The atmosphere of Saturn's largest moon, Titan is often viewed as an analog to what the Earth's atmosphere may have been like billions of years ago. Despite the 800 million miles between the two worlds, both may have had their atmospheres created through the gravitational layering and processing of asteroids and comets.
"We see Titan as a natural oasis of remarkable astrobiological significance to understand the environment in which origin of life took place on Earth," said Josep M. Trigo-Rodriguez, of the Institute of Space Sciences (CSIC-IEEC) in Barcelona, Spain.. "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."
"Titan provides an extraordinary environment to better understand some of the chemical processes that led to the appearance of life on Earth,” added Trigo-Rodriguez. “Titan’s atmosphere is a natural laboratory that, in many aspects, seems to have a strong similitude with our current picture of the pre-biotic atmosphere of Earth."
"This is remarkable, because it was thought that Earth and Titan were made from a vastly different recipe of materials in drastically different temperatures," he says.
The image above shows a portion of Titan's low latitudes taken by the Cassini probe on October 12, 2009. The view was obtained at a distance of about 296,000 kilometers (184,000 miles). The bright patches are more reflective than the dark areas and are probably made of icy materials such as water ice or frozen carbon dioxide. The dark areas likely host dune fields which are common in the low latitudes of the moon where prevailing winds seem relatively strong. A dark circular ring in the upper boundaries of a bright feature toward the center of the portion is likely the outcome of a meteoritic impact. This circular patch may also be a caldera or a cryovolcanic feature.
The research paper, "Clues on the importance of comets in the origin and evolution of the atmospheres of Titan," by Trigo-Rodriguez and F. Javier Martin-Torres (Center for Astrobiology, Madrid, Spain), offers insight into the atmospheric affinities of Earth and Titan.
Earth presumably formed from scorched, oxygen-poor rocks (planetesimals) located in the inner solar system, while Titan formed from rocks that were rich in oxygen and other volatile chemicals (cometesimals) in the outer solar system. Trigo-Rodriguez and Martin-Torres believe the vital organic ingredients in the early Earth's atmosphere were vaporized and swept away by solar winds.
The ingredients for the air we breathe today returned about 4 billion years ago, during a cataclysmic rock storm known as the Late Heavy Bombardment (LHB). During this period, oxygen- and volatile-rich materials from the outer solar system were hurled en masse towards the inner solar system.
Chris McKay, a planetary scientist at NASA's Ames Research Center, says comets may have made small contributions to the water, carbon dioxide, and nitrogen content of the Earth's early atmosphere, "but they were not the main source." This is known because the Deuterium/Hydrogen ratios of our oceans do not match the ratios found in comets. He says asteroids hurled our way during the LHB could be the main source of water on Earth.
Trigo-Rodriguez says he and McKay are basically on the same page. "We think that asteroids and comets were key sources for water and organics,” says Trigo-Rodriguez. Four billion years ago, some asteroids contained so much ice that they would have brought just as much water to our planet as comets did.
Trigo-Rodriguez and Martin Torres studied how hydrogen, carbon, nitrogen and oxygen isotopes reacted with their environments on Earth and Titan. They looked at data recorded by the Cassini-Huygens probe to better understand the isotopic ratios in Titan's dense, hazy atmosphere.
Different distances from the Sun, different sizes and different environmental conditions led to different chemical evolutions on the two worlds. Even so, both Earth and Titan were hit by similar water-rich bodies, which provided a volatile-rich source for both atmospheres during the late-heavy bombardment.
Outgassing and collisional processing on both worlds led to the production of molecular nitrogen-dominated atmospheres with similar isotopic ratios of hydrogen, carbon, nitrogen and oxygen.
Life’s Origin and Other Questions
Scientists are still wondering how Titan is able to maintain all of its atmospheric methane.
According to McKay, "Earth's atmosphere is composed of compounds that persist over billions of years. However on Titan, all of the methane should have been destroyed by sunlight on a timescale of about 30 million years. There must be a source of methane re-supply."
The methane in the atmosphere may come mainly from Titan’s lakes of liquid hydrocarbon. But to really understand what is re-supplying the methane, Martin-Torres would like to see another probe sent to the moon’s surface. (The Cassini mission sent the Huygens probe to Titan in January 2005, but the probe had limited instruments and could only transmit data from the surface for 90 minutes before the battery power ran out).
"We need a surface exploration with a lander-style mission,” says Martin-Torres. “We're still missing the most important data.” A ground probe could examine the composition of Titan's surface, the nature of its low-temperature chemistry, and search for signs of life.
Titan has many of the components for life without liquid water. But the orange hydrocarbon haze that shrouds the planet's largest moon could be creating the molecules that make up DNA without the help of water – an ingredient widely thought to be necessary for the molecules' formation according to a recent study.
As Paul Davies, a leading authority in astrobiology, director of BEYOND: Center for Fundamental Concepts in Science and co-director of the ASU Cosmology Initiative, says: "To the best of our knowledge, the original chemicals chosen by known life on Earth do not constitute a unique set; other choices could have been made, and maybe were made if life started elsewhere many times."
The researchers warn however that although Titan's atmosphere is creating these molecules, that doesn't mean that the molecules are combining to form life, But the finding could entice astrobiologists to consider a wider range of extrasolar planets as potential hosts for at least simple forms of organic life, the team of scientists from the US and France suggests.
The findings also suggest that billions of years ago Earth's upper atmosphere – not just the so-called primordial soup on the surface – may have been the sources for these "prebiotic" molecules, amino acids and the so-called nucleotide bases that make up DNA.
"We're really starting to get a sense for what kind of chemistry an atmosphere is capable of" performing, says Sarah Hörst, a graduate student in planetary science at the University of Arizona, who led the research effort.
NASA's Cassini spacecraft, which has detected large molecules at altitudes of some 600 miles above Titan's surface. But the molecules are so far unidentified because of limitations to the craft's instruments. The Cassini research team replicated Titan's atmosphere in a large chamber at the temperatures present in the moon's upper atmosphere. To play the role of the sun's ultraviolet light hitting Titan's atmosphere, they used radio energy at a power level comparable to a modestly bright light bulb. The UV light is critical because it breaks up molecules such as molecular nitrogen or carbon monoxide in Titan's atmosphere, leaving the individual atoms to choose up different partners, forming new molecules.
The experiment yielded tiny aerosol particles. The team ran the particles through a sensitive mass spectrometer, which showed the chemical formulas for the molecules that made up the aerosols.Hörst then ran the formulas past a roster of molecules biologically important for life on Earth. She got 18 hits, including the four nucleotides whose combinations form an organism's genetic information encoded in DNA. It appears to be less important that water is present to form these molecules than it is for some form of oxygen to be present in the mix of ingredients, she concluded.
On Earth, oxygen early in the planet's pre-life history would come in the form of carbon dioxide and carbon monoxide from volcanic activity, as well as from water released by volcanism and through meteor and comet impacts. On Titan, the oxygen appears to be coming from Enceladus, an ice-bound moon of Saturn in its own right because of icy geysers spewing into space from near its south pole. Some researchers think the geysers hint at a possible global subsurface sea and a potential habitat for life.
Last year, researchers showed how water molecules ejected as part of Enceladus's geysers can be carried great distances through the Saturn system, with some oxygen-bearing molecules finding their way to Titan.