Scientists who study space often seek to find the numerous similarities and differences between the Earth and other planets. While astronomers have been studying hundreds of exoplanets with varying degrees of similarity to ours, there is still a lot to be discovered in our own solar system. Just recently, NASA’s Cassini spacecraft detected propylene molecules in the lower atmosphere of Saturn’s largest moon, Titan. This is the first definitive discovery of the organic compound on a planet other than Earth.
Propylene (also known as propene) is a 3-carbon unsaturated compound with one double-bond, with the formula C3H6. Chains of propylene form the polymer polypropylene, the plastic used for reusable containers, textiles, packaging, and the new Canadian banknotes. The finding not only contributes to the human understanding of the chemical make-up of Titan, but also sheds light on the chemistry of the Earth in its early days.
Titan is a rocky and icy moon that contains large quantities of frozen water. It is larger in size than the planet Mercury. Titan is the only natural planetary satellite known to have a dense atmosphere, consisting primarily of nitrogen and hydrocarbons such as methane and ethane. Its ‘methane cycle’ forms rain, lakes, and rivers of liquid methane comparable to Earth’s water cycle. Before the discovery of propylene, other 3-carbon molecules such as the heavier, saturated propane and the lighter propyne had already been found in its atmosphere.
The Cassini-Huygens mission was launched from Earth over 16 years ago, and has since been collecting data from Saturn’s system for about a decade. It consists of the Cassini orbiter and the separable Huygens probe, named for the Dutch astronomer who discovered Titan, Christiaan Huygens. The probe landed on the surface of Titan in 2005, and is the first landing of human technology in the outer solar system, and most distant landing ever. The mission serves to characterize the behaviour, composition, origin, and geological history of the components of Saturn’s system, such as its moons, rings, and the sixth planet itself. The trace amounts of propylene were detected, in parts per billion, by Cassini’s Infrared Spectrometer (CIRS), designed to measure and analyze the composition of Titan’s atmosphere.
The discovery of propylene on Titan adds to the knowledge that humans have spent millennia gathering in the attempt to understand the world, our origins, and the interconnectedness of the universe. It also serves to show that there is still much that can be discovered, both within our own solar system, and in the entirety of the known universe.
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