Titan: Biological Birthplace?
Titan is a Mercury-sized world comprised of a 50-50 mix of ices and rock. The chemical composition of its environment resembles that of early Earth but it is far colder and lacks liquid water. Scientists think Titan may have carbon- and nitrogen-containing molecules accumulated on its surface. And these primitive precursors to life might be brought even further towards life's door if liquid water makes an occasional appearance which Lunine believes it may well do.
Given the mix of organic materials known to be available in the moon's atmosphere, and conditions believed to exist on the surface, Titan could turn out to be a "good reaction crucible" for birthing biomolecules, says Lunine, a professor at the University of Arizona in Tucson and chair of the university's theoretical astrophysics program. If a transient heat source caused liquid water to become available, the otherwise frozen giant would provide an extensive surface area and long reaction times for these molecules to form. When the water refroze, it would provide excellent preservation for these starting materials for life.
Lunine describes a two-stage process by which these molecules could form. First, solar ultraviolet radiation, along with cosmic rays and electrons from Saturn's magnetosphere, split the nitrogen and methane molecules in Titan's atmosphere into free radicals, which combine to yield simple hydrocarbons: acetylene, hydrogen cyanide, and polymers of more-complex hydrocarbons. These primitive molecules then drift down to the moon's surface.
If liquid water were present on the surface, it would donate the oxygen needed to convert these simple organic precursors into the more complex building blocks of life as we know it. That's step two. But Titan is far too cold for liquid water, registering a minus 178 degrees Celsius (minus 288 degrees Fahrenheit) in the atmosphere near its surface so cold that water ice there is as hard as granite.
If, however, transient events, such as impacts or volcanic activity, melted some of Titan's ice and generated enough heat to fuel reactions, the chains of hydrocarbons could combine with oxygen at Titan's surface to form complex organic molecules.
The tremendous heat from an impact, for example, would melt the crust, which Lunine says is mostly water ice. If a thin layer then refroze, and if ammonia also were present, acting as antifreeze, liquid water could remain pooled at the surface for hundreds, perhaps thousands, of years.
Cryovolcanism is another possible source of melting ice. On icy moons like Titan, the magma for volcanic eruptions is water, pushed up from below by the moon's internal heat. This heat in part is left over from the process that formed Titan and in part derives from the decay of the radioactive isotopes that undoubtedly exist in the cores of moons the size of Titan. Driven by these sources of heat, eruptions of melted water, "additives" like ammonia, which has a low melting point, are possible.
When this transient heat eventually dissipated, any complex organic molecules that had been produced would likely be preserved, deep-frozen as the water once again solidified.
Titan's thick atmosphere as well as its frigidity should keep safe any organics preserved in this way, Lunine says. Any ultraviolet and cosmic rays that reached Titan's atmosphere would "tend to break up polymers, and form radicals not good for building up biopolymers." But, he points out, "Titan's surface is shielded from most UV and cosmic rays, so stuff polymerizing there doesn't get destroyed."
Richard Gammon, a University of Washington chemist, agrees that Titan presents a likely reaction vessel for crafting what might resemble "freeze-dried precursors" of life on Earth. Gammon, professor of chemistry, oceanography, and atmospheric sciences at the University of Washington, says that the recent presentations by Lunine and his JPL-based coinvestigators at two recent scientific conferences (Goldschmidt / Washington)have "put Titan back on the front burner."
"The kinds of biomolecules that might form in this beaker" of Titan's environment remind Gammon of "the chemistry that must take place in interstellar space." Moreover, he says, Titan's atmospheric reactions could lead to purines and pyrimidines, the key molecules of the DNA bases. Long chain carboxylic acids, the primitive precursors to sugars and fats, might also be formed. Such reactions are thought to have occurred on prebiotic Earth, but are no longer take place here to any significant degree due to the strongly oxidizing atmosphere on our planet.
Whether or not the process theorized by Lunine actually occurs on Titan remains unknown. Surface features of the distant moon revealed so far by Earth-bound observatories and by the Hubble space telescope are limited to a resolution of a few hundred kilometers, Lunine says. Such resolution, applied to Earth, would register California or Texas as just a few tiny spots. The Cassini-Huygens mission "will be our first chance to really understand, on a global scale, what Titan is all about," says Lunine.
Giant Titan, with a diameter about two-fifths that of Earth, ranks second largest of all the solar system's moons. Only Jupiter's moon Ganymede is larger. The atmospheric pressure near its surface is 60 percent greater than on Earth at sea level. The dense Titan atmosphere is ten times as thick as Earth's gaseous covering. Unlike Earth's atmosphere, however, Titan's bears mostly nitrogen, with methane and other trace hydrocarbons a mix that resembles the smog over Earth's most polluted cities.
Titan's haze was breached by the Hubble telescope, scanning in the near infrared wavelengths, in 1994. Surface features included a bright spot the size of Australia. Early in 2005, the ESA's Huygens probe will descend through Titan's atmosphere, settling on the moon's surface. It will provide researchers with chemical analysis, spectra and images.
The Huygens probe is geared primarily towards sampling the atmosphere. "Anything else will be gravy," Lunine says. 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.
Still, the Cassini-Huygens mission should provide a wealth of data to guide a future, more mobile probe to a likely site on Titan for mining samples of biomolecules. "The strategy would be to look for places, like impact craters," Lunine says, that are "good places to look for evolved organic chemistry."
Lunine and his team will focus in the coming year on ways to detect chirality on future missions to Titan. Chirality, or "handedness," is a hallmark of terrestrial biomolecules: It's what makes living chemistry work, from assembling DNA to digesting food.
A person's two hands are chiral: Press your palms together, your hands superimpose; but place one hand palm down on the back of another, and the fingers don't match, the thumbs stick out. All of terrestrial life's critical biomolecules are also chiral, either left- or right-handed, in their atomic arrangements. Finding chiral molecules on Titan, with an excess of one arrangement over another, would suggest that organic chemistry there is evolving towards biochemistry.