For the first time, astrophysicists have arrived at concrete proof that the atmosphere surrounding Titan, the largest moon orbiting Saturn, contains a “prebiotic” chemical which is capable of forming stable structures not unlike cell membranes.
“We have made the first unambiguous identification of carbon chain anions in a planet-like atmosphere, which we believe are a vital stepping-stone in the production line of growing bigger, and more complex organic molecules, such as the moon’s large haze particles,” said Ravi Desai, lead author of a new study published in Astrophysical Journal Letters and PhD student at University College London.
The new discovery comes from data provided by the Cassini-Huygens mission, whose space probe has been in orbit around Saturn since 2004. Using results from Cassini’s plasma spectrometer, which measured the energy and electrical charge of particles as the spacecraft flew through Titan’s upper atmosphere (some 1,000 kilometres above the moon’s surface), researchers were able to detect a particular kind of negatively charged linear molecule that can serve as the building blocks for more complex organic molecules, including, scientists speculate, the earliest forms of life on Earth.
“These inspiring results from Cassini show the importance of tracing the journey from small to large chemical species in order to understand how complex organic molecules are produced in an early Earth-like atmosphere,” says Dr Nicolas Altobelli, Cassini project scientist with the European Space Agency. “While we haven’t detected life itself, finding complex organics not just at Titan, but also in comets and throughout the interstellar medium, we are certainly coming close to finding its precursors.”
The second-largest moon in the Solar System after Jupiter’s Ganymede, Titan is the only moon so far identified with a thick atmosphere, composed primarily of nitrogen and methane. Scientists say that the carbon chain anions are the product of energy from the sun interacting with these elements in the atmosphere along with traces of hydrogen and carbon, leading to more complicated “prebiotic” compounds which then drift down from the upper atmosphere and eventually to the moon’s surface.
Although conditions on Titan are much different than those on Earth (Titan’s surface temperature averages minus 179 degrees Celsius), the moon has climate cycles in some ways similar to those on Earth and is the only other space object known to have stable bodies of surface liquid, in the form of methane lakes.
The Cassini space probe is in the last stages of its mission around Saturn, making its final passes between Saturn and its rings before it runs out of fuel. Cassini used its last visit to Titan, which occurred on April 22, to provide the gravity-assisted momentum needed to slingshot itself into orbit near the innermost rings of Saturn’s atmosphere, from where scientists hope to gain more information about the rings’ age and composition.
“[Cassini’s] grand finale will be spectacular,” said project scientist Linda Spilker, with NASA’s Jet Propulsion Laboratory in Pasadena, California, to Reuters. “We’re flying in a region that has never been explored before,” she said. “I wouldn’t be a bit surprised if some of the discoveries we make with Cassini during the grand finale are the best of the mission.”