completely enveloped in an orange-colored smog thick enough to prevent them from seeing the moon's surface. Titan's atmosphere is five times as dense as the Earth's.
Only recently have we been able to peer through the orange mist and see the surface below. In October 1994, astronomers used the Hubble Space Telescope to observe Titan in the near infrared wavelengths. Looking at Titan in that part of the electromagnetic spectrum allowed the astronomers to observe a bright, Australian-sized area on Titan's surface. One theory about this bright region is that it is a mountainous continent entirely composed of water ice.
"We don't know what this region is," said Ralph Lorenz, who is now doing further research into Titan at the University of Arizona, "but one explanation is that rain falling on high ground is washing the area clean of dark sludge."
|Picture of Titan: The lighter region in the lower right illustrates the bright region that, according to one theory, is speculated to consist of a water-ice continent the size of Australia. Source: NASA/ JPL
The existence of life on Titan is not likely, even though almost all the elements necessary for life - energy, hydrocarbons, an atmosphere and water - are present there. Because of its distance from the Sun, Titan receives only one percent of the Earth's sunlight. This results in a surface temperature of about 90 degrees Kelvin (minus 180 degrees Celsius or minus 297 degrees Fahrenheit). At that frigid temperature, the water is frozen as hard as granite. Without liquid water, the hydrocarbons in Titan's atmosphere cannot develop into amino acids.
But even without substantial sunlight, Titan may have had some liquid water during its early history, when meteor impacts were more frequent. In 1992, Carl Sagan and his colleague Reid Thompson at Cornell University, New York, suggested that meteor impacts could melt Titan's icy crust and allow organic molecules from the atmosphere to react with oxygen in liquid water. So even if Saturn's moon lacked sunlight, Titan could still receive energy inputs from incoming meteor strikes.
Sagan and Thompson calculated that organics in the crust around impact sites might have been exposed to oxygen for anything up to 2 million years; enough time perhaps for simple amino acids to form. The kinetic energy from these impacts would have generated enough heat to melt frozen water. Even when the surrounding icy water in those meteor-rich areas solidified, deposits of liquid water may have persisted for thousands of years on Titan before finally freezing. This Sagan-Thompson scenario speculates an alternative possibility of amino acids forming without significant solar energy, but with hot materials bombarding Titan. In the rare chance, prebiotic chemistry takes hold when water, heat and organics get sufficient time to brew their primordial soup.
|Artist Conception of the 2004 Probe's Descent: JPL
In October 1997, the Cassini spacecraft was launched for a rendezvous with Saturn in June 2004. Cassini will have more than 30 encounters with Titan, enabling it to map the moon's surface. In November 2004, Cassini will release the European-built Huygens probe for a descent through Titan's atmosphere. The probe will land on the western edge of the bright, Australian-sized feature discovered in 1994.
Related Web Pages
Solar System Exploration, JPL NASA
The role of HCN and its derivatives in prebiotic evolution
Online articles in astrobiology